Lecture 9: Volcanic Hazards Flashcards
How many active volcanoes in the world?
596
Since 1950 there have been how many eruptions from how many volcanoes?
2208 eruptions from 347 volcanoes
Average number of eruptions per year since 1950 is
63
Volcanic eruptions can cause
loss of life and livelihoods in exposed communities, damage critical infrastructure, displace populations, disrupt business and add stress to already fragile environments.
Currently, an estimated…live within 100 km of a volcano that has the potential to erupt.
800 million people
Single events have the potential for
devastating mass casualties in a single event
Divergent boundaries
new crust is generated as the plates pull away from each other.
Convergent boundaries
crust is destroyed as one plate dives under another.
Transform boundaries
crust is neither produced nor destroyed as the plates slide horizontally past each other.
Hotspots
areas of volcanic activity away from plate boundaries.
Spreading ridge volcanism
Volcanism related to plate divergence
Composite or stratovolcano
Volcanism at an ocean-continent boundary (similar processes take place at ocean-ocean boundaries)
Shield volcano
Volcanism related to a mantle plume
Volcanic rocks, lava and ash are classified by the amount of silica they contain
Increased Si = Increased Pressure = More Explosive Volcanism
Shield volcanoes are almost exclusively
basalt, a type oflavathat is very fluid when erupted.
Shield volcanoes are most likely to produce
smaller geographically constrained eruptions – only explosive if water enter the vent
90% of the volcano is
lava rather than pyroclastic material
Largest percentage of the Earth’s individual volcanoes (~60%)
Stratovolcano
Stratovolcanoes
Most are characterised by eruptions of…
andesite and dacite lava that are more viscous than basalt – allows gas pressure to build explosive eruptions
Usually 50% lava and 50% pyroclastic material
Calderas and Caldera Complexes
Caldera complexes are the most explosive volcanoes – when they erupt they collapse in on themselves
Common eruptions types are:
Hawaiian Strombolian Vulcanian Plinian Phreatomagmatic
Hawaiian Eruptions
Type of magma
Basic, low viscosity
Strombolian eruptions
Type of magma
Moderate viscosity
Vulcanian Eruptions
Type of magma
Acid, viscous
Plinian Eruptions
Type of magma
Acid, viscous
Hawaiian Eruptions
Nature Of Effusive Activity
Thin, extensive flows from central vent
Strombolian eruptions
Nature Of Effusive Activity
Flows often absent
Vulcanian Eruptions
Nature Of Effusive Activity
Flows often absent
Plinian Eruptions
Nature Of Effusive Activity
Absent
Hawaiian Eruptions
Nature Of Explosive Activity
Very weak
Strombolian eruptions
Nature Of Explosive Activity
Weak to violent
Vulcanian Eruptions
Nature Of Explosive Activity
Moderate to violent
Plinian Eruptions
Nature Of Explosive Activity
Cataclysmic
Hawaiian Eruptions
Structures Formed Around The Vent
Broad lava domes and shields
Strombolian eruptions
Structures Formed Around The Vent
Cinder cones and lava flows
Vulcanian Eruptions
Structures Formed Around The Vent
Ash cones, explosion craters
Plinian Eruptions
Structures Formed Around The Vent
Large explosion calderas
Phreatomagmatic eruptions
Volcanic eruption resulting from the interaction between magma and water
Phreatomagmatic eruptions
Surtseyan
volcanic eruption caused by shallow-water interactions between water and lava
Phreatomagmatic eruptions
Subglacial
volcanic eruption characterized by interactions between lava and ice, often under aglacier
Volcanic Explosivity Index (VEI)
A relative measure of the explosiveness ofvolcanic eruptions.
Volume of products, eruption cloud height, and qualitative observations are used to determine the explosivity value.
The scale is logarithmic, with each interval on the scale representing a tenfold increase in observed ejecta criteria.
Hazards and Impacts
Volcanic ash and tephra Pyroclastic flows, surges and blasts Lahars and Floods Debris Avalanches and Landslides Lava Flows Volcanic gases
Tephra and Volcanic Ash
Anything that comes out of a volcano during an eruption
Blocks (>64mm)
Lapilli (64 – 2mm)
Ash (2mm
Pyroclastic flows, surges and blasts
Pyroclastic flows contain a high-density mix of hot lava,blocks, pumice,ashand volcanic gas.
They move at very high speed down volcanic slopes, typically following valleys - up to 200 m/s
Pyroclastic flows, surges and blasts
Hazards
Most lethal volcanic hazard – 1/3 of all fatalities
Destroy nearly everything in their path
Severe damage to infrastructure, vegetation and agricultural land
Extreme temperatures can ignite fires and melt snow and ice
On the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hotashand gases.
