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39-611: The Pine Valley Mountains formed from the Pine Valley Laccolith, the largest laccolith in the United States and perhaps the world. The laccolith was formed during a 20 million-year period of volcanic activity. After 4-5,000 feet of volcanic deposited on the Claron Formation, the magma vents were effectively sealed off. A final surge of magma, unable to find its way to the surface, pushed sideways along

78-466: A conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apart the host rock strata . The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith its dome-like form. Over time, erosion can expose the solidified laccolith, which is typically more resistant to weathering than

117-557: A fingered laccolith is the Shonkin Sag laccolith in Montana , US. The critical radius for the sill to laccolith transition is now thought to be affected the viscosity of the magma (being greater for less viscous magma) as well as the strength of the host rock. A modern formula for the shape of a laccolith is: where z {\displaystyle z} is the height of the laccolith roof, g {\displaystyle g}

156-407: A period of crystallization that may be very distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Later crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals. The former case

195-402: A vertical feeder dike that ends in a laccolith. Sheet intrusions tend to form perpendicular to the direction of least stress in the country rock they intrude. Thus laccoliths are characteristic of regions where the crust is being compressed and the direction of least stress is vertical, while areas where the crust is in tension are more likely to form dikes, since the direction of least stress

234-533: Is also visible in outcrops on this exposed laccolith. In Big Bend Ranch State Park , at the southwesternmost visible extent of the Ouachita orogeny , lies the Solitario . It consists of the eroded remains of a laccolith, presumably named for the sense of solitude that observers within the structure might have, due to the partial illusion of endless expanse in all directions. One of the largest laccoliths in

273-499: Is common in lavas but very rare in plutonic rocks. Muscovite is confined to intrusions. These differences show the influence of the physical conditions under which crystallization takes place. Hypabyssal rocks show structures intermediate between those of extrusive and plutonic rocks. They are very commonly porphyritic, vitreous , and sometimes even vesicular . In fact, many of them are petrologically indistinguishable from lavas of similar composition. Plutonic rocks form 7% of

312-408: Is described as miarolitic texture . Because their crystals are of roughly equal size, intrusive rocks are said to be equigranular . Plutonic rocks are less likely than volcanic rocks to show a pronounced porphyritic texture, in which a first generation of large well-shaped crystals are embedded in a fine-grained ground-mass. The minerals of each have formed in a definite order, and each has had

351-429: Is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions , such as batholiths , dikes , sills , laccoliths , and volcanic necks . Intrusion is one of the two ways igneous rock can form. The other is extrusion , such as a volcanic eruption or similar event. An intrusion is any body of intrusive igneous rock, formed from magma that cools and solidifies within

390-529: Is named and in which the town of Pine Valley is situated, was formed when lava dammed off the Santa Clara River and formed a lake. Sediments eventually filled the lake until they reached the height of the lava dam. These sediments form the floor of the present day Pine Valley, Grass Valley, and Grassy Flat. The mountains straddle the divide between the Great Basin watershed and the watershed of

429-401: Is said to be idiomorphic (or automorphic ); the latter is xenomorphic . There are also many other characteristics that serve to distinguish plutonic from volcanic rock. For example, the alkali feldspar in plutonic rocks is typically orthoclase , while the higher-temperature polymorph, sanidine , is more common in volcanic rock. The same distinction holds for nepheline varieties. Leucite

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468-420: Is the acceleration of gravity, B {\displaystyle B} is the elastic modulus of the host rock, r {\displaystyle r} is the horizontal distance from the center of the laccolith, and r 0 {\displaystyle r_{0}} is the outer radius of the laccolith. Because of their greater thickness, which slows the cooling rate, the rock of laccoliths

507-605: Is the thickness of the overlying rocks, and τ {\displaystyle \tau } is the shear strength of the overlying rock. For example, in the Henry Mountains of Utah , US, the geologist Grove Karl Gilbert found in 1877 that sills were always less than 1 square kilometer (0.4 sq mi) in area while laccoliths were always greater than 1 square kilometer in area. From this, Gilbert concluded that sills were forerunners of laccoliths. Laccoliths formed from sills only when they became large enough for

546-510: Is then horizontal. For example, the laccoliths of the Ortiz porphyry belt in New Mexico likely formed during Laramide compression of the region 33 to 36 million years ago. When Laramide compression was later replaced by extension, emplacement of sills and laccoliths was replaced by emplacement of dikes. Dating of the intrusions has helped determine the point in geologic time when compression

585-408: Is usually coarser-grained than the rock of sills. The growth of laccoliths can take as little as a few months when associated with a single magma injection event, or up to hundreds or thousands of years by multiple magmatic pulses stacking sills on top of each other and deforming the host rock incrementally. Over time, erosion can form small hills and even mountains around a central peak since

