Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Theresa Tate
The rocks of which the Earth is comprised are in a constant state of change as a result of forces on and within the planet; whether being broken down at the surface or baked far beneath it, no material is completely impervious to alteration. The process in which changes occur due to extreme heat or pressure is known as metamorphism, and the resulting rocks are called, appropriately, metamorphic rocks.
Rock cycle diagram, courtesy of Dr. Pamela Gore, Georgia Perimeter College
Metamorphism is only one of several processes by which rocks can be formed, as illustrated in the diagram above. The youngest rocks, composed of cooled magma which has risen from layers of molten rock beneath the Earth’s surface, are known as igneous rocks. These may be buried, melted again to become new igneous rock, or weathered by forces of nature, becoming sediments which, through the process of lithification, then produce sedimentary rocks. Sedimentary rocks, in turn, may also be buried or weathered again into sediments and form new sedimentary rocks. As rocks are buried, regardless of where they stand in their cycle of development, they may begin to metamorphose, becoming metamorphic rocks. This ongoing process of alteration is known as the rock cycle.
Unlike the creation of igneous or sedimentary rocks, nothing new is formed during metamorphism, only changed; in fact, the name itself comes from the Greek words “meta” and “morphos”, meaning simply “changing form”. Any preexisting rock (igneous, sedimentary, or metamorphic) which becomes buried beneath other layers of rock is prone to metamorphism. As the rock is buried below more layers and is pushed further beneath the surface, the temperature rises and the pressure being exerted on the rock grows. This change in heat and pressure eventually forces the rock to adjust to the new environment, resulting in a metamorphic rock.
Metamorphism diagrams all courtesy of Lynn S. Fichter, James Madison University
Metamorphism is not a single process, but rather a group of several distinctly different processes occurring with varying levels of heat and pressure which can produce dramatically different results from similar parent rock. The two most commonly discussed metamorphic processes are contact metamorphism and regional, or Barrovian, metamorphism.
While the presence of both extreme heat and great pressure aides in the process of metamorphism, only one may be required for the process to take place as exemplified in the case of contact metamorphism. During contact metamorphism, a quantity of magma intrudes into preexisting rock, baking those materials near the intrusion, altering their physical characteristics even at relatively low pressures. The degree of metamorphism a rock sustains in this instance relies directly upon its proximity to the magma body; those rocks nearest the intrusion are the most greatly altered while those further away, and thus less heated, are less intensely transformed. Eventually the change in temperature becomes so insignificant that the parent rock ceases to be affected.
Barrovian metamorphism, in contrast, occurs when a large region has been affected by a major tectonic event such as a collision of continents and, because of this, is far more widespread than contact metamorphism (Note: this type of metamorphism is often referred to simply as “regional metamorphism” but, because there are other metamorphic processes which also occur on a regional level, some Geologists believe “Barrovian” is the more accurate name).
In addition to being the most common form of metamorphism, Barrovian metamorphism is also extremely useful to geologists attempting to understand the history of an area. Because the high levels of both heat and pressure at work in this process often produce predictable series’ of rock materials from the same parent material, rocks altered by Barrovian metamorphism help scientists to understand how a region came to be formed. For example, knowing that shale formed from area clays will metamorphose to slate, which will then become phyllite, then schist, and finally the coarse grained gneiss, a geologist can easily determine which rocks, and thus which areas of the land, have been most severely affected by past tectonic events.
Although contact metamorphism and Barrovian metamorphism are the most commonly addressed, there are several other metamorphic processes at work on the planet.
Hydrothermal metamorphism is a chemical form of metamorphism caused by hot, chemically active waters and gases which occurs at relatively low temperatures and pressures. This process produces many important gem materials and some important mineral deposits, such as silver and copper.
Blueschist metamorphism is similar to Barrovian metamorphism but occurs when cold oceanic crust is subducted, increasing in pressure much more rapidly than it increases in heat. The name is derived from the blue color of many rocks formed by this process, influenced by the amphibole mineral glaucophane.
Finally, eclogite metamorphism takes place in ultramafic parent rock in the mantle, far beneath the Earth’s surface. Because it is so deep within the planet, however, outcrops of this material are rarely discovered.
Marble in use, courtesy of R. Weller
While the various processes of metamorphism are critically important from a geological perspective because of the insight they provide into the formation of our planet, they also have great economic implications. The wide variety of substances formed by these processes has resulted in some of the world’s most useful and sought after materials; for centuries, slate and marble have been prized building materials for their strength and beauty and many gemstones would never form without the extreme heat and pressure metamorphism provides.
Perhaps the greatest example of the economic purposes of metamorphic rocks, however, is the wide use of carbon based materials.
Diamonds, courtesy of R. Weller
Diamonds, while among the prized gemstones mentioned previously which form only under extreme heat and pressure, are also exceptionally valuable for industrial uses due to their great hardness,
Graphite, courtesy of R. Weller
… graphite’s slippery texture and distinctive black streak have made it extremely useful as both a lubricant and as the primary component in pencil lead,
Anthracite coal, courtesy of R. Weller
… and coal has been an invaluable source of energy since the Industrial Revolution.
While all the processes affecting the rocks on our planet serve important purposes, metamorphism may be among the most vital. In allowing us to study those forces that made the Earth what it is today and in providing many of the resources we have come to depend on, metamorphic rocks are a fundamental, if often overlooked, piece of our lives.