The rocks of Earth’s crust are made of various combinations
of minerals, such as silica, olivine, pyroxene and hundreds of others.
Minerals, like all substance and matter, are made of atoms, mostly joined into
molecules. There is only a limited supply of such atoms, molecules and minerals
on the planet. Rocks have been broken down by the forces of weathering and erosion,
such as wind, rain, heat, ice and waves. They are then reformed into new rocks
by heat, pressure and chemical changes. This means the same minerals go round
and round, forming one type of rock and then another, over years of geological
time. This process is known as the rock cycle.Main rock types
There are three main types of rocks:
Igneous rocks form when rock minerals are so hot that they have melted, then they cool and go solid again. The rocks formed when Lava from a volcano goes hard are igneous.
Sedimentary rocks form when tiny particles are worn or eroded from other rocks and settle into Layers, such as on the sea bed. They slowly get squashed and cemented -- glued by chemical action -- into hard rock again.
Metamorphic rocks form when other kinds of rocks are subject to great pressure and temperature, such as in the roots of mountains. They change or metamorphose without melting into new rock types.
Sedimentary rocks form in horizontal layers, at the bottom of seas, in lakes and along rivers, and also in deserts, as sandstones. These layers represent the passage of time. Only sedimentary rocks contain fossils. Any fossilized remains of plants or animals are destroyed when they are melted to make igneous rocks, or altered by pressure and temperature to form metamorphic rocks. Over time, sedimentary rocks may be bent and folded by earth movements, or worn away as particles that become future sedimentary rocks.
Rock types
There are hundreds of types of rocks. Many have special uses, especially in earlier times when people used more natural materials than we do today. Granites and basalts are igneous rocks. Sandstones, limestones and breccias are sedimentary rocks. Overall, sedimentary rocks cover two-thirds of the Earth's surface because they form on the ocean floor, overlaying the igneous rocks of seafloor spreading. Marbles, schists and gneisses are metamorphic. (World of Science)
