Plate Tectonics

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Plate Tectonics
Continental crust is also known as sial
and is less dense than sima.
Oceanic crust is known as sima and is
denser than sial.
Oceanic crust is continuous
Continental crust is discontinuous through oceans
Both parts are in a series of interlocked pieces called plates
The point between 2 plates is called a plate boundary
The plates are constantly moving (very slowly). In parts of the world where the plates are
moving apart, a constructive (or divergent or tensional) plate boundary is formed. New
oceanic crust is formed as the magma rises and solidifies e.g. Eurasian and North American
In other parts of the world the plates are moving together e.g. the Nazca and South
American plates creating destructive (convergent or compressional) plate boundaries ,
where crust is being destroyed.
Finally in areas of the world where the plates are sliding past each other, sometimes in the
same direction e.g. the Eurasian and South American plate or in opposite directions e.g. the
Pacific and the Juan de Fuca Plate. There are called conservative (transverse or passive)
plate boundaries.
Plate boundaries are associated with tectonic activity but the type of tectonic activity is
largely determined on the type of plate boundary.
Why Plates Move
Plates at our planet's surface move because of the
intense heat in the Earth's core that causes molten rock in
the mantle layer to move. It moves in a pattern called a
convection cell that forms when warm material rises,
cools, and eventually sink down. As the cooled material
sinks down, it is warmed and rises again.
Scientists once thought that Earth's plates just surfed on
top of the mantle's giant convection cells, but now
scientists believe that plates help themselves move
instead of just surfing along. Just like convection cells,
plates have warmer, thinner parts that are more likely to
rise, and colder, denser parts that are more likely to sink.

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New parts of a plate rise because they are warm and the plate is thin. As hot magma rises to the
surface at spreading ridges and forms new crust, the new crust pushes the rest of a plate out of its
way. This is called ridge push.
Old parts of a plate are likely to sink down into the mantle at subduction zones because they are
colder and thicker than the warm mantle material underneath them. This is called slab pull.…read more

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It has been proposed that the Earth's magnetic field reverses at intervals ranging from 10000s of
years to millions of years with an average interval of 250000 years. The last event
(Brunhes-Matuyama Reversal) is thought to have occurred 780000 years ago. There is no clear
theory as to how the geomagnetic reversals occur but many argue it is as a result in periodic
increases in solar activity.…read more

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Viscosity in turn controls the amount of gas that can be trapped in the magma. The greater the
viscosity, the more gas in the magma.…read more

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Dyke: magma intrusion into a vertical fault which solidifies. Not usually visible as are small
scale intrusive features. Sometimes a swarm of dykes will form.
Sill: e.g. Great Whin Sill - cooled and solidified magma between two strata (layers of rock)
along the bedding plain. These are not usually visible but can be seen at the high force
waterfall on the northern part of the River Tees.…read more

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Old and New Fold Mountains
Old Fold Mountains: more weathered and eroded (denudation) and are at lower altitudes
and smoother
o Appolation Mountains
o Scottish Highlands
o Scandinavian Mountains Formed in hercynian orogeny
o Great Dividing Range - Australia
o Urals - Russia
New Fold Mountains: less denuded and therefore more rugged and higher in altitude
o Rocky Mountains
o Andes Mountains
o Alps Formed in alpine orogeny
o Atlas Mountains
o Himalayas - formed by collision of India sub-continent with Eurasian Plate over 71…read more

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Earthquake magnitude is measured using a seismometer or seismograph on a scale of 1-8.
The Mercalli Scale
This measure the intensity i.e. the damage caused rather than the energy
This is a scale of 1-12.
Case Study: The Kashmir Quake 2005
Date: 8th October 2005
Magnitude: 7.6
Depth: 10km
74500 deaths
106000 injured
3.3 million homeless
Most deaths in rural mountainous areas - secondary causes for death e.g. hypothermia and
65% of medical facilities lost - long term spread of disease e.g.…read more

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Case Study: Indonesian Quake 2004
Date: 26th December 2004
Magnitude: 9
Eurasian and Australian Plates
250000 deaths
Strongest earthquake for 40 years
Walls of water tens of metres high hit coastal areas 1000s of miles apart as far away as
Populations were dependant on primary livelihoods meaning they lost everything
No access to clean water - contamination from sewage and decomposing bodies
Millions left homeless as buildings not made to withstand tsunamis
Tourist infrastructure destroyed
Hunger and disease were secondary impacts
Disruption of communications…read more

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Stilts used to prevent damage
Forest planting between the coastline and buildings as trees rapidly slow waves down and
cause them to lose a lot of energy
Continental-Oceanic Convergence
Where oceanic and continental plates meet, the heavier oceanic matter sinks below the
lighter continental plate in the subduction zone.
One example is the Nasca plate being subducted beneath the South American Plate.
Subduction zones are commonly marked by a long narrow trench in the ocean floor.…read more

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Case Study: Mount St. Helens 1980
Major volcanic eruption in Washington state, USA
VEI 5 event
35000 people evacuated
57 people killed
Had been being monitored for 3 months - eruption was predicted
Affected world for 2 years as affected crop production
80000 feet eruption column, depositing ash in 11 states
Case Study: Nevado Del Ruiz 1985
Nazca and S.…read more


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