Comparing Igneous, Sedimentary and Metamorphic Rocks.

Igneous rocks are formed when magma or lava cools and hardens. An igneous rock becomes a sedimentary rock through weathering and erosion of the igneous rock. The igneous rock, exposed to weathering processes at the surface, breaks into smaller pieces. Erosional processes transport these pieces to a place of deposition where they build up, layer on layer. Eventually the compaction caused by the weight of the sediments above reduces the amount of space and water between particles. In this process of rock formation known as lithification, cementing minerals are formed out of the slowly disappearing fluids, which are saturated with mineral components. Once cemented together, it is now considered sedimentary rock. Igneous rocks become metamorphic rocks once it has undergone metamorphism. Metamorphism is the change in form of a geological unit (like a rock) through metamorphic processes (must be in solid state). These processes include temperature and pressure changes as well as chemical changes. A sedimentary rock can be transformed into an igneous rock and then into a metamorphic.  First Sedimentary rock can be changed into igneous rock by undergoing melting and then solidification. At a subduction zone layers of sedimentary rock will be assimilated into the mantle. Some areas may undergo enough heating to actually melt, especially if there is water or carbon dioxide trapped in the rock. If this rock later makes it to the surface and cools, it will become an igneous rock. Then a sedimentary rock can turn into a metamorphic rock by exposure to heat and/or pressure through depth of burial, directed pressure, or proximity to a magma source. Existing minerals can recrystallize, non-hydrous minerals can replace hydrous minerals, new minerals can be formed from the introduction of new ions from heated solutions, and minerals can become flattened and aligned in parallel layers.