Tectonic Hazards 1.1-1.3 Flashcards
Why are some locations more at risk from tectonic hazards?
Tectonic hazards occur in specific locations in relation to plate boundaries
Hazard distribution is uneven
Some tectonic activity can generate multiple hazards
How old is earth?
4.6 billion years
What is the lithosphere
The crust and upper mantle
What is the asthenosphere
It’s 80-100km below the surface, semi molten, moving layer
Continental crust characteristics
Granite. Up to 70km thick, lighter. Not able to be created, much older.
Oceanic crust characteristics
Basalt. 6-10km thick, more dense. Able to be destroyed at subduction and created at divergent boundaries
The 4 theories of why tectonic plates move
Mantle convection
Slab pull
Subduction zone movement
Sea floor spreading
Divergent plate boundaries
Most common at mid ocean ridges Lots of shallow focus earthquakes New oceanic plate formed Shield or rift volcanoes (less hazardous) Rift valleys eg. Iceland
Convergent plate boundaries
Plates move towards each other and one slides under - the subduction zone.
Deep ocean trenches appear
Much larger earthquakes or tsunamis
Produces explosive volcanoes
Creates mountain ranges when they collide
Conservative plate boundaries
Consist of transform faults (weaknesses in crust)
These faults join up - locked fault
Creates friction which then releases energy = earthquake
No volcanic activity
Eg. The San Andreas Fault
Shallow focus earthquakes of high magnitude
The earth’s layers
Inner core
Outer core
Mantle
Crust
Inner core characteristics
5150km
Solid
7000 C
Radio active decay
Outer core characteristics
2890-5150km
Liquid iron and nickel
4400 to 6100 C
Mantle characteristics
Upper part is solid
Lower part is semi molten
700–2890km
870 C
Crust characteristics
Solid
Up to 400 C
Mantle plumes
Long lived areas of high levels of heat flow within the mantle. A long, thin plume with a bulbous head.
Mantle plumes formation
Upwelling long thing conduit
Bulbous head which spreads at the base of the lithosphere
Huge volumes of magma because of partial melting due to a drop in pressure
Heat from the core is passed to the mantle by conduction
Heated bits of lower mantle become less dense so rise as diapirs
Build up of magma eventually creates volcanic activity
Hot Spot examples
Galápagos Islands
Canary Islands
Hawaiian Islands
Shield volcanoes
Runny lava
Slowly sloping
Basalt (low in silica)
Eg. Mauna Kea
Volcanic hotspot formation
Mantle plume creates bulge beneath the crust
The crust is melted and forms magma
The lava flows overground and solidifies to form islands
This lava is liquid basalt
These volcanoes have gently sloping sides
Why are Hawaiis volcanoes shield shaped
The basalt lava that erupts has low viscosity and flows quickly so spreads over large distances then solidifies. This causes volcanoes to have very wide bases and gentle slopes.
Why do hotspots stay in the same place
They are long lived so stay where they are due to high heat flow
Why does magma break through the surface of the ocean floor
The build up of magma beneath the surface eventually breaks through structurally weak areas on the ocean floor due to the sheer amount and heat of magma.
Hotspots and seismic activity
Due to structural weakness at the bottom of the volcano the ‘fissures’ at the bottom become bigger, making seismic activity possible.
Focus
Point at which the earthquake happens
Epicentre
The point on the surface above the focus
Intraplate activity
Tectonic activity in the middle of the plate
Plate tectonic theory
A theory developed 60 years ago to explain the large scale movements of the lithosphere
Pangea Theory
In 1912 Alfred Wegener came up with his theory of Pangea
We were all originally one continent but separated out over millions of years
Biological evidence for Pangea
Reptile remains found in South Africa and Brazil
A plant that existed when coal was formed found in both India and Antarctica
Geological evidence for Pangea
Rocks of a similar age, type, structure and formation found in South Africa and Brazil
Climatology evidence for Pangea
Coal found under Antarctic ice cap
Evidence of glaciation in Brazil and India
Earth’s temperature gradient
The earth’s mantel has a geothermal gradient. The highest temperatures are where the mantle is next to the core so temperature increases with depth.
1940s development in tectonic theory
Harry Hess reveals the shape of ocean floors with sonar and radar
1960s development in tectonic theory
Collection of crustal samples and dating them
J. Tuzo Wilson proposes theory of sea floor spreading in 63
Then him and Harry Hess propose a combined theory of sea floor spreading and continental drift
Mantle convection
Heat which is derived from the earth’s core rises to drive convection currents (cells) which move tectonic plates
Palaeomagnetism
The study of past variations in the Earth’s magnetic field
It can be used to date the age of new tectonic crust
Sea floor spreading
Occurs at divergent boundaries under oceans
Continuous input of magma forms a mid ocean ridge
On land this would form a rift valley
Subduction
At a convergent boundary, the denser plate is subducted (every boundary except continental-continental)
Slab pull
When there is subduction, gravity begins to pull the plate down further - this is slab pull
P waves
Pressure/primary waves that travel long distances largely unnoticed. They push and pull the rock. They rapidly vibrate. Caused by compression. 8km/sec
S waves
Shear/secondary waves. They vibrate the rock side to side. They are perpendicular and slower than P waves. They can’t travel through liquids. More destructive.
