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Metamorphic Rock Formation Help

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By — McGraw-Hill Professional
Updated on Sep 1, 2011

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. Figure 8-1 illustrates how the three rock types feed into a simple rock cycle.

Metamorphic Rock

 

Fig. 8-1. The rock cycle is affected by environmental conditions of heat, pressure, and weathering.

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.

We learned that the temperature of the Earth increases the deeper you go. On average, the temperature increases 30°C/km, but can vary from 20 to 60°C/km in depth. For example, the temperature at a depth of 15 km is equal to 450°C. 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.

↑ Temperature and ↑ Pressure Metamorphic Rock High-grade metamorphic rock

↓ Temperature and ↓ Pressure Metamorphic Rock 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 10 km 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 high-temperature 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. Figure 8-2 shows the surrounding aureole effect of magma heating.

Metamorphic Rock

Fig. 8-2. Surrounding rock is heated in the area around magma pockets in a halo effect.

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 free-falling droplet, but impacts in much the same way. For example, an iron meteorite measuring 10 m across and hitting the surface at a velocity of 10 km/sec would create a crater over 300 km 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 (CaCO 3 ) 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.

Figure 8-3 shows how magma forces its way into layers of limestone, quartz sandstone, and shale. 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.

Metamorphic Rock

Fig. 8-3. Contact metamorphism happens when magma surges upward through existing rock.

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