Earth Rocks

Rocks been forming and reforming in the Earth ever since our planet was created.

There are three main types of rocks on Earth: igneous, sedimentary and metamorphic.

Igneous rocks, whose name comes from "igneus", the Latin word for "fire", are formed from molten rock, or magma.

When magma cools slowly beneath the Earth's surface, it forms rocks such as granite, which contain large crystals. These rocks called intrusive igneous rocks.

When magma emerges on the surface as lava, it cools quickly. It then forms fine-grained rocks, with small crystals. These extrusive igneous rocks are sometimes glassy, containing no crystals.

Rain, running water, wind, moving ice, solar heat, and chemicals dissolved in water are constantly attacking the rocks on the Earth's surface.

As a result, these rocks are broken down into loose material, or sediment.

This sediment is often carried long distances by rivers, glaciers and so on. Finally, it comes to rest, usually in water, and accumulates in layers.

These layers are compacted and chemically cemented together to form sedimentary rocks.

Some sedimentary rocks, which are called conglomerates, contain large pebbles.

Other sedimentary rocks, such as sandstones, are composed of sand grains.

Shales are made up of tiny particles of silt and clay.

Some sedimentary rocks consist of the remains of once-living organisms, while others form from chemicals dissolved in water.

Metamorphic rocks are rocks that have been changed by heat, pressure or chemical action.

For example, hot magma bakes the surrounding underground rocks, causing limestone, for instance, to change into marble.

Other rocks undergo incredible pressures caused by Earth movements, such as rock folding. The structure of such rocks is thus altered. For example, soft shale is pressed into hard slate.

Chemical changes occur when hot steam or water, con-taining dissolved chemicals, reacts with rocks.

Because of movements such as continental drift, rocks may be forced back into the Earth, where they melt again. The new magma that is formed may rise to the surface through volcanoes another time.

Sedimentary rocks cover about 75 percent of the Earth's land area, but form only 5 percent of the top 16 kilometers of the Earth's crust. The rest are either igneous or metamorphic.

The three types occur together where igneous rock intrudes into sedimentary rock. Metamorphic rock forms in the contact zone.