types of Rock by Justin Huang

Different types of Rock

Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them.
Rocks have had a huge impact on the cultural and technological advancement of the human race. Rocks have been used by Homo sapiens and other hominids for more than 2 million years.

Igneous rock
Igneous rock (etymology from Latin ignis, fire) is one of the three main rock types (the others being sedimentary and metamorphic rock). Igneous rocks are formed by solidification of cooled magma (molten rock). They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust.
Sedimentary rock
Sedimentary rock is one of the three main rock types (the others being igneous and metamorphic rock). Rock formed from sediments covers 75-80% of the Earth's land area, and includes common types such as chalk, limestone, dolomite, sandstone, conglomerate and shale.
Sedimentary rocks are formed because of the overburden pressure as particles of sediment are deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids in a process known as lithification ('rock formation') and the original connate fluids are expelled.
Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical or biogenic.
Organic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals, mollusks, and foraminifera, which cover the ocean floor with layers of calcite which can later form limestone.

Metamorphic rock
Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars[1]) causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (metamorphic facies).
They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above. They can be formed by tectonic processes such as continental collisions which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior.
Foliation: The layering within metamorphic rocks is called foliation (derived from the Latin word folia, meaning "leaves"), and it occurs when a rock is being compressed from one direction to a recrystallizing rock.
This results in a banded, or foliated, rock, with the bands showing the colors of the minerals that formed them.