9.1 Hazards resulting from tectonic processes Flashcards
Earthquake definition
The shaking of the ground as a result of a sudden release of energy caused by the sudden displacement or movement in the Earth’s crust along a fault
Fault definition
A zone of pre-existing weakness in the Earth’s crust
Why do earthquakes occur? (4)
- Individual plates of varying size move about the surface of the Earth at varying speeds (1-8cm per/year)
- Movement of plates creates pressure and friction caused the plates to get stuck which causes pressure to build up (increases stress of rocks, so they deform)
- When the pressure exceeds the strength of the fault, the rock fractures
- This produces a sudden release of energy, creating seismic waves that radiate away from the point of the fracture
Other causes of earthquakes (3)
- Human activity (coal mining, constructing reservoirs)
- Volcanic activity
- Landslides
Depth of focus classifications (3)
Shallow: 0-70km
Medium: 70-300km
Deep: 300-700km
Rifting definition
Stretching and thinning of the earth’s crust
Nature of earthquakes at divergent plate boundary (4)
Frequent, shallow, weak with relatively low magnitude
Generally restricted to the shallow, brittle parts of the crust up to 20km deep
Narrow belt
Don’t pose large risk to humans + do not trigger tsunamis - most are under sea
Nature of earthquakes at destructive plate boundaries
Frequent, deepest (up to 700km but greatest range of depths) and most powerful earthquakes (9 out of 10 last 100 years located here)
Hazard risk of earthquakes at destructive plate boundaries (3)
- often at coastlines
- more people + (fertile soils + volcanoes + unconsolidated sediments)
- secondary hazards due to fold mountains created
Nature of earthquakes at collision plate boundaries (3)
- deep, powerful earthquakes
- can have shallow focus too
- broader belt of earthquakes
Nature of earthquakes at conservative plate boundaries
High hazard risk
Extensive shallow focus and powerful earthquakes of considerable magnitude
Relatively narrow band
Epicentre
Marks the point on the surface of the Earth immediately above the focus of the earthquake
Fault
A fracture in the rocks that make up the Earth’s crust
Seismic waves
Waves that transmit the energy released by an earthquake
Percentage of earthquake energy released in the first 10 seconds
95%
Types of seismic waves (2)
Body waves
Surface waves
Types of body waves (2)
Primary waves
Secondary waves
Types of surface waves (2)
Love waves
Rayleigh waves
Primary waves
Longitudinal waves that travel at 8km/sec
Push + pull
Secondary waves
Transverse waves that travel slower than primary (4km/sec)
Cannot travel through liquids
Perpendicular to the direction of wave travel
Rayleigh waves
Larger, rolling, twisting
More dangerous to buildings (aseismic design)
Love waves
Side to side motion
Primary hazards definition earthquakes
Happen immediately and are caused by energy released by the earthquake
Secondary hazards definition earthquakes
Caused as a consequence of ground shaking
Number of noticeable earthquakes per year
49,000
Number of serious earthquakes per year
Around 18
Ground shaking
Following the sudden release of energy of the earthquake, the brittle crust then rebounds either side of the fracture
Example of aftershock being more powerful
Christchurch 2011 vs 2010
Surface faulting
The differential movement of the two sides of a fracture at the Earth’s surface
Ground shaking hazardous nature
Mostly related to buildings - can fall and injure/kill people
Surface faulting hazardous nature (3)
- can fracture gas pipes + damage communication lines
- can cause fires of disrupt emergence response
- common in shallow earthquakes
Soil liquefaction
Due to shaking, water-saturated sediment temporarily loses cohesion, increases in pore-water pressure, collapses and loses load-bearing strength and behave like a liquid
Soil liquefaction hazardous nature
Can cause buildings to tilt or sink into the ground (e.g. up to 60 degrees in Japan)
Loss of soil from hill slopes or the collapse of earth walls that support dams
Damage to infrastructure (e.g. power lines etc)
Primary hazards of earthquakes
Ground shaking + surface faulting
Secondary hazards of earthquakes
Soil liquefaction, mass movements, tsunamis
Evaluation of earthquake hazards (5)
Time
SPEEC
Aerial extent
Management
Speed of onset
Percentage of fatalities in earthquakes caused by building collapse
75%
Length of time that ground shaking lasts
90-120 seconds
How many people have earthquakes killed since 1900
2 million people worldwide
Examples of soil liquefaction
New Zealand Christchurch ‘10 earthquake (mostly economic damage - $28 billion + 60,000 residential buildings)
Mexico city ‘85 - amplified damage (liquefaction occurred 350km away from epicentre)
Soil liquefaction evaluation
Can be dealt with with water abstraction
Not that deadly
Requires quite specific circumstances to occur - depends on local geology + height of water table + ground shaking
Primary hazards earthquake eval
Can be significantly mitigated with aseismic design (dependant on wealth + governance)
Lasts a short amount of time
Dependant on characteristics of earthquake
Depends on geography of land + type of waves
Tsunami definition
When the vertical displacement of the sea-bed causes a series of large waves
What the magnitude of tsunamis depends on
Volume of water displaced
Reason for intra-plate seismic events
When stresses build up in the plate caused by hot spot activity, faults or human activity
Forecast definition
A relatively imprecise statement of time, place and nature of the expected event
Prediction definition
A relatively precise statement of time, place and ideally the nature and size of the event
Forecast earthquakes
Known that location of earthquake is closely linked with distribution of fault lines, therefore location of risk areas is mostly known
Foreshock definition
Small tremors before large earthquake
Prediction methods for earthquakes
Seismometers pick up vibrations in crust
Magnetometers measure variations in earth’s magnetic field
Radon gas emission
Changing water level in wells - well sensor
Horizontal movements using a laser beam
Tiltmeters measure changes in slope + ground deformation
Ground surface changes - satellites + strainmeters
Modified Mercalli Scale
- A measure of intensity - the effects of earthquake shaking on the surface, buildings and humans (intensity scale )
- Descriptive scale; qualitative and subjective
- Useful locally eg ordinary eyewitness – used sometimes to inform hazard mapping
- Ranges from I to 11
Moment magnitude scale
- Calibrated measure of the amount of energy released
- Adapted from the original Richter Scale (1930s California) – similar but more accurate for larger magnitude earthquakes
- Determined by shear strength of displaced rocks x surface area of rupture x slip distance of fault
- Logarithmic scale; no upper limit – a unit increase involves a 10-fold increase in ground motion and a 32-fold increase in amount of energy released.
Hazard mapping function
Inform people that they are at risk so they perceive the risk and escape an at-risk area as soon as possible
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Tsunami overview
· Caused by vertical displacement of the sea-bed which causes a series of waves also known as a wave train
· Tsunami magnitude depends on volume of water displaced
· Not noticed out at sea due to small wave heights and long wavelengths
· When reach coast wave is slowed due to friction with sea-bed and wave gets shorter but rises in height– height can exceed 30m and can flood areas thousands of km away from the focus
· First tsunami wave is not necessarily the largest so there is an escalation in terms of impact – waves may be over an hour apart so people may return to homes thinking waves have stopped coming – increases risk.
· Where focus near coast => little warning e.g. Aceh /Sumatra Indonesia boxing day 2004
· Only when tsunami waves approach the coast do they become deadly – wave height rises from a few cm to many metres
· Can get ‘drawback’ if trough of wave reaches coast before crest (minutes before first wave crest arrives)
Impact depends on a number of physical and human factors
· Duration of event
· Wave amplitude, how much water is displaced and distance travelled
· Local physical geography of the coast - especially water depth and gradient at the shoreline as well as shape of coastline eg in Sulawesi 2018 earthquake, the bay funneled the tsunami, heightening the wave (seiching). Shallow gradient allows greater inundation of the wave so aerial extent increases
· Degree of coastal ecosystem buffer eg protection by mangroves and coral reefs
· Timing of the event – night v day
· Quality of early warning systems
· Degree of coastal development and its proximity from the coast
Factors affecting perception of risk
- Experience – more experience could mean more adjustments to the hazard made
- Material well-being – governments /individuals with more money have more choices
- Personality – is the person a leader or follower, a risk-taker or a risk-minimiser?
- Probability of the hazard occurring
- Likely loss (cost) from hazard
- Knowledge about hazard? Frequency +
Magnitude? - Nature of hazard – warning signs?
- Level of technology
Types of responses to hazards
Prevent or modify the event; these management strategies aim to control the physical process involved by the technological fix, and therefore, modify and prevent the hazardous event, in one of two ways;
Hazard prevention and environmental control. Ideally, the event would be prevented from occurring. This is currently unrealistic. Environmental control aims to suppress the event by diffusing energy over a greater area or period of time to prevent the event occurring. E.g. lava diversion
Hazard resistant design; aims to protect people and structures from the full effects of the hazard. The focus is on the building design and engineered solutions, e.g. sea walls. Buildings can be designed to withstand hazards, and most public structures, e.g. roads, dams, bridges, will have some hazard resistant features incorporated.
(b) Modify vulnerability; this aims to change human attitudes and behaviour towards hazards, either before the event, or after it.
Prediction and warning. If a hazard is predicted, action can be taken to lessen its impact on people and property. Warnings inform people of impending hazards. They rely on adequate monitoring and evaluation of the data, then the effective dissemination of the information via various information services.
