RocksGroup4

__**media type="file" key="Sedementary Rocks Melissa and Bus.m4v" Igneous Rocks:**__
Igneous rock is formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks. This magma can be derived from partial melts of pre-existing rocks in either a planets mantel or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth's crust. These have diverse properties, depending on their composition and how they were formed.

__**Metamorphic Rocks:**__
The process of metamorphism. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, and the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they formed.

=__**Sedimentary Rocks:**__= Sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Clastic sedimentary rocks, such as limestone or sandstone, were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere. Chemical weathering is the breakdown of rock by chemical reaction. Clastic sedimentary rocks may be regarded as falling along a scale of grain size.

//__ LINKS TO SEDIMENTARY ROCK SITES;__// http://library.thinkquest.org/J002289/sed.html < Info on sedimentary rocks http://www.thefreedictionary.com/sediment < Definition of a sediment http://geology.com/rocks/sedimentary-rocks.shtml < Differant types of sedimentary rocks.

Clastic:
**__Mechanical Weathering__: the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, and the resulting expansion and contraction, also aids the process. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents.  __Chemical weathering__ is the breakdown of rock by chemical reaction. In this process the minerals within the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions. The minerals in igneous rocks may be unstable under normal atmospheric conditions, those formed at higher temperatures being more readily attacked than those which formed at lower temperatures. **Organic:** Organic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals and mollusks, which cover the ocean floor with layers of calcium carbonate which can later form limestone. Other examples include stromolites, the flint nodules found in chalk (which is itself a biochemical sedimentary rock, a form of limestone), and coal and oil shale(derived from the remains of tropical plants and subjected to heat).
 * The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size, composition of the particles, the cement, and the matrix. Types of clastic are;

Chemical:
Chemical sedimentary rocks form when minerals in solution become under saturated and precipitate. In marine environments, this is a method for the formation of limestone. Another common environment in which chemical sedimentary rocks form is a body of water that is evaporating. Evaporation decreases the amount of water without decreasing the amount of dissolved material. Therefore, the dissolved material can become over saturated and precipitate. Sedimentary rocks from this process can include the evaporate minerals halite (rock salt), sylvate, barite and gypsum.

Layers of sediments that form sedimentary rocks. Also shown is how the river carries peices of sedement to a lake or body of water (erosion) in which they sit at the bottom of and form into sedimentary rocks. (deposition)

This is an example of deposition because the sediments at the bottom of the water, after traveling through erosion, will pile on top of each other forming layers which after thousands of years will transform into sedimentary rocks.

Breccia is a clastic sedimentary rock. This is a good example because you can see the large pieces of differant minerals in the rock. You can tell it's a clastic sedimentary rock because there are large angular pieces of a mineral.