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43-573: Mount Olympus is formed of ultramafic rock, dominantly serpentinized harzburgite which is part of the Troodos ophiolite . The Mount Olympus Ski resort consists of the Sun Valley and North Face areas. Each area has its own ski lifts and runs, operated by the Cyprus Ski Club: In the Sun Valley area there is a 55m rope tow baby lift as well. Writing in the late first century BC or first century AD,

86-438: A central volcano or intrusion. Though they appear to originate in the central intrusion, the dikes often have a different age and composition from the intrusion. These radial swarms may have formed over the intrusion and were later cut by the rising body of magma, or the crust was already experiencing regional tension and the intrusion triggered formation of the fissures. In rock of the oceanic crust, pillow lava erupted onto

129-460: A curtain of fire where lava erupts along the entire length of a fissure several kilometers long. However, the length of erupting fissure diminishes over time, becoming focused on a short segment of less than half a kilometer. The minimum possible width of a dike is determined by the balance between magma movement and cooling. There may be more than one injection of magma along a given fissure. When multiple injections are all of similar composition,

172-413: A dike or dyke is a sheet of rock that is formed in a fracture of a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion , either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack. A magmatic dike

215-489: A sill is a sheet intrusion that forms within and parallel to the bedding. Mafic magma (fluid magma low in silica) usually reaches the surface through fissures, forming dikes. At the shallowest depths, dikes form when magma rises into an existing fissure. In the young, shallow dikes of the Hawaiian Islands, there is no indication of forceful intrusion of magma. For example, there is little penetration of magma into

258-425: A consequence of the direction of minimum principal stress changing as the magma ascends from deep to shallow levels in the crust. An en echelon dike set may evolve into single dike with bridges connecting the formerly separate segments and horns showing former segment overlaps. In ancient dikes in deformed rock, the bridges and horns are used by geologists to determine the direction of magma flow. Where there

301-415: A different process than typical ultramafic rocks. Metamorphism of ultramafic rocks in the presence of water and/or carbon dioxide results in two main classes of metamorphic ultramafic rock; talc carbonate and serpentinite . Talc carbonation reactions occur in ultramafic rocks at lower greenschist through to granulite facies metamorphism when the rock in question is subjected to metamorphism and

344-581: A dike is much smaller than its other two dimensions, and the opposite walls are roughly parallel, so that a dike is more or less constant in thickness. The thickness of different dikes can range from a few millimeters to hundreds of meters, but is most typically from about a meter to a few tens of meters. The lateral extent can be tens of kilometers, and dikes with a thickness of a few tens of meters or more commonly extend for over 100 km. Most dikes are steeply dipping; in other words, they are oriented nearly vertically. Subsequent tectonic deformation may rotate

387-598: A line extending for 250 km. Individual segments overlap, with the overlapping portions thinner, so that the combined thickness of the two overlapped portions is about the same as the thickness of a single segment. Other examples of en echelon dikes are the Inyo dike of Long Valley, California , US; the Jagged Rocks complex, Arizona , US; and the dikes of oceanic spreading centers . Dikes range in composition from basaltic to rhyolitic , but most are basaltic. The texture

430-809: A moon of Jupiter , because heat-mapping of Io's surface found ultra-hot areas with temperatures in excess of 1,200 °C (2,190 °F). The magma immediately below these hot spots is probably about 200 °C (360 °F) hotter, based on surface-to-subsurface temperature differences observed for lava on Earth. A temperature of 1,400 °C (2,550 °F) is thought to indicate the presence of ultramafic magma. Mercury appears to have ultramafic volcanic rock. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Dike (geology) In geology ,

473-556: A result, a distinctive type of vegetation develops on these soils. Examples are the ultramafic woodlands and barrens of the Appalachian mountains and piedmont , the "wet maquis " of the New Caledonia rain forests , and the ultramafic forests of Mount Kinabalu and other peaks in Sabah , Malaysia . Vegetation is typically stunted, and sometimes includes endemic species adapted to