Community preparedness; This involves prearranged measures and procedures which aim to reduce the loss of life and minimise damage. This includes such measures as public education and awareness programmes, evacuation procedures and provision of emergency shelters, food and medical supplies. Effective use of this has saved many lives over the years, including in the Rabaul volcanic eruption in 1994, where a emergency plan was successfully implemented to save thousands.
Hazard mapping & Land use planning; which aims to prevent hazardous areas being occupied by new settlements. Problem is that is cannot be applied to new areas. Success depends on accurate knowledge of frequency, nature, and location of hazards.
c) Modify the loss; the most passive response is to simply accept the losses incurred. This is rarely acceptable, especially after higher magnitude events. More commonly, the strategy is to share the losses. This can be achieved in two ways; aid and insurance.
Aid is provided at many levels for relief, rehabilitation and reconstruction purposes. High magnitude events are often declared disaster areas, and the losses shared nationally. At the international level, politics and pride often interfere with aid being asked for or given. In such situations, the United Nations is often involved, or charitable non-governmental organisations, e.g. the Red Cross are involved in aid. Often, sudden disasters generate more aid donations than slow onset hazards, such as droughts.
Insurance is a key strategy in HICs. The principle is that people join with a financial organisation to spread costs. An individual needs to act by purchasing a policy, and paying an annual premium. Insurance companies need to identify key areas of risk and hazards in order to secure their business. In 1994, Californian insurance companies collected $500 million in premium payments, but paid out $11.4 billion in claims resulting from the Northridge quake. Insurance for high-risk area may not be available, or come with stipulated conditions, e.g. buildings must have certain construction techniques employed. It encourages people to take preventative measures for themselves.
Long-term earthquake forecasting & warning
- Forecast = a relatively imprecise statement of time, place and nature of the expected event.
- Known that location of earthquake is closely linked with distribution of fault lines – and therefore location of risk areas is (mostly) known
EVALUATION
* However, intra-plate earthquakes aren’t known eg Gujurat, India 2001 led to upto 20,000 dead - located 300-400km from collision boundary. Even in seismically active areas like the Philippines, there are unknown faults which have caused deaths eg Bohol Fault in 2013 – 158 dead in a previously hidden fault
* Also - magnitude is also not accurately forecast - max potential earthquake in the Tohoku region was estimated as Mw 8.5. The March 2011 Tohoku earthquake was caused by thrust faulting at the plate boundary between the Pacific and North American plates as expected. However, its size was unexpectedly large (Mw 9.0). This made the forecasts less relevant.
* Seismic gap theory - Longer-term forecasting has proved relatively straightforward in many tectonic environments and is based on the premise that stress accumulates through plate movement at a relatively constant rate along many plate boundaries and so must be released by a regular pattern of seismicity.
* Scientists can find patterns in earthquake occurrence and link to rates of plate movement to calculate recurrence intervals for events of different magnitude and to identify seismic gaps that appear overdue for a major earthquake strike. EG location of the 1989 San Francisco Bay area earthquake was along a known ‘gap’
EG Turkey’s North Anatolian Fault
1300km long, running along the entire length of northern Turkey from the Aegean Sea in the west to Lake Van in the east. It is one of the fastest moving faults in the world. It slips such that central and southern Turkey are moving west relative to northern Turkey at speeds of 20-30mm a year. It is the most active and destructive earthquake-prone fault system in Turkey.
Short-term earthquake prediction, monitoring & warning
- Prediction = a relatively precise statement of time, place and ideally the nature and size of the event, i.e. a precise forecast.
Better than a forecast as it would: - provide people with a warning to enable them to prepare for an imminent earthquake or to evacuate to a safe location but how realistic is this?
- Environmental indicators linked to earthquakes can be monitored and could be used to warn people of an imminent earthquake:
- Seismic swarm foreshock activity (concentrations of minor tremors or ‘micro-earthquake activity’) – seismometers pick up vibrations in crust; clusters of small earthquakes often precede a large one
- Variations in earth’s magnetic field (due to changes of stress in rocks) is measured using magnetometer
- Radon gas emission – increases before an earthquake
- Changing water level in wells (strain changes porosity of rocks) can be measured using sensors in wells
- Horizontal movements are detected by measuring the time it takes a laser beam to move between 2 fixed points on either side of a fault
- Tiltmeters (sophisticated spirit levels) measure changes in slope and ground deformation
- Small-scale uplift or subsidence or ground surface changes can be measured by satellites and strainmeters
- E.g. Parkfield, California on the San Andreas fault is heavily monitored with instruments: strain meters measure deformation at a single point; two-colour laser geodimeters measure the slightest movement between tectonic plates; magnetometers detect alterations in Earth’s magnetic field caused by stress changes in the crust
Evaluation: - BUT to date, no universally recognised earthquake precursor or set of precursors; some major earthquakes have occurred without obvious precursors. For instance, the 1994 Northridge earthquake was not predicted by the Parkfield equipment and occurred on a fault that scientists did not know existed also Bohol in Philippines in 2013 also on undefined fault
- 1980 scientists predicted Peru earthquake – false alarm which cost $50million in lost revenue as it did not happen. In 2001 a 8.4Mg earthquake did actually happen killing 75 and costing $75million - lack of accuracy
- Reliable short-term earthquake prediction therefore remains an elusive but highly desirable objective. Many scientists think reliable prediction will be impossible
- Earthquakes are irregular in time (frequency varies from place to place) – the timing of earthquakes is very difficult to predict and whilst previous patterns and frequencies offer some clues, the size of an earthquake event is difficult to predict the maximum potential earthquake in the Tohoku region was estimated as Mw 8.5. The March 11 Tohoku earthquake was caused by thrust faulting at the plate boundary between the Pacific and North American plates as expected. However, its size was unexpectedly large (Mw 9.0).-
- Some effective predictions (e.g. Turkey Izmit 1999; Loma Prieta 1989 on San Andreas fault), but has not always resulted in effective response
- Predicting secondary hazards like liquefaction and tsunamis – more effective as spatially concentrated (90% in Pacific Basin) and early warning systems in place to transmit warnings to locals 15 mins after disruption record by permanent buoys and tide gauges monitoring all of the time. But Tsunamis tend to have slower onset especially if they occur at a distance from the coast so time to evacuate but only if population knows what to do and evacuation routes clearly sign-posted.
- Too many warnings - Some fear that false alarm leads to complacency and reduces perception of risk
Hazard mapping, zonation & land-use planning (earthquakes)
- Hazard zonation involves mapping the variation in exposure to primary and secondary earthquake hazards
- Fault location, past movements, past epicentres are used to inform hazard maps
- Possibly more useful with secondary hazards:
- Eg Landslides, liquefaction and tsunamis - because fault lines maybe unknown eg Bohol 2013 Philippines
- Japan Tsunami hazard maps effective as these are well communicated to the population who are educated on how to react: Since1990s, many local governments published tsunami hazard maps, prepared from numerical simulations. ‘Tsunami and Storm Surge Hazard Map Manual’ in 2004 - recommends hazard maps not only for residents but also for companies and fishermen. A hazard map shows the flooded area by past tsunamis and by the most likely tsunami in the near future with estimated inundation zones, the list of shelters where people could evacuate and instructions on how to survive a tsunami. In many coastal communities, people have conducted regular evacuation drills and have held workshops to learn which areas are at risk, by referring to a hazard map prepared by the local government.
- hazard maps have two functional aspects:
- inform people that they are at risk so they perceive the risk and escape an at-risk area as soon as possible, when they feel strong ground motion or hear the tsunami warning or evacuation order issued.
- On the other hand, a hazard map can function to assure residents living outside of the expected inundation zone that their area is NOT at risk. In the 2011 Tohoku earthquake, hazard maps failed to offer accurate predictions in some areas and may have increased the number of fatalities, as people believed that they did not have to evacuate immediately, even though these maps indicated the uncertainty of estimations based on past events and state-of-the-art computer simulations
- Liquefaction and subsidence – easier to map secondary hazards - map geology and water table height - In 2017 the national “Planning and engineering guidance for potentially liquefaction-prone land” was released in NZ. A key objective of the guidance is that buildings and infrastructure are located and built with appropriate consideration of the land condition
Evaluation - How reliable? - Most are based on 50-year timeframe with 10% probability that level of hazard will be achieved in any 50-year period.
- Good governance, Communication and community preparedness are essential and they must be communicated and enforced. Only effective if accompanied by other management strategies eg Maps used as basis for the following which makes them more effective than on their own:
- Longer-term proactive response to earthquake threat eg drain land susceptible to liquefaction, building codes – success in Chile in reducing deaths
- Hazard-conscious development – i.e. changing building location eg After the 2008 earthquake in Sichuan, China, geologists started searching for fault locations to map them as unsuitable locations for development
- Formulation of emergency response procedures – educating local people via drills, putting in place highly visible signage with evacuation routes to higher land to escape tsunamis etc
- Land use planning in San Francisco-San Jose California based on geological conditions (6 zones): some areas allow houses & roads; others don’t (where risk of mass movement too high)
- Wider land-use planning: Land uses that are potential fire or explosion risks are positioned away from homes and on solid rock. Land likely to liquefy is used for playing fields, nature reserves and parks, not buildings
- Moving the capital city in Iran: The devastating earthquake that killed some 40,000 people in the south-eastern city of Bam in 2003 led the government to plan to move the capital from Tehran within 25 years as the city of 12million had not experienced an earthquake for 2000 years. Is this economically viable? Still not achieved.
