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Rocks

There are three different types of rocks. You should know all about sedimentary, igneous and metamorphic rocks, so have a look at these pictures and see if you can identify each specimen. Classify these rocks. Try and work out whether they are sedimentary, igneous or metamorphic. Think about how they were formed.

Basalt

Limestone

Sheared gneiss

Granite

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Sandstone

Marble

Pitchstone

Chalk

Slate

Lava

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Conglomerate

Garnet amphibolite

Gabbro

Marble

Granite

Gritstone

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Faulted gneiss

Clay (top), mudstone (bottom)

Identifying rocks

If you are stuck and cannot decide whether a rock is sedimentary, igneous or metamorphic here are some clues. Sedimentary rocks are made up of grains of sediment that have been weathered from a pre-existing rock or mineral. The grains can be big or small, they are often rounded, and are cemented together in a matrix. Sedimentary rocks may be layered and may also contain fossils. Igneous rocks often form from volcanic activity. They are made up of interlocking crystals that form when molten rock cools. The crystals can be big (if the rock formed underground), or small (if it formed from lava that was extruded from a volcano). Igneous rocks rarely contain fossils and they are not layered, but may contain gas bubbles. Metamorphic rocks form when a pre-existing rock is subjected to heat and/or pressure. They may show evidence of re-crystallisation.

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