Hazards: Seismic Hazards (theory of tectonic plates & volcanic hazards) Flashcards
What are the 4 layers of the Earth?
Inner core = solid ball, iron and nickel (6000ºC)
Outer core = semi-molten, iron and nickel
Mantle = silicate rock, mostly rigid (top layer is called the Asthenosphere, semi-molten)
Crust = oceanic crust (thin and dense) and continental crust (thick and light), rigid top part of the mantle and crust is called the lithosphere
Why is the core and mantle so hot?
heat energy left over from when the Earth was formed and some comes from radioactive decay
- main source of internal energy because of this
Evaluate the evidence for the theory of continental drift: oldest evidence?
- Mostly geological evidence, the weakest of all evidence
- apparent closeness of the coastlines of South America and West Africa suggests they were once joined
- Mountains and rock types found to be identical from the Appalachian mountains (Eastern USA) to the UK, Scandinavia and North-West Africa, suggests joining
- overall suggests that the supercontinent Pangea existed
- no explanations of how continents move
Evaluate the evidence for the theory of continental drift: Following WW2?
- Biological evidence
- Sonar studies of seafloor -> areas of mountains & valleys found underwater in oceans
- change in Earth’s polarity creates striations on either sides of ocean ridges
- provided evidence for sea-floor spreading (only one reason)
- showed crusts were getting bigger
Evaluate the evidence for the theory of continental drift: Today?
- satellites accurately measure global movement of places, 2-20cm per year on average
- seismometer readings show clear patterns following edges of tectonic plates
- Modern technology allows us to witness plate movement in real time accurately, evidence of Wagner’s theory grows stronger
- However, this evidence doesn’t completely conclude how the plates are moving
What is the theory of continental drift?
- published by Wagner in 1912
- He suggests that 300 million years ago the supercontinent Pangea split and land masses started to move into current positions today
What are the 3 main drivers of plate tectonics?
- Convection Currents (mantle convection)
- Ridge Push (gravitational sliding)
- Slab Pull
How does convection currents drive tectonics?
- hotspots around Earth’s core generate thermal convection currents in asthenosphere
- warmer less dense material rises to cool and sink
- this causes magma to rise up towards the crust and spread before sinking
- this is the main driver of movement in the lithosphere and crust by FRICTION between asthenosphere and lithosphere which pulls/drags plates
- doesn’t account for different rates of plate movement
How does ridge push drive plate tectonics?
- rising magma pushes ridge higher than seas floor, so the thickness and weight of rock push down and out on the crust due to gravity
- this speeds up the process of sea-floor spreading
- accelerates the moving of the plate
How does slab pull drive plate tectonic?
- weight and density off descending slab at destructive plate boundary is higher than the surrounding material so it sinks
- gravity-generated force pulls subducting plate down adding to speeding up of movement
What processes are occurs at a Destructive Plate Boundary?
- Magma convection (friction) of opposite continental plate and oceanic plate so they’re pushes together
- slab pull as gravity generated force increase speed of subduction of oceanic plate (more dense so it sinks)
Friction and subduction
What are some examples of destructive plate boundaries?
Eurasian and Indian plate
Nazca and South American plate
What processes are occuring at constructive plate boundaries?
- Mantle convection creates friction between asthenosphere and lithosphere which pulls the plates
- new crust being formed by rising magma
- ridge push, gravitational force downwards of the ridge formed will create a push
- sea-floor spreading -> mid atlantic ridge
What are some examples of destructive plate boundaries
Nazca and Pacific Plates
South American and African plate
What is happening at a conservative plate boundary?
- Plates are moving at different speeds because there are different magnitudes of force of slab pull and ridge push on the plate (push and pull forces)
- they push past each other at different speeds
- lithosphere is never created or destroyed
What is the case study of a volcanic eruption in a country with high HDI?
Eyjafjallajokull (E15) Volcanic Eruption in Iceland April 2010
What were the characteristics of the hazard in Iceland volcanic eruption?
- 4 on VEI scale
- constructive plate boundary on a hotspot
- Basaltic and Andesitic lava (low viscosity)
- stratovolcano that was covered by glacier
- interaction between lava and ash creates huge ash clouds
- eruption lasted for over a month
What were the characteristics of the place/people in Iceland volcanic eruption?
