Plate Tectonics

What is the evidence for plate tectonics? (5)

• ‘fit’ of continents - South America fits nicely into West Africa

• Paleo-climate indicators - current geology does not match current climate, e.g. seashells found in limestone at peak of Mount Everest

• Truncated geological features - geology and age of Appalachians in NE North America matches that of Scottish Highlands and Scandinavia

• Far-flung fossils - fossils matched across different continents, even though they couldn’t swim; suggests continents were once joined

• Mid-Atlantic ridge - increasing age of crust moving away from middle of Atlantic; suggests continents drifting apart

What are the main parts of Earth’s structure? (4)

• crust

• Mantle

• Outer core

• Inner core

How thick is:

• oceanic crust?

• continental crust?

• 7km

• 35km

What is the:

• lithosphere?

• Asthenosphere?

• mesosphere?

• crust + very top of mantle

• Top of mantle - 1300 Celsius fluid

• Majority of mantle

What is the:

• outer core?

• Inner core?

• liquid, can reach 6000 Celsius; 5270km deep

• Solid iron and nickel - 6354km deep

What are these made of:

• oceanic crust

• Continental crust

• SIMA - Silicon and Magnesium

• SIAL - Silicon and Aluminium

What are the processes by which plates move? (2)

• Ridge push

• Slab pull

Explain:

• ridge push

• Slab pull

• molten rock in mantle pushes in between the two plates, creating new crust, and pushing the two plates apart

• Cool oceanic crust subducts (sinks under) under the denser continental crust into the hot mantle. It is helped to sink by the convection cell.

How much of plate movement is responsible by:

• ridge push

• Slab pull

• 10%

• 90%

What is a hotspot?

An area where a magma plume in the mantle has risen and has burst through oceanic crust, creating a volcanic trail.

In which direction are plates moving at constructive plate margins?

Away from each other

When plates ____ apart, partially melted ______ rises. Under the ocean, this forms mid-_____ ridges. On land, rift _______ are created.

pull, mantle, ocean, valleys

How are rift valleys formed? (3)

• two continental plates are pulling apart. The rift valley in the middle is pushed up by a mantle plume, but is lower than the two plates next to it.

• As the plates pull apart, the rift valley drops down. Seawater floods in, and a narrow sea is created.

• The two continental plates continue to pull apart, widening the sea, and creating new oceanic crust.

How are ocean ridges formed?

ridge push: oceanic plates forced to pull apart. Hot magma exits through gap. Islands constructed, e.g. Iceland. Mid-Ocean ridges formed.

What are shield volcanoes?

They are found at constrictive plate boundaries, where mantle rises to the surface as plates pull apart. The pressure on hot rock lowers; partially melts and becomes runny. Therefore, gases don’t get trapped, and nothing plugs the volcano, so eruptions are gentle. The lava flows quickly, and cools to form a gentle sloping shape. Shield volcanoes also found at hotspots, where oceanic crust is very thin.

In which direction are plates moving at destructive plate margins?

Towards each other

What are the three types of destructive plate margins, and what landforms form there?

• subduction zone - oceanic vs continental crust. Landforms include ocean trenches, fold mountains and composite volcanoes

• Subduction zone - oceanic vs oceanic crust. Landforms include ocean trenches and volcanic island arcs

• Collision zone - continental vs continental crust. Landforms include fold mountains. No volcanoes - only earthquakes.

What is the Benioff zone?

The place where the subducted oceanic crust is destroyed in the mantle.

What are the main hazards at destructive plate margins? (7)

• Composite volcanoes - explosive eruptions

• Pyroclastic flows - mixture of 400 Celsius hot gas and ash flowing at 200mph

• Tsunamis

• Earthquakes

• Volcanic bombs

• Acid rain - formed when ash / gases dissolve with rainwater in clouds

• Lahar flows - a cross between a mudflow and a flash flood.

What are composite volcanoes?

Partially melted mantle rises up (destroyed oceanic crust) - is viscous and sticky, building up pressure. When released suddenly, steam and dangerous gases erupt violently from the volcano. The sticky lava cools quickly, so it flows slowly, forming steep slopes. The alternate layering of lava and ash on the slopes creates composite volcanoes. Sometimes, cooled rock forms a plug at the top of the main vent. This leads to an enormous pressure build-up - next eruption will be even more explosive.

