Editing Geology of the Appalachians (section)

== 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" />