Marble, a metamorphic rock, has long been prized for its beauty and durability.
Geologists classify rocks as belonging to one of three categories: igneous, sedimentary and metamorphic. Metamorphic rocks are those that have undergone a major change in appearance, composition or form due to extreme heat or pressure. The original, or parent, rock may have been igneous, sedimentary or even another metamorphic rock.
Basic Geology
If you were to begin a journey to the center of the Earth, you would first pass through the Earth's crust and mantle. You would then arrive at the molten outer core before reaching the solid inner core. At the core, you would be about 6,375 miles below the surface. The temperature at the inner core is no cooler than the outer core, but the inner core is under so much pressure that it cannot remain molten.
Igneous Rocks
Magma is molten rock that is underground, and lava is simply magma that has broken through the crust. Igneous rocks form as magma or lava cools. Since the Earth was once entirely molten, all rocks began as igneous rocks. The length of time it takes the magma to cool, the pressure to which it is subjected, whether it cools in water or air and the gases within the magma, all play a part in determining the type of rock that will result. The two most common types of igneous rocks at surface level are granite and basalt.
Sedimentary Rocks
To maintain stability, the conditions under which a rock formed must never change. If conditions change, the rock can crumble, melt or harden. When it crumbles, the individual components can be washed or blown to a new location and mixed with components from a variety of rocks. Over time, more and more layers can build up, forming sedimentary rocks. River deltas and beds, shorelines and sand bars are common sites for sedimentary rocks, but they can form anywhere that is flat and conducive to layering. This is the type of rock in which fossils may be found. Examples of sedimentary rocks are limestone, sandstone and shale.
Metamorphic Rocks
Metamorphic rocks develop within the Earth under intense heat and extreme pressure, conditions that cannot occur at the surface. Most metamorphosis is either contact or regional metamorphism. Contact metamorphism is when the parent rock is exposed to magma. The magma's heat changes the rock, forming crystals that are larger or introducing additional minerals to change the rock's chemical composition. Regional metamorphism is the result of pressure exerted when tectonic plates collide to form new mountain ranges. The pressure rearranges the rocks' crystals, and many metamorphic rocks have curves or folds that reveal the direction of the pressure. Metamorphic rocks may have layers or they may show no obvious layers. One parent rock can transform into a variety of metamorphic rock, depending on the conditions under which the change occurs. Examples of metamorphic rocks include slate, marble, schist, anthracite or hard coal, quartzite and gneiss.
Tags: inner core, metamorphic rock, Metamorphic rocks, parent rock, sedimentary rocks, change rock, conditions under
