Editing Geology of the Appalachians

{{Short description|Geologic description of the Appalachian Mountains}}
{{Use American English|date=January 2023}}
{{Use mdy dates|date=January 2023}}
[[File:Appalachian Mountains of North America.jpg|thumb|upright=2|The Appalachian Mountains, as defined by physiographic classification.  This includes the Canadian classification of the Appalachian Uplands and the US classification of the Appalachian Highlands.]]

The '''geology of the Appalachians''' dates back more than 1.2 billion years to the [[Mesoproterozoic]] era<ref name="Thomas 2021">{{Cite journal |last1=Thomas |first1=William A. |last2=Hatcher, Jr. |first2=Robert D. |date=2021 |title=Southern-Central Appalachians-Ouachitas Orogen |journal=Encyclopedia of Geology |volume=4 |quote=The foundations of the Appalachian-Ouachita orogen were laid when the assembly of supercontinent Rodinia was completed. The collisional events were accompanied by high-grade metamorphism and magmatism during the Grenville orogeny in the time span of 1300–950 Ma. |via=Elsevier Science Direct}}</ref> when two continental [[Craton|cratons]] collided to form the supercontinent [[Rodinia]], 500 million years prior to the development of the range during the formation of [[Pangaea|Pangea]]. The rocks exposed in today's [[Appalachian Mountains]] reveal elongate belts of [[fold (geology)|folded]] and [[thrust fault]]ed marine [[sedimentary rock]]s, [[volcanic rock]]s, and slivers of [[ophiolite|ancient ocean floor]]—strong evidences that these rocks were deformed during [[plate tectonics|plate collision]]. The birth of the Appalachian ranges marks the first of several [[Mountain formation|mountain building]] plate collisions that culminated in the construction of Pangea with the Appalachians and neighboring [[Anti-Atlas]] mountains (now in Morocco) near the center. These mountain ranges likely once reached elevations similar to those of the [[Alps]] and the [[Rocky Mountains]] before they were eroded.<ref name="aaas">{{cite web |work=Science magazine |publisher=AAAS |url=http://news.sciencemag.org/sciencenow/2009/11/03-02.html?etoc |title=The Mountains That Froze the World |access-date=April 4, 2012 |first=Phil |last=Berardelli |date=November 2, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20130120064259/http://news.sciencemag.org/sciencenow/2009/11/03-02.html?etoc |archive-date=January 20, 2013 }}</ref><ref name="usgs2">{{cite web|publisher=USGS |url=http://geology.er.usgs.gov/eespteam/Mtleconte/website/geology.html?etoc |title=Geology of the Great Smoky Mountains |access-date=April 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130117093921/http://geology.er.usgs.gov/eespteam/Mtleconte/website/geology.html?etoc |archive-date=January 17, 2013 }}</ref>

== Geological history ==
=== Overview ===
The [[Appalachian Mountains]] formed through a series of mountain-building events over the last 1.2 billion years:<ref>{{cite web|title=Geologic Events Affecting Eastern North America|publisher=Columbia University|url=https://www.columbia.edu/~vjd1/East_NAm_geo_events.htm}}</ref><ref>{{Cite book |last=Byerly |first=Don W. |title=The Last Billion Years: A Geologic History of Tennessee |publisher=University of Tennessee Press |year=2013 |isbn=978-1572339743}}</ref>

* The [[Grenville orogeny]] began 1250 million years ago (Ma) and lasted for 270 million years.
* The [[Taconic orogeny]] began 450 Ma and lasted for 10 million years.
* The [[Acadian orogeny]] began 375 Ma and lasted 50 million years.
* The [[Alleghanian orogeny]] began 325 Ma and lasted 65 million years.

=== Proterozoic era ===
==== Grenville orogeny ====
[[File:Grenville extent.gif|thumb|Land added to Laurentia during the Grenville orogeny]]
The first mountain-building tectonic plate collision that initiated the construction of what are today the Appalachian Mountains occurred during the [[Mesoproterozoic|Mesoproterozoic era]] at least one billion years ago when the pre-North-American craton called [[Laurentia]] collided with other continental segments, notably [[Amazonian craton|Amazonia]]. All the other cratons of the Earth also collided at about this time to form the [[supercontinent]] Rodinia, which was surrounded by [[Mirovia|one single ocean]]. Mountain-building referred to as the [[Grenville orogeny]] occurred along the boundaries of the cratons. The present Appalachian Mountains have at least two areas which are made from rock formations that were formed during this orogeny: the [[Blue Ridge Mountains]] and the [[Adirondack Mountains|Adirondacks]].<ref name="Thomas 2021">{{Cite journal |last1=Thomas |first1=William A. |last2=Hatcher, Jr. |first2=Robert D. |date=2021 |title=Southern-Central Appalachians-Ouachitas Orogen |journal=Encyclopedia of Geology |volume=4 |quote=The foundations of the Appalachian-Ouachita orogen were laid when the assembly of supercontinent Rodinia was completed. The collisional events were accompanied by high-grade metamorphism and magmatism during the Grenville orogeny in the time span of 1300–950 Ma. |via=Elsevier Science Direct}}</ref>

