Plate Interactions. Convergent, Divergent and Transform Plate Boundaries. Plate Interactions.
Convergent, Divergent and Transform Plate Boundaries
We have previously discussed the evidence that supports the concept of drifting continents and plate tectonics. Because of the different directions the plates move, different processes occur at plate boundaries. We have touched on these briefly and now we will look at them in more detail.
When two plates collide, scientists call this a convergent plate boundary. These slow collisions usually result in one plate being forced beneath the
other. The overriding
plate normally grows
as folding and volcanism
produce mountain chains.
However this all depends on the types of crusts that are colliding. There are three types of collisions:
Because oceanic crust is more dense than continental crust, when these two types of crust collide the oceanic crust is always forced into the mantle (subducted).
Where the oceanic crust is bent downward, trenches form. These are the deepest parts of the ocean and are used to help indicate the edge of a plate.
As the subducting oceanic plate is forced beneath the overriding continental plate, the continental plate is lifted and folded upwards producing mountains.
An example of a oceanic-continental convergent plate boundary is along the west coast of South America. Here we find the Andes Mountains.
The Oceanic Nazac plate is slamming into the Continental South American plate
If we visit this plate boundary we can see the evidence which supports this theory.
Something else happens at this boundary which produces mountains. As the oceanic crust is forced deeper into the Earth, intense heat and pressure cause it to melt. And what happens to materials when we heat them?
They rise! The oceanic plate turns into magma and rises into the overlying continental crust. This is called a magma plume. If this plume reaches the surface we get volcanoes!
A key feature to this magma is that it is rich in silica which comes from the sediments in the oceanic crust or the melted part of the continental crust.
This high silica magma makes it more viscous (thicker) which allows it to trap gasses within it. Volcanic eruptions at such boundaries tend to be very explosive because of the intense heat and pressure build up.
This type of silica rich and explosive eruption is called andesitic volcanism (after the Andes Mountains).
But what happens if the magma never makes it to the surface? That’s a great question! The magma that has collected within the continental crust can slowly cool to form granite or similar intrusive igneous rocks.
Eventually the surrounding material is eroded away and we can see these frozen granite plumes today on the surface of the earth.
Stone Mountain in Georgia (USA) is an example of just how big these frozen granite magma plumes can get.
So what happens to the rocks on the continental crust that come into contact with these magma plumes?
They are changed ‘metamorphosed’ into a different rock. This is how we get metamorphic rocks.
When these rocks are uncovered due to weathering and erosion, we can see where this contact metamorphism has happened.
Oceanic-continental convergence boundaries also experience a lot of seismic activity. Earthquakes are quite common as the two plates are crushing into each other.
An earthquake’s focus is the exact point where the rocks in the crust break or move. At a subduction zone, scientist can measure both deep and shallow
evidence that the
oceanic plate is being
forced into the mantle.
This type of boundary occurs between two pieces of oceanic crust. Because no buoyant continental crust is involved we do not get large mountain ranges.
As one plate is forced beneath the other, it is melted and a line of volcanoes form in the same way as described in a oceanic-continental convergent boundary.
The volcanoes that are created at this boundary form a curved line out of the sea and scientists call these island arcs.
Examples of island arcs include the Philippine Islands, Japan and Indonesian Islands.
Metamorphism occurs at these boundaries and the trenches formed are the deepest on the planet. The Mariana Trench is 11 kilometres deep. These boundaries also have deep and shallow earthquakes.
This type of boundary is where two continental pieces of crust slam into each other. This boundary is different than the last because there is not complete subduction.
This is because the continental crust is so buoyant. Instead the plates become intensely folded and uplifted. An example of this is the Himalayas.
Earthquake foci are shallow at this boundary and there is no contact metamorphism. Instead there is regional metamorphism. Low heat, high pressure.
This is where two plates separate. These spreading zones are where new crust is being generated. Mid-ocean ridges mark divergent plate boundaries.
These boundaries are some of the most active on Earth. Volcanoes at these boundaries produce basalts low in silica. This means the lava has a low viscosity and releases gasses easy. Eruptions are therefore less explosive.
Lava usually erupts in long cracks in the Earth’s surface called fissures rather than mountains. This is because the crust is being pulled apart rather than forced together.
Rocks at divergent boundaries do not usually undergo metamorphism. Earthquakes are shallow as the forces cause rocks to crack and sink along fault lines.
This occurs when two plates slide past one another. Movement is parallel to the direction of the boundary so neither convergence or divergence occurs.
Volcanoes rarely occur along these boundaries. Shallow earthquakes are common because parallel movement of the plates are not smooth. Elastic energy is stored within the rocks and when the rocks give way the energy is released.
The San Andres Fault in California is an example of a transform boundary. The displacement of landforms gives a clear indication of plate movement.