- A solid
- Naturally occurring
- Is made up of minerals or mineral like matter.
Some rocks are composed of just one mineral. Pyrite and quartz are two common rocks that fit this category. Most rocks are a solid mixture of several minerals like granite.
The records are classified according to how they are formed. There are three basic groups, igneous, sedimentary and metamorphic. In These groups, tissue or grain is made in size and separation for chemical or mineral content.
Igneous rocks are volcanic and form from molten material. They include not only lava spewed from volcanoes, but also rocks like granite, which are formed by magma that solidifies far underground.
Sedimentary rocks are formed from eroded fragments of other rocks or even from the remains of plants or animals. The fragments accumulate in low-lying areas, lakes, oceans, and deserts and then are compressed back into rock by the weight of overlying materials. Sandstone is formed from sand, mudstone from mud, and limestone from seashells, diatoms, or bonelike minerals precipitating out of calcium-rich water. Fossils are most frequently found in sedimentary rock, which comes in layers, called strata.
Metamorphic rocks are sedimentary or igneous rocks that have been transformed by pressure, heat, or the intrusion of fluids. The heat may come from nearby magma or hot water intruding via hot springs. It can also come from subduction, when tectonic forces draw rocks deep beneath the Earth’s surface.
Marble is metamorphosed limestone, quartzite is metamorphosed sandstone, and gneiss, another common metamorphic rock, sometimes begins as granite.