Metamorphic Rock
Types of Rock in the Rock Cycle
Geologists classify rocks into three main types based on how the rocks form. In a continual series of
changes known as the rock cycle, rocks of any type can change into rocks of any other type.
One type of rock, sedimentary, is formed when particles of weathered rock and other matter collect in an
area and are pressed together. (These particles of matter are called sediments.) Another rock type,
igneous, forms when molten rock cools and hardens. These rocks are further classified into intrusive and
extrusive igneous rock. Intrusive igneous rock cools slowly under Earth’s surface, forming large crystals.
Extrusive igneous rock cools above Earth’s surface, where temperatures are much lower. Because
extrusive rocks harden quickly, their crystals do not have time to become large. Instead, the crystals are
small and finely textured.
When rock beneath Earth’s surface is exposed to extreme changes in temperature or pressure, its chemical
composition can transform. The new rock that results from such transformations is called "metamorphic
rock."
Properties of Metamorphic Rock
Metamorphic rock can form from sedimentary rock, intrusive igneous rock, extrusive igneous rock, or
other metamorphic rock. In the process of becoming metamorphic rock, the existing rock does not melt.
Rather, the existing rock transforms while in a solid state.
The transformation occurs because the minerals that make up rock are chemical compounds. Chemical
compounds are made up of two or more different types of atoms that are bonded together. Under extreme
heat and pressure, some of the bonds between atoms in the solid rock break. This allows those atoms to
bond in different ways, forming different compounds. The rock cycle has no starting or ending point. Rather, any
type of rock can become any other type of rock.
Because the original rock does not melt, properties of the resulting metamorphic rock are determined in
part by the properties of the original, or parent, rock. For example, granite is an intrusive igneous rock
with a coarse texture and large crystals. Subjected to heat and pressure, however, granite can change to
the metamorphic rock gneiss. Gneiss is a type of metamorphic rock known as foliated rock. The term
"foliated" refers to the bands of different colors that are visible in the rock. This happens because the
pressure on the rock pushes mostly in one direction. As the gneiss forms, the large crystals in the granite
are stretched and squeezed into long bands. G ( u l e s t d r s t s ( ) , m c m o a m s b s o r
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Another good example of this process is the metamorphic rock marble. Many people use marble for
architecture, countertops, flooring, and sculpture due to its hardness. Marble is a nonfoliated type of
metamorphic rock. This is partially due to the even texture and color of its parent rock, limestone.
The amount of heat and pressure the rock undergoes also determines some properties of metamorphic rock. For
example, shale is a relatively soft, easily broken type of sedimentary rock. Like many other kinds of sedimentary
rock, it is evenly textured. Under relatively low-pressure conditions, shale becomes the metamorphic rock slate.
Under high-pressure conditions, however, shale can also be transformed into gneiss
Granite, an igneous rock, has large visible crystals and a coarse-grain texture.
Gneiss, a metamorphic rock that originates from granite, has long bands of
color.
Gemstone Formation in Metamorphic Rock
Some types of gemstones form in metamorphic rocks. One example is garnet, a red gemstone used in
jewelry. Garnet forms when rock is under pressure from its position deep in Earth’s crust.
Another gemstone found in metamorphic rocks is jadeite. Jadeite is one of two types of rock known as
jade. It forms in areas of high pressure along plate boundaries. For example, jadeite is found in parts of
Guatemala along the Montagua fault. This fault occurs at the boundary of two tectonic plates: the North
American and Caribbean plates. As these plates collide, one plate is pushed beneath the other, causing
pressure to rise. The intense pressure causes jadeite to form. Over time, uplift caused by tectonic forces
can force the jadeite to come to the surface.
Rarer gemstones are generally more valuable than common gemstones. Gemstones such as jade and garnet are
relatively rare because these stones require a narrow range of pressures and temperatures to form. In addition,
these gemstones are long-lasting due to their hardness, and the minerals the gemstones often contain give the
stones beautiful colors.
Jade is a gemstone formed in the high-pressure areas at plate boundaries.