Orogeny - definition of orogeny by The Free Dictionary Orogeny - definition of orogeny by The Free Dictionary (ô-rŏj′ə-nē) also or·o·gen·e·sis (ôr′ə-jĕn′ĭ-sĭs) n. The process of mountain formation, especially by a folding and faulting of the earth's crust. or′o·gen′ic (ôr′ə-jĕn′ĭk) adj. or′o·gen′i·cal·ly adv. orogeny orogenesis n (Geological Science) the formation of mountain ranges by intense upward displacement of the earth's crust, usually associated with folding, thrust faulting, and other compressional processes orogenic, orogenetic adj the process of mountain formation or upheaval. [1885–90] or•o•gen•ic (ˌɔr əˈdʒɛn ɪk, ˌɒr ə-) adj. orogenesis , orogeny - Orogenesis is the formation of mountains (Greek oros) and orogeny is the process by which mountains are formed. See also related terms for mountains .

orogeny A phase of mountain building. ThesaurusAntonymsRelated WordsSynonymsLegend: orogeny - the process of mountain formation (especially by the upward displacement of the earth's crust) geologic process , geological process - (geology) a natural process whereby geological features are modified Translations Want to thank TFD for its existence? Tell a friend about us , add a link to this page, or visit the webmaster's page for free fun content . Link to this page: young References in periodicals archive ? 60 Ga magmatism in Estonia, Finland and Lithuania was in accord with one of the major stages of the Gothian orogeny at 1. Abu Dhabi - Geology This succession has been deformed into SW-NE trending folds during the Middle Devonian Neoacadian orogeny (van Staal 2007; White et al.

Stratigraphy, provenance, and tectonic setting of the Lumsden Dam and Bluestone Quarry formations (Lower Ordovician), Halifax Group, Nova Scotia, Canada Orogeny started following Neo--tethyan subsidence [11, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26] towards north and continental edges of these plates reaching the region. Flora of China, vols. 2-3, Lycopodiaceae through Polypodiaceae The EMU is known to extend over the entire NW Borneo margin and is probably related to the Sabah Orogeny which marks the cessation of sea-floor spreading in the South China Sea and collision of Dangerous Grounds block with Sabah.

Dedication: the life, career and major achievements of Brian Roy Stuckenberg (1930-2009) A total aeromagnetic intensity contour map, residual aeromagnetic intensity contour map and structural lineament map of the area being studied were obtained in this work Visual studies of total aeromagnetic intensity contour map and residual aeromagnetic intensity contour map depicted that the southern part of the area underlain by Cretaceous sedimentary rocks (Awgu shale and Lafia formation) had low and smooth magnetic intensity, whereas the northern part underlain by the Pre-Cambrian basement rocks (migmatites, gneisses, Older Granites) and Jurassic younger granites had high and complex magnetic intensity Structural lineament orientation suggested that they were products of Pan-African orogeny (NE-SW, NW-SE and NNE-SSW trends) and pre-Pan-African orogeny (NNW-SSE and E-W trend). Analysis of aeromagnetic data over Wamba and its adjoining areas in north- central Nigeria 2007); but had a main event during the Alpine Orogeny in the Late Miocence-Plieocene orogenic phase (Hessami et al.

Appalachian Orogeny Carboniferous Fossils I Permian Fossils I Triassic Fossils I Jurassic Fossils I Cretaceous Fossils I Tertiary Fossils The Appalachian Orogeny is an excellent example of a mountain building event resulting from tectonics. The Appalachian Orogeny resulted from three separate continental collisions formed the Appalachian Mountains and Allegheny Mountains at the end of the Paleozoic. The combined continents of Europe and Africa (Gondwana) collided with North America to form the supercontinent of Pangaea approximately 350 million to 300 million years ago during the Carboniferous period. The collision exerted enormous stress on what today is eastern North America, an causing an enormous upward thrusting of the entire region. The tectonic stresses also metamorphosized of the rock from igneous and sedimentary rock into metamorphic rock. These stresses concurrently caused faults (mostly thrust faults and some strike-slip faults) as well as folding. It is believed that the Appalachians once could have risen as high as or even higher than the Himalaya (i.e., more than 4 ½ miles high).

These mountains, while still impressive in places, are mere remnants of their glory days during the Paleozoic. They have been worm away by the relentless forces of erosion, forming vast amount of sediments in the process. Carbonates and sediments from mountains formed in the Appalachian Orogeny were away to form limey rocks in a shallow sea that was later uplifted and now forms the majority of Tennessee, Kentucky, Indiana and Ohio. e-WV | Appalachian Orogeny Share Appalachian Orogeny In geological terms, ‘‘orogeny’’ refers to the process of mountain building. The Appalachian Orogeny, previously called the Appalachian Revolution, describes the collective geological events that produced the belt of folded, faulted, and metamorphosed rock that stretches from Newfoundland to northern Alabama. All of West Virginia is included within this belt. It was produced during the ancient collision of the North American, European, and African continents. The current Appalachian Mountains are an erosional remnant of mountains formed millions of years ago during the Appalachian Orogeny. These ancestral mountains were much more extensive and probably appeared something like today’s Himalayas or Canadian Rockies.

