Plate Tectonics

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Plate Tectonics Theory

  • Inner core: solid ball - iron and nickle
  • Outer core: semi molten - iron and nickle
  • Mantle: silicate rocks - part nearest core is rigid - layer above is asthenosphere and is semi molten - very top is rigid
  • Crust: outer layer
  • Top mantle and crust: lithosphere

2 types of crust: continental - thicker and less dense (30-70km); oceanic - thinner and more dense (6-10km)

Lithosere divided into tectonic plates.  Moving due to convection currents in asthenosphere.  Where plates meet = boundaries/plate margins.  The idea that lithosphere made of plates moved by convection currents is theory of plate tectonics.

Plates move due to convection currents in mantle.

  • Radioactive decay generates lots of heat
  • low parts of asthenosphere heat up, become less dense, rise
  • as they move top of asthenosphere they cool, more dense sink
  • circular movements = convection currents
  • create drag on base of tectonic plate = move
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Plate Tectonics Theory

S.America and Africa looked like could fit together - Continents might have been joined.

  • 1912: Alfred Wegner = continental drift.  All continents once joined = Pangea.  Fossil evidence but no mechanism.
  • 1950s: Palaeomagnetism provided evidence.
  • 1960s: Sea floor spreading = mechanism.  Continental drift theory = Theory of plate tectonics.
  • Geology: S.America and Africa rocks of same age and composition.  Mountain ranges formed under same conditions and same place.
  • Fossil records: Match up distribution of fossils. Unlikely would've migrated. Therefore. joined together
  • Living species: Species on different continents, migration unlikely .E.G. earthworms.
  • Climatology: Past climates similar - located differently to now .E.G. glacial deposits/coal diposits
  • Paleomagnetism: Study of history of Earth's magnetic field.  Once 200000 years it reverses polarity.  Provided evidence of sea floor spreading.

Magma erupts; magnetic minerals align with Earth's magnetic field; when crust solidified, alignments fixed; magnetic minerals in normal polarity opposite direction to reverse polarity = alternating magnetic stripes along sea floor; crust is older further away from mid ocean ridge = plates moving apart.

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Types of Plate Margin - Constructive

Earthquakes and volcanoes occur at constructive margins - plates move apart.

Mantle under pressure, released when move apart.  Mantle melts = magma = less dense so rises and erupts.  Some parts move faster than others, pressure builds, plate cracks = fault line = earthquake.

2 landforms created:

  • mid ocean ridge: diverging plates are underwater.  Mid atlantic ridge = Eurasion and north American plate.  Underwater volcanoes erupt, build up, above sea level = iceland.
  • rift valley: beneath land, magma causes crust to bulge and fracture = fault lines.  Crust between parallel faults drops down, forming rift valley.  East African rift system = Nubian and Somalian.  Volcanoes found around rift valley .E.G. mt killomanjaro.
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Types of Plate Margin - Destructive and Conservati

Destructive: 2 plates move towards each other

Oceanic - continental: oceanic forced under continental = deep sea trench.

Fold mountains: sediment accumulated on continental crust, folded upwards with edge of continental crust.  Oceanic crust heated by friction and contact with upper mantle, melts into magma.  Magma less dense so rises back to surace = volcano.  One can get stuck as moves under, pressure builds up, jerks = earthquake.

Oceanic - oceanic: deep sea trench and earthquakes and volcanoes.

Volcanic eruptions under water create island arcs .e.g. mariana islands.

Continental - continental: neither subducted = earthquakes

Fold mountains = himalayas

Conservative: plates move past each other.  get locked together and pressure builds up.  jerks = earthquakes. e.g. Pacific and North American Plates.

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Volcanic Activity

Intrusive volcanic activity: beneath earth's surface, large magma chambers formed, magma forced into crust

Extrusive volcanic acitivity: on earth's surface, volcanic eruptions (major), hot springs, geysers, boiling mud pools (minor)

Chambers of magam cool underground - batholiths.  Magma flows into gaps - vertical dykes, horizontal sills.  Cracks when magma cools - cooling cracks.

Lava eruptions - extrusive

Basaltic: constructive margins, low silica content, low viscosity (runny), temp 950*C+.  Flows easily and gas can escape easily.  Non violent, frequent, long periods of time.

Andesitic: destrucive, medium, medium, 750-950*C

Rhyollitic: destructive, high, high (thick and sticky), less than 750*C

Andesitic and rhyllitic flow less easily, form blockages in vents.  Pressure builds = violent eruption.  Erupt intermittently and eruptions short lived.

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Volcanic Activity

Dome: destructive margin, rhyllitic or andesitic (composite)

Caldera: destructive margins, andesitic and rhyllitic

Shield: constructive or hotspots, basaltic

Fissure: constructive, basaltic

Hot springs: springs, groundwater emerges close to recent intrusive volcanic activity, water is heated 20-30*C, high mineral content

Geysers: groundwater heated to above boiling point by magma, water becomes pressurised and forces out through cracks, hot water and stream spray from vent, erupt periodically

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Volcanic Activity

Boiling mud pool - hot spring - form in areas with fine grained soil - hot spring mixes with soil = hot muddy pool - brightly coloured mud because of minerals - Yellowstone and Iceland

Hotspots - caused by magma plume, rises from mantle - form above magma plume - magma plume remains stationary, crust moves above it - volcanic activity that was above hotspot, decreases as moves away - new volcanoes from in crust now above hotspot - chain of volcanoes formed - Hawaii

