Rocks
By: Christina Mathew
What is a Rock?
Rocks are composed of minerals. They are classified by how they were formed (their origin).There are three types of rocks: Igneous,Metamorphic and Sedimentary.
Igneous
Igneous Rocks are formed by cooling and crystallization of molten rock. When igneous rocks solidifies it forms crystals. The cooling rate of the solidification process determines the size of the crystals. Extrusive rocks form outside the earth and cools fast. This produces smaller crystals. Intrusive igneous rocks form inside the earth and cools slower. The crystals within these rocks are bigger. Regional metamorphism is when large masses of rock are changed by the deep burial within the earth. Contact metamorphism is the chemical and physical change to a rock caused by nearby intrusion or extrusion of molten rock. Igneous rocks can be felsic or mafic. Felsic rocks are rich in silicon and aluminum. Mafic rocks are rich in iron and magnesium.
Metamorphic
Metamorphic rocks form when other rocks are exposed to heat and pressure WITHOUT melting. In order for a rock to be metamorphic, there must be foliation. Foliation is the alignment of minerals in a metamorphic rock. Most metamorphic rocks are formed deep within the earth.
Sedimentary
Most sedimentary rocks are composed primarily of weathered remains of other rocks. Sedimentary rocks are usually formed by the compaction and cementing of sediments. Three types of sedimentary rocks are clastic, bioclastic and crystalline. Sedimentary rocks are the ONLY type of rocks that contain fossils.
Examples of Igneous, Sedimentary and Metamorphic Rocks
Pegmatite
The crystals in the igneous rock pegmatite form inside the earth so the crystals are large. The texture of pegmatite is coarse and it is non-vesicular. The crystal size of pegmatite is 10mm or larger. Pegmatite is light, has a low density and is felsic. There is about 35% of quartz, 75% of potassium feldspar, 15% of biotite and 20% of amphibole.
Rock gypsum
Rock Gypsum is chemicially formed. It is crystalline and the grain size is fine to coarse crystals. The composition of rock gypsum is gypsum. The crystals are from chemical precipitates and evaporites.
Marble
Marble is fine to coarse and it is nonfoliated. The composition is calcite and/or dolomite. The type of metamorphism is regional or contact. Metamorphism of limestone or dolostone.
Rock Cycle
The rock cycle is a model showing how rocks and sediments change overtime. External processes occur on earth's surface. An example is weathering and erosion. Internal processes occur within the earth. An example is any process that requires exposure to extreme heat and/or pressure.