Community Preparedness & Emergency Response Preparedness
– collective and individual action which aims to empower people to take actions to minimise the impact of the earthquake
* Plans to help people live with earthquakes and to reduce their effects, thus saving lives, buildings and money. This is through building capacity to cope and reducing vulnerability through:
* Earthquake drills – schools, workplaces E.g. Japan’s Earthquake Awareness Day
* Public information: communities can be educated on what to do in case of an earthquake, e.g.
* Securing bookshelves
* Emergency survival kits – e.g. portable radio, flashlight, candle, matches, medication, 5-day food supply, blankets, tinned food, first aid etc.
* Securing water heater – to avoid gas leak/fire
* Protecting your home – building shear walls to protect foundations
* Family preparedness – knowing what to do during and after an earthquake:
* During: stay indoors under table/doorway; stay away from windows
* After: check for fire; put out gas; shut off power
* Education on tsunami risk
* Evacuation routes from tsunamis to high ground highly visible - Evacuation points may be placed in every neighbourhood, people are given maps of routes to follow and leaflets about how to dress and what to take
Evaluation
* Rarely an effective strategy for mitigating earthquake hazard, because it is not possible to predict the precise timing, location and size of an earthquake to a sufficiently high degree of accuracy
* This depends on good governance which determines the perception of risk by governments and individuals
* Sanriku coastal communities in Japan, people were taught the lesson or maxim of ‘Tsunami Tendenko’, which means that people should run without taking care of others, even family members. This phrase encourages people to escape by making individual decisions and taking personal responsibility; every individual effort increases the possibility of surviving.
Building regulations and structural measures(aseismic design)
- Falling buildings are by far the greatest cause of casualties during earthquakes – 75% deaths from collapsed buildings , so new buildings can be constructed to withstand their effects- ***arguably makes knowing when quakes will occur less important but depends on wealth and governance
- Earthquake-proof buildings have been constructed in many major cities, eg the Transamerica Pyramid in San Francisco. Buildings such as this are designed to absorb the energy of an earthquake and to withstand the movement of the Earth.
- Existing buildings can be retrofitted to withstand earthquakes, but this is much more expensive than doing so at the time of construction
- Roads and bridges can also be designed to withstand the power of earthquakes
- Eval : Aseismic measures add to cost and builders in some countries are more profit driven, and building regulations are not their aim
- Aseismic features:
- Strong steel frame which is also flexible stops cracking
- Spiral reinforcing on bridges
- Damper in roof acts like pendulum, reducing building sway
- Cross bracing stops floors collapsing
- Shock absorbers built into cross braces
- Strong double glazed windows stop broken glass showering down
- Very deep foundations built into underlying bedrock prevent collapse
- Basement isolation – foundations of building put on rubber mounts so ground can move under the building in isolation from building itself
- Strict building codes were credited with saving thousands of lives in February 2010 Chile earthquake M8.8. Moreover, the management following the 2010 earthquake’s effectiveness is highlighted as an 8.3 magnitude earthquake in 2014 only caused 15 deaths .
- Whilst conventional earthquake engineering may be unaffordable for some countries (e.g. Haiti), safe houses can be built cheaply (adobe, bamboo; plastic mesh; straw, old tyres) by applying aseismic principles e.g. light walls and gables (subject to smaller forces); light roofs (less damage if collapse); small windows (windows weaken walls); reinforced walls; confined masonry (brick walls connected to roof); shock absorbers between floor and foundation (p280 textbook)
- Structural measures – also used to protect against tsunamis
- Giant seawalls - the conventional approach to mitigating tsunami risk. Eg Since 2011, Japan has built hundreds of miles of concrete walls, taller than 40 feet in some places, at a cost of more than $12 billion
- EVAL: But seawalls tend to be expensive to build, tough on local tourism and fishing industries, disruptive to coastal communities and environmentally destructive – and failures can be catastrophic. Coastal infrastructure such as breakwaters and seawalls cannot always protect life and property: even great seawalls can fail.
- Seawalls should be designed with the assumption of overtopping and destruction, and communities should not rely on coastal infrastructures alone for protection.
- Sea walls -designed in Japan to protect against a 1 in 150 year recurrence interval tsunami. However, since 2011 in Japan, even when protected by great seawalls of 8–10 m, the government often prohibits the lower part of town to redevelop as a residential area, as the low land is reserved for commercial and industrial purposes. Many coastal communities on low-land devastated areas are thus moving uphill by applying for relocation and buy-out programmes.
- Cheaper/sustainable options – coastal forests to act as buffer - Mangroves can absorb 70-90 %of the energy of a normal wave. Kapuhenwala and Wanduruppa. Kapuhenwala, surrounded by 200 hectares of dense mangroves and scrub forest, the tsunami killed only two people – the lowest number of tsunami related fatalities in a Sri Lankan village. Wanduruppa, surrounded by degraded mangroves was severely affected: 5,000 to 6,000 people died in the district.
Volcanoes overview
Definition: An opening in the earth’s crust from which molten lava, rock fragments, ashes, dust, and gases are ejected from below the earth’s surface
They may be classified as active (around 550 active volcanoes around the world), dormant and extinct
Why do they occur?
· Most occur at plate boundaries although there are some exceptions where they are found intra-plate eg hotspots in the oceans eg Hawaii or continental rift valleys eg African Rift Valley
· About ¾ of the 550 historically active volcanoes are found along the Pacific Ring of Fire eg Mt Pinatubo, Mt St Helens, Nevado del Ruiz
· The rocks in the mantle are under immense pressure and so remain solid. However, at plate boundaries, this pressure is released which affects the temperature at which the rocks in the upper mantle melts so it starts to melt and creates molten magma which rises to the surface to supply the volcanoes. As this happens, gases which were previously dissolved under the earth at high pressure may become insoluble and create large bubbles of gas which may become more explosive
Classification of lava types
Basaltic Magma – Mauna Loa largest volcano .
Very hot, runny lava (non/low viscous) with a low silica content, produce gently sloping shield volcanoes with associated effusive Hawaiian eruptions. Low viscosity basaltic magma results in easy release of gases. The eruption is non-violent. Eg Hawaiian Islands and Icelandic Islands
Andesitic Magma
Cooler, viscous lava with a high silica content produces steep-sided cone volcanoes with a Plinian eruption. With andesitic magma, the high eruption columns spread gases over larger distance. As the eruption wanes, there may be a collapse of lava columns and resulting pyroclastic flows. Eg Mt Pinatubo in 1991 sent a plume of tephra 30km into the atmosphere and ash stretched for 100skm – 800km squared of farm land destroyed in the end.
Rhyolitic Magma
Silica content reaches up to 77% and creates a lava dome that plugs the volcano so that the eruption is sudden and violent through a weakness in the side of the volcano or through the top to produce a crater called a caldera. Eg Mount St Helens , Mount Pele and Krakatoa
Hot spot volcanoes
Hotspots
· A small area of the Earth’s crust where an unusually high heat flow is associated with volcanic activity away from plate margins
· E.g. Hawaii, Iceland, Yellowstone, East African Rift Valley (breaking up continent)
· Origin is debated
· Most likely due to permanent and deep-seated regions of superheating within the mantle:
· Strong convection currents raise mantle plumes, or broad, hot domes of plastic rock up to 1,000 km across up to the surface
· On nearing the surface, the plastic mantle plume encounters significantly lower pressures and becomes molten
· This molten rock may then pierce the rigid crust above it
· Faults/crustal weakness (e.g. scars from former convergence) may also play a role in allowing decompression and magma to reach the surface
EG. Hawaii
· Stretch NW across Pacific Ocean
· Comprises 19 islands and atolls in total
· All volcanic in origin
· Distinct trend in age the further you travel from the Big Island (active) e.g. , where the Big Island (only active volcanoes are here) – due to static hotspot under moving plate
· Kilauea, has been constantly erupting since 1983
· The Loihi Seamount promises to reach sea level within the next 100,000 years
· ‘Benign’ volcanism – basaltic lava
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Reason for elevate hazard risk of volcanoes
Volcanic areas are often densely populated for a number of reasons:
· The relatively flat fringes of volcanic islands and lower slopes of stratovolcanoes have fertile land due to the weathering of volcanic ash
· Many volcanoes formed at subduction zones are close to coasts which are in turn highly urbanised due to fishing, trade and transport
· They become tourist attractions - creating livelihoods
· Rapid population growth and high rates of urbanisation since the 1960s has led to the expansion of poorer districts of cities and the unplanned settlements on the lower slopes of volcanoes where land is more marginal and freely available.
· Estimated that 10% of the world’s population lives within 100km of an active volcano.
· 250,000 people have died in eruptions in the last 300 years.
· And yet, between 1990 and 1999 there were only 92000 deaths reported and a further 5 million people affected . However, in any single decade, upto 1 million people may be affected by volcanic activity.
· Why? Unlike earthquakes, volcanoes often produce a set of recognised warnings (precursors) of an eruption and modern science is increasingly good at monitoring and predicting unlike with earthquakes.
· In fact, many of the deaths associated with volcanoes are indirect hazards such as famine due to crop damage or from secondary hazards such as lahars
Volcano definition
An opening in the earth’s crust from which molten lava, rock fragments, ashes, dust, and gases are ejected from below the earth’s surface
Volcanic hazards
· Volcanoes are composite hazards. There are both primary and secondary hazards which can be caused by volcanic eruptions.
