Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite and siderite may be present. Marl was an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions, it describes a habit of coralline red alga. The term is today used to describe indurated marine deposits and lacustrine sediments which more should be named'marlstone'. Marlstone is an indurated rock of about the same composition as marl, more called an earthy or impure argillaceous limestone, it has a blocky subconchoidal fracture, is less fissile than shale. The term'marl' is used in English-language geology, while the terms Mergel and Seekreide are used in European references; the lower stratigraphic units of the chalk cliffs of Dover consist of a sequence of glauconitic marls followed by rhythmically banded limestone and marl layers.
Upper Cretaceous cyclic sequences in Germany and marl–opal-rich Tortonian-Messinian strata in the Sorbas basin related to multiple sea drawdown have been correlated with Milankovitch orbital forcing. Marl as lacustrine sediment is common in post-glacial lake-bed sediments found underlying peat bogs, it has been used as acid soil neutralizing agent. Marl was extensively mined in Central New Jersey as a soil conditioner in the 1800s. In 1863, the most common marl was blue marl. While the specific composition and properties of the marl varied depending on what layer it was found in, blue marl was composed of 38.70% silicic acid and sand, 30.67% oxide of iron, 13.91% carbonate of lime, 11.22% water, 4.47% potash, 1.21% magnesia, 1.14% phosphoric acid, 0.31% sulphuric acid. Marl was in high demand for farms. An example of the amount of marl mined comes from a report from 1880, from Marlboro, Monmouth County, New Jersey, which reported the following tons of marl sold during the year: OC Herbert Marl Pit – 9961 tons Uriah Smock Marl Pit – 4750 tons CM Conover Marl Pit – 760 tonsIn the Centennial Exhibition report in 1877, marl is described in many different forms and came from 69 marl pits in and around New Jersey.
The report identified a number of agricultural marls types, including clay marl, blue marl, red marl, high bank marl, shell layer marl, under shell layer marl, sand marl, green marl, gray marl, clayey marl. Agricultural lime D. Russell, J. and Kerry Kelts. 2003. Classification of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29: 141–154. Chalk of Kent by C. S. Harris Geochemistry and time-series analyses of orbitally forced Upper Cretaceous marl–limestone rhythmites, abstract Palaeoenvironmental Interpretation of the Early Postglacial Sedimentary Record of a Marl Lake
The Solovetsky Islands, or Solovki, are an archipelago located in the Onega Bay of the White Sea, Russia. As an administrative division, the islands are incorporated as Solovetsky District of Arkhangelsk Oblast, Russia. Within the framework of municipal divisions, they are incorporated as Solovetskoye Rural Settlement within Primorsky Municipal District; the administrative center of both divisions is the settlement of Solovetsky, located on Bolshoy Solovetsky Island. All of the population of the islands lives in Solovetsky; as of the 2010 Census, the population of the district was 861 inhabitants. The archipelago has a total area of 347 square kilometers and consists of six islands: Bolshoy Solovetsky Island, 246 km2 Anzersky Island, 47 km2 Bolshaya Muksalma, 17 km2 Malaya Muksalma 0.57 km2 Bolshoy Zayatsky, 1.25 km2 Maly Zayatsky, 1.02 km2 The islands separate the Onega Bay from the main volume of the White Sea. The closest mainland is the Onega Peninsula; the shores of the islands are indented.
They are formed of gneiss. The relief of the islands is hilly. Most of the Solovetsky Islands are covered with Scots Pine and Norway Spruce forests, which are swampy. There are numerous lakes. One interesting feature of these islands is stone labyrinths and other stone settings the Stone labyrinths of Bolshoi Zayatsky Island; such labyrinths were typical for Northern Europe, but most have perished and now Solovetsky Islands have some of the best remaining examples. The islands have been the setting of the famous Russian Orthodox Solovetsky Monastery complex, it was founded in the second quarter of the 15th century by two monks from the Kirillo-Belozersky Monastery. By the end of the 16th century, the abbey had emerged as one of the wealthiest landowners and most influential religious centres in Russia; the existing stronghold and its major churches were erected in stone during the early reign of Ivan the Terrible at the behest of St. Philip of Moscow. At the onset of the Schism of the Russian Church, the monks staunchly stuck to the faith of their fathers and expelled the tsar's representatives from the Solovki, precipitating the eight-year-long siege of the islands by the forces of Tsar Alexis.
