tectonics plates

7/30/2019 Tectonics Plates

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Tectonic Activity

Plates


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Tectonic Plates

Convection Currents

Destructive Plate Boundaries

Constructive Plate Boundaries

Conservative Plate Boundaries

Summary


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Tectonic Plates

Tectonic plates are sections of the Earths solid crust that

float on top ofthe mantle. The mantle contains hot.

molten rock (magma) heated by energy from the earth's

core

The earths crust is made up of seven main tectonic plates

and many smaller plates.


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There are two main types of tectonic plate:

Oceanic platesare relatively thin (about 6-10 km) and contain very heavy

(dense) rocks such as basalt.

Continental platesare considerably thicker (35-40 km) but contain lighter (less

dense) rocks such as granite.

The plates fit together like pieces of a large jigsaw. Tectonic plates meet along

four main types of plate boundaries.


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The tectonic plates are in constant motion travelling at a few centimetres per year.

The ocean floors are continually moving, spreading from the centre and sinking at the edges.

Where plates meet, huge forces build up causing earthquakes and volcanoes and the

formation of fold mountains and deep-sea trenches.


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Convection Currents

Hot currents of molten rock called convection currents move slowly within the earth's

mantle which moves the tectonic plates above them in different directions..

The source of heat driving the convection currents is radioactive decay which is happening

deep in the Earth. Convection occurs because the density of a fluid is related to its

temperature. Hot rocks lower in the mantle are less dense than their cooler counterparts

above. The hot rock rises and the cooler rock sinks due to gravity.


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Destructive Plate Boundaries 1 (aka Convergent or

Compressional)

a) Oceanic plate colliding with a Continental plate

When an oceanic plate and a continental plate move towards

each other the denser oceanic plate dives under the lighter

continental one creating a deep ocean trench.

As the oceanic plate goes deeper into the mantle, increased temperature and friction cause it tomelt creating asubduction zone.

Newly molten rock is lighter than the surrounding rock so it rises through the crust towards the

surface.

If molten rock (magma) reaches the earths surface, it may spread out along a fault line or itmay erupt at a single point as a volcano.

An example of an oceanic plate colliding with a continental plate is where the Pacific Plate is

moving towards the South American Plateat a rate of 9 cms per year.


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Destructive Plate Boundaries 2 (aka Convergent or Compressional)

b) Continental plate colliding with a continental plate

When continental plates collide, they crumple and create fold mountains

Where two continental crusts collide neither can sink

Instead they push into each other forcing material to be folded up into huge mountain ranges

Often this movement and pressure can cause earthquakes, but no volcanoes occur on these

boundaries

Example: Himalayas created where the Indian Plate meets the Eurasian Plate.


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Destructive Plate Boundaries 3 (aka Convergent or Compressional)

c) Oceanic plate colliding with oceanic plate

When two oceanic plate meet each other (oceanic-oceanic) this often results in the formation of an

island arc system. As the subducting oceanic crust melts,as it goes deeper into the Earth,

the newly-created magma rises to the surface and forms volcanoes. If the activity continues,

the volcano may grow tall enough to breakthe surface of the ocean creating an island.

Examples of an island arc are the West Indies and Japan


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Constructive Plate Boundaries(aka Divergent or Tensional)

When two plates move away from each other, they create a gap between them. Molten rock

(magma) rises from the mantle to fill the gap forming a mid-ocean ridge

Constructive plate boundaries cause mid-ocean ridges, volcanoes and earthquakes, though

they are often less violent than those on destructive plate boundaries.

Volcanoes can form along the edges of the plate boundary due to the rising magma. These

volcanoes are called shield volcanoes.

Example: the North American plate moving away from the Eurasian plate formed the Mid-

Atlantic Ridge and created Iceland through volcanic activity.


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Plate BoundariesConservative (aka Passive) plates

When two tectonic plates slide past each other without creating or destroying any land they

form a conservative plate boundary.

The plates often get stuck as they try to move past each other due to friction. Over time this

builds up great pressure until finally they jolt past each other. The sudden movement

causes an earthquake which may be violent and cause great damage.

Example: the San Andreas Fault where the N. American Plate meets the Pacific Plate


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Summary

Plates are sections of the Earths crust that float on top ofthe mantle

There are two types of plate - Oceanic & Continental

Convection currents cause the plates to move

Oceanic colliding with a Continental (destructive) = deep open trench, possible volcanic

eruptions

Continental colliding with a Continental (destructive) = fold mountains formed

Oceanic colliding with an Oceanic (destructive) = Island Arc system formed

Plates moving apart (constructive plate boundaries) = mid-ocean ridges, volcanoes and

earthquakes can result

Plates sliding past each other (conservative plate boundaries) = no landforms but they

can cause damaging earthquakes

tectonics plates

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