Metamorphic rocks are formed when existing rocks (igneous, sedimentary or even other metamorphic rocks) are subject to agents of change. These causes of metamorphism such as heat more than 200C (but below the boiling point of the rock) and pressures higher than 300Mpa break down the atoms in the rock's minerals and recrystalize the rock into something different with many interesting and useful properties.
Occurrence of Metamorphic Rocks
Metamorphic rocks are found all around the world, both on the surface and deep underground. However, at the earth's surface they are fairly uncommon and do not contribute much to the formation of soils. The continents tend to be granitic including the regional metamorphic rocks that make up the mountain ranges of the world, while ocean floors are made of igneous basalt.
Some of the rarest and most valuable gems are metamorphic rocks. For example, diamonds are a metamorphic rock that is formed at depths of 140 to 190 kilometers, where temperatures and pressures are high. They are brought to the earth's surface by volcanic eruptions and occur in parts of the earth's crust that have been stable for long periods.
Properties and Economic Uses of Metamorphic Rocks
Metamorphic rocks are typically hard rocks meaning that they are resistive to abrasion. They weather slowly when exposed to wind and water. On the Moh's scale of hardness metamorphic rocks are often above 6, with diamonds attaining the hardest measure relative to other minerals. The properties of metamorphic rocks give them important economic uses.
For example, the metamorphic rock slate is sought after as an extremely hard rock for building used in floors and roof tiles. Marble is a beautiful, smooth, strong white metamorphic rock used as decorative columns as far back as ancient Greece. And quartzite is a valuable metamorphic rock characterized by sparkling grains of white, red and pink used for its durability in paving blocks. Diamonds are hard gems that have exceptional optical properties making it a sought after gem for jewellery.
Particular Types of Metamorphic Rocks
Metamorphic rocks have a parent rock called its prolith which is either a sedimentary, igneous or metamorphic rock.The amount of heat and pressure that a given rock experiences determines its resulting form. For example the sedimentary rock shale is transformed progressively into slate, phyllite, mica, schist, gneiss, migmatite and eventually granite depending on the environment it experiences. For a list of different metamorphic rocks and their parent rock see the article at Buzzle that details many common metamorphic rocks, their properties and from what rock they originated.
Importance of Metamorphic Rocks
Metamorphic rocks have clear importance to human economy and life, but studying metamorphic rocks is also important. Metamorphic rocks can help reveal facts about the interior of the earth - the temperatures and pressures at different depths - showing for example that pressures are not uniform across the whole crust.
Unfortunately, the process of metamorphism destroys any fossils so metamorphic rocks aren't useful for paleontology or studies of prehistoric life. While most regionally metamorphosed rocks are old (i.e. PreCambrian), and most Pre-Cambrian rocks have been metamorphosed, any younger fossil bearing rock that is metamorphized looses its fossils.