Continental plates are ____ dense, so there is no subduction at _________ zones, and therefore, no ________ activity. The surface lifts into ____ _________. The pressure built up by the ______ pushing against each other causes the crust to crack into ______, which can trigger ___________.

less, collision, volcanic, fold mountains, plates, faults, earthquakes

For shallow focus earthquakes, state:

• depth

• Frequency

• Magnitude

• Seismic waves

• Damage

• Plate boundaries

• 0-70km

• Common

• 1-5

• Radiate horizontally

• Widespread

• Found at constructive, conservative, and upper part of destructive.

For deep focus earthquakes, state:

• depth

• Frequency

• Magnitude

• Seismic waves

• Damage

• Plate boundaries

• 70-700

• Rare

• 6-8

• Radiate vertically

• Localised

• Destructive (at Benioff zone)

In which direction are plates moving at conservative plate margins?

Alongside each other - either in same direction or opposite direction, and potentially at different speeds

Why are there no volcanoes at conservative plate margins?

There is no plate subduction

Define:

• focus

• Seismic waves

• Epicentre

• Aftershocks

• source of a quake beneath the Earth’s surface

• Fast waves of energy generated from the focus of an earthquake

• Point on Earth’s surface directly above a quake’s focus

• Series of smaller tremors after the main quake

What are the factors that affect the amount of shaking in a quake? (3)

• depth of focus

• How far one is from the epicentre

• Density of rock (geology)

_____________ study earthquakes and use ____________ to record the size of them. __________ data and other data is _______ to produce GIS (____________ ___________ _______) in order to:

• predict earthquake ______ and severity

• calculate risk of ______

• help minimise ____ in future earthquakes.

seismologists, seismometers, seismogram, layered, geographical information systems, timing, damage, risk

Define magnitude

Amount of energy released in a quake; measured using the Richter scale.

What is the Mercalli scale?

Measures damage rather than magnitude

What was the magnitude and where was the epicentre of the Japan Sendai 2011 quake?

8.9 magnitude, epicentre was off the coast in the Pacific

What triggered the earthquake and subsequent tsunami?

250 miles of sea floor thrusted up 3m (first caused earthquake). Displaced 100B cubic metres of water. Caused waves to travel away from the epicentre, e.g. Japan, US, at around 500mph. Waves reached height of 14m (some suggests 40m).

What were the impacts of the earthquake and tsunami? (10)

• huge amounts of agricultural land lost - many food shortages

• Official death toll of nearly 20,000

• 120K buildings destroyed, 1M severely damaged

• Financial impact - $200B

• Tsunami breached Fukushima Nuclear Plant - caused explosions and radioactive leaks

• 1.5M households without running water, 4.4M without electricity

• People within 20km of nuclear plant evacuated; people within 20-30km requested to stay indoors, but subject to voluntary evacuation

• Majority of deaths were 60+ people

• 24-25M tonnes of debris

• 300 hospitals destroyed, 11 damaged

Immediate responses (3)

• earthquake warning system gave people 30 seconds to prepare for quake

• Tsunami warning system issued 3 mins after quake - gave people 20 mins to prepare

• In areas hardest hit, people only had 15 minutes to prepare for tsunami

Short-term responses (9)

• workers manually cooled the 3 damaged reactors at nuclear plant

• Full effects of nuclear disaster unknown due to time it takes for radiation to take effect

• Government focused on recovering bodies, restoring water, evacuations from nuclear plant.

• Those who lost homes supported in evacuation centres, often schools. Financial support given to worst affected families

• UN immobilised humanitarian assistance

• 72 search and rescue teams

• South Korea, US, Australia, NZ all provided aid

• 91 countries offered food, blankets, and military transport

• 63 UK experts went to Japan with sniffer dogs to help search

Long-term responses (4)

• fundraising concert in Taiwan - raised $26M

• Music industry collaborated to make album - money raised went to Japanese Red Cross Society

• Red Cross provided 70K temporary homes for 280K+ people

• Improved building designs and preparation drills