Metamorphism in geology is when one type of rock is transformed into another type of rock called a metamorphic rock. The word metamorphic comes from the Greek words meaning "change" and "form." There are two main factors that can cause the conversion: heat, pressure. The presence of hydrothermal fluids at the time of transformation can also play a role in the resulting rock.
Geothermal gradients in the earth caused by radioactive decay are the most common cause of temperature differences and thus a cause of metamorphism. As a general rule, heat increases as depth into the earth increases. The gradient creates different types of rocks at different depths. Molten rock, or magma, beneath a particular geographical area results in more extreme variation in the gradients and metamorphic rock.
Magma and lava flows can literally bake rocks turning them into other rocks in what is called contact metamorphism. This happens when the molten rock touches another type of rock. A typical example of a contact metamorphic rock is hornfels.
Like heat, pressure increases as depth into the earth increases. Sedimentary rock is also formed from pressure, but when the pressure reaches a certain point the rocks become metamorphic. In many cases of metamorphism it is a combination of heat and pressure creating the new rocks. The types of rocks found below the surface depend not only upon the minerals present in the parent rock, but also upon the composition of the ground above. A part of the earth where granite is abundant causes different metamorphic changes than one with sand because granite is heavier and creates more pressure.
When tectonic plates rub and collide with one another as they shift it results in dramatic increases in pressure on the surrounding rocks. Fault lines are the cracks in the earth’s crust where the plates meet. Mylonite is often found near fault lines and is created from the pressure they cause.
Hydrothermal solutions are not a direct cause of metamorphism like heat and pressure, but they play an important role in the composition of the final rock when they are a factor. Hot water can containing dissolved ions of various elements it contacts. The flow of this water creates unique metamorphic settings in which new materials, the ions, are introduced. This most often results in metallic ore deposits within rock. Sea water can leach through the earth’s crust and dissolve any minerals that may be there, usually metallic sulfides. The water is heated in the earth, seeping into cracks and porous rock to be present at metamorphose. If hot enough it can also contribute as a heat source.