· The primary hazards include :
· pyroclastic flows, air-fall tephra, lava flows and volcanic gases.
· The secondary hazards include:
· ground deformation, lahars (mudflows), landslides, earthquakes and possibly tsunamis in ocean floor volcanic eruptions
· Important to note that volcanoes have a wider range of primary hazards and these can have a very long geographical reach away from the source
The type of hazard produced by a volcano will be determined by the type of boundary and therefore the type of lava and explosivity of the eruption.
How hazardous a volcanic eruption is will depend partly on the physical geography and nature of the boundary and eruption as well as human factors such as level of development, governance, perception of risk, sustainable management, prediction etc.
Primary impact volcano distance from source
0-10km:
- Lava dome
- Volcanic bombs
- Lateral blast
- Lava flows
- Magmatic earthquakes
10-100km:
- Pyroclastic flows
- Lahars
- Impacts of volcanic gases
- Ash fall
100-10,000km:
- Fine ash fall
- Tsunami
- Climate impacts of volcanic gases
Geologic hazards of volcano map
Volcanic explosivity index
· Intensity of volcanic is measured using the VEI (Volcanic Explosivity Index). This ranges from 0 to 8, (0=non-explosive eruption and 8=mega-colossal or apocalyptic).
· The VEI is quantified by the volume of tephra and is defined using a logarithmic scale. Each increase by one point in the VEI represents a 10 x increase in the volume of tephra.
· VEI 1= shield volcanoes in Hawaii – frequent, often daily eruptions which release pressure so explosive eruption unlikely – EFFUSIVE ERUPTIONS
· VEI 3-6 = Plinian – violent eruptions with huge amounts of magma released so that the magma chamber may become depleted, losing pressure so the volcano may collapse forming a caldera. – EXPLOSIVE ERUPTIONS
Type of eruption determining the hazards produced
· In order of how explosive they are
· Icelandic – persistent; fissure; basaltic lava - large quantities – form plateaux
· Hawaiian –still basaltic lava, gases escape easily – occasional pyroclastic flow
· Strombolian –frequent gas explosions, moderately explosive, eg Mediterranean
· Volcanian – sticky/cooled lava – more viscous lava traps gases
· Vesuvian – ash falls e.g. Vesuvius – ash over wide area
· Plinian – gas/lava, pyroclastic flows e.g. Mount Pinatubo
· Pelean – very viscous, nuee arden
Lava flows nature
Streams of molten rock that pour from erupting vent. They can destroy property by burying it, setting it alight, gases are emitted from lava which may asphyxiate
Why are lava flows hazardous?
Speed: especially if basaltic at constructive or hot spot boundaries as low Si content means lava is less viscous and very fluid. Even if less fluid, on steep slopes, some lava flows may reach 15m/sec
Temperature: can be upto 1200 C so may ignite and cause fires and can heat glaciers at summit of volcano and lead to melting of snow and ice to create lahars/ glacial bursts
Type of lava/explosivity: at destructive boundaries lava has high Si content, is viscous and gases do not dissolve – leads to violent, explosive eruptions v the effusive eruptions associated with basaltic lava such as in Hawaiian Islands. With andesitic magma, the high eruption columns spread gases over larger distance. As the eruption wanes, there may be a collapse of lava columns and resulting pyroclastic flows followed by lahars from meltwater Mt Pinatubo in 1991 sent a plume of tephra 30km into the atmosphere and ash stretched for 100skm – 800km squared of farm land destroyed in the end. Rhyolitic Magma Silica content reaches up to 77% and creates a lava dome that plugs the volcano so that the eruption is sudden and violent through a weakness in the side of the volcano or through the top to produce a crater called a caldera. Eg Mount St Helens 1981
How hazardous are lava flows?
Directly, Lava only accounts for only 1% of deaths and creates local impacts 0-10km from the vent Its impacts will be determined by viscosity related to Si content but even those with high Si content can cause huge environmental and economic impacts because of their speed:
* Mount Nyiragongo - Africa’s second most active volcano – East African Rift Valley Democratic Republic of the Congo.
* Threatening town of Goma
* Silica content of this magma is so low, speeds upto 60mph affecting Goma 2002 – 400,000 people evacuated from mining town of Goma, 140 people died from carbon monoxide poisoning given off by the lava and 15% of city affected. Despite this people live here for the longer term benefits ‘The soil on volcanic land is rich and fertile and great for farming -magma rises to the surface, it brings with it a lot of rich minerals and nutrients from deep within the Earth, including elements such as magnesium and potassium . NB that Iceland’s basaltic volcanoes are more explosive because magma interacts with ice from ice caps and glaciers and produces steam adding to explosive power eg Eyjafjallajokull. These tend to generate extensive lava flows with minor gas, ash and tephra eruptions – less hazardous
Pyroclastic flows nature
Fluidized mixture of hot rock fragments, hot gases and entrapped air that moves at high speed in thick grey-black turbulent clouds that hug the ground. Surge downhill because the heavy load of lava fragments makes them denser than the surrounding air.
Why are pyroclastic flows hazardous?
Speed: Velocity often exceeds 100kmph and can reach 160kmph so cannot outrun and destruction will be massive They pose lethal hazard from incineration, asphyxiation, burial, and impact
Temperature: can be upto 600-700C so incinerates everything – entire forests decimated in Mt St Helens in 1981 , lights fires and melts ice to create lahars , burns and kills people
Nature of the flow: Consist of two parts lower basal flow of coarse material that moves along the ground and a turbulent cloud of ash that rises above so considerable destruction. Nuee Ardentes – type of pyroclastic flow which is glowing or incandescent – result of frothing magma in the vent of the volcano
How hazardous are pyroclastic flows?
Responsible for most primary volcanic related deaths – 70%
Can have regional impacts 10-100km from vent
However - These do not occur at every volcanic eruption - Associated with explosive eruptions with a high Volcanic Explosive Index (VEI) and certain eruption types like Plinian as in the case of Mt Pinatubo in 1991 which was a 5/6 on the Explosivity Index Scale
These volcanoes are also rare with long recurrence interval times. However, these produce a wide range of hazards often occurring concurrently in a short space of time which make management challenging. They follow the topography of the land – especially their basal flow, so will affect valleys especially .
Driven by gravity, pyroclastic flows seek topographically low areas and can travel down valleys at high velocities beyond the flanks of the volcano.
Knowing that they follow topography, management by hazard mapping is possible – see Mt St Helens eg below
They are difficult or impossible to escape; therefore, evacuation of likely hazardous areas must take place before such events occur.
Example of pyroclastic flows
Mount Pinatubo, Philippines Eruption: June 15, 1991. (Plinian eruption VEI 5/6 10Km3 of tephra erupted)
- Huge avalanches of searing hot ash, gas and pumice fragments, called pyroclastic flows, roared down the flanks of Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as 200m thick.
- 840 people died, mainly from collapsed roofs of buildings smothered in ash but was considered to be a great success in prediction and evacuation
Example: Mount St Helen’s, Cascade Mountain Range, Washington State – 1980 (VEI 4, Plinian eruption)
- Most destructive volcanic eruption in US history.
- Whilst only 57 people died in the most closely observed volcano in the world, International Trade Commission estimated the damages to agriculture, timber and civil losses to be $11billion. 200 homes destroyed and thousands of animals killed
- Pyroclastic flows extended the reach of the volcano: Pyroclastic flows from the 1980 eruption ran out 8 km from the vent * Over past 4000 years numerous pyroclastic flows are known to have travelled at least as far as 10 to 15 km and one older flow reached 20 km (12 mi) from source.
- Management and hazards will be determined also by orientation of the volcano’s vent – the direction of the open crater. In 1980 57 people died partly because the prediction did not account for a lateral explosion so hazard maps did not reflect this risk.
- In this example, very rhyolitic magma caused a plug to form in the volcano which caused pressure to build up in the magma chamber and led to the unpredicted lateral blast
- Currently with the open crater to the north, distribution of pyroclastic flows into the North Fork Toutle River valley is predicted and managed with hazard maps however, all flanks of the volcano are subject to pyroclastic- flow hazard during a large explosive eruption.
Tephra/volcanic bomb nature
Tephra: Consists of all the fragmented, airborne material ejected by the volcano that subsequently falls to the ground
Classification depends on size of particle:
Volcanic bomb: largest form of tephra, .64mm in diameter. Formed when magma or rock is explosively ejected. Semi molten but solidifies before reaching ground
- Lapilli or volcanic cinders: 2-64mm
- Ash: particles <2mm
Why are volcanic bombs hazardous?
Volcanic bombs: hazard due to distance travelled from vent (can be km away), can be heavy and large and high speed with angularity so can kill
Tephra may be hot enough to ignite fires
Coarser, heavier particles will fall closer to the volcano vent and flat-roofed, unreinforced buildings tend to collapse when accumulation occurs
Depending on wind direction, finer ash may be transported and deposited far away from the vent eg within 6hrs of a modest VEI 5 eruption at Mt St Helens in 1980, ash clouds had drifted 400km
How hazardous are volcanic bombs?