Throughout the imperial period of Russian history, the monastery was renowned as a strong fortress which repelled foreign attacks during the Livonian War, Time of Troubles, the Crimean War, the Russian Civil War. In 1974, the Solovetsky Islands were designated a historical and architectural museum and a natural reserve of the Soviet Union. In 1992, they were inscribed on the World Heritage List "as an outstanding example of a monastic settlement in the inhospitable environment of northern Europe which admirably illustrates the faith and enterprise of medieval religious communities". Today, the Solovki are seen as one of the major tourist magnets in the orbit of the Russian North. After the October Revolution, the islands attained notoriety as the site of the first Soviet prison camp; the camp was inaugurated in 1921. It was closed in 1939, on the eve of World War II. By the beginning of the war, there was a naval cadet training camp for the Soviet Northern Fleet; the islands are served by the Solovki Airport.
There is regular air service to Arkhangelsk, as well as regular passenger sea connections to Arkhangelsk and Belomorsk. List of islands of Russia Архангельское областное Собрание депутатов. Областной закон №65-5-ОЗ от 23 сентября 2009 г. «Об административно-территориальном устройстве Архангельской области», в ред. Областного закона №232-13-ОЗ от 16 декабря 2014 г. «О внесении изменений в отдельные Областные Законы в сфере осуществления местного самоуправления и взаимодействия с некоммерческими организациями». Вступил в силу через десять дней со дня официального опубликования. Опубликован: "Волна", №43, 6 октября 2009 г.. Архангельское областное Собрание депутатов. Областной закон №258-внеоч.-ОЗ от 23 сентября 2004 г. «О статусе и границах территорий муниципальных образований в Архангельской области», в ред. Областного закона №224-13-ОЗ от 16 декабря 2014 г. «Об упразднении отдельных населённых пунктов Соловецкого района Архангельской области и о внесении изменения в статью 46 Областного закона "О статусе и границах территорий муниципальных образований в Архангельской области"».
Вступил в силу со дня официального опубликования. Опубликован: "Волна", №38, 8 октября 2004 г.. Brumfield, William. Solovki: Architectural Heritage in Photographs. ISBN 978-5-94607-052-2. OCLC 255613915. In English a
Volcanoes of Kamchatka
The volcanoes of Kamchatka are a group of volcanoes situated on the Kamchatka Peninsula, in eastern Russia. The Kamchatka River and the surrounding central side valley are flanked by large volcanic belts containing around 160 volcanoes, 29 of them active; the peninsula has a high density of volcanoes and associated volcanic phenomena, with 29 active volcanoes being included in the six UNESCO World Heritage List sites in the Volcanoes of Kamchatka group, most of them on the Kamchatka Peninsula. The highest volcano is the tallest active volcano in the Northern Hemisphere. Three volcanoes are visible from Petropavlovsk-Kamchatsky: Koryaksky and Kozelsky. In the center of Kamchatka is Geyser Valley, destroyed by a massive mudslide in June 2007. Owing to the Kuril-Kamchatka Trench, deep-focus seismic events and tsunamis are common. A pair of megathrust earthquakes occurred off the coast on October 16, 1737, on November 4, 1952, in the magnitude of ~9.3 and 8.2 respectively. A chain of more shallow earthquakes were recorded as as April 2006.