They were eroded nearly flat, then split in two during the opening of the Atlantic Ocean, so that rocks and structures of similar age can be found on both sides of the ocean, in North America, Europe, and Africa. In North America, these remnant structures were uplifted and eroded again to form the present Appalachians. They are pale by comparison to their rugged ancestors. The Appalachian Orogeny is generally separated into three periods, referred to as the Taconic, Acadian, and Alleghenian orogenies. In West Virginia, the evidence for these three events is limited to the thick sedimentary rock units that were deposited in the basin that formed to the west of the mountains. The Taconic Orogeny is named after the Taconic Mountains of New York. The Taconic Orogeny is thought to be the result of the collision of a volcanic arc with the east coast of North America. Extensive folding and metamorphism from New England to southeastern Pennsylvania is attributed to this event.

In West Virginia, the evidence is limited to a great wedge of Ordovician and Silurian sedimentary rocks, including the Tuscarora Sandstone and the underlying red mudstones of the Juniata Formation, both of which are exposed on North Fork Mountain along U.S. 33 in Pendleton County. Seneca Rocks is a spectacular Tuscarora formation. The Acadian Orogeny is named for Acadia, a historical term for the Canadian Maritime provinces, and resulted from deformed and metamorphosed sedimentary rocks. Extensive metamorphism and deformation in the Late Silurian and Devonian eras produced deposits of sedimentary rock often referred to as the Catskill Delta. In West Virginia, these sediments are exposed on U.S. 33 east of Elkins . The Allegheny Orogeny is named after the high ridges of the Allegheny Mountains in Pennsylvania and West Virginia. The evidence for this event is two-fold. The thick Pennsylvanian Era sediments of West Virginia, including most of the commercial coal beds, were deposited along the ancient coastal plain that flanked the highlands to the east.

These sediments consist of alternating terrestrial and marine deposits and are found throughout West Virginia west of the Allegheny Front . The other evidence can be seen in the Ridge and Valley physiographic province, which displays sedimentary rock deposited during the previous two orogenic events that were folded and faulted during the Alleghenian event. This Article was written by David Matchen Last Revised on September 17, 2014 Related Articles The Geological History of Pittsburgh - Building the Appalachian Mountains History Home Page Building the Appalachian Mountains Southwestern Pennsylvania has a long history of sedimentation punctuated by mountain building episodes ( orogenies ). Until about 250 million years ago, most of the sediment was deposited in a deltaic system feeding into a shallow marine environment. The inland sea was gone by the time that dinosaurs roamed the Earth (Mezazoic Era), as the collision of Europe and Africa with North America created high mountains and pushed southwestern Pennsylvania up well above sea level. SUMMARY OF MAJOR GEOLOGIC EVENTS Approximately one billion years ago, the eastern edge of North America slammed into another continent , resulting in the Grenville Orogeny , an episode of mountain building.

The roots of those mountains experienced intense deformation by heat and pressure, creating metamorphic rocks. The roots are all that remain of the mountains formed during the Grenville Orogeny, exposed in some parts of eastern Pennsylvania. Approximately 650 million years ago, the joined continents began to pull apart. An ocean began to form between what is now North America and Europe. This ocean, known as Iapetus, gradually widened during the next 200 million years. The ocean then began to close again. As the continents crept towards each other, the ocean basin got smaller through subduction, a process by which relatively dense lithosphere is shoved under relatively buoyant lithosphere. Subduction can involve oceanic lithosphere being shoved under continental lithosphere, resulting in mountains, or old, cold oceanic crust being shoved under young, hot oceanic lithosphere, resulting in a chain of volcanic islands in the ocean (an island arc). As the downgoing slab descends, it is heated and begins to melt. The molten rock (magma) rises towards the surface, fueling volcanoes. Examples of subduction-related volcanoes include the Cascades in Washington State, Mt.

