Metamorphic Rocks and Plate Boundaries
Text from http://www.uky.edu/AS/Geology/howell/goodies/elearning/module02swf.swf
Slate is the lowest-grade metamorphic rock that forms from a shale parent rock.
*What: Slate is made up of microscopic mica minerals, usually muscovite. These are flat minerals that are aligned parallel to each other. This orientation of the crystals causes slate to break easily in one direction. This property is called "slaty cleavage."
*Micas: Shale is made of clay minerals, and most clays are not very stable under high pressures and temperatures. As shale begins to metamorphose, heat and pressure cause the clay minerals to slowly recrystallize into flat mica flakes.- Micas are common metamorphic minerals.
*Cleavage: Slate cleaves (or breaks) in a preferred orientation because the mica crystals are flat and tend to break along the flat direction. During metamorphism, the micas grow with their flat dimension oriented at right angles to the maximum pressure. This process lines all the micas up and gives slate its characteristic cleavage direction. Slaty cleavage is one example of the metamorphic texture called "foliation," which means "layering."
*Color: The color of slate depends on the chemical composition of its parent shale or mudstone. Red slates are rich in hematite, an iron oxide; green slates contain a significant amount of chlorite; purple slates are stained by manganese oxides; and black slates contain carbon-rich organic material.
Phyllite is a low-grade metamorphic rock consisting mostly of fine-grained micas, but the mica crystals are larger than those in slate and are large enough to be just barely visible to the naked eye. Micas (especially muscovite) are very glittery, and this glitter gives phyllite its characteristic sheen.
*More: Phyllite typically develops from slate at temperatures around 300 deg C. Other common minerals found in phyllite besides muscovite include chlorite (green), graphite (dark gray or silvery), and sometimes garnets (red or brown). The characteristic wavy, layered, glittery texture of phyllite is another example of foliation in metamorphic rocks.
Schist is a medium-grade metamorphic rock that can come from a variety of parent rocks, although the most common schists are from shale parents; this means that they came through a transition from shale to slate to phyllite to schist.
*Grain size: Schist is characterized by an abundance of coarse mica mineral grains, such as muscovite, biotite, and chlorite. The mica grains are large enough to be seen without magnification; usually they can be flaked off with a fingernail.
*Texture: Schist displays "schistose" texture, which is similar to phyllitic texture except that the mineral grains are coarse enough to be seen with the naked eye. Schist typically shows little or no segregation of minerals into discrete bands or layers.
*More: Other metamorphic minerals, especially garnets, are commonly found sprinkled throughout schists. Another common type of schist is called a greenschist.- This rock forms mainly from metamorphism of a basaltic (igneous) parent rock and is marked by the presence of chlorite micas, which give greenschist its characteristic green color.
Gneiss is a high-grade metamorphic rock, only forming deep in the crust beneath active mountain chains such as volcanic arcs at subduction zones. Its prime distinguishing feature is gneissic texture, formed by segregation of minerals into distinct, and commonly colorful, bands.
*How: As temperatures approach 500 to 700 degrees C (very hot but below the typical melting temperature of the rock), the minerals in schists recrystallize and segregate into layers dominated by one mineral type. The extremes of heat and pressure destabilize the more felsic minerals, breaking them down into ions that readily migrate within the rock and recrystallize in distinct, light-colored, single-mineral layers containing larger grains of more stable felsic minerals such as quartz or feldspar. Meanwhile, ions freed from other minerals, such as biotite, amphiboles, and pyroxenes, form intervening layers of dark mafic minerals. This process of mineral separation is known as metamorphic differentiation.
*Texture: The resulting texture, showing distinct layering of dark and light minerals, is referred to as gneissic banding, a striking form of foliated (layered) texture. When looking at layered rocks in lab or in the field, be certain not to mistake gneissic banding for layering in sedimentary rocks - these are two very different processes and textures. The two main differences are that sedimentary rock layers are rarely segregated into separate minerals for each band (a characteristic of gneisses) and that the texture of sedimentary rocks is typically one of individual grains with cement in the spaces, not the interlocking crystalline texture of gneissic bands.
Migmatites are similar to gneisses except they show evidence of partial melting and recrystallization of the rock. This process indicates that the rock got VERY hot during metamorphism. At very high temperatures (usually greater than 600 deg C), and particularly in the presence of water, certain minerals within a gneiss will melt. The portions that melt are typically the most felsic portions - usually quartz and potassium feldspars. This melting forms veins that cut across the gneiss and recrystallize, creating a complex rock that is partly metamorphic and partly igneous; the term "migmatite" actually means "mixed rock."
