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natural arch

{{Short description|Arch-shaped natural rock formation}}

{{Redirect2|Natural bridge|Natural bridges|other uses|Natural Bridge (disambiguation){{!}}Natural Bridge}}

File:Delicate Arch LaSalle.jpg in Arches National Park, Utah, United States]]

File:The Great Arch Saudi Arabia.jpg

A natural arch, natural bridge, or (less commonly) rock arch is a natural landform where an arch has formed with an opening underneath. Natural arches commonly form where inland cliffs, coastal cliffs, fins or stacks are subject to erosion from the sea, rivers or weathering (subaerial processes).

Most natural arches are formed from narrow fins and sea stacks composed of sandstone or limestone with steep, often vertical, cliff faces. The formations become narrower due to erosion over geologic time scales. The softer rock stratum erodes away creating rock shelters, or alcoves, on opposite sides of the formation beneath the relatively harder stratum, or caprock, above it. The alcoves erode further into the formation eventually meeting underneath the harder caprock layer, thus creating an arch. The erosional processes exploit weaknesses in the softer rock layers making cracks larger and removing material more quickly than the caprock; however, the caprock itself continues to erode after an arch has formed, which will ultimately lead to collapse.

The choice between bridge and arch is somewhat arbitrary. The Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed.[http://www.naturalarches.org/archinfo/faq.htm Natural Arch and Bridge Society] {{webarchive|url=https://web.archive.org/web/20090831165749/http://www.naturalarches.org/archinfo/faq.htm |date=August 31, 2009 }}, FAQ. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion."American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor

The largest natural arch, by a significant margin, is the Xianren Bridge in China, with a span of {{convert|400|±|15|ft|m|0|sp=us|order=flip}}.[http://www.naturalarches.org/big-FairyBridge.htm Big 14 Tour - Fairy Bridge] {{webarchive|url=https://web.archive.org/web/20170420191440/http://naturalarches.org/big-FairyBridge.htm |date=April 20, 2017 }}, The Natural Arch and Bridge Society

Coastline

File:Malta Gozo, Azure Window (10264176345).jpg, Malta, before it collapsed in 2017]]

File:Darwinarch.jpg , Galápagos Islands, Ecuador, before it collapsed in 2021]]

On coasts two different types of arches can form depending on the geology. On discordant coastlines rock types run at 90° to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland. Two examples of this type of arch are London Bridge in Victoria, Australia, and Neill Island in the Andaman Islands, India. When these arches eventually collapse, they form stacks and stumps. On concordant coastlines rock types run parallel to the coastline, with weak rock such as shale protected by stronger rock such as limestone. The wave action along concordant coastlines breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this type of arch are the Durdle Door and Stair Hole near Lulworth Cove on Dorset's Jurassic Coast in south England. When Stair Hole eventually collapses it will form a cove.

Weather-eroded arches

File:Metate Arch, Devils Garden, Grand Staircase-Escalante National Monument, Utah, USA.jpg, a very thin arch near the end of its life]]

Weather-eroded arches begin their formation as deep cracks which penetrate into a sandstone layer. Erosion occurring within the cracks wears away exposed rock layers and enlarges the surface cracks isolating narrow sandstone walls which are called fins. Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins. The resulting holes become enlarged to arch proportions by rockfalls and weathering. The arches eventually collapse leaving only buttresses that in time will erode.{{cite web |url=http://www.nature.nps.gov/geology/parks/arch/ |title=Geology Resources, Arches National Park |publisher=National Park Service |access-date=December 23, 2015 |url-status=live |archive-url=https://web.archive.org/web/20151223190124/http://www.nature.nps.gov/geology/parks/arch/ |archive-date=December 23, 2015 }}

Many weather-eroded arches are found in Arches National Park, Canyonlands National Park, and Grand Staircase–Escalante National Monument (GSENM), all located in southern Utah, United States.

{{wide image|Hoodoo formation-Big.jpg|600px|Progressive erosion producing plateau, fin, window (or arch), and hoodoos}}

Water-eroded arches

File:Coyote Natural Bridge map.jpg of Coyote Natural Bridge in Utah shows how the meandering Coyote Gulch carved a shorter route through the rock under the arch. The old riverbed is now higher than the present water level.]]

Some natural bridges may look like arches, but they form in the path of streams that wear away and penetrate the rock. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below.

Natural Bridges National Monument in Utah protects the area surrounding three large natural bridges, all of which were formed by streams running through canyons, the largest of which is named Sipapu Bridge with a span of {{convert|225|ft|m|sp=us}}. The Rainbow Bridge National Monument's namesake was also formed by flowing water which created the largest known natural bridge in the Western Hemisphere with a span of {{convert|234|ft|m}}, based on a laser measurement made in 2007. Xianren Bridge, also known as Fairy Bridge, in Guangxi, China is currently the world's largest known natural bridge with a span recorded at {{convert|400|ft|m|sp=us}} by the Natural Arch and Bridge Society in October 2010, with a precision of ±{{convert|15|ft|m|sp=us}}.Jett, Stephen C.[http://www.naturalarches.org/china/journal.html#fairybridge China Diary] {{webarchive|url=https://web.archive.org/web/20131229025325/http://www.naturalarches.org/china/journal.html |date=December 29, 2013 }}, The Natural Arch and Bridge Society{{cite web|url=http://www.naturalarches.org/big-FairyBridgeMeasurement.htm|title=Big 17 Tour - Measurement of Fairy Bridge|website=www.naturalarches.org|url-status=live|archive-url=https://web.archive.org/web/20131112172533/http://www.naturalarches.org/big-FairyBridgeMeasurement.htm|archive-date=November 12, 2013}}

Cave erosion

File:London Bridge before collapse.jpg , Victoria, Australia, before its partial collapse in 1990]]

Natural bridges can form from natural limestone caves, where paired sinkholes collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole.

