Atmosphere The Earth's atmosphere is an “ocean of air” that surrounds the planet. It is made up of a mixture of gases, mainly nitrogen (four-fifths) and oxygen (one-fifth).

Aurora The display of coloured lights seen in the night skies close to the Earth’s magnetic poles. It occurs when high-energy particles from the Sun are trapped by the Earth’s magnetic field.

Axis An imaginary straight line that runs north to south through the centre of the Earth. The Earth rotates, or spins, on its axis.

Continent One of the seven large landmasses on the Earth. They also include the continental shelf, an area that extends beyond the seashore.

Convection current The movement of heat through liquids and gases. Heated from below, a liquid or gas will expand, become less dense, and rise. Away from the source of heat, it will cool down and sink. Convection currents in the Earth’s mantle are responsible for continental drift.

Core The innermost part of the Earth, mostly made of iron with a little nickel. It is divided into the outer and inner core.

Crust The thin, rocky outer layer of the Earth. There are two main types of crust: continental crust and oceanic crust. Continental crust is between 35 and 70 kilometres (20–40 miles) thick. Oceanic crust is only 5 to 10 kilometres (3–6 miles) thick.

Earth The Earth is a spinning ball of rock and metal. It is one of eight planets that orbit, or circle, our nearest star, the Sun. Its surface is made up of oceans and landmasses called continents. A layer of air called the atmosphere surrounds the whole planet. The Earth’s outer layer, the crust, is a thin, rocky shell. Beneath the crust lies the mantle, a thick layer made of hot, dense rock. At the very centre of the Earth is its core, a ball of metal, liquid on the outside, solid on the inside.

Inner core The very innermost part of the Earth. The inner core is a solid ball of the metal iron, about 2500 kilometres (1550 miles) in diameter. It is under so much pressure that it is solid even at temperatures of 7500°C (13,500°F).

Lithosphere A layer of the Earth made up of its crust, plus a thin layer of the upper mantle. It is split into large pieces called lithospheric, or tectonic, plates. These slide about on top of the asthenosphere.

Lower mantle The inner layer of the Earth’s mantle. It is about 2400 kilometres (1500 miles) thick. The pressure exerted by the layers above prevents the rock from melting.

Magnetic field The region surrounding a magnet, an object which has two poles and a force of attraction between them. The Earth has its own magnetic field, which stretches into space and protects it from the Sun’s high-energy particles.

Magnetic poles The two points at either end of a magnet, where its force is at its strongest. The Earth’s magnetic poles are located close to the geographical north and south poles. These are the two points where the Earth’s axis meets the surface of the Earth.

Magnetosphere The region around the Earth in which its magnetic field exerts a force. The magnetosphere stretches thousands of kilometres into space and protects us from the Sun’s high-energy particles, known as the solar wind. The solar wind “blows” the magnetosphere into a teardrop shape.

Mantle The rocky layer of the Earth that lies between the crust and the core. It is made up of the upper and lower mantle.

Moho (Mohorovicic discontinuity) The boundary between the Earth’s crust and mantle. It is recognized because of the way it affects the movement of seismic waves travelling through the Earth.

Outer core The outer layer of the Earth’s core. It is 2200 kilometres (1400 miles) thick and made up mostly of molten iron. Temperatures here rise to more than 4000°C (more than 7000°F). The liquid outer core spins around the solid inner core as the Earth rotates. This movement, known as rollers, is probably the cause of the Earth’s magnetic field.

Rollers The twisting movement of liquid iron inside the Earth’s outer core. Convection currents in the core cause liquid metal to swirl around. The currents are twisted by the spinning motion of the Earth’s rotation and form giant corkscrew-like patterns called rollers. These movements make electricity, which creates a magnetic field around the Earth.

Upper mantle The upper 600 kilometres (375 miles) of the Earth’s mantle, made of crystals of solid rock with liquid rock between the crystals in some places. The molten rock, called magma, is under great pressure and can collect to burst out of holes or cracks in the crust as volcanoes.

Van Allen Belt A doughnut-shaped region of electrically-charged particles held in place around the Earth by its magnetic field. Most of the particles are thought to come from the solar wind.

Consultant: Ian Fairchild