Topic 5 - Concepts and CS Flashcards
Hazard
Event that has a negative impact on humans and the environment, includes substantial loss of life and damage to property
constructive plate margin
two plates moving away (diverging) from each other
Destructive plate margin
Oceanic x continental
Subduction zone
(eg Pacific Ocean of South America)
Oceanic plate is denser than continental plate and so subducts beneath it. Bending oceanic plate at point of collision forms a deep oceanic trench such as the Peru=Chile trench.
As the plates converge the continental plate is uplifted, compressed and then buckles forming folded mountains like the Andes.
The oceanic plate melts then is fully destroyed, this melting zone is called the Benioff zone. The heat is caused by friction as well as the increasing heat. The friction builds tension which is released as an immediate or deep-focus earthquake.
Destructive plate margin
Oceanic x oceanic
Subduction zone
(eg marine trench, western pacific)
The faster or denser plate subducts beneath the other forming a deep ocean trench
Subducted plate melts due to heat from asthenosphere and friction
Friction causes both shallow and deep focus earthquakes
Magma from subduction is silica-rich andesitic magma with high gas content making it explosive
Explosive magma rises through cracks in the continental plate forming strata volcanoes
Destructive plate margin
Continental x continental
Collision zone
(eg Himalayas)
Continental plates have lower densities than the asthenosphere below them.
Neither plate subducts as they have similar densities, instead they collide and crumple upwards forming fold mountains
No volcanoes as there’s no subduction
Violent earthquakes due to constant friction + build-up of tension
conservative plate margin
two plates moving sideways past each other
No volcanic activity as there is no subduction, no new crust etc. Margin associated with powerful earthquakes. Friction between two moving plates builds tension which is suddenly released, as there is no subduction powerful shallow-focus earthquakes are produced
Volcanic hazards
Explosive eruptions
Associated with destructive margins and formation of composite volcanoes
Magma is viscous due to high silica content and explosive due to gas
Volcanic hazards
Pyroclasts
Large angular fragments + aerodynamic volcanic bombs ejected from explosive eruptions = pyroclasts
Can crush people, buildings and cars
Volcanic hazards
Tephra + Volcanic ash
CS Mt Sinabung
Can rise and form an eruption column
Can be carried by the wind and spread over long distances
Ash can collapse roofs and cover vegetation affecting crops + harvest
CS Aug 2010, Jan 2014 Mt Sinabung erupted, blanketing cities in North Sumatra
Volcanic hazards
Pyroclastic flow
Fuego volcano
Occurs when eruption column collapses
Flow comprised of hot rock, ash and volcanic gases, flows down flanks of volcano, can have hurricane force speeds
Most destructive volcanic hazard
CS Fuego volcano, Guatemala erupted causing flows that killed dozens.
Volcanic hazards
Nuee Ardentes
Mt Pelee
Glowing avalanches, pyroclastic flow
Caused by lateral volcanic eruptions where summit crater is blocked
Contains dense material, only travels 50km from the volcano
they are higher density than pyroclasts
CS Mt Pelee erupted, destroying St Pierre in 1902 killing 29,000 people
Volcanic hazards
Lahars (secondary)
Armero
Dangerous mudflows formed when water from rivers/snow-capped mountains mixes with volcanic ash + fragments
Creates fast flowing hazard as it slides down slopes of volcanoes
CS 20,000 people died in Armero when lahars from an eruption engulfed the town in 1985
Volcanic hazards
effusive eruptions
Mt Kilauea
Associated with constructive plate margins, hot spots and formation of shield volcanoes
Form low-viscosity basaltic lava, flows over long distances
CS Kilauea, Hawaii is the most active volcano. It inundated homes in Kalapana and destroyed over 700 homes in 2018, erupted again in Oct 2021
Volcanic hazards
volcanic gases
Mt Pinatubo
Include hydrochloric acid which condenses with water as well as sulphur dioxide
Sulphur dioxide can cause acid rain, ozone depletion and air pollution. Also forms volcanic smog eventually, which damages vegetation
CS Mt Pinatubo ejected 20 mil tonnes of Sulphur dioxide in 1991
Spatial distribution of volcanic activity
Volcanic activity is common at destructive and constructive plate margins, absent from conservative margins and collision zones. Some volcanoes occur at the centre of plates, like the Hawaiian hot spot. Volcanoes are common along rift valleys.
How can the magnitude of eruptions be measured
Can be measured using the Volcanic explosivity index:
Measures relative explosivity of an eruption
Logarithmic scale
High on the scale = high the volume of ejected material
Measures magnitude of eruption
Frequency of volcanic eruptions
St Maria
Some active volcanoes erupt once every 100 000 years, others erupt every month.