Pinatubo in the Phillipines, and, perhaps the most famous of all, Mt. Fugi in Japan. SUBDUCTION The Taconic Orogeny, the next mountain-building event that affected the Pittsburgh area, occured approximately 445 - 435 million years ago, as North America collided with an island arc (a string of subduction-related volcanic islands, like Japan and the Phillipines, formed when oceanic crust subducts under oceanic crust). When the island arc ran into North America, intense folding, thrust faulting , metamorphism, and volcanism occured. Mountains were raised in eastern Pennsylvania, and central Pennsylvania buckled to form a basin . The rising mountains began to erode and rivers carried the sediment westward into the basin. The basin first received lots of black mud, with some silt and fine sand. As erosion continued to eat away at the Taconic Mountains, huge amounts of sediment were carried into central and western Pennsylvania, and were deposited in a giant delta . Known as the Catskill delta, the sedimentary rocks formed from this delta are as much as 4000 feet of thick at their thickest point.

The delta stretched from New York through central Pennsylvania, thinning to the northwest. Approximately 1500 feet of sediment was deposited in Erie, Pennsylvania. 30 million years of tectonic quiet followed the Taconic Orogeny. During this quiet time, North America experienced simple subduction of oceanic crust as the proto-Atlantic ocean continued to shrink. During the Middle and Late Devonian (approximately 375 - 355 million years ago), the final closure of the proto-Atlantic Ocean began with the collision of North America and Europe. This collision event is called the Acadian Orogeny. Another range of mountains formed east of Pennsylvania, east of the by then deeply eroded Taconic Mountains. Pennsylvania received even more sediment from the erosion of these mountains than from the Taconian Mountains, forming an even larger delta. The Alleghenian Orogeny occurred during the Pennsylvanian and Permin Periods, approximately 300 - 220 million years ago. This orogeny resulted from collision between North America and Africa, completing the closure of the proto-Atlantic Ocean.

The sediment eroded from these new mountains formed the sedimentary rocks that are now exposed at the surface throughout most of western Pennsylvania. orogeny | geology | Britannica.com Orogeny See Article History Alternative Titles: mountain building process, mountain formation, orogen, orogenesis Orogeny, mountain -building event, generally one that occurs in geosynclinal areas. In contrast to epeirogeny , an orogeny tends to occur during a relatively short time in linear belts and results in intensive deformation. Orogeny is usually accompanied by folding and faulting of strata, development of angular unconformities (interruptions in the normal deposition of sedimentary rock), and the deposition of clastic wedges of sediments in areas adjacent to the orogenic belt. Regional metamorphism and magmatic activity are often associated with an orogenic event as well. Orogenies may result from subduction , terrane accretion (landmass expansion due to its collision with other landmasses), the underthrusting of continents by oceanic plates, continental collisions, the overriding of oceanic ridges by continents, and other causes. See plate tectonics .

Evidence of the Alleghenian orogeny can be found at this junction fault, which divides the … Dave/Pollinator mountain landform that rises prominently above its surroundings, generally exhibiting steep slopes, a relatively confined summit area, and considerable local relief. Mountains generally are understood to be larger than hills, but the term has no standardized geological meaning. Very rarely do mountains... geosyncline linear trough of subsidence of the Earth’s crust within which vast amounts of sediment accumulate. The filling of a geosyncline with thousands or tens of thousands of feet of sediment is accompanied in the late stages of deposition by folding, crumpling, and faulting of the deposits.... 13 References found in Britannica Articles Assorted References role in Triassic Period (in Triassic Period: Paleogeography ) role of tectogene (in tectogene ) occurrence during Corrections? Updates? Help us improve this article! Contact our editors with your feedback. MEDIA FOR: You have successfully emailed this. Error when sending the email. Try again later. Edit Mode Submit Tips For Editing We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.

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Close Date Published: August 07, 2009 URL: Access Date: January 17, 2017 Share Formation of the Rocky Mountains: Learn Interesting Secrets About Mountain Geology that Caused Them to Form An Introduction to the Rocky Mountains Orogeny is the process of mountain building. These processes are complex and take millions of years to shape and form mountains. The formation of the Rocky Mountains is believed to have taken place in the Early Cenozoic and Mesozoic eras some 50 to 80 million years ago during a period known as Cordilleran orogeny. The remnants of the old mountain system occurred in a series of pulses some one million years ago. Erosion has since exposed the crystalline cores and carved out valleys and peaks. One of the most important processes in forming the magnificent chain is plate tectonics. Plate tectonics is a theory that states that beneath the surface of the Earth are plates upon which continents sit. These plates are constantly moving and at some point the entire landmass of the Earth was one giant super continent called Pangaea. When Pangaea broke apart, the plates shifted and this shifting caused the collision of the plates into one another.