624-401: The Earth's crust but cools and solidifies before reaching the surface. Laccoliths are distinguished from other igneous intrusions by their dome-shaped upper surface and level base. They are assumed to be fed by a conduit from below, though this is rarely exposed. When the host rock is volcanic, the laccolith is referred to as a cryptodome . Laccoliths form only at relatively shallow depth in

663-676: The Pine Valley Mountain Wilderness , which at 50,232 acres (20,328 ha) is the fourth largest wilderness in Utah. The 2,643 acres (1,070 ha) Cottonwood Forest Wilderness is located at the southeastern end of the range. The Pine Valley Mountains support spruce-fir forests , including a large stand of virgin Engelmann spruce . The mountains also have numerous meadows up to 50 acres (20 ha) in size. Several threatened, endangered, and sensitive species live in

702-1110: The QAPF diagram . Dioritic and gabbroic rocks are further distinguished by whether the plagioclase they contain is sodium -rich, and sodium-poor gabbros are classified by their relative contents of various iron - or magnesium -rich minerals ( mafic minerals) such as olivine , hornblende , clinopyroxene , and orthopyroxene, which are the most common mafic minerals in intrusive rock. Rare ultramafic rocks , which contain more than 90% mafic minerals, and carbonatite rocks, containing over 50% carbonate minerals, have their own special classifications. Hypabyssal rocks resemble volcanic rocks more than they resemble plutonic rocks, being nearly as fine-grained, and are usually assigned volcanic rock names. However, dikes of basaltic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as diabases or dolerites. Rare ultramafic hypabyssal rocks called lamprophyres have their own classification scheme. Intrusive rocks are characterized by large crystal sizes, and as

741-717: The Virgin River , a tributary of the Colorado River . The Chinamen's Canal tunnel at the north end of Grass Valley diverts the waters of Mill Canyon Creek from the Colorado River drainage system into the Great Basin drainage system, eventually finding its way into the Newcastle Reservoir via Pinto Creek. Zion National Park can be seen to the east from the mountains. Part of the range is in

780-517: The Earth are called abyssal or plutonic while those that form near the surface are called subvolcanic or hypabyssal . Plutonic rocks are classified separately from extrusive igneous rocks, generally on the basis of their mineral content. The relative amounts of quartz , alkali feldspar , plagioclase , and feldspathoid are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in

819-831: The United States is Pine Valley Mountain in the Pine Valley Mountain Wilderness area near St. George, Utah . A system of laccoliths is exposed on the Italian island of Elba , which form a "Christmas tree" laccolith system in which a single igneous plumbing system has produced multiple laccoliths at different levels in the crust. The original shape of intrusions can be difficult to reconstruct. For instance, Devils Tower in Wyoming and Needle Rock in Colorado were both thought to be volcanic necks, but further study has suggested they are eroded laccoliths. At Devils Tower , intrusion would have had to cool very slowly so as to form

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858-639: The classic shapes of laccoliths. There are many examples of possible laccoliths on the surface of the Moon . Some are centered in impact craters and may form as part of the post-impact evolution of the crater. Others are located along possible faults or fissures. Laccoliths on the Moon are much wider but less thick than those on Earth, due to the Moon's lower gravity and more fluid magmatism. Possible laccoliths have also been identified on Mars , in western Arcadia Planitia . Intrusive rock Intrusive rock

897-481: The crust in dikes and sills). Because the solid country rock into which magma intrudes is an excellent insulator, cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained ( phaneritic ). However, the rate of cooling is greatest for intrusions at relatively shallow depth, and the rock in such intrusions is often much less coarse-grained than intrusive rock formed at greater depth. Coarse-grained intrusive igneous rocks that form at depth within

936-455: The crust of the planet . In contrast, an extrusion consists of extrusive rock, formed above the surface of the crust. Some geologists use the term plutonic rock synonymously with intrusive rock, but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock (typically formed deeper in the Earth's crust in batholiths or stocks ) and medium-grained subvolcanic or hypabyssal rock (typically formed higher in

975-423: The crust, usually from intermediate composition magma, though laccoliths of all compositions from silica -poor basalt to silica-rich rhyolite are known. A laccolith forms after an initial sheet-like intrusion has been injected between layers of sedimentary rock . If the intrusion remains limited in size, it forms a sill , in which the strata above and below the intrusion remain parallel to each other and

1014-473: The details. Both sills and laccoliths have blunt rather than wedgelike edges, and sills of the Henry Mountains are typically up to 10 meters (33 ft) thick while laccoliths are up to 200 meters (660 ft) thick. The periphery of a laccolith may be smooth, but it may also have fingerlike projections consistent with Rayleigh-Taylor instability of the magma pushing along the strata. An example of

1053-545: The headwaters of Leap Creek north of the Browse Guard Station. After erosion exposed the laccolith, volcanic activity continued and the youngest flows are 1-1.6 million years old. Many volcanic cinder cones can still be seen in the foothills of the Pine Valley Mountains and these have been dated at around 20,000 years old. As a result of these lava flows, the valley for which the mountain range