- 28th richest country
- nobody lives below the poverty line
- extensive management, e.g. land use planning and education
- rural area is sparsely populated
What are the facts about the disaster in the Iceland volcanic eruption?
- 0 deaths
- 1000 evacuation
- £130 million per day cost to the airlines that had to suspend flights
- homes and infrastructure damage
- crops damaged
What action was taken pre-disaster in Iceland?
- used the hazard response cycle: risk mapping to inform settlement planning
- scientists mitigating and monitoring the volcano
- community preparation and education
What actions were taken during the response in Iceland?
- bringing animals and securing homes in rural areas
- closing roads in case of lahars (roads designed to be swept away)
- evacuation
- exclusion zone in Iceland and air space due to ash cloud
What actions were taken post disaster in Iceland?
- rebuilding homes (insurance)
- lessons taken into account for future management
- studying the eruption to understand it more (ie. the ash plumes)
What is the case study of a volcanic eruption in a low HDI country?
Mt Nyiragongo, Congo, January 2002
What were the characteristics of the hazard at Mt Nyiragongo?
- 1 on the VEI scale
- constructive plate boundary
- large basaltic stratovolcano
- lava flow of 90km/ph
- earthquake of 5 on Richter scale
What was the evidence of a disaster at Mt Nyiragongo?
- 147 fatalities
- 1/3 Goma destroyed
- 350 000 fled to neighbouring Rwanda
- Public disorder
- drinking water poisoned
What were the characteristics of Congo?
- dense urban population in Goma 15 km away
- 228th richest country
- 68% living below the poverty line
- in the midst of recovering/living through a civil war that caused 5 million deaths, from 1994 to 2003
- eastern areas still were plagued with violence by rebel groups, UN peacekeeping mission there
What action was taken in the pre-disaster period in Congo?
- no action or monitoring taken even after tremors started
- all warning signs of eruption were ignored
- no preparation due to weak local government
What happened during the response to the eruption in the Congo?
- looting
- 350 000 fled to Rwanda (no resources)
- $15 million donated from Red Cross
- evacuation ‘red alert’ issued
- shelter and food from agencies
- UN set up emergency shelter, health care and any essentials
What happened post-disaster in the Congo?
- No long term aid for recovery due to Congolese Civil War
- extremely slow return to normality, long emotional recovery as they had to wait until lava cooled to bury bodies
- 320 000 left homeless
Q: Evaluate the effectiveness of the hazard management cycle in mitigating the impact of volcanic eruptions [20 marks]
It is/it is not effective effective…
- evidence of an eruption impact being mitigated (or not) by management
- how effective was planning in enabling this mitigation
- can you make reference to the different stages of model?
Conclusion:
Not effective in mitigating impacts
- only wealthy countries are able to utilise the cycle
- only a format, useless when not being used
Q: ‘It is possible to manage the impacts of volcanic eruptions, but the impacts cannot be prevented’ To what extent do you agree with this view? [20 marks]
Agree:
- impacts cannot be prevented
- iceland case study, still had impacts despite extensive management
Disagree:
- isn’t always possible to manage impacts
- depends on the capacity of the place to cope with the impact
- use Congo case study
How are ocean ridges formed?
- underwater constructive plate boundary move apart allowing for upwelling magma from mantle to reach surface
- magma cools and forms new crust as a ridge
- new ridge forces down on plate causing sea-floor spreading
- underwater volcanoes can form, and build up to form islands (e.g. Surtsey an Icelandic Island)
- e.g. Mid Atlantic Ridge
What are magma plumes?
- vertical column of extra-hot magma that rises up from the mantle
- form many connection cells all over the planet, these move plates in different directions
- only a few strong enough to break lithosphere and create hotspot
How do rift valleys form?
- at constructive plate boundaries across continental areas
- magma rises causes crust to bulge and fracture forming fault lines
- areas between parallel faults become brittle and drop forming rift valleys
- volcanoes form
- East African Rift valley from Mozambique to Red Sea
How do deep sea trenches form?
- at destructive plate boundary
- dense oceanic crust is forced under lighter continental crust in subduction
- forms a very deep point of ocean
- e.g. Marianans Trench
How do island arcs form?
