Metamorphic rock

rock arising from metamorphism of original rock under high heat and pressure

A metamorphic rock is a type of rock which has been changed by extreme heat and pressure. Its name is from 'morph' (meaning form), and 'meta' (meaning change).[1]

Spinel: a small spinel crystal on top of a white calcite from Mogok, Myanmar. This is a metamorphic mineral
Quartzite, a form of metamorphic sandstone
Folded strata in a metamorphic rock from near Geirangerfjord, Norway

The original rock gets heated (temperatures greater than 150 to 200 °C) and pressured (1500 bars).[2] This causes profound physical and/or chemical change. The original rock may be sedimentary rock, igneous rock or another older metamorphic rock.

There is always more pressure and higher temperature under the Earth's surface. In the root of a mountain chain or a volcano these forces will be enough to change shape of the strata and the minerals it is made of. Sedimentary rock which has been near such forces often looks as if a giant had twisted it and heated it over a fire. Examples of metamorphic rock:

The recrystallisation of minerals after heating generally causes the destruction of any fossils the rocks might have contained. These rocks are formed when igneous or sedimentary rocks are subjected to extreme heat and pressure. As a result, they undergo a complete change in their form and characteristics. Rocks who have been changed in this way are called metamorphic rocks.

Types of metamorphism


Regional metamorphism

Mississippian marble in Big Cottonwood Canyon, Wasatch Mountains, Utah

Regional metamorphism, or dynamic metamorphism, occurs in great masses of rock. Rocks can be metamorphosed just by being at great depths below the Earth's surface. There they get high temperatures and the great weight of the rock layers above.

Much of the lower continental crust is metamorphic, except for recent igneous intrusions. Horizontal tectonic movements such as the collision of continents create orogenic belts. High temperature, pressures and deformation occurs along these belts. If the metamorphosed rocks are later uplifted and exposed by erosion, they are seen as long belts or other large areas at the surface.

Contact metamorphism

A contact metamorphic rock made of interlayered calcite and serpentine from the Precambrian of Canada. Once thought to be a fossil called Eozoön canadense. Scale in mm.

Contact metamorphism occurs when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest where the magma comes into contact with the rock. The temperatures were highest there and decreased with distance from it.

Metamorphic minerals


Every mineral is stable only within certain limits. When certain minerals appear in metamorphic rocks this shows the approximate temperatures and pressures the rock experienced. These minerals are known as index minerals. Sillimanite, kyanite, staurolite, andalusite, and some garnet are examples.

Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz may be found in metamorphic rocks. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process.



  1. This originally Greek prefix has several meanings, but here it means 'change'.
  2. Blatt, Harvey and Robert J. Tracy, 1996. Petrology. 2nd ed, W.H.Freeman, p.355. ISBN 0-7167-2438-3

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