6.9), and smaller crowds at the contact of volcanic rocks. E. Galán, R.E. The contact-metamorphic rocks are usually known as hornfels. Common along the margins of small plutons (dikes, sills, etc.) Characteristics of metamorphic rocks are discussed hereunder: Contact Metamorphism: The contact metamorphism occurs when magma comes in contact with an already existing body of rock. Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780123971623000086, URL: https://www.sciencedirect.com/science/article/pii/B0122274105004336, URL: https://www.sciencedirect.com/science/article/pii/B0123693969003208, URL: https://www.sciencedirect.com/science/article/pii/B9780080982588000031, URL: https://www.sciencedirect.com/science/article/pii/B9780124095489090497, URL: https://www.sciencedirect.com/science/article/pii/S2468517819300073, URL: https://www.sciencedirect.com/science/article/pii/B9780128111598000020, URL: https://www.sciencedirect.com/science/article/pii/B0123693969003191, URL: https://www.sciencedirect.com/science/article/pii/B9780124081338000067, Encyclopedia of Physical Science and Technology (Third Edition), °C in association with plutonism or volcanic activity. It involves reduction of crystal size, mineralogical change, and partial melting. Contact metamorphism is most pronounced if the intruded rocks were previously unmetamorphosed or had only been subjected to low-grade regional metamorphism. In the study of such contact aureoles, most researchers have used isograd mapping as the preferred tool to characterise of the metamorphic (= thermal) structure. CONTACT METAMORPHISM: Contact metamorphism involves existing rocks coming into contact with intense heat. The resultant marbles and quartzites do not differ significantly with respect to the structure and composition from those generated in high-degree regional metamorphism (Section 6.5.2). The local metamorphism caused by igneous intrusion can be called either thermal metamorphism (see Pottery Making—An Analog for Thermal Metamorphism), to emphasize that it develops in response to heat without a change in pressure and without differential stress, or contact metamorphism, to emphasize that it develops adjacent to the contact of an intrusion with its wall rock. On the other hand, thermal perturbations may cause bouyancy driven fluid circulation. 9. In some areas, e.g., Montana, burning of coal has produced layers of brick-red, very fine-grained rocks at the contacts with the coal. The total organic carbon data are another interesting point to be considered. A typical rock type produced by dynamic metamorphism is a fine-grained, banded rock termed a mylonite. The type and intensity of the metamorphism and the width of the metamorphic aureole will depend on a number of factors, including country rock type, intrusion body temperature, and body size. Contact metamorphism can either happen deep underground or at the Earth's surface. The need for stability may cause the structure of minerals to rearrange and form new minerals. Contact metamorphism takes place around magma bodies in the crust, which are also most common above convergent boundaries. Now, as we mentioned earlier, there are two types of metamorphism: contact metamorphism and regional metamorphism. Fluid may transport dissolved species and isotopes. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. Flashcards. There is a systematic change in the mineralogy of the rocks with increasing depth of burial; the metamorphic facies range from zeolite to prehnite–pumpellyite (Fig. The rocks of contact metamorphism are hornfels created from clay and pelite sediments and/or tuffs. A large intrusion will contain more thermal energy and cool much slower than a small one, thus providing metamorphism with a longer time and more heat. Metamorphic contact rocks, also known as horns, are often fine-grained and do not show signs of … The skarn rocks are created from marl and clay limestone and dolomite (Table 6.1). Contact metamorphism occurs primarily as a consequence of increases in temperature when differential stress is minor. The surrounding rocks can be metamorphosed with hot solutions that originate from the magma, and such a metamorphism is called hydrothermal metamorphism. Since burning requires oxygen, combustion metamorphism takes place either at the Earth's surface or at shallow depths. The size of the aureole depends on the temperature difference between the rocks of the wall and the intrusion heat. Contact metamorphism. If the surrounding rocks are metamorphosed by hot water vapor, gases, or pneumatic gas released from magma or lava, it is pneumatolytic metamorphism. Once a significant occurrence of mineralization has been discovered the key is to define the geological controls on its host conduit zone and to follow this. Total Organic Carbon (TOC), Carbon and Oxygen stable isotopes for a Eocene-Miocene record of Campos Basin (Rodrigues, 2005). The heat is chiefly provided by silicate melts derived from melting of the upper mantle. In Brazil, at Campo Basin, the isotopic signature of those episodes was identified in bulk carbonates rocks samples by Rodrigues (2005) being possible to note a set of isotopic events in the lower Oligocene that are associated with specific deglaciation-glaciation events, which are globally identified being probably related to Milankovitch cycles of 400 Kyr. The type area for this type of metamorphism is near Taringatura, on the South Island of New Zealand. Typically, a regionally metamorphosed area is situated under a fold/thrust mountain range or along a boundary between tectonic plates. Any type of magma body, from a thin dyke to a large stock, can lead to metamorphism in contact. Several different clay minerals may form at the same time, but the deposits are usually temperature-zoned and discordant with regard to original structural features. This heat generally comes from lava or magma. Therefore commonly non-foliated. This type of metamorphism occurs when sedimentary and volcanic rocks are buried by deposit of sedimentary layers or by rock bodies from overriding thrust faults. Alteration is most common in extensional rather than convergent basins, due to high heat flow and hydrothermal activity. igneous intrusions) which raise the temperature of the surrounding rocks. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure \(\PageIndex{6}\)). Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. As these bodies cool they release heat to the surrounding country rock, leading to metamorphism. These minerals occurred in deposits of kilometre proportions, and are of economic interest (Henley and Ellis, 1983). The process is termed contact metamorphism and typical rocks are termed hornfels which are very fine grained metamorphic rocks resulting from rapid heating. G. Hoinkes, ... R. Schmid, in Reference Module in Earth Systems and Environmental Sciences, 2014. During this time, the rocks are subject to changes in temperature, pressure, and possibly fluid composition. Slate is foliated metamorphic rock while hornfels is non-foliated metamorphic rock. This will enable the heat to spread further into the country rock, creating a larger aureole. The even denser SiO2 polymorph stishovite (density = 4.2 g/cm3) has also been discovered at Meteor Crater. Contact alternation happens in the contact zone between the lava intrusion and peripheral rocks, and the evaporate components and hydrothermal solution precipitated in the later lava crystallization with the heat from the lava alternate the peripheral rocks. 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