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.
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.
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.
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.
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)