Overview of different Types of Plate Boundaries

Different Types Of Plate Boundaries

Image: Tectonic plate boundaries - Jose F. Vigil. USGS © Public Domain

In linked article 2 we started to think about the different boundaries between tectonic plates. 

Remember the structure of the earth; that the rigid plates (crust and upper mantle) move on currents in the hotter and softer rocks of the mantle below them.

 

Different Terminology

Similar plate boundaries may have different terms used to describe them; we will try and give you the main names used and what they mean.

Divergent Boundary

When two plates move apart the boundary will commonly have one of three names:

• Divergent Boundary (diverge means to move apart).

• Tensional Boundary (tension pulls apart).

• Constructive Boundary (as new crustal material can form here).

Image: Continental-continental constructive plate boundary - domdomegg © Wikimedia Commons under Creative Commons Attribution-Share Alike 4.0 International

A good example of a divergent boundary is the Mid-Atlantic Ridge.

The Mid-Atlantic Ridge lies between the North American and Eurasion Plate in the North Atlantic and between the South American and African Plates in the South Atlantic.

The distance between them grows at a speed of around 5-7cm per year.

In the North Atlantic the volcanic rocks of the ridge have pushed above the surface of the sea to form Iceland.

Image: Iceland Mid-Atlantic Ridge Fig16 - USGS © Public Domain

Volcanoes happen at tensional boundaries because as mantle rocks rise upwards the pressure on them from overlying layers decreases.

As pressure drops about 10% of the mantle rocks melt to form a magma.

The magma squeezes up along cracks to reach the surface and become lava and new crust.

These new rocks are sometimes called oceanic crust as they mostly form beneath the oceans and seas.

Oceanic crust cools quickly because of the waters; it forms cracks filled with sea water.

Convergent Boundary

When two plates move together the boundary will have one of three names:

• Convergent Boundary (converge means to move together).

• Compressional Boundary (compression pushes together).

• Destructive Boundary (as crustal material can be melted here).

If one of the plates is made up of oceanic crust then it can be forced downwards into the mantle where parts of it will melt.

Image: Continental-continental destructive plate boundary - domdomegg © Wikimedia Commons under Creative Commons Attribution-Share Alike 4.0 International

Image: Oceanic-oceanic convergence - USGS © Public Domain

When two plates made up of oceanic crust are forced together then one of them will be forced downwards into the mantle. This is called subduction.

When subduction happens a deep trench is formed as surface layers are dragged downwards.

The oceanic crust has lots of cracks within it filled with seawater.

The seawater lowers the melting point of some of the rock melts and form magma.

This magma rises and cools.

This magma is different from oceanic crust and it will form the types of rocks called continental crust.

Image: Active Margin - Booyabazooka © Public Domain

When a plate made up of oceanic crust is forced into a plate made up of continental crust the oceanic crust plate will be forced downwards into the mantle (subduction).

Rising magma will be formed and as it rises it starts to absorb the overlying continental crust. This creates a very thick, sticky magma which is explosive if it reaches the surface (becomes lava).

The overlying continental rocks will also be folded and pushed upwards to make fold mountains.

An excellent example of this is along the Western edge of the South American Plate.

In the satellite image you can see the deep blue colour showing the trench.

The reddish colours show the Andes Mountains.

Image: Peru-Chile trench - Interiot~commonswiki © Public Domain

The Andes are the longest continental mountain range in the world; they are about 7,000 km long.

The average height is about 4,000 metres. The highest mountain is called Acongagua at 6,961 metres. This is the highest mountain outside of the Himalayas.

Image: Aconcagua (aerial) - Beatriz Moisse © Wikimedia Commons under Creative Commons Attribution-Share Alike 3.0 Unported

The Andes extend from north to south through seven South American countries: Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile and Argentina.

Image: Continental-continental convergence - USGS © Public Domain

When two plates with continental crust collide they are much less dense than the underlying mantle rocks so they cannot be subducted. 

As a result the rocks are folded and forced upwards to form a fold mountain chain such as the Himalayas.

Image: Himalayas - NASA © Public Domain

The Himalayas are found in a belt between the Indian sub-continent to the South and China on the Eurasian Plate to the North.

Image: Himalayas Map - Sven Manguard © Public Domain

The Himalayas were formed as the Indian Plate moved northwards colliding with the Eurasian Plate. This is still happening today so the mountains are still getting bigger.

Image: India 71-0 Ma - Mourmine / http://comp1.geol.unibas.ch/zanskar/ © Wikimedia Commons

Conservative Boundary

When plates are not moving together or apart they are known as either a conservative or passive boundary.

If they are moving in opposite directions they may be called a transform fault.

Image: Continental-continental conservative plate boundary opposite directions - domdomegg © Wikimedia Commons under Creative Commons Attribution-Share Alike 4.0 International

The main features along this kind of boundary are small compared to the other types of boundaries. However they can have some powerful earthquakes along them so it is important to know where they are.

© British Broadcasting Corporation (BBC)