Plate tectonics

The plates and plate margins

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.Back to the top

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. 

 

4 Types of plate margins

 

1. Constructive Margins

 

At these type of plate margins two plates are moving apart  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.

 

2. Destructive Margins

At these margins 2 plates move  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.

 

3. Collision margins.

Fold mountains occur near convergent or compressional plate boundaries. Examples of fold mountains include the Alps, Rockies, Andes and Himalayas.

Formation and characteristics

The formation of fold mountains

The formation of fold mountains

1. 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.
2. When the two plates move towards each other again, the layers of sedimentary rock on the sea floor become crumpled and folded.
3. Eventually the sedimentary rock appears above sea level as a range of fold mountains.

Where the rocks are folded upwards, they are called anticlines. Where the rocks are folded downwards, they are called synclines. Severely folded and faulted rocks are called nappes 

4. 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 stress energy builds as the plates snag and grind on one another. When this stress energy 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.