Can lead to secondary hazards such as flooding and lahars
Lahars and Floods
Lahars are fast moving mixtures of volcanic debris and water
Commonly occur when intense rain moves loose volcanic rock during an eruption
Also caused by volcanic activity melting ice caps and glaciers
Can threaten an area for years after an eruption
Lahars and Floods
Hazards
Largelaharscan crush, abrade, bury, or carry away almost anything in their paths.
Buildings and valuable land may be partially or completely buried.
By destroying bridges and roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity
Confined to valleys and areas close to volcano so can identify vulnerable areas.
Debris Avalanches and Landslides
Landslides are large masses of wet or dry rock and soil that fall, slide, or flow very rapidly under the force of gravity.
Landslides are common on volcanic cones because they are tall, steep, weakened by the rise and eruption of molten rock and constructed from poorly consolidated deposits.
Can lead to lateral volcanic blasts as highly pressurized volcanic interior is exposed
Can also cause tsunamis if material enters the sea
Several conditions can trigger landslides:
Explosive eruptions
Largeearthquakedirectly beneath a volcano or nearby
Heavy or long-lived rainfall that saturates the ground.
Debris Avalanches and Landslides
Hazards
Typically destroy everything in their path
Can cause additional hazards
Dam lakes leading to lahars
Change pressure of magma chamber triggering further eruptions
Lava Flows
Streams of molten rock that pour or ooze from an eruption vent
Lava Flows
Hazards
Destroys everything in its path
Deaths are uncommon because it moves slowly
Can bury homes and agricultural land under tens of meters of hardened black rock
People are rarely able to use land buried bylavaflows or sell it for more than a small fraction of its previous worth.
The speed at whichlava moves across the ground depends on several factors
Type of lava erupted and its viscosity
Steepness of the ground over which it travels
Whether the lava flows as a broad sheet, through a confined channel, or down alava tube
Rate of lava production at the vent.
Volcanic Gases
Magma contains dissolved gases which escape during reduction in pressure as the magma moves towards the surface.
Main gas is water vapour (60-99%) which is harmless
Other gases include
Carbon Dioxide – up to 10%
Sulphur dioxide and other sulphur gases – up to 15%
Halogens, including chlorine – up to 5%
Volcanic Gases
Hazards
Fatalities and health impacts
Damage eyes and respiratory system
CO2 asphyxiation
Damage to vegetation, livestock, infrastructure and property
Impact varies widely and depends on the amount and type of gas emitted.
Environmental hazards
Cooling of the Earth and Acid rain
Volcano Monitoring
Volcanic eruptions are typically preceded by days to months of precursory activity, unlike other natural hazards like earthquakes.
Volcano observatories play a key role in monitoring and early warning due to the localised character and individuality of volcanoes around the world
Set up on all active volcanoes that threaten the human population
Designed to monitor and potentially predict the eruptive behaviour
Volcano Monitoring Methods
Seismic activity Increase in local earth quake activity Audible rumblings Ground deformation Upwelling near volcanic vent Changes in ground slope Hydrothermal phenomena Increased discharge or temperature from hot springs or fumaroles Rise in temperature in crater lakes Melting of ice and snow on the volcano Withering of vegetation of volcano slopes Chemical changes Changes in chemical composition of gas from vents
Hazard Mitigation
The process for mitigating risk differs from country to country.
Communities need to understand their risk and take action to mitigate risk.
Hazard Mitigation
Pyroclastic flows
Prior evacuation of threatened areas (identified by hazard mapping) can reduce loss of life. Effects of pyroclastic flows could be mitigated via long-term urban planning (subways, strong buildings)
Hazard Mitigation
Lava flows
Hazard maps can be developed for areas of at risk of flow-related damage
Hazard Mitigation
Lahars and Floods
Hazard mapping to identify high risk areas (valleys). Warning systems that permit evacuation could reduce the lethal effect of lahars.
Engineering measures, such construction of sediment dams, can reduce flow mass and resulting destruction. Long-term urban planning can also reduce risk by avoidance of dense population concentrations in valleys and deltas.
Hazard Mitigation
Tephra
In the developing world, tephra accumulation on roofs of poorly constructed buildings often results in collapse; campaigns to promote ash removal can reduce fatalities and damage from tephra-induced roof collapse. Humanitarian assistance can reduce potential long-term impacts of resulting crop loss, livestock deaths, and economic disruption.
Hazard Mitigation
Debris avalanches
Prior evacuation the only measure for preventing loss of life.
Hazard Mitigation
Gas
Identification and ongoing monitoring of hazardous lakes, resettlement, pre-evacuation, and establishing guidelines relative to areas of refuge.
USGS employs a nationwide volcano alert-level system.
Ranked terms to inform people on the ground about a volcano’s status
Ranked colors to inform the aviation sector about airborneash hazards.
