Hydrocarbon compounds are important because they provide an energy source on which humans depend, and collectively we call these fossil fuels . These fuels are non-renewable resources and will be consumed within the next century. Metamorphic rocks are formed when any type of igneous and sedimentary rock is altered or transformed into a new rock by tremendous pressure and high temperatures . This process occurs within the earth's crust during mountain-building processes, and changes the original texture and structure. The original chemical compounds are there from the original rocks, but some have been transformed into new mineral varieties.
Slate is formed from shale that has been super-heated and compressed.
With increased heat and pressure, slate can become schist , which represents the most advanced stage of metamorphism. Schist occasionally has scattered large crystals of newly formed minerals, such as garnet . Sandstone that becomes welded by increased temperature and pressure becomes quartzite . Limestone under high heat and pressure becomes marble . Finally, we have gneiss formed from intrusive igneous or clastic sedimentary rock types in close contact with intrusive magmas.