The Rocky Mountains are believed to have been formed during these active movements of the plates. The entire process took place in three major uplifts or mountain building stages and the final uplift, the Laramide orogeny, is responsible for raising the Rocky Mountains. Let us examine these three stages and see how the Rocky Mountains were formed. slide 2 of 6 First Uplift The first uplift generally refers to the deposition of ancestral rocks in the Rocky Mountain region. Sandstone, shale and siltstone were the original rocks deposited in the region by a prehistoric sea some two billion years ago. Precambrian metamorphic rock is the oldest rock and there are also deposits of volcanic rocks. Argilite, a sedimentary rock, is also found in the region dating back 1.7 billion years. The sedimentary rocks were trapped between the tectonic plates some 1.7 billion years ago.

As a result of the collision, these sedimentary rocks were transformed within the core of a Proterozoic mountain range into metamorphic rocks by large amounts of heat and pressure, The Proterozoic mountains, over a period of 1.3 billion to 500 million years ago, gradually eroded to expose the metamorphic rocks and granite. slide 3 of 6 Second Uplift Nearly 500 million years ago, the flattened region was covered by shallow seas. Hundreds of thousands of feet of Paleozoic sedimentary rocks were deposited onto the surface. During the Pennsylvanian sub-period, the rocks of the southern Rocky Mountains were disturbed due to mountain building processes. These processes created the Ancestral Rocky Mountains. Two large mountainous islands were created consisting of Precambrian metamorphic rock. These mountains, however, eroded leaving large deposits of sedimentary rocks. Towards the end of the Mesozoic, approximately 10,000 to 15,000 feet of sediment had accumulated in the region.

The region now known as the Rocky Mountain Park was eroded again and for a brief period covered by seas from the middle of the Permian period to the end of the Cretaceous period some 65 million years ago. slide 4 of 6 Third Uplift The main collisions of the major tectonic plates occurred some 130 million years ago on the western edge of North America. This uplifting affected the area surrounding the Colorado Rockies. The Laramide orogeny was responsible for raising the Rocky Mountains. In this process, the angle of subduction became shallow resulting in the underlying oceanic lithosphere (crust) to cause a drag on the overlying continental lithosphere. The shallow angle of the subducting plate resulted in a major increase in friction and interactions with the continental mass above. Thrust faults are breaks within the surface of the Earth where rocks of lower strata are pushed above rocks of higher strata. Due to the friction of the subducting plate with the continental mass above, huge thrusts were generated, which caused sheets of crust to pile on top of one another resulting in the formation of the extraordinary Rocky Mountains.

slide 5 of 6 Current Rockies The present state of the Rockies is due to processes that lasted millions years of years. Erosion due to wind and rain is constantly redefining the terrain. The Rockies are a popular tourist attraction and are visited by millions of visitors each year. People like to go hiking on the mountain ranges such as Pikes Peak or visit some of the popular national parks such as Yellowstone National Park and Banff National Park. Skiing is quite also popular during the winter and there are many resorts located in the United States as well as in Canada around the mountain ranges. The Rockies represent some of the ancestral rocks that formed them. There are extensive collections of granite dating back to the Ancestral Rockies such as Precambrian mudstones along with Paleozoic limestones and dolomites. They are a reminder of the awesome forces of nature that have shaped this extensive and spectacular mountain range. slide 6 of 6 1. Topinka, Lyn, "America's Volcanic Past," USGS, 2. Cameron, Ward, "The Formation of the Rocky Mountains", Mountainnature, 3.

Perkins, Sid, "How the Rocky Mountains Got its Midsection", ScienceNow, Image Credits: Orogenesis - definition of orogenesis by The Free Dictionary Orogenesis - definition of orogenesis by The Free Dictionary Related to orogenesis: plate tectonics o·rog·e·ny (ô-rŏj′ə-nē) also or·o·gen·e·sis (ôr′ə-jĕn′ĭ-sĭs) n. The process of mountain formation, especially by a folding and faulting of the earth's crust. or′o·gen′ic (ôr′ə-jĕn′ĭk) adj. or′o·gen′i·cal·ly adv. orogenesis , orogeny - Orogenesis is the formation of mountains (Greek oros) and orogeny is the process by which mountains are formed. See also related terms for mountains . orogenesis kalnodaraorogenezė orogenese Want to thank TFD for its existence? Tell a friend about us , add a link to this page, or visit the webmaster's page for free fun content . Link to this page: orogeny References in periodicals archive ?

The Andean orogenesis has been considered as a major factor affecting the patterns and levels of Neotropical species (Gentry, 1982; Young et al. Amarium Growth Strategy Materializing Due to Operations in Mexico Orogenesis and basin development: U-Pb detrital zircon age constraints on evolution of the Late Paleozoic St. Stratigraphic evolution of provenance in a thick conglomerate-dominated Carboniferous succession in the southern Isle Madame area, Nova Scotia, and paleogeographic implications Fontcuberta concibe sus googlegramas y sus paisajes, como Orogenesis (2004), ("paisajes sin memoria"), como una remediacion de la obra de artistas como Salvador Dali, Henri Rousseau, Man Ray, exhibidos como referente en pequeno formato, o distintas partes del cuerpo humano, sometiendolos a uno o varios programas informaticos de traduccion de planos topograficos en imagenes de tres dimensiones.