Volcanoes of the central rangeKluchevskaya group Shiveluch, 3307 m Klyuchevskaya Sopka Kronotsky Avachinskaya group Aag Arik Koryaksky Avachinsky Kozelsky Ksudach Ilyinsky Kambalny Karymsky Kamchatka's most active and continuously erupting volcanoOut of sequence: Tolbachik Komarov Zhupanovsky List of volcanoes in Russia Holocene Volcanoes in Kamchatka, Institute of Volcanology and Seismology Kamchatka, Russia
The Lena is the easternmost of the three great Siberian rivers that flow into the Arctic Ocean. With a mean annual discharge of 588 cubic kilometers per year, it is the second largest of the Arctic rivers, it is the largest river whose catchment is within the Russian territorial boundaries. Permafrost underlies most of the catchment, with 77% of the catchment containing continuous permafrost. Originating at an elevation of 1,640 meters at its source in the Baikal Mountains south of the Central Siberian Plateau, 7 kilometres west of Lake Baikal, the Lena flows northeast, being joined by the Kirenga River, Vitim River and Olyokma River. From Yakutsk it enters the lowlands and flows north until joined by its right-hand tributary the Aldan River and its most important left-hand tributary, the Vilyuy River. After that, it bends westward, flowing alongside the Verkhoyansk Range and making its way nearly due north to the Laptev Sea, a division of the Arctic Ocean, emptying south-west of the New Siberian Islands by the Lena Delta – 30,000 square kilometres in area, traversed by seven principal branches, the most important being the Bykovsky channel, farthest east.
The area of the Lena river basin is calculated at 2,490,000 square kilometres and the mean annual discharge is 588 cubic kilometers per year. Gold is washed out of the sands of the Vitim and the Olyokma, mammoth tusks have been dug out of the delta; the Kirenga River flows north between the upper Lena Lake Baikal. The Vitim River drains the area northeast of Lake Baikal; the Olyokma River flows north. The Amga River flows into the Aldan; the Aldan River flows into the Lena north of Yakutsk. The Maya River, a tributary of the Aldan, drains an area to the Sea of Okhotsk; the T-shaped Chona-Vilyuy River system drains most of the area to the west. It is believed that the Lena derives its name from the original Even-Evenk name Elyu-Ene, which means "the Large River". According to folktales related a century in the years 1620–1623 a party of Russian fur hunters under the leadership of Demid Pyanda sailed up Lower Tunguska, discovered the Lena, either carried their boats there or built new ones. In 1623 Pyanda explored some 2,400 kilometres of the river from its upper reaches to the central Yakutia.
In 1628 Vasily Bugor and 10 men reached the Lena, collected'yasak' from the'natives' and founded Kirinsk in 1632. In 1631 the voyevoda of Yeniseisk sent 20 men to construct an ostrog at Yakutsk. From Yakutsk other expeditions spread out to the south and east; the Lena delta was reached in 1633. Baron Eduard Von Toll, accompanied by Alexander von Bunge, led an expedition that explored the Lena delta and the islands of New Siberia on behalf of the Russian Imperial Academy of Sciences in 1885. In 1886 they investigated the Yana River and its tributaries. During one year and two days the expedition covered 25,000 kilometres, of which 4,200 kilometres were up rivers, carrying out geodesic surveys en route; the Lena massacre was the name given to the 1912 shooting-down of striking goldminers and local citizens who protested at the working conditions in the mine near Bodaybo in northern Irkutsk. The incident was reported in the Duma by Kerensky and is credited with stimulating revolutionary feeling in Russia.
Vladimir Ilyich Ulyanov may have taken his alias, from the river Lena, when he was exiled to the Central Siberian Plateau. At the end of the Lena River there is a large delta that extends 100 kilometres into the Laptev Sea and is about 400 km wide; the delta is frozen tundra for about seven months of the year, but in May the region is transformed into a lush wetland for the next few months. Part of the area is protected as the Lena Delta Wildlife Reserve; the Lena delta divides into a multitude of flat islands. The most important are: Chychas Aryta, Sagastyr, Samakh Ary Diyete, Turkan Bel'keydere, Sasyllakh Ary, Kolkhoztakh Bel'keydere, Grigoriy Diyelyakh Bel'kee, Nerpa Uolun Aryta, Misha Bel'keydere, Atakhtay Bel'kedere, Urdiuk Pastakh Bel'key, Agys Past' Aryta, Dallalakh Island, Otto Ary, Ullakhan Ary and Orto Ues Aryta. Turukannakh-Kumaga is a narrow island off the Lena delta's western shore. One of the Lena delta islands, Ostrov Amerika-Kuba-Aryta or Ostrov Kuba-Aryta, was named after the island of Cuba during Soviet times.