==== Breakup of Rodinia ==== 
After the Grenville orogeny, the direction of the [[continental drift]] reversed, and Rodinia began to break up. The mountains formed during the Grenvillian era underwent [[erosion]] from [[weathering]], [[Glacial period|glaciation]], and other natural processes, resulting in the leveling of the landscape. The eroded sediments from these mountains contributed to the formation of sedimentary basins and valleys. 

For example, in what is now the southern United States, the [[Toccoa/Ocoee River|Ocoee basin]] was formed. Seawater filled the basin. Rivers from the surrounding countryside carried clay, silt, sand, and gravel to the basin, much as rivers today carry sediment from the midcontinent region to the [[Gulf of Mexico]]. The sediment spread out in layers on the basin floor. The basin continued to subside, and over a long period of time, probably millions of years, a great thickness of sediment accumulated.<ref name=Clark>{{Cite book |last=Clark |first=Sandra H. B. |url=https://pubs.usgs.gov/gip/birth/birth.pdf |title=Birth of the Mountains: The Geologic Story of the Southern Appalachian Mountains |publisher=United States Geologic Survey |year=1996 |pages=4 }}</ref> 

Eventually, the tectonic forces pulling the two continents apart became so strong that the [[Iapetus Ocean]] formed off the eastern coast of the Laurentian margin. The rocks of the [[Ridge-and-Valley Appalachians|Valley and Ridge province]] formed over millions of years, in the Iapetus. Shells and other hard parts of ancient marine plants and animals accumulated to form limey deposits that later became [[limestone]]. This is the same process by which limestone forms in modern oceans. The weathering of limestone exposed at the land surface produces the lime-rich soils that are so prevalent in the fertile farmland of the Valley and Ridge province.<ref name=Clark/>

During this continental break-up, around 600 million to 560 million years ago, volcanic activity was present along the tectonic margins. There is evidence of this activity in today's Blue Ridge Mountains. [[Mount Rogers]], [[Whitetop Mountain]], and [[Pine Mountain (Appalachian Mountains)|Pine Mountain]] are all the result of volcanic activity that occurred around this time. <ref>{{Cite book |last=Rankin |first=James W. |title=The volcanogenic Mount Rogers Formation and the overlying glaciogenic Konnarock Formation: Two late Proterozoic units in southwestern Virginia |publisher=USGS |year=1993 |doi=10.3133/b2029}}</ref> Evidence of subsurface activity ([[Dike (geology)|dikes]] and [[Sill (geology)|sills]] [[Igneous intrusion|intruding]] into the overlying rock) is present in the Blue Ridge as well. For instance, [[mafic]] rocks have been found along the Fries Fault in the central Blue Ridge area of [[Montgomery County, Virginia]]. <ref>{{Cite book |last=Kaygi |first=Patti Boyd |title=The Fries Fault near Riner, Virginia: an example of a polydeformed, ductile deformation zone |publisher=VT Works |year=1994}}</ref>

=== Paleozoic era ===
[[File:Eastern North American Paleogeography Middle Devonian.gif|thumb|[[Palaeogeography|Paleogeographic]] reconstruction showing the Appalachian Basin area during the [[Middle Devonian]] period.<ref name="url">{{cite web |url=http://jan.ucc.nau.edu/rcb7/nam.html |title=Paleogeography and Geologic Evolution of North America |first=Ron |last=Blakey |publisher=Northern Arizona University |work=Global Plate Tectonics and Paleogeography |access-date=July 4, 2008 |archive-url=https://web.archive.org/web/20080621201253/http://jan.ucc.nau.edu/rcb7/nam.html |archive-date=June 21, 2008 |url-status=dead }}</ref>]]

[[File:Appalachian Pennsylvania salient satfoto.jpg|thumb|The "Pennsylvania Salient" in the Appalachians appears to have been formed by a large, dense block of [[mafic]] volcanic rocks that became a barrier and forced the mountains to push up around it. 2012 image from NASA's [[Aqua (satellite)|Aqua satellite]].]]
[[File:Catskill section.gif|upright=1.3|thumb|Generalized east-to-west cross section through the central Hudson Valley region. [[United States Geological Survey|USGS]] image.]]During the earliest part of the [[Paleozoic]], the continent that would later become [[North America]] straddled the [[equator]]. The Appalachian region was a [[Passive margin|passive plate margin]], not unlike today's [[Atlantic Plain|Atlantic Coastal Plain]] province. During this interval, the region was periodically submerged beneath shallow seas. Thick layers of sediment and [[carbonate rock]] were deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated.<ref name="usgs">{{USGS|url=http://geomaps.wr.usgs.gov/parks/province/appalach.html|title=Geologic Provinces of the United States: Appalachian Highlands Province|access-date=September 2, 2007|archive-url=https://web.archive.org/web/20080114232613/http://geomaps.wr.usgs.gov/parks/province/appalach.html|archive-date=January 14, 2008|url-status=dead}}</ref>

During the middle [[Ordovician]] (about 458-470 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event ([[Taconic orogeny]]) in North America. The once quiet Appalachian passive margin changed to a very active plate boundary when a neighboring [[oceanic crust]], the Iapetus, collided with and began sinking beneath the North American craton. With the creation of this new [[subduction]] zone, the early Appalachians were born.<ref name="usgs" />

[[Volcano]]es grew along the [[continental margin]], coincident with the initiation of subduction. Thrust faulting uplifted and warped older sedimentary rock laid down on the passive margin. As mountains rose, erosion began to wear them down. Streams carried rock debris downslope to be deposited in nearby lowlands.<ref name="usgs" />

Mountain building continued periodically throughout the next 250 million years (the [[Caledonian orogeny|Caledonian]], [[Acadian orogeny|Acadian]], [[Ouachita orogeny|Ouachita]], [[Variscan orogeny|Hercynian]], and [[Alleghanian orogeny|Alleghanian]] orogenies). Continent after continent was thrust and [[Suture (geology)|sutured]] onto the North American craton as Pangea began to take shape. [[Terrane|Microplates]], smaller bits of crust too small to be called continents, were swept in one by one to be welded to the growing mass.<ref name="usgs" />

By about 300 million years ago (the [[Pennsylvanian (geology)|Pennsylvanian]] period), [[Africa]] was approaching the North American craton. The collisional belt spread into the [[Ozarks|Ozark]]-[[Ouachita Mountains|Ouachita]] region and through the [[Marathon Uplift|Marathon Mountains]] area of Texas. Continental collisions raised the Appalachian-Ouachita chain to a lofty mountain range on the scale of the present-day [[Himalayas]]. The massive bulk of Pangea was completed near the end of the Paleozoic era (the [[Permian]] period) when Africa ([[Gondwana]]) plowed into the continental agglomeration, with the Appalachian-Ouachita mountains near the middle.<ref name="usgs" />

=== Mesozoic era and later ===
Pangea began to break up about 220 million years ago, in the early [[Mesozoic]] (late [[Triassic]] period). As Pangea [[rift]]ed apart a new passive tectonic margin was born, and the forces that created the Appalachian, Ouachita, and Marathon Mountains were stilled. Weathering and erosion prevailed, and the mountains began to wear away.<ref name="usgs" />

By the end of the Mesozoic, the Appalachian Mountains had been eroded to an almost-flat plain.<ref name="usgs" /> It was not until the region was [[tectonic uplift#Isostatic uplift|uplifted]] during the [[Cenozoic]] era that the distinctive topography of the present formed.<ref>{{cite journal|first1=C. Wylie|last1=Poag|first2=William D.|last2=Sevon|title=A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic continental margin|journal=Geomorphology|volume=2|issue=1–3|date=September 1989|pages=119–157|doi=10.1016/0169-555X(89)90009-3|bibcode = 1989Geomo...2..119P }}</ref> Uplift [[River rejuvenation|rejuvenated]] the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams [[downcutting|downcut]] so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures.<ref name="usgs" /> The ridges of the Appalachian Mountain core represent erosion-resistant rock that remained after the rock above and beside it was eroded away.<ref name="usgs" />

== Physiographic provinces==
The geographic boundaries of the Appalachian Mountains follow a definition that accounts for all the land mass in the United States and Canada used by the [[US Geological Survey]] and the Geologic Survey of Canada using the science of physiography. The US uses the term Appalachian Highlands, and Canada uses the term Appalachian Uplands, to define contiguous regions that have similar geology, topography, history, and native plant and animal communities. (The Appalachian Mountains are not synonymous with the [[Appalachian Plateau]], which is one of the provinces of the Appalachian Highlands).{{cn|date=December 2023}}

=== Appalachian Basin ===
The Appalachian Basin is a [[foreland basin]] containing Paleozoic [[sedimentary rock]]s of early [[Cambrian]] through early [[Permian]] age. From north to south, the Appalachian Basin province crosses [[New York (state)|New York]], [[Pennsylvania]], eastern [[Ohio]], [[West Virginia]], western [[Maryland]], eastern [[Kentucky]], western [[Virginia]], eastern [[Tennessee]], northwestern [[Georgia (U.S. state)|Georgia]], and northeastern [[Alabama]]. The northern end of the Appalachian Basin extends offshore into Lakes [[Lake Erie|Erie]] and [[Lake Ontario|Ontario]] as far as the United States–Canada border. The province covers an area of about {{convert|185500|mi2|km2}} and is {{convert|1075|mi|km}} long from northeast to southwest and between {{convert|20|and|310|mi|km|-1}} wide from northwest to southeast.<ref name="appBasin">{{USGS|url=http://certmapper.cr.usgs.gov/data/noga95/prov67/text/prov67.pdf|title=Appalachian Basin Province (067)|author=R.T. Ryder}}</ref>

The northwestern flank of the basin is a broad [[homocline]] that dips gently southeastward off the [[Cincinnati Arch]]. A complexly thrust faulted and folded [[terrane]] (Appalachian Fold and Thrust Belt or Eastern Overthrust Belt), formed at the end of the Paleozoic by the [[Alleghanian orogeny]], characterizes the eastern flank of the basin. Metamorphic and igneous rocks of the [[Blue Ridge Thrust Belt]] that bounds the eastern part of the Appalachian Basin Province were thrust westward more than {{convert|150|mi|km}} over lower Paleozoic sedimentary rocks.<ref name="appBasin" />

==== Coal, oil, and gas production ====
The Appalachian Basin is one of the most important [[coal]] producing regions in the U.S. and one of the largest in the world. [[Bituminous coal]] has been mined throughout the last three centuries. Currently, the coal primarily is used within the eastern U.S. or exported for [[Electricity generation|electrical power generation]], but some of it is suitable for [[metallurgy|metallurgical]] uses. Economically important coal beds were deposited primarily during Pennsylvanian time in a southeastward-thickening foreland basin. Coal and associated rocks form a [[Clastic rock|clastic]] wedge that thickens from north to south, from Pennsylvania into southeast West Virginia and southwestern Virginia.<ref name="coal">{{USGS|url=http://pubs.usgs.gov/pp/p1625c/CHAPTER_A/CHAPTER_A.pdf|title=Executive Summary—Coal Resource Assessment of Selected Coal Beds and Zones in the Northern and Central Appalachian Basin Coal Regions|author=Leslie F. Ruppert}}</ref>

Discovery of oil in 1859 in the [[Drake Well]], [[Venango County, Pennsylvania]], marked the beginning of the oil and gas industry in the Appalachian Basin. The discovery well opened a prolific trend of oil and gas fields, producing from upper [[Devonian]], [[Mississippian (geology)|Mississippian]], and Pennsylvanian [[sandstone]] reservoirs that extend from southern New York, across western Pennsylvania, central West Virginia, and eastern Ohio, to eastern Kentucky.<ref name="appBasin" />

A second major trend of oil and gas production in the Appalachian Basin began with the discovery in 1885 of oil and gas in lower [[Silurian]] Clinton sandstone reservoirs in [[Knox County, Ohio]]. By the late 1880s and early 1900s, the trend extended both north and south across east-central Ohio and included several counties in western New York where gas was discovered in lower Silurian Medina Group sandstones. About 1900, large oil reserves were discovered in Silurian and Devonian carbonate reservoirs in east-central Kentucky. Important gas discoveries from the lower Devonian Oriskany Sandstone in [[Guernsey County, Ohio]], in 1924; [[Schuyler County, New York]], in 1930; and [[Kanawha County, West Virginia]], in 1936 opened a major gas-producing trend across parts of New York, Pennsylvania, Maryland, Ohio, West Virginia, Kentucky, and Virginia.<ref name="appBasin" /> Another drilling boom occurred in the 1960s in [[Morrow County, Ohio]], where oil was discovered in the Upper Cambrian part of the Knox [[Dolomite (rock)|Dolomite]].<ref name="appBasin" />

=== Crystalline Appalachians ===
[[File:Mid-Atlantic Appalachian rock types.gif|thumb|upright=2|Geological map of the southern Crystalline Appalachians]]
The Blue Ridge, Piedmont, Adirondack, and New England Provinces are collectively known as the Crystalline Appalachians because they consist of Precambrian and Cambrian igneous and metamorphic rocks.<ref name="nyc">{{cite web |title=The Sedimentary Appalachians |work=NYC Regional Geology |url=http://3dparks.wr.usgs.gov/nyc/valleyandridge/sedimentaryapp.htm |archive-url=https://web.archive.org/web/20110516070439/http://3dparks.wr.usgs.gov/nyc/valleyandridge/sedimentaryapp.htm |archive-date=May 16, 2011 |access-date=May 20, 2010 |url-status=dead }}</ref>

The Blue Ridge Thrust Belt Province underlies parts of eight states from central Alabama to southern Pennsylvania. Along its western margin, the Blue Ridge is thrust over the folded and faulted margin of the Appalachian basin, so that a broad segment of [[Paleozoic]] strata extends eastward for tens of miles, buried beneath these subhorizontal crystalline thrust sheets.<ref>{{cite journal|last1=Harris|first1=L.D.|last2=Harris|first2=A.G.|last3=de Witt, Jr.|first3=W.|last4=Bayer|first4=K.C.|year=1981|title=Evaluation of the southern eastern overthrust belt beneath Blue Ridge-Piedmont thrust|journal=American Association of Petroleum Geologists Bulletin|volume=65|issue=12|pages=2497–2505|doi=10.1306/03b599ef-16d1-11d7-8645000102c1865d}}</ref> At the surface, the Blue Ridge consists of a mountainous to hilly region, the main component of which are the Blue Ridge Mountains that extend from Georgia to Pennsylvania. Surface rocks consist mainly of a core of moderate-to high-rank crystalline metamorphic or igneous rocks which, because of their superior resistance to weathering and erosion, commonly rise above the adjacent areas of low-grade metamorphic and sedimentary rock. The province is bounded on the north and west by the Paleozoic strata of the Appalachian Basin and on the south by Cretaceous and younger sedimentary rocks of the [[Gulf Coastal Plain]]. It is bounded on the east by metamorphic and sedimentary rocks of the [[Piedmont (United States)|Piedmont Province]].<ref name="otherProv">{{USGS|title=Blue Ridge Thrust Belt (068), Piedmont Province (069), Atlantic Coastal Plain Province (070), Adirondack Province (071), and New England Province (072)|url=http://certmapper.cr.usgs.gov/data/noga95/prov71/text/prov71.pdf|author=Robert C. Milici}}</ref>

The Adirondack and New England Provinces include sedimentary, meta-sedimentary, and [[Intrusive rock|plutonic]] igneous rocks, mainly of Cambrian and Ordovician age, similar [[lithologically]] to rocks in the Blue Ridge and Piedmont Provinces to the south. The uplifted, nearly-circular Adirondack Mountains consist of a core of ancient Precambrian rocks that are surrounded by upturned Cambrian and Ordovician sedimentary rocks.<ref name="otherProv" />

== Notes and references ==
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{{refbegin}}
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* {{Cite journal |last1=Loewy |first1=S. L. |last2=Dalziel |first2=I. W. D. |last3=Pisarevsky |first3=S. |last4=Connelly |first4=J. N. |last5=Tait |first5=J. |last6=Hanson |first6=R. E. |last7=Bullen |first7=D. |title=Coats Land crustal block, East Antarctica: A tectonic tracer for Laurentia? |year=2011 |journal=[[Geology (journal)|Geology]] |volume=39 |issue=9 |pages=859–862 |url=https://www.researchgate.net/publication/232551854 |access-date=24 January 2016 |doi=10.1130/G32029.1 |bibcode=2011Geo....39..859L}}
{{refend}}

== External links ==
* [https://web.archive.org/web/20030209053037/http://tapestry.usgs.gov/ USGS.gov: Tapestry]
* [http://pubs.usgs.gov/dds/dds11/ USGS.gov: dds]

[[Category:Appalachian Mountains|*]]
[[Category:Geology by mountain range|Appalachians]]
[[Category:Regional geology of the United States|Appalachians]]
[[Category:Geology of Newfoundland and Labrador|Appalachians]]
[[Category:Geology of Quebec|Appalachians]]
[[Category:Geology of Nova Scotia|Appalachians]]
[[Category:Geology of New Brunswick|Appalachians]]
[[Category:Allegheny Plateau|.]]
[[Category:Cenozoic United States]]
[[Category:Cenozoic Canada]]
[[Category:Ordovician United States]]
[[Category:Ordovician Canada]]