PNW Geology Lecture 7 intrusion ( igneous intrusions up to the size of batholiths ) metamorphism (deep burial and regional metamorphism of large volumes of continental crust and contact metamorphism of crust adjacent to igneous intrusions) erosion (as the mountains are uplifted they increasingly become exposed to the forces of erosion ) These processes combine to thicken the continental crust. The average crustal thickness of most of North American is about 15 miles (25 kilometers). Right now, the thickest continental crust in the continental U.S. is the Cascade Range. The distance from the highest peaks to the base of the continental crust beneath them is about 25 miles (40 kilometers). The roots of this orogenic belt reach down into the mantle somewhat like the keel of a boat. Erosion is always working to reduce areas that rise above sea level back down to sea level. Because the highest elevations above sea level occur in orogenic belts (mountain ranges), those areas are where rates of erosion are highest. Erosion removes sediments from the mountains. Sediments shed from the eroding mountains are deposited in basins between and beyond the ranges.

In the long run, volcanoes that were on top of the mountain ranges will be completely eroded away, but evidence of the volcanoes' existence will be preserved in layers of sediment. Two types of sandstone are particularly indicative of orogenies: graywacke , from active volcanic arcs eroding into a nearby trench or marine basin, and arkose from erosion of granite and gneiss that was formed deep in the crust and uplifted into mountains. Orogenies - Opportunities for Learning The most recent orogenies provide abundant evidence in the geologic record of the events that occurred to build mountains. The events of older orogenies may be more difficult to piece together but analysis of rocks and structures combined with an understanding of plate tectonics can shed light on even the oldest orogenies. Most orogenies can be explained in terms of processes that occur at convergent plate boundaries . However, some orogenies seem to take place far inland from convergent plate boundaries. Mountain building can also occur along continental transform plate boundaries . The Basin and Range and Rocky Mountain landscape regions are examples of orogenies that are not near plate boundaries.

The timing and geologic structures of the Basin and Range indicate that the region began to undergo extension and uplift at the same time the San Andreas fault began to form. This timing suggests that mountain building in the Basin and Range is related to the change in the character of the plate boundary along the coast of California--from a convergent to a transform plate boundary. The continued uplift of ranges in the Rocky Mountains suggests that the effects of a convergent plate boundary may reshape the continental crust much farther inland than the volcanic arc . The Cretaceous North Cascades orogeny is an example of an orogeny that seems to have involved more than simple plate convergence. On the one hand, the North Cascades contain thrust faults, accreted terranes , folds, intrusive rocks and regional metamorphic rocks that would be expected in a plate boundary setting in which the oceanic plate converges directly into the continent. On the other hand, paleomagnetic data indicate that some of the rocks in the North Cascades moved north, possibly as much as 1,000 miles, since the mid-Cretaceous. Such movement occurs at a transform plate boundary, rather than at a convergent plate boundary.

Orogenic styles seen in the Pacific Northwest include volcanic arcs associated with subduction zones (Cascadia orogeny), block mountain ranges offset along steep faults (Laramide and Basin and Range orogenies), and thrust fault belts (Sevier and North Cascades orogenies). Geologists continue to debate the details of these orogenic styles and what each style tells us about how continents form. Pre-Mesozoic Orogenies of the Pacific Northwest During Archean time orogenies occurred in the part of the North American craton that is now the Rocky Mountains. Evidence of these long-ago orogenies includes the granites that formed in cores of these ancient mountains, the metamorphosed volcanic rocks that they erupted, and the sedimentary rocks that formed as they eroded. The North American craton was flat and stable after the Archean mountains were leveled by erosion. During much of the Paleozoic era, the edge of the craton in the Pacific Northwest was a passive continental margin , with no plate boundaries nearby and no mountain building. There were a few exceptions to this passive plate margin during the Paleozoic era and the geologic record indicates some short-lived mountain-building activity at the western edge of the craton.

These Paleozoic orogenies seem to have involved temporary subduction zones along the coast of the growing North American continent where terranes accreted and thrust inland. One of these was the Antler Orogeny, which affected parts of Nevada, Utah, and Idaho in the Pennsylvanian period. The Sevier Orogeny The Rocky Mountains run from the interior of Alaska and western Canada to Mexico. In parts of Nevada and Utah there are huge thrust faults that stacked up large slabs of rock, being pushed from west to east, during the Cretaceous period, starting about 140 million years ago. Similar styles and ages of thrust faults also occur in the Rocky Mountains of Montana, Canada, and Alaska. The process that created this stacking up and eastward pushing of thrust-fault slabs is called the Sevier orogeny, named after the Sevier River in Utah. The Sevier orogeny overlaps in time and space with the Laramide orogeny (see next section).

