Geology

The rock cycle is often depicted as a diagram that is given to help explain the changes that the earth's rocks can go through. There are three categories of rocks that are discussed in the rock cycle: igneous, sedimentary, and metamorphic. Each of these rock types can change to a different rock type through the processes of heat, pressure, and time - additionally you require erosion.

An igneous rock forms from cooling magma. This can happen quickly as when it reaches the earth's surface after traveling up through a volcano. Igneous rock may also cool slowly when it is trapped under the earth's crust but close enough to the surface for the temperature to decrease.

Sedimentary rock forms from fine particles of rock (igneous, sedimentary, or metamorphic) that have been eroded from the earth's surface. These particles are so small that they can be carried by wind or water - as you will see later that Sedimentary rocks can have partials the size have your fist!. Sedimentary rock is formed when these particles form layer upon layer on the earth's surface and become compacted and cemented together.

Metamorphic rock is formed when heat is applied to either sedimentary or igneous rock. This type of rock is formed deep within the earth's crust and is usually found in the form of crystals -this is not true... the minerals within a metamorphic rock re-crystallize... but the rocks are not crystals . Metamorphic rock can travel up to the earth's surface with magma to become igneous rock - no... metamorphic rock must be come magma or lave before it can become igneous rock... metamorphic rock that travels to the surface of the earth will be exposed to erosion and the particles that erode will become part of sedimentary rock. It can also be brought to the surface through earthquakes and tectonic plate movements. After reaching the surface it can be eroded by the earth's weather patterns and become sedimentary rock. Sedimentary and igneous rock can then become metamorphic when they are pressed into the earth's crust and heated to a melting point - one sedimentary or igneous rock melts it is magma... then magma can become igneous rock... if any rock melts it will become magma... melting is not required or possible to form metamorphic rock.

References

Plummer, McGeary, and Carlson. 2004. Physical Geology, tenth edition. The