Rocks

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Chemistry Revision

Explaining the Earth

Rocks

are made out of a mixture of minerals.

The shape of rocks can be changed by weathering

and erosion. Weathering can be

chemical, physical or biological.

The Rock Cycle

The

Earth is continually changing. Rocks are weathered and eroded and new rocks are

being formed. The processes that make rocks, weather them and change them are

linked together in the rock cycle.

The earth movements that bury rocks as part of the cycle are caused when the plates that form the surface of the

earth push into each other.

Types of rock

Sedimentary

Igneous

Metamorphic

Sedimentary Rocks

Rock

fragments, formed as a result of weathering and erosion, are transported by moving water, wind or

ices, and the fragments get worn down. Small rock fragments are called grains. When the water, wind or ice

slows down, some of the grains are deposited

at the bottom of rivers, lakes or seas, as sediment. Layers

of sediment collect on the sea bed, and the bottom layers get squashed. The

grains of sediment are forced closer together (compacted). Eventually, sedimentary

rock is formed. The composition and texture of sedimentary rocks vary and

depend on the way the rocks are formed. For example, the sediments that made a

rock with large grains must have been moved by a fast-flowing river.

If

any animals or plants get trapped in the sediment, they may form fossils.

Igneous Rocks

Molten

rock is called magma. If the molten

rock flows out of volcanoes it is called lava.

Igneous rocks are formed when magma cools down. Lava

cools down quite quickly, and forms igneous rocks with small crystals (like basalt). Magma underground cools down

much more slowly and forms rocks with bigger crystals, like granite.

Metamorphic Rocks

Sedimentary

or igneous rocks can be changed by heat or pressure into new kinds of rock,

called metamorphic rocks. Metamorphic

rocks have different properties from the sedimentary or igneous rocks they were

made from.

Rocks

Sedimentary Rocks

Igneous Rocks

Metamorphic Rocks