Marbles are the type of metamorphic rock that results. Quartzo-Feldspathic - Rocks that contain an abundance of quartz and feldspar fall into this category. Protoliths are usually granites, rhyolites, or arkose sandstones and metamorphism results in gneisses containing an abundance of quartz, feldspar, and biotite.
Metamorphism can take place in several different environments where special conditions exist in terms of pressure, temperature, stress, conditions, or chemical environments. We here describe several diff rent types of metamorphism that are recognized. A map of a hypothetical regionally metamorphosed area is shown in the figure below.
Most regionally metamorphosed areas can be divided into zones where a particular mineral, called an index mineral, is characteristic of the zone. The zones are separated by lines surfaces in three dimensions that mark the first appearance of the index mineral. These lines are called isograds meaning equal grade and represent lines really surfaces where the grade of metamorphism is equal. A map of a regionally metamorphosed areas are can be seen in figure 8.
Hydrothermal Metamorphism - Near oceanic ridges where the oceanic crust is broken up by extensional faults, sea water can descend along the cracks. Since oceanic ridges are areas where new oceanic crust is created by intrusion and eruption of basaltic magmas, these water-rich fluids are heated by the hot crust or magma and become hydrothermal fluids.
The hydrothermal fluids alter the basaltic oceanic crust by producing hydrous minerals like chlorite and talc. Because chlorite is a green colored mineral the rocks hydrothermal metamorphic rocks are also green and often called greenstones. Subduction Related Metamorphism - At a subduction zone, the oceanic crust is pushed downward resulting in the basaltic crust and ocean floor sediment being subjected to relatively high pressure.
But, because the oceanic crust by the time it subducts is relatively cool, the temperatures in the crust are relatively low. Under the conditions of low temperature and high pressure, metamorphism produces an unusual blue mineral, glaucophane. Compressional stresses acting in the subduction zone create the differential stress necessary to form schists and thus the resulting metamorphic rocks are called blueschist.
Shock Metamorphism - When a large meteorite collides with the Earth, the kinetic energy is converted to heat and a high pressure shock wave that propagates into the rock at the impact site. The heat may be enough to raise the temperature to the melting temperature of the earth rock. The shock wave produces high enough pressure to cause quartz to change its crystal structure to more a dense polymorph like coesite or stishovite.
Ancient meteorite impact sites have been discovered on the basis of finding this evidence of shock metamorphism. In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism. Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to.
This pressure and temperature environment is referred to as Metamorphic Facies. The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism. A high geothermal gradient such as the one labeled "A" in the figure shown here, might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies.
Under a normal geothermal gradient, such as "B" in the figure, rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism or depth of burial increased. Before moving on to the rest of the course, you should read Interlude C in your textbook pages Now that we have discussed the three types of rocks, it is important to understand how the atoms that make up these rocks cycle through the earth.
This cycling involves process that will be discussed in detail throughout the remainder of this course. Since the rock cycle links the rock forming processes to tectonic process and to surface process most of which will be discussed throughout the rest of the course , it is important to understand the concept of the rock cycle and the various linkages involved.
We here start our discussion with Volcanoes and Volcanic eruptions and processes that are involved in the production of igneous rocks at the earth's surface. Metamorphism and Metamorphic Rocks. Factors that Control Metamorphism Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids.
Rounded grains can become flattened in the direction of maximum stress. Minerals that crystallize or grow in the differential stress field can have a preferred orientation. This is especially true of the sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine. These sheet silicates will grow with their sheets orientated perpendicular to the direction of maximum stress. Preferred orientation of sheet silicates causes rocks to be easily broken along approximately parallel sheets.
Such a structure is called a foliation. Fluid Phase. This fluid is mostly H 2 O, but contains dissolved ions. The fluid phase is important because chemical reactions that involve changing a solid mineral into a new solid mineral can be greatly speeded up by having dissolved ions transported by the fluid. If chemical alteration of the rock takes place as a result of these fluids, the process is called metasomatism. Time - Because metamorphism involves changing the rock while it is solid, metamorphic change is a slow process.
During metamorphism, several processes are at work. Recrystallization causes changes in minerals size and shape. Chemical reactions occur between the minerals to form new sets of minerals that are more stable at the pressure and temperature of the environment, and new minerals form as a result of polymorphic phase transformations recall that polymorphs are compounds with the same chemical formula, but different crystal structures.
Laboratory experiments suggest that the the sizes of the mineral grains produced during metamorphism increases with time.
Thus coarse grained metamorphic rocks involve long times of metamorphism. Experiments suggest that the time involved is tens of millions of years. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.
Low-grade metamorphism takes place at temperatures between about to o C, and relatively low pressure. Low grade metamorphic rocks are characterized by an abundance of hydrous minerals minerals that contain water, H 2 O, in their crystal structure. Examples of hydrous minerals that occur in low grade metamorphic rocks: Clay Minerals Serpentine Chlorite High-grade metamorphism takes place at temperatures greater than o C and relatively high pressure.
As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H 2 O and non-hydrous minerals become more common. Examples of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks: Muscovite - hydrous mineral that eventually disappears at the highest grade of metamorphism Biotite - a hydrous mineral that is stable to very high grades of metamorphism. Pyroxene - a non hydrous mineral.
