Published by Australian Academy of Science
Discovering Australia's evolutionary past Box 1 | Plate tectonics

The structure of the Earth

The Earth is a rocky planet 12,700 kilometres in diameter. Deep in the centre of the Earth lies the core, which has a diameter of about 6900 kilometres. The core probably consists mostly of iron and nickel. The temperature in the inner part is estimated to be about 4000°C. The pressure is intense since the weight of the rest of the planet is pushing down on it.

The core is surrounded by a rocky layer called the mantle, about 2900 kilometres thick, which constitutes about 80 per cent of the planet's volume. Due to the heat and pressure, the minerals in the mantle can move slowly, rather like thick putty. In some places the top part of the mantle is partly molten. Above the mantle lies the lithosphere, the outermost shell of the Earth. The lithosphere is about 100 kilometres thick and is rigid and strong. The upper part of the lithosphere is the Earth's crust.

There are two types of crust: oceanic and continental. The oceanic crust consists of lava flows that are about 5 kilometres thick. They form at the mid-ocean ridges where the lava wells up from the interior. The crust spreads away from the ridges as it forms and dives back into the mantle when it collides with a continent or another plate. As it sinks back down, the pressure converts the minerals in the rock to denser ones, like garnet. This makes the slab of rock heavier, so that it sinks deeply into the mantle. This seems to be the main driving force for plate tectonics. The sea floor spreads away from the mid-ocean ridges because it is dragged back down into the interior, pulled rather than pushed.

As the thin oceanic crust dives down beneath the continents, the high temperatures 100 kilometres below the surface cause parts of it to melt. The mixture of molten rock (magma) and water that is formed erupt at the surface as spectacular explosive volcanoes, like the Rabaul volcano in Papua New Guinea that erupted on 19 September 1994. These volcanoes not only add ore deposits but also contribute to the thick continental crust. In contrast to the dense oceanic crust, the continents are typically about 40 kilometres thick and are made up of lighter rocks like granite. The lighter, thicker continental crust elevates the continents above the level of the ocean basins.

The Earth's surface is made up of moving plates

Although it feels solid enough, our planet's rocky surface, on land and under the sea, is a restless jigsaw of slowly moving pieces.

The fact that the lithosphere is rigid, and that the mantle can move a little, is important in explaining this. During the 1960s geologists came to realise that parts of the lithosphere are in constant motion relative to one another and that they carry the continents with them. These moving parts are called plates.

Each plate is about 100 kilometres thick. The plates move extremely slowly, creeping along at a rate of about 1-12 centimetres per year. Although slow, such movements are driven by great forces and dramatic events occur when two plates are pushed together or pulled apart.

The movements and collisions of plates account for the existence of folded mountain ranges, earthquakes, volcanoes and continental drift. Over millions of years, the movement of plates can make entire continents split, come together or drift apart. When looking at a map of the world, you might have noticed that the outlines of some continents suggest that they once could have fitted together.

Why the plates move

The Earth's plates move because of the heat inside it. Within the mantle, convection currents circulate, slowly mixing its material. The slow rising and falling of these currents goes on continuously. In the process, parts of the lithosphere are moved apart by the sideways movement of the currents underneath. The convection currents in the mantle also bring heat to the surface.

Earthquakes

When plates try to slide past each other, friction between them stops their movement at first. Tremendous strains then build up. Eventually, the friction is overcome and the plates suddenly snap past each other, moving by a few metres at a time. Earthquakes are the result.

Mountain building

Where the movement of plates has caused land masses to collide (although in slow motion), huge mountain ranges are pushed up over millions of years, like wrinkles on a tablecloth. The Himalayas were formed in this way as the plate carrying India slowly pushed into the one carrying Asia. These plates are still moving together, at the rate of about 11 centimetres per year, causing the surface to buckle and the ridges of the Himalayas to continue rising.

Related sites

• When the Earth moves: Seafloor spreading and plate tectonics (Beyond Discovery, National Academy of Sciences, USA) (A PDF file of the complete article is available.)

• Layers of the Earth (Volcano World, USA)

• Plate tectonics (Wheeling Jesuit University, USA)

• This dynamic Earth: Historical perspective (US Geological Survey)

Other boxes

Box 2. The name game – Australia's eucalypts and acacias