*Water: The presence of small amounts of water in a gneiss significantly enhances partial melting. The reason is that water lowers the melting temperature of many minerals, enabling them to melt more easily than they would if the rock were dry.
*Flowing: Many migmatites, such as this one, show evidence of folding of veins and foliation. Migmatites commonly are folded during metamorphism because they are so hot that some of the minerals behave plastically, allowing flowage (movement) within the rock.
Pure sandstone lacks the platy minerals (micas and clays) necessary for foliation. Therefore, metamorphosis of pure sandstone, even under directed pressure, results in the nonfoliated rock quartzite. In some cases it is hard to distinguish quartzite from a hard, well-cemented sandstone.
*Strength: However, recrystallization changes the way the rock breaks. Sandstone typically breaks between individual sand grains, because the cement connecting the sand grains is weaker than the grains themselves. Quartzite, however, is commonly so thoroughly recrystallized that original grain boundaries are unrecognizable, and breakage occurs randomly across and around quartz crystals, making it a very durable rock.
Marble is formed from relatively pure limestone or dolostone, both of which lack platy silicate minerals such as micas or clays. Therefore, foliation is not very likely to form, even when these rocks are metamorphosed under directed pressure. Instead, the result is a recrystallized coarse-grained rock with interlocking equant mineral grains (that is, grains having sides of roughly equal length).
*Color: The purest marbles are usually white, but tiny amounts of impurities such as iron and manganese can add beautiful swirling colors to these rocks. Because calcite is a soft mineral, marbles are ideal for sculpting and shaping into building stones.
Contact metamorphism is entirely a result of the heat from the magma and from circulation of associated hot water; pressure is not generally a significant factor here. Because rocks are poor conductors of heat, the effect of contact metamorphism is strongest near the magma and decreases away from the intrusion. Thus rocks in direct contact with the magma will be highly metamorphosed, and those farther away are only slightly altered.
*Examples: One common contact metamorphic rock is hornfels - a dark, hard, fine-grained rock that has a shale for parent material. A piece of hornfels in your hand will look much like a piece of basalt. Where a limestone is "baked" next to an igneous intrusion, a skarn will result - skarn is a contact metamorphic marble containing garnets and other common metamorphic (especially hydrothermal) minerals.
Regional metamorphism alters rocks for thousands of square kilometers. It is responsible for the vast regions of exposed metamorphic rock found in the Appalachians of New England, the Rockies, and the Cascades. There are two types of regional metamorphism: burial metamorphism and dynamothermal metamorphism.
*Burial: Burial metamorphism occurs when rocks are overlain by more than 10 kilometers (6 miles) of rock or sediment. The confining pressure and geothermal heat at these depths begins to cause recrystallization and alteration of the rock's minerals.
*Dynamothermal: Dynamothermal metamorphism occurs during mountain building, when rocks are cooked and compressed between converging plates and deep beneath rising mountains. The lateral pressure of the converging plates creates foliated rocks such as slate, phyllite, schist, and gneiss. Dynamothermal metamorphism is responsible for the vast regions of metamorphic rock that can be found at the cores of many mountain ranges.
The pressure and heat in the immediate vicinity of an active fault can be high enough to produce foliation and metamorphic recrystallization. Usually fault metamorphism creates gneisses. These effects taper off a short distance from the fault zone. Most fault metamorphism produces very fine-grained gneisses, with sharp, fine foliation. However, if hot water is locally available to aid the movement of ions through cracks in the rocks, fault metamorphism may produce unusually large grains due to recrystallization - some the size of grapefruits! In rare instances, enough frictional heat is produced during fault metamorphism to locally melt some of the rock, creating small veins of glassy igneous rocks within the metamorphic gneisses. Such very high temperatures appear to be created in association with rapid fault movement, such as occurs along the San Andreas fault in California during large earthquakes.
The water responsible for hydrothermal metamorphism may come from magma directly, it may be stripped from the molecular structure of the nearby rocks during metamorphosis, or it may be groundwater that has percolated down from the surface and been heated by contact with deep subsurface rock.
*Sea-floor spreading: Most metamorphism of this type occurs within ocean floors, when seawater circulates through cracks near a hot, divergent plate boundary at which sea-floor spreading occurs. Superheated water can dissolve or help recrystallize many kinds of rocks, and hot water readily transports ions needed for metamorphic mineral growth.
*Hot Spots: Yellowstone National Park is located above a continental hot spot; a plume of hot rising, mantle rocks that causes volcanic activity in the area. Yellowstone has many mineral springs that occur at points where superheated groundwater carries ions upward from hydrothermal metamorphism in the rocks beneath, to the surface where chemical deposits are forming in striking and beautiful patterns.