Geography

Things I need to know for the exam

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  • Created by: Rhiannon
  • Created on: 15-04-12 18:42

Tectonic Plates

  • We live on the earth's crust, its only 20km thick
  • At the centre of the earth there is a core made of solid nickel and iron
  • around the core is the mantle which is semi-molten rock which moves slowly
  • the crust is divided into tectonic plates, 2 types
  • oceanic- thinner and more dense (5km thick)
  • continental- thicker and less dense
  • the plates are moving because the rock underneath the mantle is moving due to convection currents
  • the places where plates meet are called plate boundaries
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Destructive Plate Margins

  • push together- moving together
  • denser oceanic crust sinks below the lighter continental crust
  • oceanic crust sinks into the magma in mantle where it melts in the subduction zone
  • energy builds up, can be released as an earthquake
  • or the molten magma rises up causing a volcanic eruption- composite cones
  • the continental crust becomes crumpled into fold mountains or ocean trenches

(http://upload.wikimedia.org/wikipedia/commons/a/af/Destructive_plate_margin.png)

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Constructive Plate Margins

  • Moving in opposite directions- apart
  • mostly happens under oceans
  • plates move apart and the gap is filled by magma rising from the mantle which cools creating a new crust, shield volcanoes and volcano islands

(http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCSE/bytesize%20images/techPlateBoundaries1.gif)

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Conservative Plate Margins

  • moving past each other (sideways)
  • move in same direction
  • or if one moving slightly faster, pressure builds up along the fault line until one plate jerks past the other which causes an earthquake
  • e.g. along west coast of USA

(http://4.bp.blogspot.com/-zrhRd2VNC8I/TZ32exvtFUI/AAAAAAAAABU/xiuHrVlzVqA/s1600/transform.jpeg)

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Ocean Trenches

  • mostly around sides of pacific ocean
  • subduction zone is an ocean trench- one wall is formed by sub-ducted ocean plate, the other by overriding continental plate
  • trenches are very deep
  • inaccessible to humans
  • ocean area of greatest importance- continental shelf

(http://media.tiscali.co.uk/images/feeds/hutchinson/ency/0008n057.jpg)

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Formation Of Fold Mountains

Where an area of sea separates two plates, sediments settle on the sea floor in depressions called geosynclines. These sediments gradually become compressed into sedimentary rock.

When the two plates move towards each other again, the layers of sedimentary rock on the sea floor become crumpled and folded.

Eventually the sedimentary rock appears above sea level as a range of fold mountains.

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Problems with Fold mountains

Relief: high and steep

  • mountain valleys= narrow and gorge like
  • little flat land for forming settlements

climate:, increasing height becomes colder, windier, wetter and more snow

  • short growing season
  • often impossible to grow crops at such high levels

soils- stoney, thin and infertile

Accessibility, roads and railways= expensive and difficult to build

  • travel disrupted by frequent rock falls, avalanches and bad weather
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Uses of Fold Mountains

Farming: higher mountain slopes aren't good for growing crops so used for grazing animals like mountain goats

  • lower slopes used for growing crops, steeper slopes sometimes terraced to make easier

Mining: major source of metal ores

  • steep slopes make access difficult so zig-zag roads have to be carved out the sides of the mountain

Tourism: spectacular scenery which attracts tourists, winter- skiing, snowboarding, summer- walkers

  • tunnels have been cut through fold mountains to make straight fast roads improving communications

Hydro-electric power- steep sided slopes and high lakes makes them ideal

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Fold Mountain CASE STUDY -

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Fold Mountain CASE STUDY -

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Fold Mountain CASE STUDY -

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Fold Mountain CASE STUDY -

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Fold Mountain CASE STUDY -

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Volcanoes

  • found at destructive and constructive plate margins
  • a volcano= cone shaped mountain formed by surface eruptions from magma chamber
  • At destructive margins
  • The oceanic trench moves down into the mantle, where its melted and destroyed
  • a pool of magma forms
  • the magma rises through cracks in the crust called vents
  • the magma erupts onto the surface (lava) forming a volcano
  • At constructive margins
    magma rises up into the gap created by the plates moving apart
  • The worlds distribution of volcanoes shows almost perfect fit with tectonic plate margins
  • each time there is a volcanic eruption a new layer of lava is added to the surface of the volcano
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Types of Volcanoe



Composite volcanoes- e.g.

  • lava is usually thick and flows slowly it hardens quickly forming a steep sided symmetrical volcano
  • made up of ash and lava in alternate layers, lava cools inside and next eruption there will be an explosion
  • subsidiary cones and vents form

  Shield Volcano- eg

  • lava is runny and flows quickly and spreads over a wide area forming low, flat volcano, made only of lava, frequent non violent eruptions
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Types Of Volcanoes (Images)

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Super Volcanoe

A supervolcano is much bigger than a standard volcano, the erupt with massive volumes of material and develop over hotspots e.g. yellowstone national park

How?

