Plate tectonic revision notes

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Plate tectonics
Earth structure:
The core: Roughly the size of
Mars. Made of rocks rich in iron
and nickel so is the densest part
of the Earth, roughly 6000C.
Inner Core: Solid
Outer Core: Semi-molten
Mantle: composed of molten and
semi-molten rocks containing
lighter elements such as silicon
and oxygen. 2900km
Lithosphere: more rigid upper
mantle approximately 80-90km
thick.
Asthenosphere: semi molten
layer below the lithosphere on which the plates float and move.
Crust: Coolest (1200C at deepest) least dense rock rich in silicon, oxygen, aluminium, potassium
and sodium. Oceanic: basaltic, 6-10km deep. Continental: igneous, metamorphic and sedimentary can
be up to 70km deep.
Plate tectonic theory:
1. Continental fit: Continents such as western Africa and eastern South America seem to fit
together particularly if you use the continental shelves.
2. Geological evidence: Rocks of the same type and age found in south-east Brazil and South
Africa. Secondly, the trends of mountains in eastern USA and north-west Europe. Also similar
rock sequences found in Eastern Canada and Scotland suggesting being laid down in similar
conditions
3. Glacial deposits: Evidence of the glaciation in the late carboniferous period in
south-America, Antarctica and India. These deposits cannot be explained in their current
location so this explains that they must have been formed together and then moved.
Striations showing the same orientation when the continents fit together also.
4. Climatological evidence: Coal deposits that are formed in tropical climate zones of similar
ages were all found in Antarctica, North America, Svarlbard and the UK.
5. Biological evidence: Similar fossil remains found on either side of the Atlantic For example a
reptile called a mesosaur found only found in south America and southern Africa.
Oceanic Plate Continental Plate
Denser Less dense
Newer Older
Basaltic Andesitic
6. Sea floor spreading: Paleomagnetism can be seen at the mid-Atlantic ridge as the
alternating polarity of the rocks that form the ocean crust. Similar thickness of striping of the
magnetism either side of the ridge. There is roughly the sea distance between the
continental plates. Also, the further from the mid-Atlantic ridge the older the basaltic rocks
became.
Convection currents:
"Zones of hotter, more liquid
magma exhibit a continuous
circulatory motion in the

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Hot mantle rises up as it is less dense from the
core that has been heated by radioactive decay in the core and transfers its heat the base of the
lithosphere. The mantle that has lost heat in this way becomes slightly denser and sinks downwards
again.…read more

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Formation: Where convection currents rise and diverge they create high temperatures that cause
up doming of the crust and tensional forces that pull it apart when the plate pull apart the magma
from the core (1200C) rises up to fill the gap forming new land.…read more

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Formation: When convection currents converge they pull plates towards each other they meet. If
the plates are oceanic and continental as the oceanic plate is denser than the continental plate the
oceanic plate is sub-ducted underneath into the upper mantle.…read more

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Associated Landforms:
Island arcs: The volcanoes that form are usually over laid with ocean so therefore the will eventually
form islands. They have been formed by magma from the Benioff zone. They usually form in curving
lines of new volcanic land.
Ocean trenches: The oceanic plate pulled down being sub-ducted to form a deep trench.
Characteristics:
Explosive Volcanoes
Shallow, deep/intermediate Earthquakes
3. Continental vs Continental (Indian vs Eurasian) ­Himalayas
Formation: Where the sub-duction of oceanic crust draws two continental masses together a
collision margin.…read more

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Hot spots: "A site of enhanced long-term volcanic activity, typically where a rising plume within the
mantle gives rise to large volumes of basaltic magma." Result of plumes of magma that originate
deep within the mantle. Hawaii is 3,200km from the nearest plate boundary so geologists think that
the formation of the volcanic islands are due to hot spots. A point on the surface of the Earth above a
plume of rising magma. E.g. the Hawaiian Islands.…read more

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Major extrusive:
Plate Margin Constructive Destructive
Type of Magma Basaltic Rhyolite
Lava Low Viscosity High Viscosity
Characteristics Low Silica High Silica
From Core (1200C) From melted plate (600C)
Type of eruption Little violence: gases escape easily as Potentially explosive: lava shatters
the more fluid magma allows for gas into pieces.…read more

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Shield volcanoes
Shape: Gently sloping cones built up from
numerous less viscous lava flows
Location: Hot spots and where oceanic crust
meets oceanic crust. Mauna Loa, Hawaii.
Rock type: Basaltic
Eruptions: Gentle and predictable
3. Composite volcanoes
Shape: Layers of ash from initial explosive
phases and subsequent layers of lava from main
eruption phases
Location: At destructive margins, Mt. Etna and
Mt. Vesuvius
Rock type: Andesitic
Eruptions: Explosive and unpredictable
4. Acid/Dome
Shape: Steep sided volcanoes from very viscous
lava
Location: Puy de Domes, France.…read more

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Lahars: Volcanic mud flows when the volcanic ash mixes with rain water. Pinatubo eruption
(1991)
Flooding: Melting of glaciers of ice caps. Grimsvotn glacial burst on Iceland (November 1996)
Tsunamis: Giant sea waves generated after violent caldera forming eruptions. Krakatoa
(1883) drowned 36,000 people
Landslides:
Climatic change: Ejection of volcanic debris into the atmosphere can reduce global
temperatures and been an agent in past climatic change. Pinatubo ash reduced global
temperatures by 0.…read more

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Collapsing buildings
Destruction of road surfaces and other forms of communication.
Water and electricity supplies disrupted.
Fires from ruptured gas pipes.
Flooding
Disease
Food shortages
Impact on economy.
Tsunamis: giant sea waves generated by shallow-focus underwater earthquakes or
landslides from volcanic eruptions.
Causes of earthquakes away from plate boundaries:
1. Readjustment from glaciers causing isostatic change.
2. Old faults lines
3. Movement from historic collisions
4. Fracting mining of oil
5. Reservoirs putting stress on rocks
6. Nuclear testing
7.…read more

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