Metamorphic rocks are made by physically and chemically changing the characteristics of sedimentary or igneous rocks. Heat, pressure under the Earth's surface or friction from colliding tectonic plates can cause these changes.
Direct contact with hot magma is called contact metamorphism. When a sedimentary or igneous rock meets magma, fluids and vapors are created in the existing rock, changing the rock's chemical composition. Regional metamorphism occurs due to the extreme pressure built up from the process of the Earth thrusting upward to form mountains. This intense pressure causes a change in temperature, which can chemically alter the contents of the rock. Dynamic metamorphism also occurs due to intense pressure and is linked to sudden geographic upheavals.
Due to the intense heat and pressure required to create metamorphic rocks, the grains in the rock may become flattened into bands. When these grains are aligned, the metamorphic rock is said to be foliated. Examples of foliated metamorphic rocks include slate, schist and gneiss.
Nonfoliated metamorphic rocks do not have grains that are aligned in bands. These rocks are created either by the presence of heat without any pressure or because the grains in the original rock would not align. Marble and quartzite are examples of nonfoliated metamorphic rocks.