Basalt A dark grey igneous rock. The entire ocean crust and much of the Earth’s solid surface is made of basalt.

Batholith A large mass of igneous rock formed when an upwelling of magma spreads through rocks underground, but never reaches the surface. It may be revealed later when erosion wears away the rocks above it.

Bedrock The solid rock that lies beneath the soil.

Clay Very fine-grained sediments or sedimentary rock. It consists of a mixture of minerals and some organic fragments and is formed on lake and sea beds. It becomes plastic when moist, but hardens on heating, such as when it is fired in a kiln. Hardened clay is used in pottery and in the manufacture of tiles and bricks.

Concretion A rock formed by the accumulation of minerals in the spaces between layers of sedimentary rock. They form around a nucleus, such as a tooth, piece of shell or fossil, before the rest of the sediments have hardened into rock. Often spherical, oval or lens-like in shape, concretions are usually harder and more resistant to weathering than the surrounding rock layers.

Crystal A substance with a regular arrangement of atoms (the basic building blocks of matter). Crystals form when molten rock cools and solidifies. The more slowly the rock cools, the larger the crystals will be. If the rock cools too quickly the crystals may not have time to form.

Dyke A sheet of rock formed by magma rising vertically through the Earth’s crust and cutting across surrounding rock strata.

Erosion The transportation by water, wind or ice of rock fragments that have been broken down by weathering.

Gemstone A type of mineral that is valuable because it is rare and beautiful. Diamonds, rubies and topaz are all types of gemstone.

Geode A rock, roughly spherical in shape, that contains hollow cavities lined with crystals. The name comes from a Greek word, meaning "Earth-like". Geodes are created in the bubbles in volcanic rock. Over time, dissolved minerals form an outer shell. Groundwater continues to seep into the hollow chamber inside. The chemicals dissolved in the water form crystals of quartz, including the purple variety amethyst, and calcite.

Granite A hard, pink-grey igneous rock, made up of large, clearly visible crystals.

Igneous rock A type of rock formed when magma rises to the Earth’s surface and solidifies. Intrusive igneous rock, such as granite, is formed when magma solidifies beneath the ground. Extrusive igneous rock, such as basalt, is formed when it solidifies above the ground.

Laccolith A dome of igneous rock that lies between layers of sedimentary rock. It is formed when magma pushes between the sedimentary rock strata.

Limestone A type of sedimentary rock, often made up of the remains of tiny sea creatures that lived millions of years ago. It is made up of calcium carbonate, which dissolves easily in water.

Marble A type of metamorphic rock formed when limestone is heated to high temperatures. Marble has a smooth structure and can be almost any colour from pink, to blue, to black.

Metamorphic rock A rock that has changed due to intense heat and pressure underground. Sedimentary rocks, igneous rocks and other metamorphic rocks may all undergo this change. When a rock is changed in this way, its minerals recrystallize into different shapes.

Mineral A naturally occurring solid chemical substance. Rocks are made up of various different types of minerals, such as quartz, feldspar and mica.

Moh’s scale A scale used to measure how hard a mineral is, where 1 is the softest and 10 is the hardest.

Nodule A small, irregular lump of mineral that has different chemical composition from the surrounding rock. Examples include chert in limestone, flint in chalk and iron pyrites (also called "fool's gold") found in a variety of sedimentary or metamorphic rocks. The word nodule is also used to describe lumps of manganese found on the floors of the world's oceans. Like many nodules, including flint, these can also be called concretions.

Obsidian A dark, glassy rock formed when lava cools down so quickly that its minerals do not have time to crystallize.

Ore Rock that contains sufficiently high quantities of one or more useful metals to allow them to be extracted.

Porosity The ability of rocks to hold water in pores, the tiny spaces between the mineral grains of which they are made. Sandstone and chalk are examples of porous rocks.

Rock The hard substance that makes up the Earth’s crust. Rocks lie beneath both the soil and the depths of the ocean. They are made up of a solid mixture of minerals, consisting of different chemical substances. There are many different kinds of rocks, but they can all be divided into three main groups. These are: igneous rocks, sedimentary rocks and metamorphic rocks.

Rock cycle The process by which rocks change from one type to another. Rocks on the surface of the Earth are constantly worn away by wind and rain. The fragments of rock left behind may be pressed together to make sedimentary rock or may be baked and compressed to form metamorphic rock. Alternatively, the molten rock may rise, cool and solidify to form igneous rock.

Sandstone A sedimentary rock made up of grains of sand compressed and cemented together.

Sedimentary rock A type of rock that is formed by compressing rock fragments, called sediments, over millions of years. These fragments, such as sand, gravel and mud, are formed when other rock types are worn away by wind and rain. They settle in layers in lakes, rivers and seas. As the layers build up, the particles are compressed and cemented into sedimentary rock.

Shield A large area of exposed igneous and metamorphic rocks, mainly formed during the Precambrian, between 2 to 3.5 billion years ago. Shields are relatively flat, low-lying regions, lying far from the edge of tectonic plates and little changed by folding. They were once made up of highlands formed billions of years ago before being eroded.

Sill A flat sheet of igneous rock formed as magma pushes through the Earth’s crust and squeezes between older horizontal layers of rock.

Slate A dark grey metamorphic rock. The crystals that make it up are very flat, so slate easily splits into thin slabs. This makes it a useful material for tiling roofs.

Stratum A layer of rock. When a cliff face is exposed by erosion or faulting, different strata can often be seen in the rocks.

Stratification The formation of rock layers, or strata.

Weathering Weathering is the breaking down of rocks due to the weather. Water seeps into tiny cracks in the ground and dissolves away the rock. In cold weather, ice forms in the cracks and expands, splitting off pieces of rock. Tree roots can make the rock break apart even faster.

Consultant: Ian Fairchild