Accounts for less than 5% of deaths but even a light covering of ash can contain toxic chemicals which contaminate farmland and water supplies. Mt Pinatubo 1991 – livelihood of 5000000 farmers affected as ash fell 30km away
Even at boundaries triggering low VEI eruptions, impacts maybe amplified under certain conditions – eg Icelandic eruptions may combine with meltwaters to produce ash with considerable, far reaching and prolonged impacts
Can have local to global impacts – 100-10,000km away and even affect weather patterns and these impacts can have very much longer term impacts even years long
Can have very long term impacts – described by Nixon as creating ‘slow violence’ towards affected communities. EG Heimaey, Iceland. 1973 eruption produced thick basaltic ash deposits on the eastern side of the island which continue to be eroded by the wind. Strong North Atlantic winds cause the ash to be entrained in the air; it then abrades house exteriors, contaminates interiors as well as causing psychological distress and breathing problems
Lupins with nitrogen fixing root nodules now used to reclaim land by reducing ash erosion
Volcanic bombs examples
Example: Mount Pinatubo, Philippines 1991 VEI 5/6
- Mt. Pinatubo is a stratovolcano at subduction zone * Violent, explosive eruption
- The ash plume or cloud height reached more than 40 km high and ejecting more than 10 km3 of ash formed downwind of the volcano
- A complicating factor in the dispersal of ash was at the same time as the eruption, Typhoon Yunna channelled the ash from the usual dispersal out to the ocean toward the island of Luzon. This combination gave rise to wet ash, increasing loading on structures with a large proportion of the 847 death toll due to roof collapse.
- The eruption cost $700 million in damage, $100 million of which was damage to 16 aircraft flying at the time of the eruption and $250 million in property with the rest a combination of agriculture, forestry and land.
Example: Eyjafjallajökull Ash Cloud, Iceland – March 2010
- Relatively small eruption, no direct deaths but in the wrong place and demonstrates the far-recaching impacts ash may have as a hazard
- Began as a normal effusive, basaltic lava volcano typical of divergent plate boundary - low in Si so effusive rather than explosive and should not have been that hazardous.
- But volcano occurred under the glacier which generated ash which became a major hazard with global impacts.
- As the ice started to melt, glacial water began flooding into the volcano where it met the bubbling magma at the centre of the eruptions. This rapid cooling caused the magma to shear into fine, jagged ash particles.
- Large plumes of volcanic ash quickly spread above the volcano, moving eastwards with the jet-stream towards the Faroe Islands, Norway, and northern Scotland.
- By April, much of Europe affected – more than 100,000 flights affected as aircraft grounded
- Wind direction and velocity influenced the impacts
- Iceland responded by declaring a state of emergency and European airspace was closed as a safety precaution. It is estimated that airlines lost an estimated £130m every day that airspace remained closed, while millions of passengers were left stranded.
- With airfreight impacted , world Bank estimated that African countries may have lost $65 million due to impacts of shutting down airspace and transport of perishable goods like fruit and flowers from countries like Kenya, Zambia and Ghana
Volcanic gases nature
Associated with explosive eruptions and lava flows – making less hazardous lava eruption volcanoes more hazardous
Mix normally includes water vapour (steam), SO2, Hydrogen, CO and CO2
Why are volcanic gases hazardous?
EG In 1986 - CO2 emitted from Lake Nyos in Cameroon killed 1700 people
EG Sudden eruption of Mount Ontake Japan , 2014, killed 60 – phreatic steam eruption without warning. Despite small scale of this volcano, it was the deadliest in Japan for 90 years. Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits). The intense heat of such material (as high as 1,170 ° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and bombs
Impact of gas as a hazard Some gases, such as carbon dioxide, are greenhouse gases that promote global warming, while others, like Sulphur dioxide, can cause global cooling, ozone destruction, and polluted air known as volcanic smog or “vog”.
Health hazards can range from minor to life threatening. Exposure to acid gases such as sulphur dioxide, hydrogen sulphide, and hydrogen chloride can damage eyes and mucous membranes along with the respiratory system and, under extreme conditions, can lead to death.
A very serious hazard can occur under certain conditions from volcanic emissions of carbon dioxide. Carbon dioxide is heavier than air and can collect in low and poorly ventilated places. Nearly two thousand people have died of carbon dioxide asphyxiation near volcanoes in the past two decades, most of them in Cameroon, Africa, and in Indonesia.
Gases may severely damage vegetation. Direct exposure to concentrated volcanic gas or long-term exposure to dilute volcanic gas is lethal to most types of foliage. Since 1990, areas of dying forest around Mammoth Mountain have grown in size and number because of high concentrations of carbon dioxide in the soils and now occupy more than 100 acres
How hazardous are volcanic gases
- Make less hazardous lava eruption volcanoes more hazardous
- Most deaths associated with CO2 – gases are colourless, odourless, heavier than air so dense and accumulates in valleys undetected by people so less or no warning and may not be detected prior to eruption
- Can have regional impacts – 10-100km and even global impacts when greenhouse gases enhancing global warming and climate change
Lahars nature
Volcanic mudflows composed of fine sands and silts. Secondary hazard associated with heavy rainfall as a trigger of old tephra deposits on steep slopes which are remobilised with rainfall.
Volcanic deposits combine with water from heavy rainfall or glacial melt - Water increases pore water pressure in the sediment, increases lubrication whilst reducing cohesion. Water adds weight which combined can trigger a mudflow.
Why are lahars hazardous?
They can carry large boulders, cars, or even bridges and can destroy or bury almost anything in their paths.
How hazardous are lahars?
Jökulhlaups (glacial dam bursts) with lahars combined are the second greatest volcanic hazard after pyroclastic flow. People living in tropical regions eg Indonesia and in glaciated environments such as Iceland can be particularly susceptible.
Regional impact 10-100km
Made worse and faster with topography of the land:
Degree of hazard depends on steepness of slope which determines the gravitational pull and velocity (they may be 80km/hr) volume of material and particle size.
Ideal conditions as steep slopes, unlimited supply of unconsolidated, loose material and rainfall/melting ice due to altitude and heat generated by the eruption
They will be influenced by topography of the land, flowing through river valleys where settlements may be built, increasing the hazard risk (degg)
Lahars can flow many KM downstream from the volcano (upto 100km), making this the most threatening hazard in some areas
When lahars reach low-lying areas, they spread out, slow down, and deposit their vast loads of debris over many square miles – increasing the area affected. Villages like Lourdes 15 miles from Mt Pinatubo were buried by lahars in the months after the 1991 eruption.
They can have longer term and recurring effects as ash gets remobilised during rainfall events years after the eruption – Mt Pinatubo 1991 below
HOWEVER, because of their predictable course, they can be managed
Volcanic landslides nature
Landslides are common on volcanic cones because they are tall, steep, and weakened by the rise and eruption of molten rock.
Magma releases volcanic gases that partially dissolve in groundwater, that weakens rock by altering minerals to clay.
Volcano landslides (debris avalanches) range in size from less than 1 km3 to more than 100 km3
Lahar examples
Example: Mount Pinatubo, Philippines 1991
- Eruption deposited more than 5 km ³of volcanic ash and rock fragments on the volcano’s slopes.
- Within hours, heavy rains began to wash this material down into the surrounding lowlands in giant, fast-moving mudflows called lahars.
- In the next four rainy seasons, lahars carried about half of the deposits off the volcano, causing even more destruction in the lowlands than the eruption itself.
- The village of Lourdes, built on lowlands 15 miles northeast of Mount Pinatubo, was inundated by giant, fast-moving mudflows of volcanic debris (lahars) in the months following the volcano’s cataclysmic June 1991 eruption. Since that eruption, lahars have destroyed the homes of more than 100,000 people in the area surrounding Pinatubo
- Effects of lahars lasted for 6 years – long term impact of these hazards.
However, they can be quite well-managed:
Mount Rainier, Cascade Mountains, Washington State, USA
· The lahar warning system includes a detection component and a warning-dissemination component.
· The detection component is automated and identifies lahars with a network of tripwires and small sensors called acoustic flow monitors (AFMs). AFMs are embedded underground and measure ground vibrations made by passing lahars.
· Because lahars from Mount Rainier can reach populated areas in a matter of minutes, warning messages are intended to trigger immediate, preplanned emergency-response actions.
· Warnings transmitted by sirens and via the Emergency Alert System (EAS) which is the primary means for providing the public with critical alert information about an emergency or disaster.
· Washington State agencies have developed an evacuation plan with marked evacuation routes to aid residents and visitors. Parts of some communities rely on evacuation to high ground by foot, especially in areas where highways may become clogged with traffic.
Why are volcanic landslides hazardous?
The high velocity and great momentum of landslides allows them to cross valley divides and run up slopes several hundred metres high.
EG the landslide at Mount St. Helens on May 18, 1980, had a volume of 2.5 km3, reached speeds of 50-80 m/s , and surged up and over a 400-m-tall ridge located about 5 km (3 mi) from the volcano.
Large landslides often bury valleys with tens to hundreds of metres of rock debris, forming a chaotic landscape marked by dozens of small hills (hummocks) and closed depressions.
If the deposit is thick enough, it may dam tributary streams to form lakes; the lakes may eventually drain catastrophically forming lahars and floods downstream.
How dangerous are volcanic landslides?
Speed and volume make them especially hazardous
They can reach considerable distances from the vent
Can trigger other secondary hazards - generate some of the largest and most deadly lahars
These may even reach coastlines and trigger tsunamis
Jökulhlaups nature
Type of catastrophic glacial outburst flood
Why are jökulhlaups hazardous?