Seismic Activity - earthquakes caused by tectonic plate movement

  • by tension that builds up at each plate margin
  • plates jerk past eachother, sends out seismic waves = earthquake
  • seismic waves spread from focus (where earthquake starts in lithoshere)
  • epicentre is point on earth's surface where felt first - straight above focus
  • damage - structs collapse, tsunamis, avalanches etc

Also caused by: reactivation of old fault lines unactive for long time; subsidence as result of deep mining; pressure on surface rocks from water in large resovoirs

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Seismic Activity

Seismic waves through interior of Earth:

  • P waves (primary) - travel through solids and liquids.  waves push and pull earth in same direction as wave travelling.  fastest
  • S waves (secondary) - through solids.  move earth 90 degrees in direction of travel (vertically).  lots of damage

Others near Earth's surface (crust), surface waves - slower:

  • love waves - solids.  surface move side to side.  lots of damage (shearing effect)
  • rayleigh waves - liquids and soilds.  move surface in rolling motion
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Seismic Activity

Energy released measured by seisometer. Measures magnitude, duration and direction.  Seismic records look at frequency of earthquakes.

Richter scale

  • magnitude
  • doesnt have upper limit and is logarithmic. magnitude 5 amplitude 10x more than mag 4
  • each value is 30x more energy released than previous
  • don't feel maag 1-2. major ones are 7+

Mercalli scale

  • impacts
  • measured using observations
  • scale 1-12. 1 = only by instruments. 12 = total destruction
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Seismic Activity

Cause tsunamis

  • large waves caused by displacement of water
  • triggered by underwater earthquakes.  waves radiate from epicentre.  greater movement of sea floor, bigger wave
  • volcanic eruptions and landslides can cause
  • more powerful if close to coast as lose energy towards lands
  • very fast in deep watert without much warning = high death toll

Tsunamis can affect whole column of water.  In open ocean where deep water, waves are fast and have long wavelength and small amplitude.  Closer to land, water is shallowed so waves compressed and energy is more concentrated.  Before tsunami hits coast, water withdraws down shore = draw back.  Hits with great force, only travels short distance.

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Managing the impacts of tectonic hazards

Hazard: potential threat to human life or property.  Tectonic hazards are caused by movement of tectonic plates.  Many impacts on people can be reduced by prediction, building techniques, planning and education.

Earthquakes

  • impossible to predict but clues (tremors, cracks, animal behaviour)
  • earthquake warning systems detect P waves after it has begin - Japan's earthquake early warning system on TV and radio
  • predict future earthquakes will happen using old data

Volcanoes

  • roughly predict; allows people to be evacuated
  • difficult to evacuate as people dont want to abandon

Tsunamis

  • rely on earthquake warning systems
  • warning centres all around world
  • time to evacuate - relying on good communication system. if close to land, happens too quickly
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Managing the impacts of tectonic hazards

Building techniques can protect

Earthquakes

  • designed to withstand - strong materials, absorb energy
  • construction laws stricts - so new buildings withstand

Volcanoes

  • cant withstand lava flows. strengthened so less likely to collapse
  • divert lava from settlements using barriers .e.g. Etna 1983 - rubble barrier diverted slow moving lava

Tsunamis

  • buildings designed with raised, open foundations made from concrete
  • Hokkaido (Japan) tsunami walls built around settlements

Planning and education

  • future developments planned to avoid at risk areas; evacuation routes
  • emergency services train and prepare for disasters
  • educate people on what to do and how to evacuate - tsunami drills
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Managing the impacts of tectonic hazards - Some fa

Development of a country

  • impacts higher on ledc.  dont have money for prep or respoonse
  • buildings poorer quality, easily damaged
  • infrastructure poorer, difficult to reach affected areas
  • health care poorer
  • depend on agriculture, badly affected
  • economic impact higher in medc's as worth lots

Population

  • more people, more affectedr
  • densely populated areas - buildings - collapse
  • difficult to evacuate large numbers - routes limited

Timing

  • when during the year
  • middle of night - more injuries
  • winter - freeze to death
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Volcanic Eruption - Montserrat

Soufriere Hills Volcano - composite -- destructive - N.American plate under Carribean plate.  1995, earthquake, small eruptions and lahars started until 1997.  25th June 1997 small earthquakes, followed by pyroclastic flows.  Main eruption 4-5million m cubed of material released over 20 mins.  Pyroclastic flows reached 50m of airport, 5.5km of volcano.  large ash cloud.  further eruptions over next few months, airport destroyed 21sts Sept 1997

Economic

  • loss £1bn
  • 20 villages 2/3 homes destroyed
  • tousists stayed away; schools, hospitals, airport, port destroyed

Social

  • 19 dead 7 injured
  • 100s lost homes; fires
  • population declined, 8000 have left since 1995

Environmental

  • large areas covered with volcanic material - plymouth 12m under mud and ash
  • vegetation and farmland destroyed; volcanic ash improved soil fertility
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Volcanic Eruption - Montserrat

Responses

  • people evacuated
  • shelters
  • temporary infrastructure (roads and electricity supplies)
  • UK fave £17mil in emergency aid
  • local emergency services to search & response survivors
  • risk map - exclusion zone in place. S is off limit - volcano still active
  • UK £14mil long term aid - new docks, airport, houses built
  • Montserrat Volcany Observatory to predict

Little management before increased severity

  • scientists studied volcano in 1980s, report not noticed
  • no disaster management plan, responses slow
  • 1990s key infrastructure built in areas at risk
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