Metamorphism does not cause a rock to melt completely. It will also sound different to a piece of shale if you tap it with something hard! 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. These pressures are particularly noteworthy in that they are recorded in rocks derived from sedimentary rather than basaltic protoliths. Formed when shale, mudstone and other clay rich rocks are exposed to moderate heat and pressure, causing the clay minerals to convert to our platy minerals such as mica. The photos in Figures 8.4 and 8.5 below show two outcrops of regional metamorphic rocks. The dark material is a block of amphibolite which is metamorphosed dolerite. NOTE: If the protolith is not shale but some other rock the resultant metamorphic rocks will be different because the chemical make up of the protolith minerals has a major influence on the chemical make up - and thus the mineralogy - of the resultant metamorphic rocks. At an even higher grade of metamorphic pressure and temperture phyllite will change into schist.The schist shown below is an example of this metamorphic rock type. Continued intrusion of magma over a period of time would cause an increase in crustal temperatures at relatively shallow depths and produce the high-temperature rocks adjacent to the high-pressure rocks generated in the subduction zone. The key diagnostic feature of regional metamorphic rocks is the development of a foliation due to the differential stresses. These rocks were heated to temperatures above 600 degrees Celsius. Metamorphism acts at two scales: regional and local. Regional metamorphism: We find metamorphic rocks exposed over regions of the Earth's surface, either in the cores of mountain belts or the roots of what were once mountain belts. It has grown during metamorphism. In some instances, metamorphic rocks produced during much earlier events are simply unroofed and exposed by the faulting but show little or no recrystallization related to extension. This educational product is designed for Yr 7-10 secondary students to complement the earth and space componentof the Australian National Science Curriculum and all Australian State and Territory curricula, The content and design of this educational product is based upon materials previously published by, This is best demonstrated by the protolith mud-rich sedimentary rock with distinct laminations called, Under low grade metamorphic pressure and temperture conditions shale is changed into, Under a slightly higher grade of metamorphic pressure and temperture slate will change into, At an even higher grade of metamorphic pressure and temperture phyllite will change into, At the highest grade of metamorphic pressure and temperture schist will change into. Metamorphic Rocks Changed rocks- with heat and pressure But not melted Change in the solid state Textural changes (always) Mineralogy changes (usually) Metamorphism The mineral changes that transform a parent rock to Most regional metamorphism takes place within continental crust. Learn vocabulary, terms, and more with flashcards, games, and other study tools. This is best demonstrated by the protolith mud-rich sedimentary rock with distinct laminations called shale. Foliation in geology refers to repetitive layering in metamorphic rocks. Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly those associated with convergent tectonic plates or the roots of previously eroded mountains. The differential stress usually results from … Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Rapid subduction of the cool oceanic lithosphere perturbs the thermal regime in such a way that high pressures can be obtained at relatively low temperatures, thereby generating blueschists and eclogites (high-pressure facies series) from ocean-floor basalts transported down the subduction zone. Slaty cleavage: type of foliation that is a … garnet-mica-schist). Under low grade metamorphic pressure and temperture conditions shale is changed into slate.The slate shown below is typical of this metamorphic rock type. The two main types of metamorphism are both related to heat within Earth: Regional metamorphism: Changes in enormous quantities of rock However the planar foliation is now forced to wrap around new metamorphic minerals that are not platy and so appear to form large bumps within the foliated mica. For example a basalt or a dolerite will form an amphibole rich rock called an amphibolite, not a gneiss, even though both rocks form at the same metamorphic grade. The prismatic crystals in the rock below are the mineral andalusite. While rocks can be metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths. Figure 7.4.2 Regional metamorphic zones in the Meguma Terrane of southwestern Nova Scotia. The rocks were originally shales, limestones, diabase sills, and basalts that had been emplaced in the Precambrian to early Cambrian. The shale shown below is typical of this sedimentary rock type. Such rocks cover large areas of the Earth's crust and are therefore termed regional metamorphic rocks. Some form during mountain-building by forces of others from the heat of igneous intrusions in regional metamorphism others from the heat of igneous intrusions in contact metamorphism. Look it up now! It is a distinctly different looking rock to shale and slate.The clay minerals in the shale/slate have been changed into mica minerals, all aligned to give the rock an obvious foliation. Local metamorphism happens at a much smaller level, usually from nearby igneous intrusions. Although the processes that formed each of these mountain belts are broadly similar, in almost all such crustal events at different times and places, there is uniqueness as well as conformity to a general pattern. Most metamorphic rocks occur in fold mountain belts or cratonic areas. Regionally metamorphosed rocks are also exposed in areas where the crust has been thinned by extensional faulting, such as the Basin and Range Province of the western United States.
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