The Earth's structure and plate tectonics
Home learning focus
Learn about the Earth's structure and plate tectonics including plate movement and boundaries.
This lesson includes:
two videos explaining the main features of plate tectonics
Watch this animation for an introduction to plate tectonics, and how their movement impacts on Earth.
The Earth's structure
The Earth has four main layers - the inner core, the outer core, the mantle and the crust.
- The inner core is 5,500°C - extremely hot. It is a very dense solid made from iron and nickel.
- The outer core is 2,000 km thick and is a liquid.
- The mantle is semi-molten (partly melted) and about 3,000 km thick.
- The crust is the rocky outer layer. It is thin compared to the other sections, approximately 5 to 70 km thick.
The crust is made up of pieces called plates. There are two types of crust: oceanic and continental crust. The oceanic crust is found under the sea and is thinner and more dense (crowded closely together) than the continental crust.
Heat from the core causes convection currents (a movement within the Earth's mantle caused by the heat of the core) in the mantle.
These currents slowly move the crust around. In some places the crust is destroyed. In other places new crust is formed.
Earthquakes and volcanoes are primarily found at plate boundaries (the region where two or more plates meet). The plates are like giant rafts that slowly move around.
Their movement is driven by convection currents in the mantle. The mantle is much hotter than the crust and its rock is molten (a liquid substance formed by heating a solid eg. a rock).
At the boundaries between plates, molten magma is able to force its way to the surface and escape as lava (molten rock that is released is from the Earth's core in a volcano or fissure).
There are a number of different types of plate boundary.
A destructive plate boundary is sometimes called a convergent or tensional plate margin. This occurs when oceanic and continental plates move together. The oceanic plate is forced under the lighter continental (belonging to a continent, eg Europe) plate. Friction causes melting of the oceanic plate and may trigger earthquakes. Magma rises up through cracks and erupts onto the surface.
An example of a destructive plate boundary is where the Nazca plate is forced under the South American Plate.
Collision zones form when two continental plates collide. Neither plate is forced under the other, and so both are forced up and form fold mountains.
A constructive plate boundary, sometimes called a divergent plate margin, occurs when plates move apart. Volcanoes are formed as magma wells up to fill the gap, and eventually new crust is formed.
An example of a constructive plate boundary is the mid-Atlantic Ridge.
A conservative plate boundary, sometimes called a transform plate margin, occurs where plates slide past each other in opposite directions, or in the same direction but at different speeds.
Friction is eventually overcome and the plates slip past in a sudden movement. The shockwaves created produce an earthquake (a sudden shaking of the ground which releases energy and results from underground movement along a fault plane) .
This occurs at the San Andreas Fault in California.
In this short film Liz Bonnin explains how our oceans are constantly changing due to the underlying plate tectonics.
Here are a few activities to help you remember what you've learnt about the Earth's structure and plate tectonics.
There's more to learn
Have a look at these other resources around the BBC and the web.