Earth's layers

The crust and the rigid top part of the mantle together form the lithosphere. The lithosphere is broken into large pieces called tectonic plates, which move slowly over a hotter, softer layer of the upper mantle (the asthenosphere). Plates move a few centimetres each year, about as fast as your fingernails grow.

Why do plates move?

Heat from deep inside Earth keeps the mantle moving very slowly. Hot rock rises, spreads out and sinks again as it cools, forming convection currents. Gravity also plays a big part: at some boundaries the edge of a cold, dense plate sinks into the mantle and pulls the rest of the plate along behind it (slab pull), and new crust at ocean ridges sits higher and slides away from the ridge (ridge push).

Plate boundaries

Earthquakes

As plates move, rocks along their edges get stuck. Stress builds up until the rocks suddenly break or slip along a fault, releasing energy as seismic waves that shake the ground. The point underground where the slip starts is the focus; the point on the surface directly above it is the epicentre. Seismic waves are recorded on instruments called seismometers, which produce traces called seismograms.

Earthquake size is described by its magnitude, today usually on the moment magnitude scale (the older Richter scale works in a similar way). Each increase of 1 in magnitude means the ground shaking recorded is about 10 times larger and about 32 times more energy is released.

A large earthquake under the sea can suddenly lift or drop the sea floor, pushing up a huge volume of water and creating a tsunami: a series of very long waves that travel fast across the ocean and grow taller as they reach shallow water near the coast.

Volcanoes

Most volcanoes form at plate boundaries. At subduction zones, the sinking plate releases water into the mantle above it, which helps the rock melt; the magma rises to form explosive volcanoes. At divergent boundaries, magma rises where plates pull apart, usually producing runny lava and gentler eruptions.

Some volcanoes form far from any plate boundary, above a hotspot, where unusually hot mantle rock rises. As the plate moves over the hotspot, a chain of volcanoes forms. The Hawaiian Islands formed this way, and the oldest islands are furthest from the hotspot.

The Ring of Fire is a horseshoe-shaped zone of subduction zones around the edges of the Pacific Ocean. Most of the world's earthquakes and active volcanoes are found there.

Evidence for plate tectonics

• Fit of the continents: the coastlines of South America and Africa fit together like jigsaw pieces. In the early 1900s Alfred Wegener proposed that the continents were once joined in a single supercontinent, Pangaea, which began to break apart about 200 million years ago.
• Fossils: fossils of the same land reptiles and plants are found on continents now separated by wide oceans.
• Rocks and mountains: matching rock types and mountain ranges line up across oceans.
• Sea-floor spreading: rocks on the ocean floor are youngest at mid-ocean ridges and get older with distance from the ridge, and magnetic stripes in the rocks form matching patterns on each side of the ridge.
• Earthquakes and volcanoes: these are concentrated along narrow belts that mark the plate boundaries.

Check yourself

1. Name Earth's four main layers, from the outside in, and state whether each is solid or liquid.
2. What is the difference between the focus and the epicentre of an earthquake?
3. At what type of boundary do plates move apart, and what forms there?
4. Explain why the oceanic plate sinks when an oceanic plate meets a continental plate.
5. Why do fold mountains such as the Himalayas form where two continental plates collide?
6. Why are there earthquakes but no volcanoes along the San Andreas Fault?
7. Hawaii is in the middle of the Pacific plate, far from any boundary. How did its volcanoes form?
8. A plate moves 3 cm per year. How far does it move in 2 million years? Give your answer in kilometres.
9. How much more energy is released by a magnitude 7 earthquake than a magnitude 6 earthquake?

Answers: 1. Crust (solid), mantle (solid, but flows very slowly), outer core (liquid), inner core (solid). 2. The focus is the point underground where the rock first breaks or slips; the epicentre is the point on the surface directly above the focus. 3. A divergent boundary; new crust forms there, building mid-ocean ridges or rift valleys. 4. Oceanic crust is denser than continental crust, so it sinks beneath it (subduction). 5. Continental crust is too thick and not dense enough to sink easily, so the crust crumples, folds and is pushed upwards. 6. It is a transform boundary: the plates slide past each other, so no crust is destroyed and no magma is produced, but stress builds and is released as earthquakes. 7. It sits above a hotspot, where hot mantle rock rises and melts; as the plate moves over the hotspot, a chain of volcanic islands forms. 8. 3 cm × 2 000 000 = 6 000 000 cm = 60 000 m = 60 km. 9. About 32 times more energy.