Like all rock formations, natural bridges are subject to continued erosion, and will eventually collapse and disappear. One example of this was the double-arched Victorian coastal rock formation, London Bridge, which lost an arch after storms increased erosion.{{cite web|url=http://www.trekearth.com/gallery/Oceania/Australia/photo206866.htm|title=Port Campbell|website=www.trekearth.com|url-status=live|archive-url=https://web.archive.org/web/20080908102543/http://www.trekearth.com/gallery/Oceania/Australia/photo206866.htm|archive-date=September 8, 2008}}

Moon Hill in Yangshuo, Guizhou Province, China, is an example of an arch formed by the remnant of a karst limestone cave.

Arches as highway or railway bridges

File:Natural Bridge State Park (31044316221).jpg, Virginia]]

In a few places in the world, natural arches are utilized by humans as transportation bridges with highways or railroads running across them.

In Virginia, US Route 11 traverses Natural Bridge. Two additional natural arch roadways are found in Kentucky. The first, a cave erosion arch made of limestone, is in Carter Caves State Resort Park and has a paved road on top.{{cite web |url=https://parks.ky.gov/parks/resortparks/carter-caves/trails.aspx?id=105 |title=Natural Bridge Trail: Carter Caves State Resort Park |publisher=Kentucky State Parks |access-date=November 19, 2019}} The second, a weather-eroded sandstone arch with a dirt road on top, is on the edge of Natural Bridge State Park in Kentucky. The latter arch is called White's Branch Arch (also known as the Narrows) and the road going over it is usually referred to as the Narrows Road.{{citation needed|date=November 2019}}

In Europe, the Romanian village of Ponoarele has a road segment called God's Bridge that is {{cvt|30|m}} long and {{cvt|13|m}} wide, passing over a stone arch {{cvt|22|m}} high and {{cvt|9|m}} thick.{{cite web |url=https://www.romaniajournal.ro/travel/gods-bridge-the-natural-bridge-of-mehedinti-romania/ |title='God's Bridge', the Natural Bridge of Mehedinti, Romania |first=Ramona |last=Ciortescu |date=December 10, 2014 |publisher=Romania Journal |access-date=November 19, 2019}}

The railroad from Lima, Peru crosses the Rio Yauli on a natural bridge near kilometer 214.2 as it approaches the city of La Oroya.{{citation needed|date=November 2019}}

Notable natural arches

=Africa=

File:TadrartRouge3.jpg range, Algeria]]

File:Hole In The Wall.jpg

=Antarctica=

  • The Kerguelen Arch, Christmas Harbour, the Kerguelen Islands (collapsed ca. 1910){{cite web |first= |title=Kerguelen Islands, French Southern and Antarctic Lands (Part 1) - Iles Kerguelen, TAAF |url=http://www.discoverfrance.net/Colonies/Kerguelen.shtml |url-status=live |archive-url=https://web.archive.org/web/20120117194429/http://www.discoverfrance.net/Colonies/Kerguelen.shtml |archive-date=January 17, 2012 |website=www.discoverfrance.net}}
  • Scott Island in the Antarctic has a natural arch

=Asia=

File:Natural stone arch in tirumala.JPG, Tirumala, India]]

File:Arches in Timna Park in summer 2011 (7).JPG Park, Negev Desert, Israel]]

=Europe=

File:Praia da Marinha (2012-09-27), by Klugschnacker in Wikipedia (31).JPG, Caramujeira, Lagoa, Algarve, Portugal]]

File:Es pontas 2.jpg is a natural arch on the coast of Mallorca, Spain]]

File:Durdle Door Dorset Sunset.jpg, Dorset, the United Kingdom]]

File:Мала прераст у нањону Вратне.jpg in eastern Serbia]]

=North America=

==Canada==

File:QC Gaspesie RocherPerce tango7174.jpg, Quebec, Canada]]

==Caribbean==

==Mexico==

File:CaboSanLucasLandsEnd.JPG, Mexico]]

==United States==

{{See also|Arches National Park}}

File:Hawaii Volcanoes National Park 02.jpg, Hawaii Volcanoes National Park, Hawaii, United States]]

File:Landscape Arch Utah.jpg, Utah, United States – one of the longest natural arches in the world{{cite web |url=http://www.naturalarches.org/big9.htm |title=The Worlds Longest Natural Spans |website=www.naturalarches.org |publisher=Natural Arch and Bridge Society |access-date=January 10, 2017 |url-status=live |archive-url=https://web.archive.org/web/20161225013414/http://www.naturalarches.org/big9.htm |archive-date=December 25, 2016 }}]]

File:Utah Rainbow Arch.jpg, Utah, a natural bridge formed by a meandering watercourse]]

=Oceania=

File:Hole in the Rock at Motukokako Island (Piercy Island).jpg, Cape Brett, New Zealand]]

File:Rock arch south of Tunnel Beach.jpg, Dunedin, New Zealand]]

==Australia==

==New Zealand==

=South America=

File:20030924PPedraFurada9.jpg, Piauí, Brazil]]

Image:Morro da Igreja - Pedra Furada - Zoom.jpg, Santa Catarina, Brazil]]

See also

References

{{reflist}}

External links

{{Commons category|Natural arches}}

  • [http://www.naturalarches.org/ The Natural Arch and Bridge Society]
  • {{Cite AmCyc|wstitle=Bridge, Natural |short=x}}
  • [http://arcg.is/2noo9Vl World map of many natural arches]

{{Coastal geography}}

Category:Coastal and oceanic landforms

Category:Coastal geography

Category:Erosion landforms