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Physics - How Rocks are Made

There are many hundred kinds of rock. All come directly or indirectly from magma- the molten rock which lies beneath the hard rock’s of the Earth’s crust.

Fiery rocks

‘Fiery’ or igneous rocks occur where magma has welled up, then cooled and hardened. Sometimes it happens underground. Granite forms like this. Great blobs of granite grow like giant underground blisters that show up when the rocks above are gradually worn away.

Basalt and obsidian are igneous rocks that form when magma cools and hardens on the surface.

Granite cools so slowly its ingredients have time to form crystals big enough for us to see. Obsidian cools too fast for crystals to grow; it looks like glass.

The crystals found in rocks like granite are called minerals. There are more than 2000 kinds altogether. Different minerals form at different temperatures. They ‘freeze’ out of magma one by one as the magma cools.

Sedimentary rocks

Sedimentary rocks are made from loose particles (sediments) that have piled up on land in water. The pressure of the sediments above squashes those below. Then natural cements glue the squashed sediments together to form solid rock.

Sediments and sedimentary rocks cover more than two-thirds of the Earth’s surface. Some sedimentary layers are several hundred metres thick. In many places different sedimentary rocks lie on top of each other like layers in a sandwich.

Different sedimentary rocks have different ingredients. Sandstone is often made of grains of quartz from broken down granite. Limestone comes largely from the hard shells and skeletons of billions of tiny sea animals.

Most sedimentary rocks are formed from broken down igneous rocks-they are igneous at second hand.

Metaphoric rocks

Metaphoric or ‘changed form’ rocks are igneous or sedimentary rocks changed into new kinds of rock by great heat or enormous pressure.

When a great mass of molten rock is pushed up through the Earth’s crust is alters the rocks around it. In this way, limestone changes into marble as its shelly mass of calcium carbonate becomes hard calcite crystals.

Metaphoric rocks are also formed in the buckling that builds fold mountains. Heat changes soft mudstone into mica-schist. If things get hotter still, mica-schist produces rock called gneiss.

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