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What are the three primary layers of earth?
Core
Mantle
Crust
Characteristics of the core?
Inner core is solid and is 5000km down
Outer core is liquid and is 2800km
Characteristics of the mantle?
Asthenosphere 300km down and semi molten
Lithosphere 100km down rigid layer
Characteristics of the crust?
Continental crust, 35km thick and granitic
Oceanic crust, 10km thick and basaltic
What is continental drift and who came up with it?
Alfred Wagner 1912
He proposed Pangea which is a single large plate
This then split into north and south and then split again until continents formed
People were sceptical because he failed to provide an explanation
Geological evidence for continental drift
Continents fit together
Effects of glaciation on continents suggests they were joined
Mountain chains and rock types that would have joined
Biological evidence for continental drift
Similar marine and reptile fossils found in adjacent continents
Fossils from thousands of years ago show few similarities which suggests evolution
What is palaeomagnetism?
Ancient record of changes in rocks
How can technology measure variations in earths magnetic field?
Lava erupts and cools
Magnetic iron particles lock into rock
Magnetic field showed stripes across ocean floor
What is sea floor spreading?
Lateral movement of new oceanic crust away from the mid ocean ridge
Moves material across the ocean floor
Sea floor reaches ocean trench and material subducts
How can age of sea floor be found?
Ocean drilling investigated ocean sediment on ocean floor
Revealed spatial pattern of sediment due to sea floor spreading
Thickest and oldest sediment was nearest to the continents
But no sediment older than 200 million yrs so ocean crust constantly recycled
How were tectonic plates discovered?
1960’s nuclear tests were conducted underground
They released vast amounts of energy that were picked up by seismometers
Most earthquakes were shown to be in narrow bands
This suggested a tectonic plate boundary
What happens at a divergent/constructive plate boundary?
Plates move apart
Magmas rises through the asthenosphere and forces its way to surface
What forms at divergent plate boundaries?
Mid ocean ridges with a combined length of 60,000km
Mid ocean ridges are broken by transform faults
What are transform faults?
Large faults in the crust that form at right angles to the mid ocean ridge
Are there volcanoes/earthquakes at divergent boundaries?
Earthquakes when vast amounts of energy is released
Volcanoes from seafloor spreading
3 types of spreading ridges and their characteristics?
Fast spreading form broad and smooth mountains and have no central rift valleys
Medium spreading form smooth mountain chains and poorly defined valleys
Slow spreading form steep slopes and clearly defined central rift valleys
What happens when magma eruption is mostly uderwater?
Cools rapidly forming pillow lavas
As magma then rises to surface overlying rocks are forced upwards
Lithosphere is under stress and eventually fractures
What happens at a convergent/destructive plate boundary?
Plates move towards each other
What are the three ways plates can combine?
Oceanic-Continental
Continental-Continental
Oceanic-Oceanic
What happens at the oceanic-continental boundary?
Denser oceanic plate submerges under the continental
It causes subduction
What does subduction do?
Deepens the ocean causing an ocean trench
What is an ocean trench?
An ocean trench is a long narrow depression and the steepest side lies towards the continent
How do mountain chains form at oceanic-continental plate boundaries?
Layers of rock develop on oceanic plates
As they converge rock crumples and is lifted along the continental plate
Magma is also uplifted
And a mountain chain forms
How do earthquakes form at o-c plate boundaries?
As oceanic plate descends it comes under immense pressure
Faulting and fracturing occur in the Benioff zone
And energy is released resulting in an earthquake
How do volcanoes form at o-c plate boundaries?
The oceanic plate melts when it subducts
Because it is less dense than the surroundings it rises towards the surface as magma plutons
This can cause volcanoes
What happens at oceanic-oceanic plate boundaries?
The slightly denser one submerges under the other
As it descends the plate melts and magma rises forming island arcs
What are island arcs?
Volcanic islands
E.g. The Antilles formed by North America Plate subducting under the Caribbean plate
What happens at a continental-continental plate?
Little to no subduction takes place
Due to similar densities
Collision of the two plates can they force them upwards resulting in mountain chains
E.g. African and Eurasian plates have created the Alps
What happens at conservative plate boundaries?
They slide past each other in a shearing motion
Friction causes pressure and rocks sometimes fracture causing earthquakes
E.g. San Andreas fault system
No volcanoes though
Features of explosive volcanic eruptions?
