(48) Earth Science

Intro to Metamorphic Rocks

ES46 

Metamorphic Rock

 Igneous rock is formed as a result of the Earth’s internal ‘‘engine,’’ while sedimentary rock formation depends on external climate and conditions. Metamorphic rock, however, takes place after these rock types have already formed. It is created by transforming igneous or sedimentary rock into something new.

Of the three major rock types, igneous, metamorphic, and sedimentary, metamorphic rock is the chameleon rock. It transforms into different types of rocks depending on the factors that it is exposed to within the Earth. This rock type is both a wonder and a headache to geologists. Since metamorphic rock begins originally as something else, it can be confusing as to whether it is the original rock or a transformed version. To solve this problem, geologists gather clues from the surrounding area or an outcrop from which the sample rock is found.

Besides being intruded upon by magma regularly, the Earth’s crust is subjected to stresses within the crust and mantle that cause it to break and bend forming fault folds. These forces often center along thin, winding belts when folding. They also combine with magma intrusion and extrusion while pushing up mountain ranges. The rocks within a mountain range are not onlyunder extreme pressure, but heated by magma intrusion as well. These stresses deform and recrystallize rock to different degrees. Pressure and temperature can also change previously metamorphosed rocks into new types.

Rock-forming and destroying processes have been active since the Earth was first formed. When sedimentary and igneous rocks are exposed to extreme pressure or medium heat, they are changed. They become metamorphic rocks, which form while deeply buried within the Earth’s crust. It is important to remember that metamorphism does not just melt existing igneous or sedimentary rock, but transforms it into a denser, compacted rock.

Metamorphic rocks are formed from rocks that were originally another type and were changed into a different form.

 

The name metamorphic comes from the Greek words, meta and morph, which mean ‘‘to change form.’’ Geologists have found that nearly any rock can become a metamorphic rock. When existing rock is shoved and pressurized, its minerals become unstable and out of equilibrium with the new conditions, causing them to change.

Remember the chameleon? When a chameleon moves from a gray rock to a bright green leaf, he changes his skin color to the same as his environment. By adjusting to his new conditions, the chameleon protects himself and comes into equilibrium with his surroundings. The process of metamorphism is similar. When a rock is slowly moved through tectonic processes to a new temperature or pressure environment, its original chemical and physical conditions are changed. In order to regain stability in the new conditions, chemical and physical changes take place. With metamorphism, mineral changes always move toward reestablishing equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble with many grades in between.

Most of the time, metamorphic rock is buried many kilometers below the crust which allows increasing temperatures and pressures to affect it.

However, metamorphism can also happen at the surface. When geologists study soils under hot lava flows, they find metamorphic changes. The three main forces responsible for the transformation of different rock types to metamorphic rock are:

* Internal heat from the Earth,

* Weight of overlying rock, and

* Horizontal pressures from rock that changed earlier.

Temperature

Temperature increases in sedimentary layers that are found deeper and deeper within the Earth. The deeper the layers are buried, the more the temperature rises. The great weight of these layers also causes an increase in pressure, which raises the temperature even more.

This cycle of heat and pressure that describes the transformation of existing rock is called the rock cycle. It is a constantly changing feedback system of rock formation and melting that links sedimentary, igneous, and metamorphic rock.

The pushing down of rock layers at subduction zones causes metamorphism in two ways: the shearing effect of tectonic plates sliding past each other causes the rocks to be deformed that are in contact with the descending rocks. Some of the descending rocks melt from this friction. These melted rocks are considered igneous rock not metamorphic. Then secondly, nearby solid rock that lies alongside melted igneous rock can be changed by high heat to also form metamorphic rock.

The temperature of the Earth increases the deeper you go. On average, the temperature increases 308C/km, but can vary from 20 to 608C/km in depth. For example, the temperature at a depth of 15km is equal to 4508C. At the same depth, the pressure of the overlying rock is equal to 4000 times the pressure at the surface.

This heat and pressure gradient, changing with depth, allows metamorphism to happen in a graded way. The deeper you go, the hotter the temperature and pressure, the greater the metamorphic changes. Depending on the conditions under which rock is changed, the rock gradient forms new metamorphic rock into high-grade or low-grade metamorphic rock.

As rock adjusts to new temperature or pressure conditions, the crystal structure of its minerals are affected. Ions and atoms are energized. They begin breaking their chemical bonds and creating new mineral linkages and forms. Sometimes, crystals grow larger than they were in the original rock.

