Isn’t it funny to think that the Earth is moving! If we stand perfectly still and look into the distance, the Earth appears to be perfectly still, too. But the Earth is actually moving in many different directions. The Earth rotates around its own axis, and we experience this as day and night. The Earth is also in motion as it orbits the Sun, and we experience this movement as the seasons change. We don’t feel the movement as the Earth spins and rotates, but we know it is happening.
There is another type of movement that affects the Earth. This movement happens underneath our feet. We don’t usually feel this movement because it is quite gradual – just a few millimetres every year. With time, the pressure of this movement builds up, and there is a sudden shift inside the Earth that we feel as an earthquake.
Picture the Earth as if it were a hardboiled egg. The yolk is the core of Earth and the white is the mantle. The thin shell around the outside of the egg is like the thin crust of the Earth. If you bump the egg against a plate, the shell develops cracks. The Earth’s crust also has cracks. Scientists call these cracks tectonic plate boundaries.
Tectonic plate boundaries
The huge tectonic plates that cover the Earth fit together a bit like pieces in a global puzzle. The Earth’s mantle under the crust is hot and flexible so the plates (puzzle pieces) are able to move, but they do so very, very slowly. There are three different ways the plates move: the plates can move past each other, they can move apart from each other or they can move towards each other.
Scientists have special names for the way the plates move
Transform boundaries are where the plates meet and try to move past each other. Friction holds the plates in place, so they cannot simply glide past one another. Stress builds up and is released as an earthquake. New Zealand’s Alpine Fault is an example of a transform boundary.
Divergent boundaries are where the plates slide apart from each other, and the space that this creates is filled with magma and forms new crust. This often happens below the sea, for example, the Pacific Ocean is growing wider by about 18 cm per year.
Convergent boundaries are where the plates slide towards each other. Sometimes this creates mountains, for example, the collision between the Australian plate and the Pacific plate formed the Southern Alps. When two plates under the ocean collide, they usually create an island as one plate moves beneath the other. The Solomon Islands were created this way.
On the move for billions of years
Scientists now think the tectonic plates have been on the move for around 3 billion years, but only 50 or 60 years ago, people thought that the continents were set in the same position forever. Scientists try to find out how and why things work. By studying rocks, fossils and earthquakes, they came up with the new theory of plate tectonics.
Nature of science
Science knowledge changes when new evidence is discovered. Scientists first used fossils and other geological evidence to show that the continents are on the move. Today, they use GPS to track tectonic plate movement.