Form at convergent plate boundaries
Lava is acidic
High viscosity
Lower temperature
Violent bursts of gas bubbles
Gas, dust, ash and lava bombs occur
Strato volcanoes
Don’t erupt frequently
Features of effusive eruptions?
Occur at Divergent plate boundaries
Lava is basaltic
Low viscosity
Higher temperature
Limited explosive force
Gas and lava flows
Happen more frequently
Shield volcanoes
What is a product of an explosive eruption?
A caldera
How does a caldera form?
The acidic magma doesn’t flow easily so vents are often filled with solidified magma
Enormous pressure builds up resulting in an explosive eruption
This destroys the cone of the volcano and empties magma chamber
Without support of the magma the sides collapse resulting in a depression known as a caldera
Example of a cladera?
Krakatoa eruption of 1883 left a 7km wide caldera
2 products of effusive eruptions
Lava Plateux
Shield volcanoes
What is a lava plateux?
Flat elevated platform
Basic magma erupts and areas are covered by free flowing lava
This is called flood basalts
Weathering of the earths surface creates varied relief
What is another word for weathering of the earths surface?
Deundation
How do effusive eruptions cause shield volcanoes?
Basaltic lava results in gentle sloping sides which can extend for tens of kilometres
Most occur unseen on ocean floor in mid ocean ridges
What is a hot spot?
A fixed area of intense volcanic activity where magma from a rising plume reaches the earths surface
What is a super volcano and give an example?
A volcano that erupts more than 1000km³ of material
Yellowstone in Wyoming is 75km in diameter
Facts about Yellowstone super volcano?
Volcanic activity began 2 million years ago
Since then 2 major eruptions
It decreased global temperatures due to ash cover over the sun
There is a hot spot underneath but the probability of eruption is very low
Misconceptions about Yellowstone?
People say its due an eruption but it has no particular pattern
People say it will have global implications but it will have little impacts outside of the Yellowstone area
People say its growing larger but there is no evidence of this
How is volcanic activity measured?
Volcanic Explosivity Index (VEI)
Measures magnitude (amount of material erupted)
Measures intensity (speed at which material erupts)
0 (least) to 8 (most)
Also includes, height material reaches, the duration, volume of erupted material and many qualitative descriptions
What are the 7 hazards of a volcano?
Lava flows
Pyroclastic flows
Tephra
Eruptions
Lahars
Floods
Tsunamis
How are lava flows a hazard?
Everything in the lavas path is burned, buried or bulldozed
Destroys infrastructure, crop, residents
But rarely lead to fatality and injury
How are pyroclastic flows a hazard?
500 degrees + of hot gases and ash
Rock travels at 100km/hour
Destroy everything in their path
Inhalation of gas causes instant death
e.g. Pompeii AD79
Why is tephra a hazard?
Material ejected from the air
Buries farmland
Disrupts transport
Why are eruptions a hazard?
Emit a wide range of toxic gases like CO2, CO and SO2
Can pose deadly threat to humans if inhaled
When rain and SO2 combine acid rain
Why are lahars a hazard?
Mud flows of about 50km/hour
Snow, ice, ash, soil all mix to create a large flow
Destroy everything in their path
Why are floods a hazard?
Volcanic eruptions below glaciers can cause rapid melting and floods
Water accumulates until it finds an exit
This is known as a jokulhlaup in Iceland
Why are tsunamis a hazard?
Can cause massive displacement of water resulting in 600km/hour waves
Can cause drowning
What is an earthquake?
A release of stress that has built up in the earths crust caused by tension, compression and rock shearing
Where is an earthquakes epicentre?
A series of seismic waves originates from the focus
The epicentre is directly above the focus
Where do earthquakes tend to happen although they can anywhere?
Mid ocean ridges
Ocean trenches
Collision zones
Conservative plate margins
What are the 3 types of seismic waves?
Primary (P) waves
Secondary (S) waves
Surface (L) waves
Characteristics of primary waves?
Low frequency
Fast travelling
Happen everywhere
Characteristics of secondary waves?
Half the speed of primary
High frequency
Cant pass through the outer core as its liquid
Characteristics of surface waves?
Slowest waves
Low frequency
Occur in outer crust
What are characteristics of an earthquake with a shallow depth focus?