1092-464: The host rock. The exposed laccolith then forms a hill or mountain. The Henry Mountains of Utah , US, are an example of a mountain range composed of exposed laccoliths. It was here that geologist Grove Karl Gilbert carried out pioneering field work on this type of intrusion . Laccolith mountains have since been identified in many other parts of the world. A laccolith is a type of igneous intrusion , formed when magma forces its way upwards through

1131-448: The individual crystals are visible, the rock is called phaneritic . There are few indications of flow in intrusive rocks, since their texture and structure mostly develops in the final stages of crystallization, when flow has ended. Contained gases cannot escape through the overlying strata, and these gases sometimes form cavities , often lined with large, well-shaped crystals. These are particularly common in granites and their presence

1170-437: The intrusion remains sheetlike. The intrusion begins to lift and dome the overlying strata only if the radius of the intrusion exceeds a critical radius, which is roughly: where P m {\displaystyle P_{m}} is the pressure of the magma, P l {\displaystyle P_{l}} is the lithostatic pressure (weight of the overlying rock), T {\displaystyle T}

1209-463: The intrusive rock is usually more resistant to weathering than the host rock. Because the emplacement of the laccolith domes up the overlying beds, local topographic relief is increased and erosion is accelerated, so that the overlying beds are eroded away to expose the intrusive cores. The term was first applied as laccolite by Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875. The word laccolith

Pine Valley Mountains - Misplaced Pages Continue

1248-410: The lodgepole pine ( Pinus contorta Douglas) is missing. There are numerous opportunities for recreation in the mountains, including hiking, rock climbing, camping, wildlife watching, fishing, hunting, horseback riding, and boating on a reservoir. Download coordinates as: Laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by

1287-777: The mountains, including Bonneville cutthroat trout , Townsend's big-eared bat , and the pygmy rabbit , among others. Botanically speaking, Pine Valley Mountain is the most diverse mountain range in Utah. A floristic survey was conducted between 1985 and 1987 and 967 species were collected. Since then, additional species have been added bringing the total to nearly 1,000. Three species new to Utah were identified including bitter cherry ( Prunus emarginata (Dougl.) Walpers), common sandweed ( Athysanus pusillus (Hook.) Greene), and Lemmon's onion ( Allium lemmonii S. Wats.). Nineteen ferns and fern allies are present, 18 gymnosperms (conifers and Mormon teas), 150 species of monocots (including 86 grasses, 10 orchids, and 2 agaves, among others), with

1326-408: The pressure of the magma to force the overlying strata to dome upwards. Gilbert also determined that larger laccoliths formed at greater depth. Both laccoliths and sills are classified as concordant intrusions, since the bulk of the intrusion does not cut across host rock strata, but intrudes between strata. More recent study of laccoliths has confirmed Gilbert's basic conclusions, while refining

1365-473: The remainder being dicots. The three largest families represented in the flora of Pine Valley Mountain include the Asteraceae with 160 species, Poaceae with 85 species, and Fabaceae with 63. The three largest genera and number of species collected are Eriogonum , 23, Cryptantha , 20, and Astragalus , 20. Some of the finds include: All but one of Utah's conifers grow in the Pine Valley Mountains. Only

1404-490: The slender pencil-shaped columns of phonolite porphyry seen today. However, erosion has stripped away the overlying and surrounding rock, so it is impossible to reconstruct the original shape of the igneous intrusion, which may or may not be the remnant of a laccolith. At other localities, such as in the Henry Mountains and other isolated mountain ranges of the Colorado Plateau , some intrusions demonstrably have

1443-531: The weak seam between the Claron and the overlying volcanic layers. This injected a 3,000 foot-thick layer of monzonite porphyry to form the Pine Valley laccolith. The contact between the top of the Claron and the bottom of the laccolith can be seen in several locations, most notably near the headwaters of Cottonwood Creek along the southwest corner of the mountain range (see photo below in the gallery), and at

1482-517: Was derived in 1875–1880, from Greek lákko(s) 'pond' plus -lith 'stone'. Laccoliths tend to form at relatively shallow depths and in some cases are formed by relatively viscous magmas, such as those that crystallize to diorite , granodiorite , and granite . In those cases cooling underground may take place slowly, giving time for larger crystals to form in the cooling magma. In other cases less viscous magma such as shonkinite may form phenocrysts of augite at depth, then inject through

1521-739: Was replaced with extension. In addition to the Henry Mountains, laccolith mountains are found on the nearby Colorado Plateau in the La Sal Mountains and Abajo Mountains . The filled and solidified magma chamber of Torres del Paine ( Patagonia ) is one of the best exposed laccoliths, built up incrementally by horizontal granitic and mafic magma intrusions over 162 ± 11 thousand years. Horizontal sheeted intrusions were fed by vertical intrusions. The small Barber Hill syenite -stock laccolith in Charlotte, Vermont , has several volcanic trachyte dikes associated with it. Molybdenite

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