The Sevier orogeny may be an expression of how the upper crust of western North America responded to tectonic processes driven by plate interactions at that time, while the Laramide orogeny may be an expression of how the deep crust and underlying lithosphere responded to the same tectonic processes. Learn the details of the Sevier Orogeny on Focus Page 8 . The Laramide Orogeny Many individual mountain ranges make up the overall Rocky Mountains. The highest mountain ranges in the Rockies reveal deep crustal metamorphic and plutonic rocks in their cores. These are called "basement cored" mountain ranges. The orogeny that produced these mountain ranges gets its name from the Laramie Mountains in Wyoming, which are an example of such a mountain range. The Laramide ranges show evidence of having been folded, thrust-faulted, and, in some of the big blocks of Laramide rock, reverse-faulted upward. This style of uplift in which deep-crustal rocks are uplifted to form basement-cored mountain ranges is called "thick-skinned" thrusting. Learn the details of the Laramide Orogeny on Focus Page 8 .

The Basin and Range Orogeny You enter the Basin and Range landscape region if you go south of the Central Oregon Plateau and the Snake River Plain. The region is defined by mountain ranges oriented in a north-south direction separated by desert basins. Crustal extension dominates the tectonics of the region as seen in the normal faults along the base of many of the mountain ranges and the detachment faults and metamorphic core complexes present in some of them. Learn the details of the Basin and Range Orogeny on Focus Page 8 . The North Cascades Orogeny There is no generally agreed-upon name for the orogeny of the North Cascades during the Late Cretaceous period, so we will simply call it the North Cascades orogeny. The North Cascades can be divided into three zones, separated from each other by major faults or fault zones. In the east is the Methow Valley, in the center is the North Cascades crystalline core and to the west is the Northwest Cascades. The rocks of the crystalline core formed deep in the earth's crust, as plutonic and high-grade metamorphic rocks.

The rocks of the Methow Valley and Northwest Cascades formed mainly in the upper crust, as volcanic and sedimentary rocks. As discussed in Focus Page 8, the North Cascades seem to result from a combination of subduction and transform faulting. A simple model would describe the North Cascades orogeny as a subduction-driven orogeny at a convergent plate boundary with a resulting package of rocks that was shifted northward when the convergent plate boundary changed to a transform plate boundary. Reality appears more complicated, including the possibility that subduction occurred beneath the older rocks of the Cascades even as the rocks were being shifted northward along the coast. The North Cascades provides an unusual exposure of many parts of an orogeny, from its upper crustal levels to its deep crustal interior. Unresolved questions about when and where the rocks formed and how they were assembled into their present configuration continue to attract geologists seeking to explain how the earth makes mountain ranges. Learn the details of the North Cascades Orogeny on Focus Page 8 . The Cascadia Orogeny The Cascadia orogeny is occurring today in association with the Cascadia subduction zone.

The volcanoes of the Cascades, the unseen magmas that are intruding and solidifying in the crust beneath the volcanoes, and the accretionary complex of the Coast Ranges, including the Olympic Mountains, are all part of the Cascadia orogeny. Learn the details of the Cascadia Orogeny on Focus Page 8 . Driving Through Orogenies Making a geologic traverse of the North Cascades Mountains in Washington State allows a good look inside two orogenies: the North Cascades Orogeny which occurred in the Cretaceous and Paleogene periods, and the Cascadia Orogeny, which began in the Neogene period and continues today. Wenatchee to Seattle (SR 97 and I-90) Traveling on State Route 97 and Interstate 90 from Wenatchee to Seattle, you traverse sedimentary rocks that formed in the area before the modern Cascade range near Blewett Pass and Cle Elum. Approaching Blewett Pass you also pass through part of an accreted terrane. East of Snoqualmie Pass are volcanic rocks that erupted from volcanoes in the Cascade volcanic arc.

At Snoqualmie Pass you enter the Snoqualmie Batholith, which intruded during the Miocene Epoch. In the forested foothills surrounding I-90 before you reach the Puget lowland, you travel through metamorphosed volcanic and sedimentary rocks of one or more accreted terranes. Stevens Pass (U.S. 2) In the Stevens Pass traverse you pass through metamorphic rock that used to be seafloor sediment and was buried deeply and metamorphosed at high temperature and pressure (east of Stevens Pass), and a large batholith of Cretaceous age (the Mt. Stuart batholith, at Stevens Pass and west of there). Once you pass Skykomish, headed west on U.S. 2, you drive past Neogene intrusive rocks (including the Index batholith). West of the Index batholith near U.S. 2 are accreted terranes consisting of pieces of the ocean floor and probably one or more island arcs. North Cascades Highway (SR 20) The North Cascades Highway takes you through a variety of sedimentary rocks related to orogeny, plutonic rocks , and high-grade regional metamorphic rocks.

