1 08_00.jpg metamorphism: : The process by which one kind of rock transforms into a different kind of rock. metamorphic rocks: a rock that forms from the modification of another rock. protolith: : preexisting rock that was deformed and modified into a metamorphic rock.
2 Metamorphic rocks are either foliated: : Layering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock. foliation or nonfoliated: : Rock containing minerals that recrystallized during metamorphism, but which has no layering has appeared.
3 parent rock is the same thing as protolith
4 08_01b.jpg foliation garnet Metamorphic minerals: : are the new minerals that form in metamorphic rocks due to increase in temperature and/or pressure. Metamorphic textures: : are the ways that metamorphic minerals grow in a metamorphic rock. Above this metamorphic rock has a strong foliation with garnet as its distinctive metamorphic mineral. The garnet grew in a pyroclastic metamorphic texture.
5 Formation of a metamorphic rock take a very long time that involves several different processes: Recrystallization: : to change the shape and size of a mineral without changing its original composition. Phase change: : to change one mineral into a different mineral Metamorphic reaction: : growth of new minerals out of the protolith due to metamorphic processes. This is a type of diffuse mineral growth. Pressure solution: : process of dissolution at points of contact, between grains, where compression is greatest, producing ions that then grow into new minerals. Plastic deformation: : deformational process in which mineral grains behave like plastic and, when compressed or sheared, become flattened or elongate without cracking or breaking.
6 08_02ab.jpg Recrystallization Metamorphic reaction
7 08_02cd.jpg Pressure solution Plastic deformation
8 Phase change occurs at depth in the earth when the mineral olivine converts into a different mineral structure called a spinel type structure.
9 Metamorphism is caused by heating of a rock and subjecting a rock to high pressures.. In the earth, as we have seen with igneous rocks, pressure and temperature change together. A good example of this is the Al SiO 2 5 mineral system. The 3 minerals below all have the same chemical formula (Al SiO 2 5 ); but, the molecular structure of each mineral is different. Temperature and pressure are the controlling factors.
10 08_04.jpg Metamorphic rocks are commonly deformed as a result of their metamorphism. This leads to a preferred orientation of mineral grains in the rocks. The rocks become deformed by... differential stress: : a condition causing a material to experience a push or pull in one direction of a greater magnitude than the push or pull in another direction; in some cases, differential stress can result in shearing.
11 08_04.jpg There are two kinds of differential stress: 1) Normal stress: : The push or pull that is perpendicular to a surface.
12 08_04.jpg 2) Shear stress: : A stress that moves one part of a material sideways past another part.
13 08_06.jpg These diagrams show how differential stress cause preferred mineral orientations.
14 08_05.jpg Metamorphic minerals tend to be equant, elongate and platy after they form in a metamorphic rock.
15 Metamorphic reactions usually take place in the presence of very hot fluids called hydrothermal fluids.. These fluids react chemically with the rocks during metamorphism. Supercritical fluid is a hydrothermal fluid that is at great depth so it has the physical characteristics of both a liquid and a gas. This process by which a metamorphic rock s chemical composition changes due to the interaction with hydrothermal fluids is called metasomatism.
16 Foliated metamorphic rocks are classified based on the intensity of the metamorphism. The hotter the metamorphic reaction, the more well developed the foliation will be in a metamorphic rock.
17 08_08b.jpg Slate: : Fine-grained, low-grade metamorphic rock, formed by the metamorphism of shale.
18 08_09.jpg slaty cleavage: : The foliation typical of slate, and reflective of the preferred orientation of slate s clay minerals, that allows slate to be split into thin sheets. Forms perpendicular to differential stress applied to the rock.
19 08_10a.jpg Phyllite: : A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained mica. Forms at higher temperatures and pressures than slate.
20 08_10c.jpg Schists: : A medium-to-coarse-grained metamorphic rock that possesses schistosity. At higher temperatures and pressures than slate and phyllite. Schistosity: : Foliation caused by the preferred orientation of large mica flakes.
21 08_11a.jpg Gneiss: : A compositionally banded metamorphic rock typically composed of alternating dark- and light-colored layers. Formed at higher temperatures and pressures than slate, phyllite, and schists. Usually formed from the melting of a protolith. Compositional layering: : A type of metamorphic foliation, found in gneiss, defined by alternating bands of light and dark minerals.
