{"id":3783,"date":"2020-01-12T01:20:04","date_gmt":"2020-01-12T01:20:04","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/?post_type=chapter&p=3783"},"modified":"2020-07-07T15:06:49","modified_gmt":"2020-07-07T15:06:49","slug":"mountains","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/chapter\/mountains\/","title":{"raw":"MOUNTAINS","rendered":"MOUNTAINS"},"content":{"raw":"

Mountains<\/h1>\r\n
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How do plate motions create mountains?<\/h3>\r\n\"A\r\n

Plate tectonic processes create some of the world's most beautiful places. The North Cascades Mountains in Washington State are a continental volcanic arc. The mountains currently host some glaciers and there are many features left by the more abundant ice age glaciers. Changes in altitude make the range a habitable place for many living organisms.<\/p>\r\n\r\n<\/div>\r\n

Converging Plates<\/h1>\r\nConverging plates create the world's largest mountain ranges. Each combination of plate types\u2014continent-continent, continent-ocean, and ocean-ocean\u2014creates mountains.\r\n

Converging Continental Plates<\/h4>\r\nSome of the biggest mountain ranges in the world are created by the convergence of continental plates.\u00a0 The collision of the Indian sub-continent with Asia began with the subduction intervening oceanic crust beneath Asia, and significant generation of magma and \"arc-volcanism\" and volcanic mountains (much like today's Cascade and Andes mountains), as per the section below (\"subducting oceanic plates\").\u00a0 After the oceanic crust that sat between India and Asia was subducted, then India literally rammed into Asia and was pushed beneath.\r\nOceanic crust is easier to subduct than continental crust, largely because it is denser.\r\nSo when the continental crust of India was thrust beneath Asia, huge stresses developed from the collision and the subducted continental crust was buoyant enough to push the overlying mountains even higher!\r\n\r\nSee Figure 5, below.\r\n\r\nThis sort of collisional event and uplift will cause folds, reverse faults, and thrust faults, effectively raising and shortening the crust.\r\n\r\nAs noted previously there is currently no mountain range of this type in the western U.S., but at some point in the history of the Appalachian mountains (east part of U.S.), continental collision was involved and very likely quite high mountains!\r\n\r\n[caption id=\"attachment_1995\" align=\"aligncenter\" width=\"481\"]\"The Figure 5. (a) The world\u2019s highest mountain range, the Himalayas, is growing from the collision between the Indian and the Eurasian plates. (b) The crumpling of the Indian and Eurasian plates of continental crust creates the Himalayas.<\/span>[\/caption]\r\n

Subducting Oceanic and Continental Plates<\/h3>\r\n[caption id=\"attachment_1996\" align=\"alignright\" width=\"300\"]\"The Figure 6. The Andes Mountains are a chain of continental arc volcanoes that build up as the Nazca Plate subducts beneath the South American Plate.<\/span>[\/caption]\r\n\r\nAs with the pre-continent collision of India and Asia, oceanic crust can be subducted beneath continental crust.\u00a0 This, of course, is a subduction zone, and the ocean crust almost always is the subducted material, since it has a higher density.\r\nSubduction of oceanic lithosphere beneath continental lithosphere,at convergent plate boundaries, also builds major mountain ranges.\r\nExamples of this would include the Andes of South America (see Figure 6) and the Cascades of the U.S. Pacific Northwest region.\r\nThese mountains form as a result of both rising magma (above the subducted slab) and the compressional stresses of collision.\r\n

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Subduction of Ocean Lithosphere beneath Ocean Lithosphere<\/h3>\r\nAnother collisional plate circumstance is that of oceanic lithosphere (and the crust that sits atop) being subducted beneath another plate of oceanic lithosphere.\r\nThis occurs in numerous places around the world, but a great example is that of the Aleutian Islands off Alaska.\r\nIt is a curved group of islands, and this feature is characteristic of ocean-ocean convergence, and ultimately results because of subducting slabs on a curved surface-- i.e. the spherical shape of planet earth!\r\nBecause of the curvature, ocean-ocean convergence generates volcanic islands that are commonly referred to as volcanic arcs.\r\n\r\n[caption id=\"attachment_3765\" align=\"alignnone\" width=\"300\"]\"\"<\/a> Kanaga Volcano, an Aleutian Arc volcano-- a consequence of subduction of Pacific Plate beneath the Bering Sea Plate<\/span>[\/caption]\r\n

Diverging Plates<\/h3>\r\n

Amazingly, even divergence can create mountain ranges. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults. The result is alternating mountains and valleys, known as a basin-and-range (Figure 7). In basin-and-range, some blocks are uplifted to form ranges, known as horsts, and some are down-dropped to form basins, known as grabens.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1997\" align=\"aligncenter\" width=\"660\"]\"A) Figure 7. (a) Horsts and grabens. (b) Mountains in Nevada are of classic basin-and-range form.<\/span>[\/caption]\r\n\r\n

Watch this quick animation of movement of blocks in a basin-and-range setting<\/a>.<\/div>\r\n