- hotspots cause magma to eat into plate above
- lava breaks through and creates underwater volcanoes which build up to form islands
- plate moves over hotspot creating a line of volcanoes (islands)
- e.g. Hawaiian Islands
How do young fold mountains form?
- when 2 continental plates are moving towards each other
- lower density than underlying layers of plate so no subduction
- sediments are forced up forming mountains
- no volcanic activity only earthquakes
- e.g. Himalayas
How do volcanoes form?
- the deeper a plate descends into a subduction zone the hotter the surroundings become in mantle
- this alongside friction oceanic plates melts into magma in Benioff zone
- less dense material in asthenosphere rises as plutons reaching the surface to form volcanoes
What is explosivity of a volcano determined by?
Type of lava:
basaltic = low viscosity = more gentle (less silica)
andesitic = high viscosity = explosive (more silica)
rhyditic = high viscosity = explosive (more silica)
What is the magnitude of a volcanic eruption measured by?
Volcanic explosivity index (VEI) 0 to 8 (logarithmic scale)
- doesn’t take gas emissions into account
What is hazard intensity?
Measure the scale of the effects on people, human structures and the natural environment of the hazard
What is areal extent?
The size/scale of the area impacted by a high
Q: Assess the scale of an eruption.
Things to write about…
- areal extent of the eruption
- hazard intensity (effect on people, human structures and natural structures and the extent of this all, ie severe moderate etc)
- VEI
- physical characteristics or impacts
Categorise the types of volcanoes…
Destructive: Dome volcano (moderate explosivity -> tend to have a high silica content) Composite volcano (high explosivity -> lava has a high silica content and can build up high gas pressure)
Constructive: Fissure volcano (low -> basaltic lava) Ash-cinder (moderate -> cones form on the flanks of shield, strato and shield volcanoes)
Hotspots:
Shield volcano (low -> basaltic lava)
Caldera (High -> old cone collapsed in to orm a new cone with magma chamber below)
What is an example of a caldera volcano?
Yellowstone super volcano
Do volcanic hazards show a relationship between hazard intensity and areal extent?
- primary hazards tend to have a small areal extent but a high hazard intensity
- secondary hazards tend to have a large areal extent and a lower hazard intensity
- lahars and tsunamis are the exceptions: lahars have a small areal extent and tsunamis having a high hazard intensity
What are the primary hazards of a volcanic eruption?
Lava flows, tephra, volcanic gases and pyroclastic flows
What are the secondary hazards of volcanic eruptions?
Landslides, Lahars, Tsunamis, Flooding, Acid Rain, Climatic Change
What are pyroclastic flows?
- aka nuee ardente
- mixture of super-heated gas, ash and volcanic rock flowing down the side of a volcano at high speeds (80km/h) for long distances (10-15 km)
What are lava flows?
lava flowing from the vent of the volcano, speed and distance depending on the viscosity of the lava
What are volcanic gases?
CO2 and sulfur dioxide that can be humans and animals
What is tephra?
solid material that is ejected by a volcano into air ranging from large volcanic bombs to fine ash
What are lahars?
- formed by volcanic ash mixing with water and flowing downhill
What is acid rain?
sulphur combining with atmospheric moisture and acid rain results
Q: Analyse the role of plate margin type in influencing the risk posed to people by volcanic hazards?
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What are the warning signs and monitoring techniques of volcanic eruptions?
- Lots of small earthquakes as magma rises through crack = seismometers detect earthquakes
- Temperatures around the volcano starts to rise = thermal imaging and satellite cameras detect heat
- Higher sulphur content in gases = gas samples taken and chemical sensors
What is the importance of monitoring of volcanoes?
allows for prediction and allows identify a time when to evacuate safely
What are some preparation techniques of volcanic eruptions?
- plan of evacuation
- producing series of threat levels (done in NZ)
- individual preparation
- community search and rescue teams
What are some protection techniques of volcanic eruptions?
- identifying high risk areas (land use planning), e.g. risk assessment made for an area around Mt Rainier in USA protects 3.5 million
- digging trenches, dropping blocks and using explosives to divert lava
What are some adaptation techniques of volcanic eruptions?
- strengthened buildings to reduce chance of collapse
- capitalise on factor of living next to volcano: farming with fertile soil or tourism industry