New minerals are created either by rearrangement of ion bonding or by reactions with fluids that enter the rocks.

There are five main ways that metamorphic rocks are created. These different metamorphic rock processes include contact, regional, dynamic metamorphism, cataclastic, hydrothermal, and burial metamorphism. A closer look at each one of these will show how they are different.

Contact Metamorphism

Contact metamorphism takes place when igneous intrusion of magma heats up surrounding rock by its extreme temperatures. This surrounding rock is called country rock. When igneous intrusion happens, the country rock’s temperature heats up, and becomes filled with fluid brought along by the traveling magma. The area affected by hot magma contact is usually between 1 and 10km in size.

When contact metamorphism happens on the surface because of an outpouring of lava, it is restricted to a fairly thin rock layer. Since lava cools quickly and gives heat little time to penetrate the underlying country rock, the metamorphism that takes place is limited.

An aureole or rock halo is formed by metamorphosed rock around a hightemperature source. The metamorphic rock close to the magma pocket contains high-temperature minerals, while rock found further away has lower-temperature minerals. These heat sources are commonly closer to the surface crust in contact metamorphism than other types.

When a plutonic magma pocket is rimmed by a contact ring of metamorphic rock, it is known as an aureole.

A special type of contact metamorphism, impact metamorphism, is caused by the high-speed impact of a meteorite. As the meteorite hits the Earth’s surface, it causes shock waves. These are sent out from the impact site as a way to scatter the energy from impact. Depending on the speed and angle of impact, the surface at impact is immediately compacted, fractured, melted, and may be vaporized. Following the initial slam and shock wave, the rock decompresses sending rock flying in all directions and forming an impact crater.

Have you ever seen high-speed photography of a droplet of water hitting the surface of a still pool? The impact compresses the water’s surface downward for an instant, followed immediately by a rebounding ring of droplets shooting upward. The shock-wave impact is absorbed throughout the liquid as ripples.

Unlike deep mantle metamorphism, shock metamorphism happens in the instant of a high-velocity impact.

A meteorite impact has much greater velocity and energy than a freefalling droplet, but impacts in much the same way. For example, an iron meteorite measuring 10m across and hitting the surface at a velocity of 10 km/sec would create a crater over 300km in diameter.

The shock wave from a meteorite impact causes high-pressure shock metamorphism effects such as specific fracture patterns and crystal structure destruction. In fact, the formation of polymorphs, or in-between shock-related minerals like coesite or stishovite, not commonly found on the surface, helps geologists to find ancient impact craters.

Contact metamorphism produces nonfoliated rocks (without any lines of cleavage) such as marble, quartzite, and hornfels.

Nonfoliated rock is made up of crystals in the shape of cubes and spheres that grow equally in all directions

Marble is formed from metamorphosed limestone or dolomite that has recrystallized into a different texture after contact with high heat. It is made up of calcite, but if it contains a large amount of dolomite, then it is called dolomitic marble. Both limestone and dolomite have large amounts of calcium carbonate (CaCO3) and many different crystal sizes. The different minerals present during the formation of marble give it many different colors. Some of marble’s colors include white, red, pink, green, gray, black, speckled, and banded.

Since marble is much harder than its parent rock it can be polished. Marble is used as a building material, for kitchen and bathroom countertops, bathtubs, and as carving material for sculptors. Grave stones are made from marble and granite because they weather very slowly and carve well with sharp edges.

Quartzite is the product of metamorphosed sandstone containing mostly quartz. Since quartzite is formed from sandstone that contacted hot, deeply buried magma, it is much harder than its parent rock. As it is transformed, the quartz grains recrystallize into a denser, tightly packed texture. Unlike matte-finished sandstone, quartzite has more of a shiny, glittery look. While sandstone shatters into many individual grains of sand, quartzite fractures across the grains.

Hornfels is a fine-grained, nonfoliated, large crystal metamorphic rock formed at intermediate temperatures by contact metamorphism. These can be further defined as pyroxene-hornfels and hornblende-hornfels formed at still lower temperatures.

The high heat coming from the deep magma chamber changes these sedimentary rocks into the metamorphic rocks, such as marble, quartzite, and hornfels. These changed rocks are listed to the right of the figure in relation to their original rock types.

 

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