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516-533: A ring fracture. Magma rising into the ring fracture produces a ring dike. Good examples of ring dikes and cone sheets are found in the Ardnamurchan peninsula of Scotland. A feeder dike is a dike that acted as a conduit for magma moving from a magma chamber to a localized intrusion . For example, the Muskox intrusion in arctic Canada was fed by a large dike, with a thickness of 150 meters. A sole injection

559-560: A scarcity or absence of feldspar and quartz . Rare occurrences may include unusual surface deposits of maars of kimberlites in the diamond fields of southern Africa and other regions. Technically ultrapotassic rocks and melilitic rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO which can be considered "ultramafic". Ultrapotassic, ultramafic igneous rocks such as lamprophyre , lamproite and kimberlite are known to have reached

602-411: A shallow magma chamber. Cone sheets form when magma is injected into a shallow magma chamber, which lifts and fractures the rock beds above it. The fractures take the form of a set of concentric cones dipping at a relatively shallow angle into the magma chamber. When the caldera is subsequently emptied by explosive volcanic activity, the roof of the magma chamber collapses as a plug of rock surrounded by

645-474: Is a sheet of igneous rock that cuts across older rock beds. It is formed when magma fills a fracture in the older beds and then cools and solidifies. The dike rock is usually more resistant to weathering than the surrounding rock, so that erosion exposes the dike as a natural wall or ridge. It is from these natural walls that dikes get their name. Dikes preserve a record of the fissures through which most mafic magma (fluid magma low in silica) reaches

688-457: Is concentrated on the tip of the propagating fracture. In effect, the magma wedges apart the brittle rock in a process called hydraulic fracture . At greater depths, where the rock is hotter and less brittle, the magma forces the rock aside along brittle shear planes oriented 35 degrees to the sides of the dock. This bulldozer-like action produces a blunter dike tip. At the greatest depths, the shear planes become ductile faults, angled 45 degree from

731-408: Is rapid flow of molten magma through a fissure, the magma tends to erode the walls, either by melting the wall rock or by tearing off fragments of wall rock. This widens the fissure and increases flow. Where flow is less rapid, the magma may solidify next to the wall, narrowing the fissure and decreasing flow. This causes flow to become concentrated at a few points. At Hawaii, eruptions often begin with

774-555: Is the Mackenzie dike swarm in the Northwest Territories , Canada. Dike swarms (also called dike complexes ) are exposed in the eroded rift zones of Hawaiian volcanoes. As with most other magmatic dikes, these were fissures through which lava reached the surface. The swarms are typically 2.5 to 5 km in width, with individual dikes about a meter in width. The dike swarms extend radially out from volcano summits and parallel to

817-691: Is the P-wave velocity of the host rock (essentially, the speed of sound in the rock). This formula predicts that dikes will be longer and narrower at greater depths below the surface. The ratio of thickness to length is around 0.01 to 0.001 near the surface, but at depth it ranges from 0.001 to 0.0001. A surface dike 10 meters in thickness will extend about 3 km, while a dike of similar thickness at depth will extend about 30 km. This tendency of intruding magma to form shorter fissures at shallower depths has been put forward as an explanation of en echelon dikes. However, en echelon dikes have also been explained as

860-415: Is the thickness of the dike; b {\displaystyle b} is its lateral extent; P e x {\displaystyle P_{ex}} is the excess pressure in the magma relative to the host rock; ρ h o s t {\displaystyle \rho _{host}} is the density of the host rock; and V P {\displaystyle V_{P}}

903-483: Is typically slightly coarser than basalt erupted at the surface, forming a rock type called diabase . The grain size varies systematically across the dike, with the coarsest grains normally at the center of the dike. Dikes formed at shallow depth commonly have a glassy or fine-grained chilled margin 1 to 5 cm thick, formed where the magma was rapidly cooled by contact with the cold surrounding rock. Shallow dikes also typically show columnar jointing perpendicular to

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946-416: Is usually dense, with almost no vesicles (frozen bubbles), but vesicles may be seen in the shallowest part of a dike. When vesicles are present, they tend to form bands parallel to walls and are elongated in direction of flow. Likewise, phenocrysts (larger crystals) on the margins of the dike show an alignment in the direction of flow. In contrast to dikes, which cut across the bedding of layered rock,

