Metamorphic rocks are formed due to extreme temperature, pressure, and hot fluids with high mineral content or any combination of these factors. They are formed from preexisting rocks. Their formation typically occurs at the boundaries between tectonic plates, deep in the earth, and near volcanoes. The rocks are not melted but are heated and compacted into denser rocks. This process is called metamorphism. Metamorphic rocks can be folded and flattened, but if they were to melt, they would become igneous rocks.
There are two types of metamorphic rocks, foliated and non-foliated metamorphic rocks. Foliated rocks have layers because of direct heat and pressure. Examples of foliated metamorphic rocks include slate, schist, and gneiss. Non-foliated metamorphic rocks, on the other hand, do not have visible layers or bands. Non-foliated metamorphic rocks include marble, quartzite, and hornfels.
Metamorphic rocks can show evidence of plate tectonic activity and past volcanic eruptions. They can show plate collisions and evidence of heat from volcanic activity. When plates collide, they cause pressure and heat, forming metamorphic rocks.
Metamorphic rocks have several uses, specifically, their mineral deposits. Marble and slate are used for construction. Marble is a common countertop material similar to granite. Granite, however, is an igneous rock; when subjected to heat and pressure, it forms into gneiss, a foliated metamorphic rock.
Metamorphic rocks can also be crushed into abrasives which can be used for sandpaper, grinding wheels, and polishing compounds. Metamorphic minerals can include graphite and talc. Metamorphic rocks can also house emeralds, rubies, spinel and tanzanite gemstones.
