Plate Tectonics

Convection Currents

The plate tectonics that make up the Earth’s crust are not stationery lumps of rock, they are continually moving at around 2-3cm per year.  The position of the plates as they are today is shown in the map below.

Image courtesy of USGS, via Wikipedia

They ‘float’ on the magma beneath them in the mantle, which acts like a conveyor belt for the plates due to the convection currents that occur within the molten rock.  At the surface, the plates are dragged in different directions depending on the direction the convection currents are flowing in.

The heat from the core causes the mantle to be heated at its base.  The hot rock rises gradually towards the crust.  As it rises, it moves further away from its heat source at the core and starts to cool down.  When it reaches the crust, it is forced out sideways because it cannot easily pass through the solid rock above it.  It continues to cool and as it cools, it begins to sink back down towards the core.  As it reaches the solid outer core, it is again forced sideways because it cannot pass through the solid iron/nickel layer and at the same time it is heated up once again and rises to the surface.  This process of heating, rising, cooling and sinking forms currents within the magma called convection currents.

Image courtesy of USGS, via Wikipedia

Continental Drift

Before the World looked as it does today, it was all one large supercontinent called Pangea.  Over time, the convection currents that created Pangea eventually caused it to break apart.  The new land masses moved around the World in a process called Continental Drift.  Eventually, some collided with each other and other broke apart leaving the World to look like it does today.  Eventually, the continents are predicted to collide once again to form a new supercontinent.  The animation below shows how Pangea was separated and the World in its present state was formed.

Image courtesy of Dr Ron Blakey, via Wikipedia

At the edge of each plate, there is a plate margin where interactions with another plate occur.  There are 3 main types of plate margin and these are outlined below.

Destructive Plate Margins

Where plates are moving towards each other at destructive margins, rock is destroyed.  Where oceanic crust pushes into continental crust there is a collision.  Due to the continental crust being less dense, the more dense oceanic crust forces its way underneath it.  This process is called subduction and occurs at a subduction zone.  As the oceanic crust is subducted down into the mantle, it begins to melt.  Similarly, as it rubs against the continental crust, friction and therefore heat is created also causing the rock to melt.  Sometimes these collisions also occur between 2 continental plates or 2 oceanic plates.

Image courtesy of Worldlywise Wiki

Constructive Plate Margins

Where plates are moving apart, rock is created.  Constructive margins often occur between 2 pieces of continental crust that is pulled apart by the convection currents in the mantle.  As the pieces of crust separate, a gap is left that is filled by magma from the mantle.  As this magma cools and solidifies it creates new igneous rock (rock created by molten rock cooling), which overtime can lead to the creation of ocean ridges such as the Mid-Atlantic Ridge.

Image courtesy of Worldlywise Wiki

Conservative Plate Margins

The final type of common plate margin is the conservative margin.  Here the plates slide past each other, either in opposite directions or at different speeds in the same direction.  The sliding motion often causes the plates to get stuck due to the rough nature of the rock.  This leads to a build up of pressure, which when released often causes very powerful earthquakes.

Image courtesy of Worldlywise Wiki