Generally, less frequent eruptions have a greater magnitude and more damaging.
CS - Saint Maria, Guatemala is a frequently erupting stratovolcano
However, it had eruptions ranking 6 on the VEI + lahars during rainy seasons
Randomness vs regularity of volcanic eruptions
Kilauea
Some volcanoes erupt at regular intervals; others may be dominant for thousands of years but randomly erupt multiple times in a quick succession.
Kilauea erupts once every 2-3 years
CS - Mt Merapi, Java
most active volcano in Indonesia, 2010
Most active volcano in Indonesia
Stratovolcano with frequently violent eruptions
Tens of thousands of people farm on the fertile flanks, at risk of its eruption
Oct 2010, after more than 5000 earthquakes were recorded beneath the volcano, evacuation of 20,000 was advised
Powerful pyroclastic flows down the flanks of the volcano followed a series of eruptions
Fires, respiratory failures, and blast injuries caused 350 fatalities
350,000 were displaced as their homes were destroyed and farmland was covered in thick ash. Ash also led to aviation problems across Java
Hazard mitigation
Anything done to reduce severity or impacts of a hazard
spatial distribution of earthquakes
Location of earthquakes is closely associated with plate margins. There is a high frequency of earthquakes along the Pacific great ring of fire, extremely concentrated where the Pacific plate meets the Indo-Australia plate at a conservative and destructive margin.
Earthquakes at destructive margins
Subduction results in deep focus earthquakes, subduction also results in a wider horizontal spread as the oceanic plate sinks at an angle, deep into the asthenosphere. These earthquakes are powerful as compressional forces are greatest in the Benioff zone. They can occur under the sea close to populated coastal zones, making the threat of tsunamis more likely. Tend to have higher concentration of earthquakes compared to conservative margins, eg N and W edge Pacific ring of fire
earthquakes at conservative margins
Earthquakes have a shallow focus. Continental plates are dragging past each other, and compressional forces are high. They are powerful and can be extremely damaging if they occur to densely populated areas. High frequency of earthquakes associated with this margin
CS - Earthquake resulting from human activity, China
Quasi-natural earthquakes most commonly associated with crustal deformation caused by the weight of water stored in reservoirs
The Sichuan earthquake, 2008 which killed 80,000 people has been associated with the construction of the Zipingpu dam and the pressure it exerted on underlying rocks once at full capacity
Richter scale
1-10 logarithmic scale, 7 is 10x more powerful than 6 and 100x than 5
Earthquakes ranking 8+ tend to occur once or twice a decade
Accuracy of scale decreases for larger earthquakes
Moment magnitude scale
Measures total distance a fault has moved and the force needed to generate it aka the moment release of an earthquake
Modified mercalli intensity
Measure of the effect on people and human infrastructure
A moderate earthquake under a major city can register higher on the MMI than a strong one in an unpopulated area
Earthquakes
Release of energy in the form of seismic waves from the focus, where the pressure is released from
Tsunamis
Japan Tsunami
Generated by seismic waves produced by a large submarine earthquake
Rapid deformation of seabed can uplift a column of water as oceanic crust is thrust up
The column of water collapses, radiating energy outwards from the focus
CS – Japan Tsunami, 2011
Tokuoka earthquake generated Tsunami reaching heights of 40m
4.4 million homes left without electricity and 16,000 died in north-east Japan
Soil Liquefaction
Seismic waves travel through soft sediments causing them to behave like a liquid due to an increase in pore water pressure
Affects saturated, unconsolidated sediments at a depth of less than 10m
As a result, buildings become unsupported causing them to sink or topple over
Landslides
CS Nepal landslides
Sudden shaking of the ground creates slope failure, even of gentle slopes
Landslides can be more dangerous than primary ground shaking, particularly in mountainous zones
CS – Nepal landslides, 2015
300 cubic meters of landslides occurred as a result of the Gorkha earthquake
Avalanche triggered by quake travelled through Mt Everest base killing 17 and injuring 61
CS – Indian ocean tsunami, 26th Dec 2004
Third largest earthquake in history 240km north of Indonesia, ranking 9 on the Richter scale triggering a huge displacement of the seabed resulting in the large tsunami waves
290,000 missing/dead
Affected 15 countries, all were low lying and surrounded by sea, vulnerable to the tsunami
CS – Port-au-Prince earthquake, Haiti, 2010 (LDE CS)
approximately 3.5 million people lived in the area that experienced shaking intensity of MM 7 to 10, a range that can cause moderate/heavy damage even to earthquake-resistant structures.
