Science Rocks

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Igneous rocks

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Rocks that form from the cooling of lava or magma below the Earth’s crust

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extrusive rocks

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The igneous rocks that form from red-hot lava above the Earth’s surface are called extrusive rocks

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Pumice

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Pumice is a pale-coloured rock. It is very light because it is full of holes. It floats on water and sometimes washes up on beaches. Powdered pumice is used in some abrasive cleaning products.

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Scoria

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Scoria is heavier than pumice, and darker because it has more iron. Scoria is a reddish-brown or grey rock that can be used in garden paths or around drainage pipes.

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Basalt

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Basalt is an extrusive rock that can take on many appearances. One big difference between samples of basalt is the size of the crystals that make up the rock.

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Pillow Basalt

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Rocks formed from underwater volcanoes are smooth and round. The crystals in this basalt are so small that they are difficult to see

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Obsidian

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Obsidian is a smooth, black rock that looks like glass. It is formed when lava cools almost instantly.

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Instrusive rocks

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Those that have formed from magma that cooled below the surface are called intrusive rocks.

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batholiths

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Large bodies of intrusive rock are called batholiths. They can stretch over distances of up to 1000 kilometres.

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Granite

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Granite is a common intrusive rock. The crystals in granite form over long periods of time and grow large enough to make them easy to see with the naked eye. Granite is very hard and can be used for building

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Sedimentary rock

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Rocks that are formed from the particles of sediments are called sedimentary rocks.
Most sedimentary rocks are formed from weathered rock that has been deposited due to erosion. Grains of sediment are cemented together to form a solid rock.

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Chalk

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Chalk is a sedimentary rock. It is similar to limestone, but not as hard. Chalk is formed from very fine grains of calcium carbonate that have separated from sea water

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sandstone

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Sandstone is formed from grains of sand that have been cemented together over a period of time.

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Mudstone + Shale

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Mudstone and shale are formed from finer grains of sediment.

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Siltstone

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Siltstone has grains slightly larger than those of mudstone

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Limestone

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Limestone is a sedimentary rock that is formed from deposits of the remains of sea organisms such as shellfish and corals.

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Coal

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Coal is formed from the remains of dead plants that are buried by other sediments. In dense forests, layers
of dead trees and other plants build up on the forest floor. If these layers are covered with water before rotting is completed, they can become covered with other sediments.

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Uses of sedimentary rocks

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Sandstone and limestone are often used as external walls of buildings. Shale can be broken up and crushed to make bricks. Limestone is broken up to produce a chemical called lime. Coal is used as fuel.

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metamorphic rock

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Igneous and sedimentary rocks deep below the Earth’s surface are buried under the huge weight of the rocks, sediments and soil above them. This heat and pressure can change the composition and appearance of the minerals in rocks.

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Slate

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When shale is exposed to moderate heat and pressure, it forms slate.

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Marble

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Marble (pictured below) forms from limestone under heat and pressure.

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Ways metamorphic rocks get a different appearance

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- the type of original rock
- the amount of heat to which the
- original rock is exposed
- the amount of pressure caused by the weight of the rocks above
- how quickly the changes take place.

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native elements

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Elements found naturally in their uncombined form are also minerals. These elements, called native elements, include diamonds (pure carbon) and gold.

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Minerals

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Most minerals in rocks are compounds of the elements oxygen and silicon, together with one or more of the metal elements. The colours, shapes and textures of the minerals in rocks tell us what they are made of and how they were formed

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Crystals

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The atoms that join together to form minerals make up regular geometric shapes in particles called crystals

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Lustre

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lustre of a mineral describes the way that light is reflected from it. Minerals could be described, for example, as dull, pearly, waxy, silky, metallic, glassy or brilliant.

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streak

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The streak is the powdery mark left by a mineral when it is scraped across a hard surface like an unglazed white ceramic tile.

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Hardness

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The hardness of a mineral can be determined
by trying to scratch one mineral with another. The harder mineral leaves a scratch on the softer mineral.

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Flint

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sedimentary rock. Wheitn flint breaks, it
rsaphleaves a razor-sharp edge, so it was ideal for making sharp tools like knives, axes and spearheads.