It is on the northern edge of the delta. Alexander von Bunge & Baron Eduard Von Toll, The Expedition to the New Siberian Islands and the Yana country, equipped by the Imperial Academy of Sciences. Lena Pillars List of rivers of Russia List of longest undammed rivers William Barr, writer of The First Soviet Convoy to the Mouth of the Lena; this article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed.. "Lena". Encyclopædia Britannica. 16. Cambridge University Press. Arctic Great Rivers Observatory NASA Earth Observatory page on flooding on the Lena River Information and a map of the Lena's watershed Permafrost in the Lena Delta Alfred Wegner institute Publications, Berichte zur Polar- und Meeresforschung - free, downloadable research reports on the biology, oceanography, paleontology, fauna, soils, so forth of the Lena Delta, Laptev
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
Yakutsk is the capital city of the Sakha Republic, located about 450 kilometers south of the Arctic Circle. Yakutsk, with an average temperature of −8.8 °C, is the second coldest city with more than 100,000 inhabitants in the world after Norilsk, although Yakutsk experiences colder temperatures in the winter. Yakutsk is the largest city located in continuous permafrost and one of the largest that cannot be reached by road. Yakutsk is a major port on the Lena River, it is served by the Yakutsk Airport as well as the smaller Magan Airport. The Yakuts known as the Sakha people, migrated to the area during the 13th and 14th centuries from other parts of Siberia; when they arrived they mixed with other indigenous Siberians in the area. The Russian settlement of Yakutsk was founded in 1632 as an ostrog by Pyotr Beketov. In 1639, it became the center of a voyevodstvo; the Voyevoda of Yakutsk soon became the most important Russian official in the region and directed expansion to the east and south. With an extreme subarctic climate, Yakutsk has the coldest winter temperatures for any major city on Earth.
Average monthly temperatures in Yakutsk range from +19.5 °C in July to −38.6 °C in January, only Norilsk has a lower mean annual temperature than any other settlement of over 100,000. Yakutsk is the largest city built on continuous permafrost, many houses there are built on concrete piles; the lowest temperatures recorded on the planet outside Antarctica occurred in the basin of the Yana River to the northeast of Yakutsk, making it the coldest major city in the world. Although winters are cold and long – Yakutsk has never recorded a temperature above freezing between 10 November and 14 March inclusive – summers are warm, with daily maximum temperatures exceeding +30 °C, making the seasonal temperature differences for the region the greatest in the world at 105 °C; the lowest temperature recorded in Yakutsk was −64.4 °C on 5 February 1891 and the highest temperatures +38.4 °C on 17 July 2011 and +38.3 °C on 15 July 1943. The hottest month in records going back to 1834 has been July 1894, with a mean of +23.2 °C, the coldest, January 1900, which averaged −51.2 °C.
Yakutsk has a distinct inland location, being 1,000 kilometres from the Pacific Ocean, which coupled with the high latitude means exposure to severe winters and lack of temperature moderation. July temperatures soar to an above-normal average for this parallel, with the average being several degrees hotter than such more southerly Far East cities as Vladivostok or Yuzhno-Sakhalinsk; the July daytime temperatures are hotter than some marine subtropical areas. The warm summers ensure; the climate is quite dry, with most of the annual precipitation occurring in the warmest months, due to the intense Siberian High forming around the cold continental air during the winter. However, summer precipitation is not heavy since the moist southeasterly winds from the Pacific Ocean lose their moisture over the coastal mountains well before reaching the Lena valley. With the Lena River navigable in the summer, there are various boat cruises offered, including upriver to the Lena Pillars, downriver tours which visit spectacular scenery in the lower reaches and the Lena delta.