989-550: The Archaean and are essentially restricted to the Neoproterozoic or earlier. Subvolcanic ultramafic rocks and dykes persist longer, but are also rare. There is evidence of ultramafic rocks elsewhere in the solar system. Examples include komatiite and picritic basalt . Komatiites can be host to ore deposits of nickel . Ultramafic tuff is extremely rare. It has a characteristic abundance of olivine or serpentine and

1032-653: The Phanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic. Many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subduction zones. Serpentine soil is a magnesium rich, calcium, potassium and phosphorus poor soil that develops on the regolith derived from ultramafic rocks. Ultramafic rocks also contain elevated amounts of chromium and nickel which may be toxic to plants. As

1075-451: The geographer Strabo reported that on one of its promontories was a temple to Aphrodite Acraea ( Ancient Greek : Ἀφροδίτης Ἀκραίας ) which means Aphrodite of the Heights, which women were forbidden to enter. This Cyprus location article is a stub . You can help Misplaced Pages by expanding it . Ultramafic Ultramafic rocks (also referred to as ultrabasic rocks , although

1118-685: The center of the dike. If the previous dike rock has cooled significantly, the subsequent injection can be characterized by fracturing of the old dike rock and the formation of chilled margins on the new injection. Sometimes dikes appear in swarms, consisting of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event. Dike swarms are almost always composed of diabase and most often are associated with flood basalts of large igneous provinces . They are characteristic of divergent plate boundaries . For example, Jurassic dike swarms in New England and Paleogene swarms in

1161-476: The center of the rift zone before abruptly dropping to very few dikes. It is likely that the number of dikes must increase with depth, reaching a typical value of 300 to 350 per kilometer at the level of the ocean floor. In some respects, these dike swarms resemble those of western Scotland associated with the flood eruptions that preceded the opening of the Atlantic Ocean. Dikes often form as radial swarms from

1204-471: The chemistry of the magma from which they crystallized. The ultramafic intrusives include the dunites , peridotites and pyroxenites . Other rare varieties include troctolite which has a greater percentage of calcic plagioclase. These grade into the anorthosites . Gabbro and norite often occur in the upper portions of the layered ultramafic sequences. Hornblendite and, rarely phlogopite , are also found. Volcanic ultramafic rocks are rare outside of

1247-438: The dike is described as a multiple dike . However, subsequent injections are sometimes quite different in composition, and then the dike is described as a composite dike . The range of compositions in a composite dike can go all the way from diabase to granite , as is observed in some dikes of Scotland and northern Ireland. After the initial formation of a dike, subsequent injections of magma are most likely to take place along

1290-415: The long axis of the volcanic shield. Sills and stocks are occasionally present in the complexes. They are abruptly truncated at the margins of summit calderas. Typically, there are about 50 to 100 dikes per kilometer at the center of the rift zone, though the density can be as high as 500 per kilometer and the dikes then make up half the volume of the rock. The density drops to 5 to 50 per kilometer away from

1333-688: The magma fractured and disintegrated the rock at its advancing tip rather than prying the rock apart. Other dikes may have formed by metasomatism , in which fluids moving along a narrow fissure changed the chemical composition of the rock closest to the fissure. There is an approximate relationship between the width of a dike and its maximum extent, expressed by the formula: 2 w 2 b = 2.25 P e x ρ h o s t V P 2 {\displaystyle {\frac {2w}{2b}}={\frac {2.25P_{ex}}{\rho _{host}V_{P}^{2}}}} Here w {\displaystyle w}

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1376-401: The margins. Here the dike rock fractures into columns as it cools and contracts. These are usually 5- to 6-sided, but 3- to 4-sided columns are also common. These are fairly uniform in size within a single dike, but range from a few centimeters to over 0.3 meters across in different dikes, tending to be thicker in wider dikes. Larger columns are likely a consequence of slower cooling. Dike rock

1419-414: The metamorphic fluid has more than 10% molar proportion of CO 2 ( carbon dioxide ). When such metamorphic fluids have less than 10% molar proportion of CO 2 , reactions favor serpentinisation, resulting in chlorite - serpentine - amphibole type assemblages. The majority of ultramafic rocks are exposed in orogenic belts, and predominate in Archaean and Proterozoic terranes. Ultramafic magmas in

