Physical Geology 2004

 

The Rock. Photo courtesy of http://www.rsverlag.ch/afrika.htm

The Rock of Gibraltar and Surroundings

Gibraltar is a fascinating place for many reasons. It is home to ardently British people living within sight of Africa. Its peak is home to a colony of wild apes. But most importantly, it is a spectacular rock rising nearly 1400 feet out of the sea.

Formation

The formation of the Rock of Gibraltar began during the Jurassic Period, when shells of marine organisms accumulated in an Atlantic Ocean much narrower than today and formed a layer of limestone. Compression at the boundary of the African and Eurasian plates then uplifted this layer above sea level. This uplift, combined with a lower sea level due to larger polar ice caps, was large enough to block the Strait of Gibraltar from about 6 to 4.5 million years ago, causing the Mediterranean Sea to dry up. Water from the Atlantic refilled the Mediterranean about 4.5 million years ago, roaring in through the Strait of Gibraltar.

Reliable information on how the uplifted limestone became the isolated rock it is today is surprisingly hard to find. It was obviously due to some sort of weathering process which eroded the surrounding limestone and left the Rock. I will speculate that what is now Gibraltar was once a promontory, and that its base was eroded by the ocean, leaving the Rock of Gibraltar as a gigantic sea stack. Sea level would have needed to be slightly higher than today for this to happen, as Gibraltar is now a peninsula, not an island. If anyone reading this can enlighten this undergrad geology student, please e-mail samueke@earlham.edu

The Rock and Its Features

A nice view of Gibraltar, showing its overall shape. Photo courtesy of http://www.tlfq.ulaval.ca/axl/europe/gibraltar.htm

Today, the Rock of Gibraltar forms a peninsula jutting out into the Strait of Gibraltar from the southern coast of Spain (Gibraltar itself is a British colony). The whole peninsula is about 3 miles long and somewhat under a mile wide. The Rock's highest point stands 1398 feet above the strait. Its east face forms a very impressive near-cliff, while its west slope is relatively gentle.

Calcite, the mineral which makes up limestone, dissolves slowly in rainwater. Over time, this process can form caves. (For more information on cave formation, click here.) Since the Rock of Gibraltar is made of limestone, it is not surprising that it contains a number of caves. St. Michael's Cave, located halfway up the western slope of the Rock, is a popular tourist attraction.

St. Michael's Cave. Photo courtesy of http://stevefazzio.com/portspainmoroc.htm

Gorham's Cave is located near sea level on the steep eastern face of the Rock. It is noteworthy because archaeological excavations in the cave have found evidence that Neanderthals used it as far back as 30,000 years ago. It is especially significant because plant and animal remains found in the cave (and others nearby) indicate that the Neanderthals had a highly varied diet.

Gorham's Cave is the opening furthest to the right. Photo courtesy of http://www.gib.gi/museum/gor.htm

Regional Geology

Gibraltar is located near the boundary between the Eurasian and African plates. Earthquakes occur in the region frequently.

There has been some interesting research done recently into the nature of the plate boundary near Gibraltar. One study suggested that the Eurasian and African plates are colliding along a line west of Gibraltar (the Azores-Gibraltar line), causing the oceanic lithosphere to undergo compressional deformation without subducting. This is unusual because most plate collisions involving oceanic lithosphere cause one of the oceanic plates to subduct.

It has more recently been theorized that Gibraltar lies on top of an active subduction zone, at which Atlantic oceanic lithosphere subducts under the western Mediterranean Sea. It does not follow any currently recognized plate boundary. Evidence for this conclusion includes deep-focus earthquakes east of Gibraltar, and an extinct volcanic arc around the edges of the southwestern Mediterranean.

Ocean Currents

The Strait of Gibraltar is the only natural entrance from the ocean to the Mediterranean Sea. Because of this, water flowing both into and out of the Mediterranean must pass through it. This flow forms two currents in the strait. A surface current brings water in from the North Atlantic. Mediterranean water, saltier because of that sea's high evaporation rate, sinks under the surface current and flows out into the North Atlantic below it. This lower current is thermohalene and is called Mediterranean Outflow water, or MOW.

As the MOW is exiting the Mediterranean, near the western end of the Strait, it encounters a sill, or sudden rise in the sea floor. This forces the flow up, where it runs into the water flowing in from the Atlantic. This collision generates a series of 'internal' waves which travel not along the surface, but along the boundary between the two currents. These travel back into the Mediterranean.

Photo from space. Internal waves are visible as lighter bands on lower right. For reasons I do not know, north and south are flipped. Photo courtesy of http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Features/Ocean_Features/internal_waves_gibraltar_strait.htm

At least one scientist, R. G. Johnson, controversially theorizes that we must dam the Strait of Gibraltar in order to avert a new Ice Age. His theory states that a higher evaporation rate in the Mediterranean, due to global warming, would lead to a stronger MOW. This would trigger a westward current which would divert the Gulf Stream towards eastern Canada. This warm water would cause more evaporation, leading to increased snowfall in eastern Canada. This snow would cause more of the sun's energy hitting the Earth to be reflected back into space, leading to lower temperatures and the expansion of the Arctic ice cap. Damming the Strait of Gibraltar, so the theory goes, would prevent this from happening. This theory does not seem to be widely accepted.

Impacts

The strategic value of an enormous rock commanding the entrance to the Mediterranean has shaped the human history of Gibraltar. It has been besieged at least 14 times. Its most recent rulers, the British, took over in 1713, and they have managed to hold onto it since then - giving rise to the expression, "safe as the rock of Gibraltar".

Literature Cited

Bryson, William. 1996 November. Gibraltar. National Geographic vol. 190 no. 5 pp. 54-71. Academic Search Elite. Accessed 2004 April 18.

Sartori, R. et. al. 1994 June. Eastern segment of the Azores-Gibraltar line (central-eastern Atlantic): An oceanic plate boundary with diffuse compressional deformation. Geology 22:555-8. GeoRef. Accessed 2004 April 19.

Gutscher, M.-A. et. al. 2002 December. Evidence for active subduction beneath Gibraltar. Geology 30:1071-4. GeoRef. Accessed 2004 April 19.

Price, J. et. al. 1993 February 26. Mediterranean Outflow Mixing and Dynamics. Science 259:1277-82. JSTOR. Accessed 2004 April 20.

Weisburd, S. 1985 March 9. New wave at the Rock: Standing in the Strait of Gibraltar. Science News 127:149. Academic Search Elite. Accessed 2004 April 20.

Marshak, Stephen. Essentials of Geology. New York: W. W. Norton, 2004.

http://www.arch.ox.ac.uk/db-powell/NB%20Projects%20Gibraltar%20Caves.htm

http://www.ifm.uni-hamburg.de/ers-sar/Sdata/oceanic/intwaves/gibraltar/intro/

http://www.clas.ufl.edu/users/pciesiel/gly3150/med_story.html

http://www-das.uwyo.edu/~geerts/cwx/notes/chap11/mow.html

 

Some links for non-geological information on Gibraltar

CIA World Factbook: http://www.cia.gov/cia/publications/factbook/geos/gi.html

Government of Gibraltar: http://www.gibraltar.gov.gi/

Author: Kevin Samuels
Creation/revision date: April 20, 2004

Link to other Student Webpages for 2004 Earlham Physical Geology

This website was prepared as an assignment for Geosciences 211 (Physical Geology) taught in the spring of 2004 at Earlham College, Richmond, Indiana.

Earlham College Geosciences Department Earlham Geosciences 211: Physical Geology

Copyright 2004 Earlham College. Revised April 20, 2004 . Send corrections or comments to mailto:samueke@earlham.edu