Yakutia Airlines has its head office in the city. There are several theaters in Yakutsk: the State Russian Drama Theater, named after A. S. Pushkin. There are a number of museums as well: the National Fine Arts Museum of Sakha; the annual Ysyakh summer festival takes place the last weekend in June. The traditional Yakut summer solstice festivities include a celebration of the revival and renewal of the nature and beginning of a new year, it is accompanied by national Yakut rituals and ceremonies, folk dancing, horse racing, Yakut ethnic music and singing, national cuisine, competitions in traditional Yakut sports. There is a local punk scene in Yakutsk, with many bands. Shows can bring up to 300 people, young but older too. Yakutsk is the capital of the Sakha Republic; as an inhabited locality, Yakutsk is classified as a city under republic jurisdiction. Within the framework of administrative divisions, it is, together with the settlement of Zhatay and eleven rural localities, incorporated as the city of republic significance of Yakutsk—an administrative unit with a status equal to that of the districts.
As a municipal division and the eleven rural localities are incorporated as Yakutsk Urban Okrug. The settlement of Zhatay is not a part of Yakutsk Urban Okrug and is independently incorporated as Zhatay Urban Okrug. Divisional source:Population source:*Administrative centers are shown in bold Yakutsk is a destination of the Lena Highway; the city's connection to that highway is only usable by ferry in the summer, or in the dead of winter, by driving directly over the frozen Lena River, since Yakutsk lies on its western bank, there is no bridge anywhere in the Sakha Republic that crosses the Lena. The river is impassable for long periods of the year when it contains loose ice, when the ice cover is not thick enough to support traffic, or when the water level is too high and the river is turbulent with spring f
Metamorphism is the change of minerals or geologic texture in pre-existing rocks, without the protolith melting into liquid magma. The change occurs due to heat and the introduction of chemically active fluids; the chemical components and crystal structures of the minerals making up the rock may change though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism occurs between diagenesis, melting; the geologists who study metamorphism are known as "metamorphic petrologists." To determine the processes underlying metamorphism, they rely on statistical mechanics and experimental petrology. Three types of metamorphism exist: contact and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism. Metamorphic rocks can change without melting. Heat causes atomic bonds to break, the atoms move and form new bonds with other atoms, creating new minerals with different chemical components or crystalline structures, or enabling recrystallization.
When pressure is applied, somewhat flattened grains that orient in the same direction have a more stable configuration. The temperature lower limit on what is considered to be a metamorphic process is considered to be 100 – 200 °C; the upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is 700 – 900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not segregated from the refractory residue. Since the 1980s it has been recognized that rocks are dry enough and of a refractory enough composition to record without melting "ultra-high" metamorphic temperatures of 900 – 1100 °C; the metamorphic process has to be over pressure of at least 100 mega pascals but below 300 mega pascals, the depth of 100 mega pascals varies depending on what type of rock is applying pressure. Regional or Barrovian metamorphism covers large areas of continental crust associated with mountain ranges those associated with convergent tectonic plates or the roots of eroded mountains.
Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism; these orogenic mountains are eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone produce regional metamorphic effects, characterized by paired metamorphic belts; the techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane. Contact metamorphism occurs around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock; the area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole.
Contact metamorphic rocks are known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are fine-grained. Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact; the size of the aureole depends on the heat of the intrusion, its size, the temperature difference with the wall rocks. Dikes have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have thick and well-developed contact metamorphism; the metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is related to the metamorphic temperatures of pelitic or aluminosilicate rocks and the minerals they form; the metamorphic grades of aureoles are sillimanite hornfels, pyroxene hornfels. Magmatic fluids coming from the intrusive rock may take part in the metamorphic reactions. An extensive addition of magmatic fluids can modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism.
If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest. A special type of contact metamorphism, associated with fossil fuel fires, is known as pyrometamorphism. Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition; the difference in composition between an existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic, circulating ocean water. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas