{{Short description|Mountain-forming event that formed the Appalachian and Allegheny Mountains}} [[File:Hercynides EN.svg|thumb|upright=1.6|Location of the Hercynian/Variscan/Alleghanian mountain chains in the Carboniferous period. Large labels are continents that joined during these orogenies. Present day coastlines in gray. Sutures are red.]] The '''Alleghanian orogeny''' or '''Appalachian orogeny''' is one of the geological mountain-forming events that formed the [[Appalachian Mountains]] and [[Allegheny Mountains]]. The term and spelling '''Alleghany orogeny''' was originally proposed by H.P. Woodward in 1957.
The Alleghanian [[orogeny]] occurred approximately 325 million to 260 million years ago<ref>{{cite web|last=Hatcher|first=R.D.|title=Tracking lower-to-mid-to-upper crustal deformation processes through time and space through three Paleozoic orogenies in the Southern Appalachians using dated metamorphic assemblages and faults|url=http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_150729.htm|work=Geological Society of America Abstracts with Programs|publisher=Geological Society of America|access-date=2012-01-23|archive-url=https://web.archive.org/web/20180806090332/https://gsa.confex.com/gsa/2008AM/finalprogram/abstract_150729.htm|archive-date=2018-08-06|url-status=dead}}</ref> over at least five deformation events<ref>{{cite book |last1=Bartholomew |first1=M.J. |last2=Whitaker |first2=A.E. |year=2010 |chapter=The Alleghanian deformational sequence at the foreland junction of the Central and Southern Appalachians |editor1-last=Tollo |editor1-first=R.P. |editor2-last=Bartholomew |editor2-first=M.J. |editor3-last=Hibbard |editor3-first=J.P. |editor4-last=Karabinos |editor4-first=P.M. |title=From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region, GSA Memoir 206 |pages=431–454 |publisher=Geological Society of America |isbn=9780813712062 |chapter-url=https://books.google.com/books?id=CQoeMsyDNPYC&dq=Bartholomew,+M.J.,+and+Whitaker,+A.E.,+2010,+The+Alleghanian+deformational+sequence+at+the+foreland+junction+of+the+Central+and+Southern+Appalachians+in+Tollo,+R.P.,+Bartholomew,+M.J.,+Hibbard,+J.P.,+and+Karabinos,+P.M.,+eds.,+From+Rodinia+to+Pangea:+The+L&pg=PA431 |access-date=24 August 2021}}</ref> in the [[Carboniferous]] to [[Permian]] period. The orogeny was caused by Africa's collision with North America. At the time, these continents did not exist in their current forms: North America was part of the [[Euramerica]] super-continent, while Africa was part of [[Gondwana]]. This collision formed the super-continent [[Pangaea]], which contained all major continental land masses. The collision provoked the orogeny: it exerted massive stress on what is today the [[East Coast of the United States|Eastern Seaboard]] of North America, forming a wide and high mountain chain.<ref name="nps">{{NPS|url=https://web.archive.org/web/20050422074237/http://www.nps.gov/mana/pphtml/subenvironmentalfactors13.html|title=Geology of Manassas National Battlefield Park}}</ref> Evidence for the Alleghanian orogeny stretches for many hundreds of kilometres on the surface from [[Alabama]] to [[New Jersey]] and can be traced further subsurface to the southwest. In the north, the Alleghanian deformation extends northeast to [[Newfoundland (island)|Newfoundland]]. Subsequent erosion wore down the mountain chain and spread sediments both to the east and to the west.
==Continental collision== [[Image:Appalachian orogeny.jpg|thumb|upright=1.4|The Alleghanian orogeny, a result of three separate continental collisions. USGS]] The immense region involved in the continental collision, the vast temporal length of the orogeny, and the thickness of the pile of sediments and igneous rocks known to have been involved are evidence that at the peak of the mountain-building process, the Appalachians likely once reached elevations similar to those of the [[Alps]] and the [[Rocky Mountains]] before they were eroded.<ref name="aaas">{{cite web|publisher=AAAS |url=http://news.sciencemag.org/sciencenow/2009/11/03-02.html?etoc |archive-url=https://web.archive.org/web/20130120064259/http://news.sciencemag.org/sciencenow/2009/11/03-02.html?etoc |archive-date=2013-01-20 |title=The Mountains That Froze the World |work=Science magazine |first=Phil |last=Berardelli |date=2009-11-02 |url-status=dead }}</ref><ref name="usgs2">{{cite web|publisher=USGS |url=https://geology.er.usgs.gov/eespteam/Mtleconte/website/geology.html?etoc |archive-url=https://web.archive.org/web/20130117093921/http://geology.er.usgs.gov/eespteam/Mtleconte/website/geology.html?etoc |archive-date=2013-01-17 |title=Geology of the Great Smoky Mountains |access-date=2012-04-04 |url-status=dead }}</ref>
As the continents collided, the rock material trapped in-between was crushed and forced upward. With nowhere to go, rocks along the eastern margin of the North American continent were shoved far inland (the same occurred in the opposite direction along the margin of the African continent, forming the [[Atlas Mountains]] of [[Morocco]] and the western [[Sahara]]).<ref name=usgs/> Close to the boundary between the colliding plates, tectonic stresses contributed to the metamorphism of the rock (i.e. the transformation of [[igneous rock|igneous]] and [[sedimentary rock]] into [[metamorphic rock]]).
The sedimentary rock in the eastern [[Appalachian Basin]] region was squeezed into great [[Fold (geology)|folds]] that ran perpendicular to the direction of forces. The greatest amount of deformation associated with the Alleghanian orogeny occurred in the Southern Appalachians ([[North Carolina]], [[Tennessee]], [[Virginia]], and [[West Virginia]]). In that region, a series of great [[Fault (geology)|faults]] developed in addition to the folds. As the two continents collided, large belts of rock bounded by [[thrust fault]]s piled one on top of another, shortening the crust along the eastern edge of North America in the North Carolina and Tennessee region by as much as {{convert|200|mi|-2|order=flip}}. The relative amount of deformation gradually diminishes northward. The fold belt extends northward through [[Pennsylvania]] and gradually fades in the vicinity of the [[New York (state)|New York]] border. The [[Kittatinny Mountain]]s in northwestern New Jersey mark the northeasternmost extension of the high ridges of the [[Ridge-and-Valley Appalachians|Valley and Ridge Province]]. The influence of Alleghanian deformation on the regions east of the Valley and Ridge Province must have been even more intense; however, there is little evidence preserved. Rocks of [[Mississippian (geology)|Mississippian]], [[Pennsylvanian (geology)|Pennsylvanian]], and [[Permian]] age are missing along the Eastern Seaboard.<ref name=usgs>{{USGS|url=https://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm|archive-url=https://web.archive.org/web/20110722154205/http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm|url-status=dead|archive-date=2011-07-22|title=NYC Regional Province: Valley and Ridge Province}}</ref>
==Subsequent erosion== [[Image:Junction fault 0112.jpg|thumb|300px|Major fault at the dividing line between the Allegheny Plateau and the true Appalachian Mountains ([[Williamsport, Pennsylvania]]).]] The mountains formed by the Alleghanian orogeny were once rugged and high<ref>{{cite web|title=Mesozoic Basins|work=Geology of National Parks, 3D and Photographic Tours|publisher=USGS|date=21 Aug 2013|url=https://3dparks.wr.usgs.gov/nyc/mesozoic/mesozoicbasins.htm}}</ref><ref>{{cite web|url=http://csmres.jmu.edu/geollab/vageol/vahist/K-LatPal.html|title=Cross Section K, The Late Paleozoic Alleghanian Orogeny|work=The Geological Evolution of Virginia and the Mid-Atlantic Region|publisher=James Madison University|first=Lynn S.|last=Fichter|year=1999|access-date=2013-12-20|archive-date=2014-12-14|archive-url=https://web.archive.org/web/20141214093757/http://csmres.jmu.edu/geollab/vageol/vahist/K-LatPal.html|url-status=dead}}</ref> during the [[Mesozoic]] and late [[Paleozoic]] but in our time are eroded into only a small remnant: the heavily eroded hills of the [[Piedmont (United States)|Piedmont]]. [[Sediment]]s that were carried eastward formed the [[coastal plain]] and part of the [[continental shelf]]. Thus, the coastal plain and Piedmont are largely the byproducts of erosion that took place from 150+ million years ago to the present. Sediments that were carried westward formed the [[Allegheny Plateau|Allegheny]] and [[Cumberland Plateau|Cumberland]] plateaux. Although people living in those respective areas sometimes refer to them as mountains, they are more accurately [[dissected plateau|uplifted and eroded plateaux]].<ref>{{cite book|title=The Historic Cumberland Plateau: An Explorer's Guide|first=Russ|last=Manning|year=1999|publisher=University of Tennessee Press|page=6}}</ref>
A portion of the Alleghanian mountain system departed with Africa when Pangaea broke up and the [[Atlantic Ocean]] began to form. Today, this forms the [[Anti-Atlas]] mountains of Morocco. The Anti-Atlas have been geologically uplifted in relatively recent times and are today much more rugged than their Alleghanian relatives.
==See also== *[[Geology of the Appalachians]] *[[Central Pangean Mountains]] *[[Mauritanide Belt]] *[[Inliers and outliers (geology)]]
==References== {{reflist}}
[[Category:Orogenies of North America]] [[Category:Carboniferous orogenies]] [[Category:Permian orogenies]] [[Category:Carboniferous North America]] [[Category:Permian North America]] [[Category:Geology of North America]] [[Category:Appalachian Mountains]] [[Category:Allegheny Mountains]]