Magnitude 7 earthquake was released after decades of built-up tension, shallow focus at 13km
3 million people were affected, 220,000 deaths
1,3 million people leave homeless due to collapsed houses
CS – Tohuku earthquake, Tokyo, 2011 (HDE CS)
20,000 killed, 6,000 injured
People only had minutes to evacuate after the JMA sent a TV broadcasted earthquake warning
The 2011 earthquake and following tsunami resulted in 20,000 deaths and several thousand unrecovered persons
Most expensive natural disaster Japan has faced, 98% of the damage was attributed to tsunamis
On Sept 1 each year, the Japanese practice earthquake dills on a national training day. It marks the anniversary of the Tokyo earthquake, 1923 which killed 156,000
Tropical storm
A low pressure, spinning storm with high winds and torrential rain
Conditions for tropical storms
Air pressure
Must be an area of unstable air pressure – usually an area where high and low air pressure meet (convergence) so warm air rises more readily forming clouds, air must also be humid
Conditions for tropical storms
Temperature
Ocean temperatures must be around 26 - 27°C and at least 50 metres deep. Warm water provides the storm with energy
Conditions for tropical storms
rotation
Tropical storms only form around the equator, but no less than 5° on either side. The Coriolis Effect is the effect of the Earth’s rotation on weather events. The storm spins because the Earth is spinning; but there is no Coriolis Effect at the equator, hence why these storms will only form a certain distance away from it
Conditions for tropical storms
A trigger
A pre-existing thunderstorm, a spot of very high sea surface temperature, an area of low pressure and many other factors can act as a trigger for a storm to develop, which will only further develop when the other conditions are present
storm hazards
Heavy precipitation
Can rapidly increase river discharge resulting in flash floods
Water can soak into raised relief resulting in landslides which entail huge loss life + expensive damage to property
storm hazards
High winds
Some may exceed 150mph
Property destroyed, can also lift objects turning them into projectiles
Can also drive coastal waves forward producing storm surges
storm hazards
Landslides
Flooding can cause material on slopes to be loosened via undercutting resulting in mass movement down slope onto typically inhibited areas below
storm hazards
river flooding
Storm surges can flow up river channels causing flash floods further inland – areas where the surge would not otherwise reach
storm hazards
coastal flooding
This is particularly problematic in countries where large populations living along the coast as the human death toll and economic cost can be immense
Saltwater contamination can fail entire crop harvests and make agricultural land unstable for months after
storm surges
Storm surges can sweep over lowland areas destroying property and people, typically most densely populated
Winds push the raised coastal waters which are a result of the low pressure atmospheric conditions of the storm
Can be as high as 7m
CS – Hurricane Katrina, HDE storm hazard
Hit USA’s east coast 2005, one of the wealthiest countries in the world
It was a category 3 hurricane when it hit Mississippi and Louisianna
200km/h windspeeds, storm surge of 8.5m in Mississippi (bay area)
The delta was right at sea level, easily flooded. The bay increased storm surge height
Storm surge and heavy rainfall flooded 80% of New Orleans, the hazards had the following
CS – Cyclone Nargis, LDE storm hazard
Nargis was a category 4 cyclone when it reached Myanmar, one of the least developed countries. Wind speeds of 215km/h and storm surge heights of 7m
Irrawaddy delta was the worst hit, very low lying, majority was flooded
Wildfires
CS – Cyclone Nargis, LDE storm hazard
conditions for wildfires
fuel
Any flammable material surrounding the fire: trees, grass, soils even buildings. The greater the fuel load the greater the fire’s intensity
conditions for wildfires
air
Supplies oxygen the fire needs to burn. Initially supplied by prevailing winds. A fire can self sustain oxygen supply once fire-tornado conditions are reached – fire pulls air in
conditions for wildfires
combustible heat
Provided by lightning, campfires, volcanic eruptions, cigarettes or decomposing vegetation. They can also be started by humans - arson
CS – Australia’s Black Friday wildfire
The wildfire coincided with the IOD positive phase, the climate was drier than normal which increased the risk of an outbreak. Scientists used this data to predict hazard risk will increase with a warming climate.
CS – Wildfires associated with El Nino
Alberta wildfire, Canada 2016
Ash Wednesday, Australia1983
NASA found links between El Nino warming and increased CO3, associated with forest fires. Indonesia, 1990s
CS – “the beast”, Canadian Wildfire, 2016
The wildfire swept across parts of Canada’s Alberta Province, 600 000ha of land was burned
No one was injured or killed as a direct result of the fire, however analysis suggests it may have been Canada’s most economically taxing disasters
CS – Australian Wildfires, 2019-2020
The fires raged from June 2019 to March 2020 with the fires occurring in December and January
The worst affected regions were/are New South Wales and Victoria
More than 10 million hectares of land were burnt, 33 lives were lost and thousands of building were destroyed