In the southern Methow Valley you drive through accreted oceanic crust and island arc rock. Continuing north on the highway, you pass through ocean floor sedimentary rock from the nearby Methow Ocean. The sequence of marine sediments from the Methow Ocean is overlain by sedimentary rocks deposited after the area had come above sea level. These continental sediments were deposited by rivers that flowed and discharged to the coast of North America. This took place during the latter half of the "Age of Dinosaurs." Therefore, it is possible that someday dinosaur fossils will be found in the sedimentary rocks of the Methow Valley, in the rocks that were originally deposited above sea level along the coast of North America as it existed back then. Heading from the Methow Valley to the North Cascades Mountains, you pass metamorphic rocks in the crystalline core of the range. The rocks were metamorphosed in the middle to late Cretaceous Period, and most batholiths and smaller plutons in the crystalline core intruded during the Late Cretaceous period as well.

West of the town of Marblemount, you are west of the crystalline core in a zone of thrust-faulted, accreted terranes that were added to North America late in the Cretaceous period. In contrast with the rocks in the Cascades crystalline core, accreted terranes west of Marblemount are not metamorphosed at high temperatures and not intruded by batholiths. The accreted terranes west of Marblemount include basalt and mudstone from the ocean floor that was metamorphosed during subduction and accretion, and dirty sandstones shed from active volcanic arcs. These rocks were formed before the North Cascades orogeny, some of them at considerable distance from North America.

In sum, you can encounter the following on any of these drives across the central or north Cascades Mountains: "dirty sandstones" made of sediment shed from eroding volcanic mountain ranges large igneous intrusions made of granitic rock metamorphic rocks of various types, formed at various pressures and temperatures in the crust major faults that may have been the result of tectonic plate interactions accreted terranes, made of rock that did not originate in the North American continent young volcanic rocks of the modern Cascade volcanic arc however, in the North Cascades, the young volcanic rocks are only found at or near the currently active volcanoes, apparently because the North Cascades have been uplifting and eroding so rapidly that the slightly older volcanic cover has been stripped away, revealing the deeper metamorphic and plutonic rocks, which are the source of the name "crystalline" When you drive through the North Cascades and pay attention to the rock types, structures, and landforms, you are driving through an active orogen. Rocks of the earlier North Cascades orogeny continue to uplift and erode, capped by the volcanoes of the ongoing Cascadia orogeny.

Orogeny Along the outer edge of most currently active mountain belts is a narrow, deep oceanic trench. Seismic Activity Shallow great earthquakes along the inner wall of the trench, then deeper earthquakes along a planar zone dipping beneath the mountain range, reaching depths of up to 700 kilometers. Why Mountains Form Mountains form at subduction zones. Shown below is a true-scale cross-section of the Andean subduction zone in northern Chile (roughly 25 S). The vertical change of 15 kilometers in only a few hundred kilometers distance is the largest elevation change on Earth in such a short distance. Plates consist not only of the crust, but about 150 kilometers of the underlying mantle as well. Collectively the crust and associated mantle are termed the lithosphere. Oceanic crust is typically 5 kilometers thick. The continental crust thickens from its normal 40 kilometers to 70 beneath the high Andes. When the descending slab reaches a depth of about 100 kilometers, it begins to melt, causing, directly or indirectly, many of the events associated with mountain-building.

Why Mountains are High Mountains are high because orogeny shortens and thickens the crust, and isostasy causes the thicker crust to rise. Some of the processes are shown above: Even uniform materials, when compressed from one direction, tend to expand in the direction of least resistance. Layered rocks shorten by folding, but the stack of layers also becomes thicker. Thrust-faulting thickens the crust by stacking slices of crust atop one another. Intrusions add volume to the crust. A great deal of magma never invades the crust but accumulates at its base, a process called underplating. Where the crust is heated, thermal expansion causes the rocks to become lighter and more buoyant. Types of Subduction Zones Present day: Alps, Himalaya, Persian Gulf Past: Appalachians, Urals Anatomy of an Orogenic Belt Shown here is a simple continent-ocean orogenic belt. We can divide an orogenic belt into parallel zones defined by their deformation, lithology, or metamorphism. These zones may approximately coincide with each other but somewhat overlap, so it's necessary to have distinct names for them.