70km down
Very common
Release low energy but sometimes can be high
Occur in cool brittle rock
What are characteristics of an earthquake with a deep depth focus?
70-700km down
Pressure high
Temperature high
not very well known as they can explore that far down
How are earthquakes assessed?
Richter scale made in 1935
1(least) to 9(highest recorded) but there is no limit
Anything under a 2 not usually felt
What affect do earthquakes have on landforms?
Associate with the formation of mountain chains
Create large fold mountains
Rift valleys occur along mid ocean ridges
Fault scarps form where tension has been from an earthquake
What are the 4 hazards of earthquakes?
Ground shaking an displacement
Liquefaction
Landslides/Avalanches
Tsunamis
Why is ground shaking/displacement a hazard?
Its the vertical and horizontal movement of the ground
Rips apart pipes and railways
Buildings can better withstand vertical than horizontal displacement
Disrupts groundwater and can cause it to become contaminated
Why is ground liquefaction a hazard?
When an earthquake strikes an area with high water content vibrations can cause solid materials to act like liquids
Materials lose their strength, slopes collapse and places shrink
Why are ground landslides/avalanches a hazard?
Ground shaking and liquefaction can both cause landslides
Steeper slopes more vulnerable
Deforestation and heavy rain also increases vulnerability
Block transport and rivers
Can cause flooding
Why are tsunamis a hazard?
underwater earthquakes displace water
Wave height increases as they approach the shore
Tsunamis can spread far inland if no precautions have been taken against them
What is an active volcano?
One that has erupted in the last glacial period or within the last 10,000 years
What is a dormant volcano?
One that has not erupted in the past 10,000 years but is expected to erupt again
What is an extinct volcano?
One that is not expected to erupt again
There are exceptions though as Yellowstone hasn’t erupted in 70,000 years but is not considered extinct
What are the 5 reasons people choose to live in tectonically active places?
Fertile soil
Tourism
Geothermal energy
Minerals
Poverty
Why does fertile soil make people want to stay in an area?
Fertile soil is produced by weathered lava
Its possible to grow 2-3 crops on the same piece of land in a year
Places that rely on agriculture tis brings in more economy and food into the country
Why does tourism make people want to stay in an area?
Places like Sakurajima is low magnitude eruptions so attracts tourists for sight seeing
Brings economy into the country
Why does Geothermal energy make people want to stay in an area?
Japan has had power stations since 1960s
promotes renewable energy and helps countries progress
However much of the energy is in conservation sites so can’t be accessed
Why do minerals make people want to stay in an area?
Minerals produced by earthquakes are used in manufacturing
Such as sulphur in chemicals
Why does poverty make people want to stay in an area?
They can’t leave due to travel costs
Or they can’t pay for education to get a better job somewhere else
So they must rely on primary industry such as agriculture for a source of income
What does WAPMERR stand for?
World Association of Planetary Monitoring and Earthquake Risk Reduction
Formed in 2001
What does physical exposure to hazards depend on?
Frequency of earthquakes and volcanoes
Magnitude
Type of hazard in a particular area
Number of people that live there
What is a recurrence interval?
Average time between 2 events of equal magnitude
What is vulnerability?
Ability of a person or community to withstand a hazard
What is resiliance?
Rate of recovery from a hazard or event
It links with effectiveness of measures designed to protect people
What are the 5 stages of disaster response?
Modify the cause of the event
The even occurs
Search and rescue
Relief and rehabilitation
Nature of recovery and reconstruction
What does the graph for this look like?
How can you mitigate against the event for volcanoes?
Lava diversion channel
Spraying lava so it cools
Slowing the flows by putting obstacles
How can you mitigate against the event for earthquakes?
nothing can be done
How can you mitigate against vulnerability for volcanoes and earthquakes?
Education in recognising signs of eruption and what to do along with evacuation drills
Preparedness as in building shelters and strengthening buildings
Warning as in using technology to monitor
Hazard resistant buildings such as steep sloping rooves
How can you mitigate against loses for volcanoes and earthquakes?
Emergency aid
Disaster response teams
Search and Rescue
Insurance
Resources for rebuilding
What is aseismic design?
Buildings being built adjacent so they can sway and bump into each other
Stepped buildings with a larger bottom on them to offer support
Excavating weak material so you can build on flat land
Reinforcements below buildings
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