L waves
Love waves. A surface wave. They are slower, larger, in a twisting motion. High amplitude
Factors affecting the severity of an earthquakes
Vulnerable population Vulnerable location Poor quality buildings Buildings that aren’t earthquake proof Poor governance
4 stages of how an earthquake happens
1 Earthquakes are caused by sudden movements near the Earth’s surface along a fault. Plate movement creates a build up of tectonic strain + a store of energy
2 the pressure exceeds the strength of the fault so the crust fractures
3 there is a sudden release of energy which creates seismic waves which radiate outwards
4 the crust rebounds near the fracture which causes the ground shaking on the surface
Primary hazards of an earthquake
Ground shaking
Secondary hazards of an earthquake
Liquefaction Crustal fracturing Landslides/avalanches Tsunamis Aftershocks
Composite volcano characteristics
Steep sides
Formed from very thick viscous kava
Lava builds up around vent
Explosive eruptions
Shield volcano characteristics
Gentle slope, shield shape
Low viscosity, lava flows easily and quickly
Gentle, non explosive more frequent eruptions
Primary hazards of volcanoes
Pyroclastic flow
Lava flows
Tephra and ash cloud
Gas eruptions
Secondary hazards of volcanoes
Lahars and Jokulhlaups
Lahar
A severe landslide or mudslide due to rapid melting of ice during eruptions
Jokulhlaups
A volcano erupting under a glacier which melts it and causes huge floods
Example of Pyroclastic flows
Mt St Helens 1980 caused a highly explosive eruption and Pyroclastic flow. 57 died.
Example of tephra and ash fall
E15 Iceland volcano affected 100,000 flights - airlines lost £130 million per day. Kenya lost £1.3 million a day in lost shipments of flowers and fruits to Europe.
Lahar case study
After Mount Pinatubo’s eruption, there were giant fast moving mudflows of volcanic debris that destroyed the homes of over 100,000 people
Jokulhlaups case study
E15 eruption was under an ice sheet. Glacier melt water meant nearby areas were flooded and agricultural land was damaged.
Tsunami define
Unrelated to tidal processes, it is the movement of energy through the water.
Caused by a movement of crust.
Tsunami characteristics
Low in height out at sea (less than a metre)
Grow in height as they reach shallow water due to friction at the bottom
A series of waves, a wave train
Long wavelength- an hour between waves
High risk to humans
4 stages to a tsunami creation
Energy of an earthquake means the crust is uplifted, displaces water
Gravity means water is pulled down and forms distinct waves
Shallow shorelines means the wave slows down and gets bigger
They can travel up to a mile inland
Japanese 2011 Tsunami 3 basic facts
Magnitude 9
Over 15,000 deaths
Costliest natural disaster in history ($235 billion)
Japan seismic facts
30% of all earthquakes are in Japan
It is at the meeting of 4 tectonic plates
They have a tsunami warning system
2011 Tsunami case study
A 10m column of water ‘beat’ the 10m sea wall because the coast had dropped 1m
10 billion tonnes of water hit Japan
The residents of Tokyo only had a few minutes’ warning due to Japan’s warning system
2011 Tsunami Nuclear Crisis
The Fukushima Daiichi nuclear power plant had an automatic shutdown of the reactors.
They began to overhead because the cooling systems were damaged by the tsunami
They released radioactivity into the air
80,000 had to evacuate a 20km zone
It was a ‘major accident’ - same level as Chrernobyl
Areas even now have 15x higher radiation levels than before.
2011 Tsunami impact on food supply
Fear of contamination of food and water by nuclear material
Radiation detected in fish - and may be inedible for 10 years
Farmers are destroying crops or animal product that have high radiation levels so are hazardous
Bam Earthquake 2003 basic facts
Magnitude 6.6
26,000 deaths
Shallow earthquake only 7km below epicentre
Bam 2003 factors influencing primary
It happened at 5:26AM so most people were in their unsafe homes
Iranian seismic building code hadn’t been enforced
Recent constructions were poor quality
Wooden structures had been damaged by termites
Bam 2003 factors influencing secondary
3 main hospitals destroyed, 20% health professionals killed
Lack of training in large scale trauma
Emergency services’ infrastructure was destroyed
Cold winter temperatures led to hypothermia
Nepal Earthquake 2015 basic facts
Magnitude 7.8
9000 deaths
Over 22,000 injured
Nepal factors influencing primary
High population density at 13,000/km2
Vulnerable population
Poverty led to people building their own homes (92% in Kathmandu Valley) - built without the correct building code
Poor earthquake science, current hazard map is 20 years old
Nepal factors influencing secondary
85%of the country’s population is rural so its main economy is the primary industry which was destroyed by the earthquake
Landslides and avalanches because Nepal is a multiple hazard zone
This meant search/help teams couldn’t get there
