What is a Rock

What is a Rock?
A rock is many things. It is a collection of the particular chemical elements that make it up. The elements form compounds or a mineral assemblage that usually occurs in crystalline form. The minerals make up the rock in a great variety of arrangements. In coarse grained rocks the minerals are large enough to be seen with the naked eye. In fine grained rocks the individual grains are so small that they can only be viewed with a magnifying glass or microscope.	A rocks history begins with its birth. It may subsequently be deformed mechanically, become chemically and minerally changed, and moved laterally or vertically in response to external forces.
Rock Type
Igneous	Any rock made by the cooling of magma. This origin is reflected in their texture which is like a mosaic of crystals. Igneous rocks are divided into two types. intrusive: rocks that were emplaced within rocks that surround them. They originated as hot magma that pushed it way into cracks and other openings. extrusive: rocks form form lava that flowed out of volcanoes or from volcanic ash that was released high into the air and fell back to the ground.
Sedimentary	Sediment is gravel, sand, or clay laid down in riverbeds, ponds, lakes, and oceans. When sediment is under great pressure for millions of years sedimentary rock is the result. One of the most obvious characteristics of sedimentary rocks is their layering or bedding derived form the settling of particles form water or air forming stratified sediments. Sediments are a consequence of erosion or fragmentation of pre-existing rocks, trasportaiton of the erosional products, and finally deposition.
Metamorphic	Rocks that have been altered by by the action of heat and pressure after formation. Sedimentary and igneous rocks may become metamorphosed by being buried deeply in the earth and being subjected to heat and pressure. A major characteristic of metamorphic rocks is the platy or leafy structure call foliation. This is the alignment of minerals along straight or way planes.

Mountain Building
Folds	The term fold implies that a structure that originally was flat, like a sedimentary bed, has been bent. The deformation may be produced by horizontal or vertical forces in the crust. Folding is the most common form of deformation of layered rock and is typically found in mountain belts. Folds can be gentle of the bending can be severe. Types of folds are: asymmetrical overturned recumbent
Fractures	Rocks yield to deforming forces in several ways: Some layers will crumple into folds, some will break, some will fold first and then break if the applied stresses build up beyond the strength of the rock. Fractures mau be divided in two categories, joints and faults. A joint is a crack alon which no appreciable movement has occured. If there is displacement of the rocks on both sides of a fracture and parallel to it, the fracture is a fault.
Faults	normal fault is when the rocks above the fault plane move down relative to the rocks below. reverse fault is when the rocks above the fault plane move up relative to the rocks below. thrust fault is a reverse fault where the overlaying block is pushed horizontally. strike slip fault is when the movement is horizontal and parallel to the strike of the fault plane. oblique slip fault is when the movement is horizontal and parallel to the strike of the fault plane but with vertical displacement.

Volcanos
When eruptions are relatively quiet and most of the extruded material is a low viscosity lava, large broad mountains called shield cones are formed.
Composite cones are the result of more violent eruptions, in which both rock fragments (pyroclasts> and lava are ejected from the volcano. These cones are smaller and steeper than that of a shield cone. Eruptions of composite cones follow a sequence of 1)explosion, 2)eruption of pyroclasts, 3)eruption of lava which may pour out from fissures on the side of cone rather than the central crater. By alternating these layers a cone is built up.
Cinder cones are much like composite cones but without the intervening lava. Only pryoclasts and gasses are thrown out resulting in a steep sided cone. The sides are at the angle of repose for jagged material of various sizes. Cinder cones are smaller that composite cones and generally have a shorter life. Larger fragments fall near the summit while the finer particles are carried farther from the vent and form gentle slopes at the base of the cone.

  source: http://www.geotech.org/survey/geotech/basicgeo.html