Hazard to people and infrastructure and can cause widespread landform modification through erosion and deposition – when Katla in Iceland erupted in 1918, the glacial burst deposited so much material that it extended the coastline by 5km.
Occur suddenly with rapid discharge of large volumes of water, ice and debris from a glacial source. EG Eyjafjallajökull 2010
How hazardous are Jökulhlaups?
Short lived.
However, they can be managed - Iceland
· Currently, scientific investigation and monitoring of jökulhlaups contribute to an island-wide early warning system designed and maintained by the Icelandic Meteorological Office
· The network includes >115 automatic weather stations, 100 manned stations, 170 hydrological gauges, 55 seismic stations, and 70 GPS stations
· Reports are generated daily and transferred to the MET for evaluation by scientists (Sigurðsson et al., 2011). If the MET detects any unusual activity or possible hazards, it can issue a warning to the Civil Protection Authorities, which could then trigger various civilian hazard management procedures
- Fig. 3 shows a hazard map of a volcano.
- Describe the nature of the erupted materials shown in Fig. 3 (4)
- Explain their hazardous impacts. [6]
.
Describe the pattern of volcanic hazard zones shown in Fig. 7.1 (4)
Suggest two reasons why some places have a very high hazard level (6)
Describe the pattern of ash fallout shown in Fig. 7.1 (4)
Suggest two reasons for the pattern you described in (a)
Volcanoes monitoring + predictions
Key difference between earthquakes and volcanoes is that volcanoes have some precursor activity which can help predict and forecast with some certainty eg
Visible observations Eg melting glacial ice and snow
Seismometers Measure tremors which may indicate movement of magma beneath the volcano
Extensometers Measure ground deformation and flank bulging
Gas spectrometers Measure composition of gas emissions
Airborne gravimeters Measure changes in ground density that indicate upwelling magma
Thermometers and thermal imaging Measure changes in lake water and ground temperature
Success:
· Since 1980, 19 of Mt St Helens’ 22 eruptions in Washington State, USA, have been accurately predicted.
· Even some developing countries have effective monitoring and warning organisations eg Philippines Institute for Seismology and Volcanology (Phivolcs) which accurately predicted the eruption of Mt Pinatubo in 1991 and evacuated 60000 people from an area of 40km from the summit and the VEI 6 eruption only killed 600-800 even though 100000 were displaced.
· Monitoring and prediction said to have saved 5000 lives in Pinatubo.
· Montserrat – Soufriere Hills volcano - predictions led to 60% of island being declared as uninhabitable in a designated exclusion zone – only deaths (19) from people who returned to Exclusion Zone
Limitations to prediction and monitoring:
Cost of this technology means monitoring is mostly centred around volcanoes in the developed world – this means that only 20 are permanently monitored worldwide which limits the accessibility and impact of prediction as a way of reducing the impacts of an eruption.
· Success of prediction depends on subsequent evacuation which is affected by Perception and sense of risk: some people around Mount Pinatubo in the Philippines refused to be evacuated because:
- They under -estimated scale of eruption
- Didn’t want to leave their property, livestock etc
- Had no transport or couldn’t walk long distances
- Had traditional belief that the volcano was a god and wouldn’t harm them (Aeta tribes people)
· And the evacuation was made harder by a category 3 typhoon (Typhoon Yunya) which moved in over Luzon Island and increased the lahar risk which lasted for upto 6 years / 4 rainy seasons and extended the affected area as monsoon rains remobilized the ash deposits . 100 people were killed by the lahars and more damage was caused by these than
the eruption itself (20000 people homeless, 1991 harvest destroyed and buried farmland unusable for many years) but following this eruption, a new lahar hazard map and monitoring equipment was installed.
· Despite the success in evacuation and lives lost, 847 people died, many from longer term impacts of disease as a result of poor sanitation in the evacuation centres. 1 million people lost their homes and the cost of the eruption reached $700,million
· Even if prediction and forecasting is possible – it must be acted upon:
EG Nevado del Ruiz, Colombia 1985.
· Easy to locate volcanoes but not easy to say exactly when will erupt e.g. Nevado del Ruiz 1985 – last volcano to kill significant number of people even though it had a low VEI of 3 and warnings issued. Only small scale activity first (people didn’t evacuate => 23,000 deaths in town of Armero 40km away within 4hrs of the eruption; lahars – 50m deep lahars when 10% of snow and ice cover melted by pyroclastic flows, travelled at 60km /hr, evacuation notices were too late or not acted upon, no drills and people had no where to go)
· Predictions can be wrong which affects perception of risk
· EG Mammoth Lakes tourist resort, California, USA 1982
· USGS issued a volcano hazard alert in 1982 but no eruption followed.
However, the economy in the Mammoth area crashed, according to the town’s Chamber of Commerce.
Since the alert was issued, property values fell 20-70%, new house construction fell by 75%; school enrolment was down 26 %, and a town with 5,000 people in 1982 had only 3,000 by 1984.
· Inaccurate predictions may lead to complacency second time around and affect individual perception of risk
Moreover, Some types of eruption may be difficult to predict eg Limnic Eruptions at Niraygongo - gas emitted from lake which was not visible and could not be smelled and Phreatic Eruptions (steam eruptions) at Ontake , Japan 2014
· Some dangers are not directly due to a volcanic eruption eg Mt Merapi where pyroclastic materials are reactivated to form lahars with as little as 50mm rainfall/hr and lahars in Mt Pinatubo recurred for 6 years after the 1991 eruption
Hazard risk mapping volcanoes
Whilst prediction is arguably one of the most important methods of reducing death tolls from eruptions, they must be communicated to the people who can act on them in a timely way. This will hopefully include hazard maps drawn up in advance of an eruption – a sort of risk assessment to inform and alert people of the need to evacuate should an eruption occur.
Effective
Hazard maps identify areas at risk from different volcanic hazards eg Mount Rainier in Washington state, USA is an andesitic stratovolcano. It is part of the Cascadian subduction zone (along with Mount St Helens) which hasn’t erupted since the mid 19th Century. It is capped by 25 glaciers. The hazard map is separated into:
- A central high risk zone 20km from summit at risk from pyroclastic flows and lava flows
- Outer lahar-risk zone within populated river valleys extending to 100km of the summit. 80000 people live in this zone. The western side of the volcano is deemed to be most at risk eg Nisqually River Valley
This hazard mapping has fed into:
- Mount Rainier Volcanic Hazards Response Plan -operates at national, state and local levels
- Published volcanic hazard evacuation routes which are signed on roads leading to high ground
- Educated local communities and schools on lahar risks
- Automated lahar warning system which detects ground movement from a passing lahar- which is crucial in this area as a lahar could reach the major town of Orting just 40minutes after it is triggered.
- Warning sirens tested twice a year and schools have evacuation drills. After sirens, warnings are transmitted to radio and phones and evacuation is key to protect life if not possessions. Residents are advised to evacuate on foot to avoid bottle necks and take a grab bag with first aid and water – community preparedness is key here.
Hazard mapping without doubt is one of the most important aspects of reducing death tolls from eruptions but they must be communicated to the people who can act on them in a timely way.
· Taal, Philippines 2020 – following successful monitoring and warnings, a 14km exclusion zone meant no deaths
· Mount Merapi, 2020 – following successful monitoring by Yogyakarta’s Volcanology and Geological Hazard Mitigation Centre,, evacuation routes planned in a 6 mile zone and lights erected along routes and at evacuation centres and livestock sheds to ensure people could evacuate at night. 1300 people evacuated
Limitations
· Less effective for hazards like pyroclastic flows – they may travel short OR long distances and are not necessarily linked to height of volcano
· Eruptions can also take an unexpected route which contradicts hazard map– the plinian style eruption at Mt St Helen’s in 1980 saw a side vent blast laterally extending 8km and lengthening the impacts of the pyroclastic flow beyond the exclusion zone that had been mapped and killing 57 people.
· Moreover, whilst may lives were saved in this HIC with a volcano that was monitored 24 hrs per day as one of the most closely observed volcanoes in the world, the impacts on the economy were considerable despite lives saved - International Trade Commission estimated the damages to agriculture, timber and civil losses to be $11billion. 200 homes destroyed and thousands of animals killed
· Some hazards eg lahars and ash travel a long distance so are more difficult to map as are affected by global circulation
· Ash - Accounts for less than 5% of deaths but even a light covering of ash can contain toxic chemicals which contaminate farmland and water supplies.
· Mt Pinatubo 1991 – livelihood of 5000000 farmers affected as ash fell 30km away
· Even at boundaries triggering low VEI eruptions, impacts maybe amplified under certain conditions – eg Icelandic eruptions may combine with meltwaters to produce ash with considerable, far reaching and prolonged impacts
· Relatively small eruption in Iceland in 2010, no direct deaths but in the wrong place and demonstrates the far-recaching impacts ash may have as a hazard which cannot be hazard mapped
· Began as a normal effusive, basaltic lava volcano typical of divergent plate boundary - low in Si so effusive rather than explosive and should not have been that hazardous.
· But volcano occurred under the glacier which generated ash which became a major hazard with global impacts.
· As the ice started to melt, glacial water began flooding into the volcano where it met the bubbling magma at the centre of the eruptions. This rapid cooling caused the magma to shear into fine, jagged ash particles.