23 08_12.jpg This diagram shows how compositional layering can form in a gneiss through progressive deformation and recrystallization under high pressures and temperatures..
24 08_13.jpg Migmatite: : A rock formed when gneiss is heated high enough so that it begins to partially melt, creating layers, or lenses, of new igneous rock that mix with layers of the relict gneiss. Formed at the highest temperatures and pressures of any foliated metamorphic rock.
25 08_17a.jpg As you increase temperature and pressure, you increase metamorphic grade.
26 08_17b.jpg Metamorphic grade: : the amount or intensity of change in a protolith as the result of temperature and pressure increase. Metamorphic mineral assemblage: : A group of minerals that form in a rock as a result of metamorphism.
27 08_18.jpg Metamorphic grade can range from low grade to high grade.
28 Nonfoliated metamorphic rocks include: Amphibolites: : metamorphic mafic rocks that consists of the minerals hornblende and plagioclase.
29 08_15b.jpg Quartzite: : A metamorphic rock composed of quartz and transformed from a protolith of quartz sandstone.
30 08_23.jpg Hornfels: : Rock that undergoes metamorphism simply because of a change in temperature, without being subjected to differential stress. This is due to metamorphic aureole: : The region around a pluton, stretching tens to hundreds of meters out, in which heat transferred into the country rock and metamorphosed the country rock.
31 08_24ab.jpg An example of hornfels produced from the contact with a pluton is the Onawa pluton in Maine. Far way from the pluton as slates that were deformed before the pluton was emplaced.
32 08_24cd.jpg Closer to the pluton, low-grade hornfels minerals start to form. And just adjacent to the pluton occurs the highest-grade hornfels mineral assemblages.
33 Marble: : A nonfoliated metamorphic rock composed of calcite and transformed from a protolith of limestone.
34 08_20.jpg Metamorphic grade and metamorphic mineral assemblages leads to the classification of different metamorphic temperatures and pressures into metamorphic facies: : A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures.
35 At a convergent plate boundary, multiple metamorphic facies occur as a result of the differing temperatures and pressures created at these zones.
36 08_19.jpg Prograde metamorphism: : metamorphism that occurs with increasing temperature and pressure. Retrograde metamorphism: : metamorphism that occurs with decreasing temperature and pressure.
37 08_21.jpg Metamorphic sequences are the result of index metamorphic minerals that reflect different metamorphic temperatures and pressures.
38 08_17a.jpg As more rocks form on top of each other, rocks become buried deeper into the earth. Due to the geothermal gradient (~24 C/km) rocks will undergo burial metamorphism as a result of this process..
39 Metamorphism that occurs along faults results from shearing forces applied to the rock. This results in dynamic metamorphism: Metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure. A metamorphic rock type that is produced as a result of dynamic metamorphism are mylonites (Fine-grained rock formed by the nonbrittle subdivision of larger grains). 08_25.jpg
40 08_26.jpg Dynamothermal metamorphism: : Metamorphism that involves heat, pressure, and shearing. This type of metamorphism is the result of collisions of one tectonic plate with another tectonic plate. This type of metamorphism effects large areas so it is also called regional metamorphism.
41 08_27.jpg When cold sea water interacts with the hot rising magma at mid-ocean ridges, the sea water heats up and produces a low-grade hydrothermal metamorphism.
42 08_28.jpg exhumation: The process (involving uplift and erosion) that returns deeply buried rocks to the surface. This is how metamorphic rocks become exposed at the surface of the earth. This process occurs over a long period of time.
43 08_29a.jpg Shield: : An older, interior region of a continent. This is typically where metamorphic rocks are exposed.
44 Now that we have learned about igneous, sedimentary and metamorphic rocks, it is important to know that they are related together in what is known as the rock cycle: : The succession of events that results in the transformation of Earth materials from one rock type to another, then another, and so on. Sedimentary rocks can consists of fragments derived from older metamorphic, igneous and sedimentary rocks. Metamorphic rocks can form through the deformation of older sedimentary, igneous and metamorphic rocks. Igneous rocks can form from the partial melting of older sedimentary, metamorphic and igneous rocks.