The Earth is an enormous place. There is so much that we don’t yet know about what happens on the surface of the Earth, so how could we possibly know about what happens deep down below the Earth’s surface? Scientists are still working out some of the details, but with the help of equipment used when studying earthquakes, they know the Earth is made up of layers.
Let’s start at the very centre of the Earth. The word ‘core’ means the central part of something, for example, an apple has a core. The Earth has an inner core and an outer core. Scientists think that the inner core is solid and mainly made of iron. Iron is very hard – that’s why knights used it to make their armour. The outer core is also mostly iron, but this layer is liquid (melted) iron.
The next layer is called the mantle. It is made of rock. It is very hot in this part of the Earth, causing the rock to behave a little bit like a liquid and a solid.
The crust is the part of the Earth that we walk on. Compared to the other layers, it is quite thin and breakable. You can think of the crust of the Earth as being a bit like the crust of a pie – although this crust is 5–70 km deep depending on whether you are under the ocean or on top of a mountain! The Earth’s crust is not one continuous surface like on a tennis or soccer ball. The crust is ‘broken’ into enormous pieces called tectonic plates.
Scientists think that the crust of the Earth is made up of six large (major) tectonic plates and a few smaller ones. These plates fit together like puzzle pieces and float on the partially molten mantle. They slowly move and bump against each other at a rate of a few millimetres to up to 20 cm per year. It is this bumping and rubbing that causes earthquakes and volcanoes.
Volcanoes and tectonic plates
The hot, melted rock in the mantle doesn’t usually make it through the many kilometres of crust that forms the ground we walk on. It’s only at places like the edges of tectonic plates that magma starts to creep through. Think of our Earth as being a bit like a hot mince pie. The filling stays in place until there is a break in the crust. Then, the hot gravy can come to the surface. Volcanoes are a bit different to pies, though. Often there is pressure, steam or gases that force the magma from the Earth, causing the powerful explosions we often associate with volcanic eruptions.
Earthquakes and tectonic plates
Although the tectonic plates fit together like puzzle pieces, they still move a bit. Think about how you can wiggle a puzzle once it’s put together. The plates build up energy when they push against each other. When the pressure gets too much, something has to give (like when you bend a stick until it breaks). The sudden movement of these underground pieces causes earthquakes.
Nature of Science
Scientists can’t journey to the centre of the Earth! The ideas they get about how the Earth is made up come from piecing evidence together from many areas of science such as seismology, astronomy and geology. After gathering the evidence, they make a model (comprised of the Earth’s crust, mantle and core) to explain earthquakes and volcanoes that we see at the Earth’s surface.
This animation shows the movement of the tectonic plates that make up the Earth’s crust. Watch how New Zealand was formed.