· Large plumes of volcanic ash quickly spread above the volcano, moving eastwards with the jet-stream towards the Faroe Islands, Norway, and northern Scotland.
· By April, much of Europe affected – more than 100,000 flights affected as aircraft grounded
· Wind direction and velocity influenced the impacts
· Iceland responded by declaring a state of emergency and European airspace was closed as a safety precaution. It is estimated that airlines lost an estimated £130m every day that airspace remained closed, while millions of passengers were left stranded.
· With airfreight impacted , world Bank estimated that African countries may have lost $65 million due to impacts of shutting down airspace and transport of perishable goods like fruit and flowers from countries like Kenya, Zambia and Ghana
· Ash Can have local to global impacts – 100-10,000km away and even affect weather patterns and these impacts can have very much longer term impacts even years after
· Topography eg valleys and steepness of slopes will affect some hazards eg lava flows and lahars– their speed of onset and how far they run out
· Higher volcanoes will affect how far volcanic landslides travel
Gas is notoriously difficult/impossible to hazard map:
Most deaths associated with CO2 – gases are colourless, odourless, heavier than air so dense and accumulates in valleys undetected by people so less or no warning and may not be detected prior to eruption so impossible to hazard map unless the hazard is known about.
Moreover, gas may have regional or even global impacts making hazard mapping less relevant and specific– 10-100km and even global impacts when greenhouse gases enhancing global warming and climate change
· EG In 1986 - CO2 emitted from Lake Nyos in Cameroon killed 1700 people (Limnic eruption)
· EG Sudden eruption of Mount Ontake Japan , 2014, killed 60 – phreatic steam eruption without warning. Despite small scale of this volcano, it was the deadliest in Japan for 90 years. Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits). The intense heat of
such material (as high as 1,170 ° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and bombs
Overall, the success of hazard mapping depends on action :
· To save lives vulnerable people must act on hazard mapping advice and prepare for an eruption– Mount Vesuvius, Italy – Naples lies only 15km away and 600000 people live in the 18 towns in the so-called ‘red zone’ at greatest risk. Estimates suggest it would take 3 days to evacuate people along narrow and congested roads. No evacuation drill has ever been carried out and plans assume precursors would provide 4 weeks warning time.
· Currently, no evacuation plan as assumed it will only be a VEI4 eruption and that prevailing winds will carry most debris to SE away from Naples. However, there is an argument that it could be a VEI6 eruption which could send pyroclastic flows 20-30km from Vesuvius
Overall, the success of these will depend, however, on type of volcanic hazard, local geography and degree of development and governance. Even with HICs, there are limitations to the success of hazard mapping and it is severely impacted by perception of risk. Moreover, whilst hazard mapping may save lives, it can do little to save property and assets especially once settlements are built
Diversion + mitigation volcanoes
– structural measures designed to modify the eruption and its hazards
Cannot do anything to manage pyroclastic flows but some hazards may be tamed and mitigated against. These strategies may protect against lava flows but will be less or not effective against other volcanic hazards like lahars and especially ash.
- Concrete and steel reinforced buildings with pitched roofs can withstand tephra
- In Japan – concrete tunnels – shelter from lava bombs
- In areas of lahar risk, concrete diversion channels may be built, dams checked and sediment traps constructed – these are however costly and only able to cope with small lahars.
- Use of explosives to divert lava flow, rock embankments to divert and trap and contain in a narrow channel – used successfully in Mount Etna in 1983 where lava flows were bombed and diverted away from settlements by channels at cost of $3million but did protect from $10million of damage had the lava engulfed settlements.
- 1992 Zafferana – large concrete blocks dropped into the lava tube – stopped lava from reaching the town 6. Hawaiians used explosives dropped from planes in 1935 and 1942 to try to disrupt lava flows from Mauna Loa volcano that were threatening the town of Hilo on the Big Island. The idea was to disrupt the channels or lava tubes in the volcano that were supplying lava to the surface. Neither attempt was successful. The explosions created new channels, but the newly formed lava flows soon rejoined the original lava channel.
- Salt water spraying was also used in Eldfell in 1973 protecting the port of Heimaey – took 5 months of 24 hour monitoring, more than 2 billion gallons of saltwater and only effective due to coastal location and the Aa lava which is typical of these areas and is much slower and so more easily managed by salt water as it cools quickly and solidifies so stops the flow. It also cost £1.5million in 1973 so not accessible to lower income countries. HOWEVER, these strategies only copes with small, slow-moving lava flows and always needs a back up plan if it doesn’t work
- In January 2024 A human-constructed barrier diverted some of the flow from one fissure away from town of Grindavik in Iceland, but lava from a fissure closer to Grindavík engulfed several homes.
· Good:
Lava diversion good to protect areas of economic and residential importance and is teh only way of protecting against the impacts on property and assets
· Bad:
All this can be costly and may not work where there is lots of tephra or very large lahars – can stretch 50 miles and long term
Requires pre-planning AND a lot of money and excellent governance with teh relevant technology to achieve this.
Lava diversion not possible where:
- Volume of lava erupted is v. large
· Lava erupts rapidly from fresh fissures and flows very quickly
· The area lacks the necessary technical and financial capabilities
How is an earthquake measured?
A seismometer measures the amount of ground shaking during an earthquakes, recording both the horixontal and vertical waves
- uses moment magnitude scale - adapted from old Richter scale
- Logarithmic scale - increase 1, 10x ground shaking, 32x more energy
Mercalli scale measures intensity (descriptive)
Nature of volcanoes at divergent plate boundaries
Melting of upper mantle produces basaltic magma as some minerals melt before other (dry-partial melting).
This alters the composition of the molten rock produced and creates basaltic magma at constructive boundaries. Magma rises through fissure to form new oceanic crust and supplies volcano.
Basaltic lava has low viscosity so bubbles of gas rise to surface and don’t get trapped
More effusive + less explosive - especially when they occur under water deep in the ocean floor. Generally less hazardous however hazards like ash may have far reaching effects and runny lava may travel at speed
Nature of volcanoes at destructive boundaries
Subduction in the ocean produces chains of volcanoes known as volcanic island arcs. Subduction of cold, denser oceanic late - slide down subduction zone, warming u0p slowly - leads to flux/wet partial melting - water and CO2 leave the oceanic slab and rise up into the mantle, lowering its melting point due to impurities in expelled water so it melts creating more silica rich viscous andesitic lava which is more viscous + traps gas enough so that pressure builds enough to create a volcanic eruption
These appear on the surface directly above the magma that forms directly above the down-thrust plate
Around 80% of the world’s volcanoes lie on subduction zones
Most explosive composite cone volcano
Hazards:
- lava
- tephra
- pyroclastic flows
- nuee ardent
Less frequent but more destructive
Eyjafjallajökull, Iceland 2010
HIC , well monitored and prepared and should not be an especially hazardous volcano but had global impacts
Plate boundary detail: constructive boundary – Eurasian diverges from North American
Explosiveness: Generally less explosive than at destructive due to basaltic lava low in Si and less viscous. However, heat from lava melted the glacier above creating a glacial burst or jökulhlaup rivers of water and mud which flooded local farmland and water combined with the magma to create a phreatic steam eruption which sent a plume of steam and ash 11km high with a VEI (Volcanic Explosivity Index) of 4 The plume was driven SE by the jet stream and westerly prevailing winds across the North Atlantic to Northern Europe
Even though there were no fatalities due to this hazard event, the Eyjafjallajökull eruption shows how a hazard like this can impact negatively on the economies of businesses and countries.
Impacts on people:
Flooding caused by the glacial burst affected the Markarfljót river and 800 local people were evacuated.
Ash caused decline in air-quality –caused schools in southern Iceland to close. Minor respiratory and other health problems as a result of the ash-fall, such as eye irritations and dry throats. Those vulnerable to respiratory conditions such as asthma were advised to be vigilant. The effects, however, were short-lived.
Impacts of flight ban: Tourism – the impacts on tourism were mixed. Initially the eruption in March attracted tourists to Iceland. Tourist organisations quickly exploited the situation by offering helicopter, bus and jeep tours to view the eruption. By the end of March, 10,000 people had visited the eruption. Travel agents in the UK lost an estimated £6 million in business each day. The impacts continued into the summer period as potential travellers delayed booking holidays. Resorts in Greece, Spain and Portugal were particularly affected by reticent potential customers – wide impacts.
Losses in economic productivity – about 7 million passengers worldwide were stranded as the result of airport closures. Many of these were on holiday and unable to return to work, which left businesses without employees. One estimate put the loss in productivity at £400 million a day. There were wider impacts on trade – World Bank estimated that African countries like Zambia and Kenya lost $65 million due to the flight ban which affected transport of their perishable cash crops like fruit and flowers
Impacts on environment
Disruption to farming – in order to prevent livestock eating grass or drinking water poisoned by fluorine-tainted ash, farmers were advised to keep their animals indoors.
Pastures, which were just beginning to grow again after the winter, suffered where ash-fall was deep – more than 10 cm. Lighter ash-falls, however, were soon washed away by rain and caused little damage to the vegetation. In the longer term, nutrients released from the ash may improve soil fertility.
Mudflows (lahars) – ash mixed with meltwater and rain created mudflows or lahars. Mudslides falling into rivers raised channel beds, increasing the flood risk. Later in the summer, heavy rain falling on ash caused renewed concerns about flooding and created longer term impacts as lahars recur where ash deposits are remobilised by rainfall.