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The Western U.S. Mountains!<\/h2>\r\n[caption id=\"attachment_3768\" align=\"alignleft\" width=\"300\"]\"\"<\/a> USGS Digital Elevation Model of United States<\/span>[\/caption]\r\n\r\nThis is a digital elevation image of the United States.\u00a0 It shows NOTHING but elevation.\u00a0 It is color coded for elevation.\r\nThe most striking feature of this sort of map is that the east margin of the U.S. has some ripples, called the Appalachians, but the real action (at least in terms of elevated terrain!) is in the western U.S.\r\n\r\nThe Appalachians are a consequence of various mountain building events (called \"orogenies\") that for the most part occurred during the Paleozoic, i.e. pre-dinosaur time, from about 500 million years ago to 250 million years ago.\r\n\r\n \r\n\r\n[caption id=\"attachment_3777\" align=\"alignleft\" width=\"388\"]\"\"<\/a> from Phil Stoffer and Paula Messina, Hunter College == http:\/\/www.geo.hunter.cuny.edu\/bight\/highland.html ==<\/span>[\/caption]\r\n\r\n \r\n\r\nIn the adjacent image, we see that the Appalachians were all about various kinds of collisional tectonics.\u00a0 There was subduction of ocean crust under ocean crust, forming island arcs.\r\nThe island arcs were slammed up against proto-North-America during ocean-ocean convergence, and then the final phase of mountain building involved the collision of continental plates, namely North America and Africa, during the Late Paleozoic.\r\n\r\nBut, a very big idea here is that although there has been some degree of post-Paleozoic mountain building in the eastern U.S., it's been largely a time of erosion and denudation.\u00a0 This is one reason why the Appalachians are relatively low in elevation compared to the western mountains of the U.S.\r\n\r\nThere is more to it though!\r\n\r\nSeismic probing of the lithospheric mantle beneath the western U.S. suggests that relatively low density lithosphere exists beneath the Sierra Nevada Range, the Basin and Range region, and the Rocky Mountains.\u00a0 This low density, and relatively buoyant, lithosphere acts to \"hold up\" the overlying crust and allows for very high mountain elevations (compared to other parts of the U.S.).\r\n

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<\/h3>\r\nThe colorful image, on the right, of the U.S. is based on seismic wave velocities.\r\nThe image uses S-wave velocities in the upper-most mantle.\u00a0 This would be the lower part of the what we continental lithosphere (remember, lithosphere is mantle plus overlying crust), just beneath the crust.\r\nQuite interestingly,\r\nIn this image we see a lot of \"warmer\" colors of red and yellow.<\/span>in the western U.S., coincident with the high elevation terrain of the Colorado Plateau, the Rocky Mountains, and the Sierra Mountains.\u00a0 The red\/yellow colors indicate SLOWER seismic velocities, and therefore lower density material.\u00a0 Of course, low density mantle material (although still rock!), is relatively buoyant and will try to rise up relative to surrounding mantle.\r\nThe POINT OF ALL THIS-- is that the velocities suggest quite a bit of warmer and lower density upper mantle in the western U.S., and most likely, this warmer mantle is helping to sustain the high mountains and high overall terrain in this part of North America.<\/em>\r\n

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MAKING CONTINENTAL CRUST<\/h1>\r\n

How Do \"Plates\" of Continental-type FORM in the first place?<\/h2>\r\nAfter all this discussion of plates bumping into each other (convergence) or getting pulled apart (divergence) or sliding past one another (transform), one might ask \"How do Plates form?\"\r\n\r\nIn fact, another good question would be-- \"The earth seems to be mostly covered by OCEANIC CRUST and less so by CONTINENTAL CRUST.\u00a0 Why is this the case?\"\r\n\r\n(Remember in elementary school--- we learned that the earth is mostly covered by oceans... somewhere between 2\/3 and 3\/4 of earth surface.)\r\n\r\nIt turns out that Plate Tectonics is a key to answering this question.\r\nThe most \"primary\" sort of crust is oceanic crust.\u00a0 It forms when the deep mantle (asthenosphere) wells up at mid ocean ridges and generates NEW crust of the basaltic, or oceanic, variety.\r\nLater, this basaltic crust SUBDUCTS, typically at the margins of big ocean basins (but not always).\u00a0 And where subduction occurs, NEW magma rises of a more granitic type, and this is how mountains and new continental crust is made.\r\n\r\nIn the diagram below, note how subducting ocean crust generates bubbles of magma that rises to make the (speckled\/dotted) continental crust.\u00a0 As the intervening ocean crust is removed by subduction, larger and larger masses of continental crust form!\r\nThis is what we think was occurring during the earliest times of continental crust formation-- in the Precambrian.\r\n\r\n\"\"<\/a>\r\n\r\n \r\n

Age Provinces of North America<\/h2>\r\n\"\"<\/a>\r\n\r\nSo, as per the above discussion on the formation of continental crust-- we'd expect that early continents had new continental fragments added on with time.\r\n\r\nThis is exactly what we see in North America (and on many other continents)-- the oldest continental crust is in the north part of the continent, in what we call the Canadian Shield region.\u00a0 In the adjoining diagram, the Canadian Shield is composed of very old rock (purple and pink), in excess of 2.3 billion years.\u00a0 Around this \"proto-continent\" we find younger and younger crust, e.g. the green and yellow and blue material, younger than 1 billion years.\r\n\r\nTo be clear-- much of the old crust, or \"basement rock\" as it is sometimes called, can be covered by younger sediments.\u00a0 So the colored map here of North American Age Provinces refers to the deep roots of the continent.\u00a0 These roots are called the continental CRATON<\/em><\/span>, and where it is exposed (with no overlying sediment of younger age) it is called a SHIELD<\/em><\/span> region.\r\n

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Summary<\/h2>\r\n