Garnet - a non hydrous mineral. Retrograde Metamorphism As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one might expect metamorphism to a follow a reverse path and eventually return the rocks to their original unmetamorphosed state.
Metamorphic Rock Types There are two major subdivisions of metamorphic rocks. Non-foliated Metamorphic Rocks Non-foliated rocks lack a planar fabric. Absence of foliation possible for several reasons: Rock not subjected to differential stress. Hornfels, with its alternating bands of dark and light crystals is a good example of how minerals rearrange themselves during metamorphism. In this case, the minerals separated by density and became banded. Gneiss forms by regional metamorphism from both high temperature and pressure.
Quartzite and marble are the most commonly used metamorphic rocks. They are frequently chosen for building materials and artwork. Marble is used for statues and decorative items like vases Figure 4. Ground up marble is also a component of toothpaste, plastics, and paper. Quartzite is very hard and is often crushed and used in building railroad tracks Figure 4. Schist and slate are sometimes used as building and landscape materials. Skip to main content. Search for:. Index minerals are used by geologists to map metamorphic grade in regions of metamorphic rock.
A geologist maps and collects rock samples across the region and marks the geologic map with the location of each rock sample and the type of index mineral it contains. By drawing lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region.
The lines are known as isograds. Regional metamorphism occurs where large areas of rock are subjected to large amounts of differential stress for long intervals of time, conditions typically associated with mountain building. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism.
As the rocks become heated at depth in the Earth during regional metamorphism they become ductile, which means they are relatively soft even though they are still solid.
The folding and deformation of the rock while it is ductile may greatly distort the original shapes and orientations of the rock, producing folded layers and mineral veins that have highly deformed or even convoluted shapes. The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress.
The photograph below shows high-grade metamorphic rock that has undergone several stages of foliation development and folding during regional metamorphism, and may even have reached such a high temperature that it began to melt. Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated.
Where intrusions of magma occur at shallow levels of the crust, the zone of contact metamorphism around the intrusion is relatively narrow, sometimes only a few m a few feet thick, ranging up to contact metamorphic zones over m over feet across around larger intrusions that released more heat into the adjacent crust. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. The rocks closest to the contact with the intrusion are heated to the highest temperatures, so the metamorphic grade is highest there and diminishes with increasing distance away from the contact.
Because contact metamorphism occurs at shallow to moderate depths in the crust and subjects the rocks to temperatures up to the verge of igneous conditions, it is sometimes referred to as high-temperature, low-pressure metamorphism. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids.
The difference in composition between the existing rock and the invading fluid drives the chemical reactions. The hydrothermal fluid may originate from a magma that intruded nearby and caused fluid to circulate in the nearby crust, from circulating hot groundwater, or from ocean water.
If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rock—changed its chemical composition.
Ocean water that penetrates hot, cracked oceanic crust and circulates as hydrothermal fluid in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to mid-ocean spreading ridges and other ocean-floor volcanic zones. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist.
Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite.
The fluids eventually escape through vents in the ocean floor known as black smokers, producing thick deposits of minerals on the ocean floor around the vents. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form.
Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. Burial metamorphism is the lowest grade of metamorphism. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals.
It usually requires a strong microscope to see the small grains of zeolite minerals that form during burial metamorphism. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle.
In most subduction zones the subducting plate is relatively cold compared with the high temperature it had when first formed at a mid-ocean spreading ridge.
Subduction takes the rocks to great depth in the Earth relatively quickly. This produces a characteristic type of metamorphism, sometimes called high-pressure, low-temperature high-P, low-T metamorphism, which only occurs deep in a subduction zone. In oceanic basalts that are part of a subducting plate, the high-P, low-T conditions create a distinctive set of metamorphic minerals including a type of amphibole, called glaucophane, that has a blue color.
Blueschist is the name given to this type of metamorphic rock. Blueschist is generally interpreted as having been produced within a subduction zone, even if the plate boundaries have subsequently shifted and that location is no longer at a subduction zone. The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combination labratory experiments, physics-based theoretical calculations, along with evidence in the textures of the rocks and their field relations as recorded on geologic maps.
The knowledge of temperatures and pressures at which particular types of metamorphic rocks form led to the concept of metamorphic facies. Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. Even though the name of the each metamorphic facies is taken from a type of rock that forms under those conditions, that is not the only type of rock that will form in those conditions.
For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth.
Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. One MPa equals nearly 10 atmospheres. A pressure of MPa corresponds to a depth of about 35 km inside the Earth. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth. Temperature depends on the heat flow, which varies from location to location.
The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. In the diagram below, three different geotherms are marked with dashed lines. The three geotherms represent different geological settings in the Earth. High-pressure, low-temperature geotherms occurs in subduction zones.
As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions.
High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. Rocks that have their pressure and temperature conditions increased along such a geotherm will metamorphose in the hornfels facies and, if it gets hot enough, in the granulite facies.
Blueschist facies and hornfels facies are associated with unusual geothermal gradients. The most common conditions in the Earth are found along geotherms between those two extremes.
Most regional metamorphic rocks are formed in conditions within this range of geothermal gradients, passing through the greenschist facies to the amphibolites facies.
0コメント