Mount Pinatubo, Northern Island of Luzon, Philippines 1991
(Middle income but excellent management and especially evacuation but complicated by tropical cyclone and lahars that lasted 6 years- shows long term effects)
Management was very effective because so many precursors: Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey had forecast Pinatubo’s 1991 climactic eruption, resulting in the saving of at least 5,000 lives and at least $250 million in property.
There was a Relocation of settlements from the hazard mapped zones which saw more than 200,000 people evacuated safely away from the threat of collapsing buildings, ash columns and magma. In total, 58,000 people were evacuated from the high-risk areas. This was followed by a new scheme of an establishment of a lahar monitoring-warning-evacuation system to deal with the lahar problem on an emergency basis
Mount Pinatubo is an active volcano found in the Zambales Mountains on the northern island of Luzon and is part of the Luzon island arc chain. Subduction zone : between Eurasian and Philippine Plates It was the second-largest volcanic eruption of the 20th century with a big impact on a very densely populated area. Lava is andesitic (viscous) and eruption was explosive with a large volume of gas, bombs, ash, tephra (lumps of volcanic rock) and pyroclastic flows (hot mixtures of gas and rock). In total, the eruption ejected more than 5 km³ of material. Pyroclastic deposits were 200m thick in valleys This quantity of volcanic rock dramatically changed the landscape of Luzon, filling some valleys while emptying others. Some sections of the volcano expanded, while the top collapsed into a 2.5 km diameter caldera. Tropical Storm Yunya was passing 75km North-East of Pinatubo – rainfall combined with ash to create lahars that recurred for 6 years
Impacts on people:
Eruption caused $700 million in damage - $250 million in property with the rest a combination of agriculture, forestry and land The economy of central Luzon was greatly affected, with an estimated $18.5 million of business lost & 650,000 workers lost jobs. 58,000 people had to be evacuated from a 30km radius of the volcano and around 75,000 homes were destroyed or damaged by the eruption During the eruption, 350 people were killed, mainly the Aeta people who refused to evacuate their area due to fatalistic response and perception of risk, and these people were killed by the magma and ash and collapsing roofs that resulted from the eruption. 20,000 indigenous Aeta people were displaced previously living on the slopes of the volcano. Secondary impact: An epidemic of respiratory and gastric diseases and measles broke out in the temporary housing and mortality rates increase in the first few weeks, especially amongst the Aeta tribe where death rate increased from 7 per 1000 to 26 per 1000. Secondary impacts on education: 700 school buildings with 4,700 classrooms were destroyed displacing an estimated 236,700 pupils and 7,009 teachers
Impact on environment:
20 million tonnes of SO2 were discharged into the atmosphere - short term global coolant because of the effect it has on our ozone layer, resulting in a decrease in the temperature (dropped by 0.5*C) worldwide over the next few years The aerosol cloud spread around the earth in two weeks and covered the planet within a year. During 1992 and 1993, the Ozone hole over Antarctica reached an unprecedented size The eruption is believed to have influenced such events as 1993 floods along the Mississippi River and the drought in the Sahel region of Africa. The United States experienced its third coldest and third wettest summer in 77 years during 1992. The mud storms and mudslides covered 50000 hectares of cropland, destroying all crops and over 100,000 homes. And many farmlands of rice-paddy fields are out of use for years to come.
Mount St Helens, Cascade Mountain range, Washing State, USA 1980
HIC well prepared but still hazardous due to unpredicted lateral blast Hazard mapping: A zone of restricted access had been established, which minimised the amount of people living/walking in the endangered area A hazard warning was issued The authorities were able to evacuate people from the surrounding areas before the landslides
Phreatic eruption Ash column rose 15 miles into the atmosphere The Mount St Helens eruption rated at a VEI of 5, but only just due to its small magma output Hot material melted the mountains glaciers, creating huge lahars which travelled as fast as 90mph. The lahars affected 3 of the 4 stream drainage systems on the mountain Glacier and snowmelt mixed with tephra on the volcano’s northeast slope to create much larger lahars
Impacts on people: (economic and social) 57 people died, 198 people had to be rescued. 200 houses, 47 bridges, 24 km of railways, and 185 298 km of highway were destroyed Ash caused £100 million of damage to farm machinery and crops.
Impacts on environment The sudden lateral blast—heard hundreds of miles away—removed 1,300 feet off the top of the volcano, sending shock waves and pyroclastic flows across the surrounding landscape, flattening forests, melting snow and ice, and generating massive mudflows All plant and animal life within a 25km radius of the volcano was killed, including fully grown trees Hundreds of square miles were reduced to wasteland, causing over $1 billion in damage (equivalent to $3.5 billion in 2020), thousands of animals were killed, and Mount St. Helens was left with a crater on its north side In the longer term, Bridges were taken out at the mouth of Pine Creek and the head of Swift Reservoir to accommodate the nearly 14,000,000 metres cubed of additional water, mud, and debris as a result of the debris flow
Nevado del Ruiz , Colombia 1985
–good example of poor management , lack of warning and lack of community preparedness and failure to follow advice and lahars as major secondary hazard Armero tragedy was exacerbated by the lack of early warnings, unwise land use, and the unpreparedness of nearby communities, the government of Colombia created a special program (Oficina Nacional para la Atencion de Desastres, 1987) to prevent such incidents in the future. All Colombian cities were directed to promote prevention planning in order to mitigate the consequences of natural disasters, and evacuations due to volcanic hazards have been carried out. SUCCESS: About 2,300 people living along five nearby rivers were evacuated when Nevado del Ruiz erupted again in 1989
Large strato volcano—a cone-shaped volcano built from successive layers of lava, ash, and pyroclastic flow deposits. The volcano is fed by magma generated above the boundary between the subducting Nazca and overriding South American tectonic plates. Melted ice and snow at the summit of the volcano - Mudflows (lahars) swept tens of kms down river valleys along the volcano’s flanks Volcano was monitored but too late to evacuate – 1 hr after detecting likelihood of eruption, lahars had travelled to villages in lower valleys . Problem was disseminating evacuation order and warnings to population – the villages including Armero had no power so no communication lines
Impacts on people: (economic and social) Lahars swept tens of kms down river valleys- killing at least 23,000 people – 70% of the population About 7,500 people were left homeless. Most of the fatalities occurred in the town of Armero which was completely inundated by lahars – this was 40km away from vent . Management before: CO2 and SO2 found at surface - officials in the region predicted an eruption. Warnings and preparedness guides were sent out to all those living in the region. Warnings of a low VEI of 3 and warnings issued. Only small scale activity first so people didn’t evacuate => complacency led to 23,000 deaths in town of Armero 40km away within 4hrs of the eruption lahars – 50m deep lahars when 10% of snow and ice cover melted by pyroclastic flows, travelled at 60km /hr, evacuation notices were too late or not acted upon, no drills and people had no where to go to escape so lack of community preparedness Ruiz is an absolutely seminal event in modern volcanology by virtue of demonstrating the hazards of long-reaching lahars from snow- and ice-clad volcanoes
Impacts on environment The lahars destroyed everything in their paths: roads, bridges, farm fields, aqueducts and telephone lines. They wiped out 50 schools, two hospitals and more than 5,000 homes. The region lost 60 % of its livestock, 30 % of grain and rice crops, and half a million bags of coffee.
Merapi, Java, Indonesia 2010
(LIC, hazard mapping and evacuation) Quite well-managed- Volcanic hazards in Indonesia are managed by National Disaster Mitigation Agency and the Centre for Volcanology and Geological Hazard Mitigation Risks assessed and hazard mapped - local authorities evacuated everyone living within 10km of the summit due to precursors of unusual eruptive behaviour (gas emissions and ground movement) Twenty-three This saved between 10,000 and 20,000 lives. Indonesian Red Cross and Red Crescent had 398 volunteers disseminating information to communities to warn of Merapi’s level IV volcanic activity
Strato-volcano located on the island of Java (Indonesia). Part of the Ring of Fire. Magnitude VEI 4 Plates involved / Boundary type: Subduction zone (Indo-Australian plate beneath Sunda plate). Boundary: convergent Lava type: andesitic – explosive eruption Pyroclastic flow= 100km/h; ash fell more than 30km away Ash, rock and lava deposited on the sides of the volcano was washed down into towns by rainfall creating a lahar due to heavy rain Runout distances of lahars exceeded 15km. Volcanic bombs and hot gases (sulphur dioxide) of up to 800°C spread over 11km away. Sulphur dioxide was blown across Indonesia as far south as Australia.
Impacts on people: (economic and social) Primary Deaths: 353 (many due to severe burns). Injuries: 577 Total cost: US$781 million Secondary 200,000 people made homeless. Evacuation centres were overcrowded leading to poor sanitation, no privacy and serious disease risk. People, particularly farmers, lost their homes and livelihoods Schools were closed up to 120km from the volcano. Financial losses of US$781 million. Tertiary Vegetable prices increased because of damage to crops from ash e.g. potatoes and spinach specifically (they doubled in price) leading to food insecurity. Health problems amongst the evacuees including respiratory issues. Decline in tourism (tourist numbers in Yogyakarta fell as much as 70 %).
Impacts on environment Lava flows closed many roads and other infrastructure was closed off for safety reasons. Lahars destroyed bridges and impacted communication lines. In some places, ash was up to 30cm deep. Water contaminated with acidic lava and ash