1462-431: The rock, always opening a path along a plane normal to the minimum principal stress . This is the direction in which the crust is under the weakest compression and so requires the least work to fracture. At shallow depths, where the rock is brittle, the pressurized magma progressively fractures the rock as it advances upwards. Even if the magma is only slightly pressurized compared with the surrounding rock, tremendous stress

1505-429: The sea floor is underlain by sheeted dike complexes that preserve the conduits through which magma reached the ocean floor at mid-ocean ridges . These sheeted dikes characteristically show a chilled margin on only one side, indicating that each dike was split in half by a subsequent eruption of magma. Ring dikes and cone sheets are special types of dikes associated with caldera volcanism. These are distributed around

1548-527: The sequence of strata through which the dike propagates so that the dike becomes horizontal. It is common for a set of dikes, each a few kilometers long, to form en echelon . This pattern is seen in the Higganum dike set of New England. This dike set consists of individual dikes that are typically four kilometers in length at the surface and up to 60 meters wide. These short segments form longer groups extending for around 10 km. The entire set of dikes forms

1591-560: The sides of the dike. At depths where the rock is completely plastic, a diapir (a rising plug of magma) forms instead of a dike. The walls of dikes often fit closely back together, providing strong evidence that the dike formed by dilatation of a fissure. However, a few large dikes, such as the 120-meter-thick Medford dike in Maine, US, or the 500-meter-thick Gardar dike in Greenland, show no dilatation. These may have formed by stoping , in which

1634-417: The soils. Often thick, magnesite - calcrete caprock , laterite and duricrust forms over ultramafic rocks in tropical and subtropical environments. Particular floral assemblages associated with highly nickeliferous ultramafic rocks are indicative tools for mineral exploration . Weathered ultramafic rocks may form lateritic nickel ore deposits . Ultramafic lava may have been detected on Io ,

1677-950: The surface of the Earth. Although no modern eruptions have been observed, analogues are preserved. Most of these rocks occur as dikes , diatremes , lopoliths or laccoliths , and very rarely, intrusions. Most kimberlite and lamproite occurrences occur as volcanic and subvolcanic diatremes and maars ; lavas are virtually unknown. Vents of Proterozoic lamproite ( Argyle diamond mine ), and Cenozoic lamproite ( Gaussberg , Antarctica ) are known, as are vents of Devonian lamprophyre ( Scotland ). Kimberlite pipes in Canada, Russia and South Africa have incompletely preserved tephra and agglomerate facies . These are generally diatreme events and as such are not lava flows although tephra and ash deposits are partially preserved. These represent low- volume volatile melts and attain their ultramafic chemistry via

1720-417: The surface. They are studied by geologists for the clues they provide on volcanic plumbing systems . They also record ancient episodes of extension of the Earth's crust , since large numbers of dikes ( dike swarms ) are formed when the crust is pulled apart by tectonic forces. The dikes show the direction of extension, since they form at right angles to the direction of maximum extension. The thickness of

1763-780: The terms are not wholly equivalent) are igneous and meta -igneous rocks with a very low silica content (less than 45%), generally >18% MgO , high FeO , low potassium , and are usually composed of greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks . Intrusive ultramafic rocks are often found in large, layered ultramafic intrusions where differentiated rock types often occur in layers. Such cumulate rock types do not represent

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1806-400: The walls of dikes even when the walls consist of highly porous volcanic clinker, and little wall material breaks off into the molten magma. These fissures likely open as a result of bulging of the rock beds above a magma chamber that is being filled with magma from deeper in the crust. However, open fractures can exist only near the surface. Magma deeper in the crust must force its way through

1849-488: The west of Scotland and running into northern England record the early opening of the Atlantic Ocean. Dike swarms are forming in the present day along the divergent plate boundary running through Iceland. Dike swarms often have a great cumulative thickness: Dikes in Iceland average 3 to 5 meters in width, but one 53-kilometer stretch of coast has about 1000 dikes with total thickness of 3 kilometers. The world's largest dike swarm

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