Plate Tectonics Explained

Earth's surface is not one solid shell. It is cracked into huge slabs of rock called tectonic plates, which fit together like the pieces of a cracked eggshell. The British Geological Survey (BGS) counts about 15 major plates, plus many smaller ones. Some carry a continent, some lie under the ocean, and most carry both.

Plate tectonics is the theory that explains how these plates move and what happens where they meet. Their edges, called plate boundaries, are where most earthquakes, volcanoes, deep ocean trenches and young mountain ranges are found. The rest of this series looks at each of those in turn.

To understand the plates, start with what lies beneath them. Earth has four main layers:

• Crust: the thin outer skin. Continental crust is about 25 to 70 km thick and forms the land; oceanic crust under the sea floor is only about 5 to 10 km thick, but its rock is denser.
• Mantle: a hot, dense layer of rock about 2,900 km thick. It is solid, yet over millions of years parts of it slowly flow.
• Outer core: about 2,200 km of liquid iron and nickel.
• Inner core: a ball about 1,250 km thick at the centre, made mostly of iron and nickel, which is solid despite temperatures of around 5,200 °C because of the enormous pressure.

In the diagram, the half-Earth is drawn with these thicknesses in proportion. At that scale the crust would be thinner than a pencil line, so the top panel zooms in on it.

Geologists also divide the outer layers by how the rock behaves, not just what it is made of.

The lithosphere is the crust plus the very top of the mantle, welded together into a stiff, brittle layer. The US Geological Survey (USGS) says it averages at least 80 km thick over much of the Earth. The lithosphere is the layer that is broken into plates.

Below it is the asthenosphere, a narrower zone of the mantle where hot rock is close to melting. It is still mostly solid, but it can soften and flow very slowly, rather like warm toffee. The rigid plates of lithosphere rest on this softer layer, which lets them move.

Heat from deep inside Earth keeps the mantle slowly churning. Hot rock rises, spreads out, cools and sinks again in giant loops called convection currents. Scientists describe three forces that work together:

1. Mantle convection drags on the base of the plates.
2. Ridge push: at mid-ocean ridges, new hot plate sits high and slides away downhill.
3. Slab pull: where an old, cold, dense plate sinks into the mantle at an ocean trench, it pulls the rest of the plate after it. The BGS describes slab pull as the main driving force.

Plates move at about 2 to 15 cm a year, according to National Geographic. That sounds tiny, but it adds up. The Atlantic Ocean is widening at about 2.5 cm a year along the Mid-Atlantic Ridge. Over one million years that is:

2.5 cm × 1,000,000 = 2,500,000 cm = 25 km.

Over the 100 to 200 million years since the Atlantic began to open, this slow spreading has built an entire ocean.

Nobody has drilled through the crust, so scientists use earthquakes as a giant X-ray. Seismic waves change speed and direction as they pass through different materials. One kind, the S wave, cannot travel through liquid. S waves never get through the outer core, which is how we know it is liquid. The geologist R. D. Oldham first identified Earth's core from earthquake records in 1906.