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Plate Tectonics and the Earths structure
Types of plate and crust
Plate margins
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Tectonics is a theory that tries to explain how the Earth is structured and what it is made up of.  The human race has never been to the centre of our own planet so scientists have had to use various pieces of evidence to show the structure of the Earth.  To the left is an idealised diagram of the Earth's interior (middle bit).  The Earth formed approximately 4.5 billion years ago following a huge explosion of a star.  The materials that make up our earth slowly gathered together due top gravity, to create a ball of hot molten material.  This material has slowly cooled over geological time, forming a crust at the Earth's surface of rocks.  These rocks are fractured into huge segments called Tectonic plates.  These tectonic plates are moving about very slowly, pushed and shoved around from underneath by currents within the mantle called convection currents.  Beneath the crust temperatures starts to rise as you descend into the second of the Earth's zones, the Mantle, a zone of molten Silicates and other minerals.  The Earth does have a solid core of Iron and Nickel, which is solid despite temperature of 3700°C because of the intense pressure there. Click here to see an interactive site on the structure of the Earth.

The structure of the Earth's interior

The plates and plate margins
Map of the World's tectonic plates. Click on the image to go to

The Earth's crust is divided up into chunks of solid rock called tectonic plates.  they vary in size and the Earth's surface can be likened to that of a boiled egg which has been cracked.  The major plates include the Pacific, Eurasian, African, Antarctic, North American and South American, and the Indo-Australian.  There are other smaller plates however, such as the Philippines and Cocos plates.  The plates are made up of different materials, and there are 2 broad types:
Continental crust is thicker, older and lighter, and is composed mainly of Granite. It is
22 mi (35 km) thick on average and less dense than oceanic crust, which accounts for its mean surface elevation of about 3 mi (4.8 km) above that of the ocean floor. Continental crust is more complex than oceanic crust in its structure and origin and is formed primarily at subduction zones at destructive plate margins

Oceanic crust is younger and heavier, and is mainly composed of basalt and Gabbro.  it is mainly formed at constructive margins or spreading mid ocean ridges.

Analysis of seismic waves passing through the Earth's crust has revealed this to us. You can watch the break up of Pangea here, and watch predicted futures movements of the Earth's plates here.
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Location of the World's plate margins

Types of plate margin

Volcanoes and earthquakes mainly occur along plate boundaries where magma can escape from the earth’s mantle or where stresses build up between 2 plates rubbing together. An exception to this includes Hawaii, which is found in the middle of the Pacific plate over a hot spot. Examples of plate margin types are linked to the map.

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USGS image of plate margins types


Constructive or Divergent Margins

At these type of plate margins two plates are moving apart (DIVERGE) from each other in opposite directions.  Convection currents moving in opposite directions (caused by the intense heat of the Earth's interior) in the mantle move two plates apart.  As these plates move apart this leaves cracks and fissures, lines of weakness, that allows magma from the mantle to escapes from the highly pressurised interior of the planet. This magma fills the gap and eventually erupts onto the surface and cools as new land.  this can create huge ridges of undersea mountains and volcanoes such as the Mid-Atlantic Ridge, and where these mountains poke above the level of the sea islands are created. Both earthquakes and volcanoes can result at these margins, the earthquakes caused by the movement of magma through the crust. A really good example of this is the mid Atlantic Ridge, where the Eurasian plate moves away from the North American plate at a rate of around 4cm per year.  Iceland owes its existence to this ridge. Watch an animation of this process here.


Destructive or Convergent Margins

At these margins 2 plates move or CONVERGE together and the Destruction of some of the Earth's crust results.  An oceanic plate (denser) is pushed towards a continental plate (less dense) by convection currents deep within the Earth's interior. The oceanic plate is subducted (pushed under) the continental plate at what is called a subduction zone, creating a deep ocean trench.  It is the Oceanic crust which sinks down into the mantle because it is denser (heavier). As it descends friction, increasing pressure and heat from the mantle melt the plate.  Some of this molten material can work its way up through the continental crust through fissures and cracks in the crust to collect in magma chambers.  This is often some distance from the margin where magma can eventually re-emerge at the surface to create a range of mountains.  The movement of the plates grinding past one another can create earthquakes, when one plate eventually slips past the other releasing seismic energy. There are several really good examples of Destructive plate margins, including along the West coast of the Americas and Japan, where the Philippines sea plate is pushed under the Eurasian plate. Watch an animation of this process here.



At these margins 2 plates of similar density are forced toward each other.  Neither plate descends into the mantle because of the similar density of the plates.  Instead, the 2 plates crumple into one another and fold upwards into fold mountains. At these margins we get fold mountains and earthquake activity, and a fantastic example of this is the Himalayan mountains.  Here, the Indo Australian plate is colliding with the Eurasian plate and has done so for millions of years.  Originally, there used to be a sea called the Tethys sea between India and Asia, but over time India has collided into Eurasia creating huge fold mountains rich in marine (sea) fossils! See an Animation of India at a Collisional Margin


Conservative margins

At conservative margins mountains are not made, volcanic eruptions do not happen and crust is not destroyed.  Instead, 2 plats either slide past each other in opposite directions, or 2 plates slide past each other at different speeds. As they move past each other friction builds as the plates snag and grind on one another. When this friction is eventually released it sends shock waves through the earth’s crust. We know these shock waves as earthquakes, and a good example of this is the San Andreas fault in California, where the Pacific plate is moving NW at a faster rate than the North American plate.

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Find out more

1) Copy into Word then complete the chart below using information gathered in class or using the web sites below

Dynamic Earth - Complicated look at Plate Tectonics
Internet Geography - notes on Plate Tectonics



Name: Name: Name:
Conservative.gif (23645 bytes) Constructive.gif (10459 bytes) Destructive.gif (50812 bytes)
Describe what happens here.





Describe what happens here.





Describe what happens here.





Does it result in Earthquakes or volcanoes?



Does it result in Earthquakes or volcanoes?




Does it result in Earthquakes or volcanoes?




World Examples?



World Examples?



World Examples?



2) Have a go at a jigsaw activity on geography all the way!

3) Take a tectonics quiz

4) Geography4kids quiz

5) Have a go at a tectonics labelling game!

6) Try this Post it note interactive labelling exercise

You Tube Videos

How the early Earth formed Explanations of Plate Tectonics An explanation of the Wilson Cycle

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