  • magma rises through cracks in the crust forming large magma basin below the surface, the pressure of the magma causes a circular bulge on the surface
  • the bulge cracks creating vents for lava to escape through, lava erupts out of vents causing earthquakes and sending up gigantic plumes of ash and rock
  • as the magma basin empties the bulge is no longer supported so collapses
  • when the eruption finishes there is a big crater (caldera) left where the bulge collapsed, sometimes these get filled with water to form a large lake e.g. Lake Toba in indonesia

characteristics: flat, cover a large area, have caldera

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Consequences of a super volcano

Last known eruption about 75,000 years ago

  • so much dust would circulate in the atmosphere it could lead to a 'volcanic winter' which would lower global temperatures because less sunlight would reach the surface- mini ice age
  • a thick cloud of super-heated gas and ash would flow at high speed from the volcano, killing, burying and burning everything it touches
  • releases a lot more material

 

  • Mount St Helens in USA in 1970 released 1km  (cubed) of material but if a supervolcano erupted it would alter the landscape of hundreds if not thousands of kilometres
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Volcano - negative effects

Primary effects- immediate impact

  • people die/injured
  • houses/buildings/businesses/ farm land destroyed
  • communications disrupted/destroyed
  • public services disrupted

Secondary Effects-

  • healthcare not able to access- more people die
  • loss of income/homeless/ no food- more die
  • economic problems- cost of rebuilding
  • grief- family stress
  • water contamination- disease and death
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Volcano - positive effects

POSITIVES

fertile soil, tourism (hot springs)

industry (minerals- sulphur, borax, pumice),

generating power and heating things (geothermal power)

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Volcano - negative effects

Primary effects- immediate impact

  • people die/injured
  • houses/buildings/businesses/ farm land destroyed
  • communications disrupted/destroyed
  • public services disrupted

Secondary Effects-

  • healthcare not able to access- more people die
  • loss of income/homeless/ no food- more die
  • economic problems- cost of rebuilding
  • grief- family stress
  • water contamination- disease and death
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Volcano - positive effects

POSITIVES

fertile soil, tourism (hot springs)

industry (minerals- sulphur, borax, pumice),

generating power and heating things (geothermal power)

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Warnings of A volcanic erruption

  • Small earthquakes
  • increased emissions of steam and gases
  • visual signs of bulging around crater
  • in more developed countries there are special machines that give advanced warnings
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Volcano (LEDC)- CASE STUDY - Montserrat

The soufriere hills Montserrat

  • main eruption 25th June 1997, started in july 1995
  • 4-5 million m3 of rocks and gas released
  • 19-23 people killed

Cause:

  • above a destructive plate margin where the Atlantic plate is being forces under the Caribbean plate
  • magma rose up at weak points under the soufriere hills forming underground pool of magma
  • rock above pool collapsed, opening the vent and causing an eruption
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Volcano (LEDC)- CASE STUDY - Montserrat Timeline

July 1995- first eruption- signs of volcanic activity

August 1995- 50% population evacuated

1996- finally erupted-mudflows then pyroclastic flows, part of dome collapsed

April 1996- eruption continues

June 1997- eruption killed 23 people and lava became more explosive

Few months later- Plymouth burned to ground and covered with ash

Only 39/103 square kilometres were considered safe

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Volcano (LEDC)- CASE STUDY - Effects

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Volcano (LEDC)- CASE STUDY - Responses

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Earthquakes

Earthquake= vibrations in the earth's crust which shake the ground surface- some too small to feel

happen on all 3 plate margins

  • destructive- tension builds up when one plate gets stuck as its moving down past the other into the mantle
  • constructive- tension builds along cracks within the plates as they move away
  • conservative- tension builds up when plates are grinding past each other get stuck

Pattern= around tectonic plates

  • lots around west side of south American plate
  • lots around Philippine plate
  • on African plate marks out practically the whole margin
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The Richter Scale

  • Measures magnitude (energy released) of an earthquake
  • measured using a seismograph
  • doesn't have an upper limit (normally between 1-10)
  • its logarithmic- means that an earthquake with magnitude of 5 is 10 times more powerful than one with a magnitude of 4
  • most people don't feel earthquakes of magnitude 1-2
  • major earthquakes are above
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The Mercalli Scale

Used to indicate intensity/effects of an earthquake

effects are measured by asking eye witnesses of what happened, and takes into account effects on Earth's surface, people and buildings

given a number between 1-12, sometimes Roman numerals

I (1)- rarely felt by humans

II-IV (2-4)- feeble to moderate effects- felt by people indoors

V-VII (5-7) strong effects, causing panic felt by everyone, structural damage

VIII (8) destructive to poorly built structures only slight damage to those well designed

IX-XII (9-12) ruinous, disastrous, total destruction

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Why earthquakes happen

90% occur where plates are colliding

focus= point at which the earthquake originates- waves stronger and do more damage

Epicentre= the point on the surface directly above the focus where greatest force of the earthquake is felt

Shock-waves= radiate out in all directions gradually becoming less strong as you get further away from the epicentre
(http://www.bbc.co.uk/schools/gcsebitesize/geography/images/tec_007.gif)

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Effects of Earthquakes

collapsing buildings, bridges and roads- people are killed by being trapped

severity of primary effects depends on human and physical factors

chance effect- time of day-

were fewer people closer to epicentre?

Fires- caused by fractured gas pipes, bringing down electricity cables- spread quickly in areas of poor quality housing

Tsunamis- giant sea wave- really dangerous in areas of low lying coasts

landslides- most likely on steep slopes and in areas of weak rock

disease- typhoid and cholera spread easily when pipes burst- leads to shortage of water and contamination

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