Structural Zones The Accretionary Prism Sediment eroded from the orogenic belt accumulates in the trench and is intensely deformed as the plates converge. Like the wedge of earth ahead of a bulldozer, the sediment thickens until it is capable of resisting further deformation. The Igneous Arc When the descending plate reaches about 100 kilometers depth, it begins to melt. Magma invades the crust, creating batholiths and a volcanic mountain chain. The intrusions also produce metamorphism, and by making the crust more ductile, make it easier to deform. This is the belt of greatest deformation, metamorphism and igneous activity. The Foreland Here, metamorphism is mild but compression of the crust results in folding and thrust-faulting. Often this deformation is "thin-skinned", meaning that rock layers near the surface become detached from deeper layers much the way a carpet wrinkles when a piece of furniture is pushed over it. This process is called decollement. Usually the layer where separation occurs is made up of weak rocks like salt, gypsum, or shale. The Craton This is the stable interior of the continent.

It may be thinly mantled with sedimentary rocks or have large areas of ancient igneous and metamorphic rocks. Lithologic Zones The Eugeocline The rocks of the accretionary prism and much of the igneous arc consist of great thicknesses of immature deep water sediment. Often these rocks show evidence of deposition in an unstable setting, such as evidence of turbidity flows or submarine landslides. This sort of deposit is known as flysch. Such rocks are typical of a continental rise or trench setting. Accompanying these rocks are often submarine volcanic rocks, pillow lavas. The Miogeocline The rocks of the foreland are typically shallow-water sedimentary rocks typical of a continental shelf, which become thinner toward the interior of the continent. Igneous rocks are uncommon. As the mountain belt rises, great thicknesses of sandstone and conglomerate are deposited on its flanks and frequently bury much of the foreland. These rocks are typically shallow-water or terrestrial, often red in color, and are called molasse deposits. The Platform The stable interior of the continent will often be covered with thin layers of shallow-water or terrestrial sedimentary rocks.

This thinly-mantled region, part of the craton, is the platform The Shield Areas where ancient crystalline rocks are exposed over wide areas are called shields. Every continent has at least one shield. The shield and adjacent platform together make up the craton. Driving from Green Bay to Wausau takes you from a platform into a shield. Metamorphic Zones One of the best indicators of former subduction is the presence of paired metamorphic belts, a belt of typical Greenschist and Amphibolite metamorphism flanked by a belt of Blueschist metamorphism. Greenschist-Amphibolite Metamorphism The rising magma from the descending plate heats the crust, resulting in greenschist and amphibolite metamorphism in the igneous arc. At very high temperatures, rocks become very dehydrated; even muscovite mica breaks down to potassium feldspar and amphibole to pyroxene. This sort of metamorphism, called granulite metamorphism, occurs deep in the crust just about everywhere simply due to the normal geothermal gradient. At 25 degrees per kilometer, the temperature at the base of the crust, 40 kilometers deep, is 1000 degrees C.

Of course, unusually intense heating can cause it to occur at shallower levels. Blueschist Metamorphism At high pressures but low temperatures, rocks are metamorphosed to blueschist grade. The reason temperatures are abnormally low is that the descending slab is still cool and helps keep adjacent rocks cool as well. Normally sodium is the most predictable major element; it occurs just about exclusively in plagioclase. At high pressure and low temperature, though, albite feldspar breaks down and forms the pyroxene jadeite and amphiboles like glaucophane and aegerine. The amphiboles are bluish, hence the term "blueschist" It's a bit puzzling that there are very few blueschist rocks older than Mesozoic. Possibly older mountain belts have been eroded to depths where temperatures were too high for blueschist metamorphism. Or perhaps, in most orogenic belts these rocks eventually get heated to greenschist grade, and we only see the places where it hasn't happened yet. Some people have suggested that the geothermal gradient was higher in the past, meaning the deep earth was too hot for blueschist metamorphism.

Eclogite Metamorphism At about 100 kilometers depth, pyroxene, olivine and plagioclase recrystallize to a denser form to produce sodium-bearing pyroxene and garnet. The result is one of the most beautiful of rocks, eclogite, a mass of light green pyroxene enclosing pink garnets. Note that the boundary of eclogite metamorphism rises upward within the descending slab. This happens because the rocks are relatively cool. High temperatures inhibit the recrystallization of rocks to denser forms because high temperatures cause materials to expand. Thus eclogite metamorphism occurs at shallower depths in the descending slab. The slab in that area is denser than the surrounding mantle, and its greater density assists it in sinking. This mechanism is called slab pull and is one of the driving mechanisms of plate tectonics.

Question: What are formed by the process of ‘orogeny’?
Answer: Mountains