Three types of rocks- igneous, sedimentary and metamorphic- make up the Earth's crust. All three types of rocks are subject to the processes that change one type of rock to another.
Igneous rocks form when magma is expelled from beneath the crust of the Earth through a volcano, is cooled and crystallizes. It is grainy in texture, whose coarseness is relative to the cooling process.
The slower the cooling, the larger the crystals are in the rock. The faster the cooling, the smaller the crystals formed in the rock. Common igneous rocks are quartz, basalt, granite and feldspar.
Sedimentary rock begins forming when small pieces of rock, minerals or calcified animal bones and shells settle in layers. As more layers build up, the weight of the uppermost layers cement the lower layers.
The three types of sedimentary rocks are clastic, chemical and organic. Clastic sedimentary rocks are formed of chips of other rocks of all three types. Chemical sedimentary rocks form by the minerals left behind when water evaporates. When animal shells, bones and teeth settle to the sea floor, the calcium from them form organic sedimentary rocks. Common sedimentary rocks are shale and sandstone.
Metamorphic rock is formed when rock is exposed to extreme heat and pressure which forces the tiny mineral particles to fuse together. Many of these rocks are found near the edges of the Earth's crust plates as a result of the plates colliding and rubbing over one another.
Other metamorphic examples are found in the craters and ridges formed by earthquakes and volcanoes. Metamorphic rocks can be changed into other metamorphic rocks by exposing them and other rocks to heat and pressure. Common metamorphic rocks include schist, gneiss, slate and marble.
Each kind of rock can be formed into another. Rain, wind and rivers chip away all three rocks and transport and deposit then into layers which become sedimentary rock. All three rocks can be melted and expelled through a volcano to be crystallized into igneous rock. All three rocks can be heated and crushed to form metamorphic rocks.
Reference: classzone.com; jersey.uoregon.edu; athro.com