Plate Tectonics and Landform Processes
The Aleutian Islands, Alaska
The Aleutian Islands are located along the southwestern coast of Alaska, ad at the northern edge of the Pacific plate. This plate runs along the Pacific coast of North America, with its well-known faults — the San Andreas and the Denali — causing the strike-slip plate motion that is familiar to residents of the Pacific coastal areas. However, at the Aleutian Island location — a convergent boundary — the strike-slip behavior changes to that of subduction. So, rather than the grinding of the North American plate and the Pacific plate against one another, at its northern edge, the Pacific plate drives under the North American plate. The 150 islands that make up the Aleutian arc create a sort of barrier between the Bering Sea to the north and the Pacific Ocean to the south. The Aleutian Islands are part of the emergent peaks of the Pacific Ring of Fire, which is a submarine mountain range that is the result of movement of the Pacific plate as it shifts downwards under the Bering Sea plate.
The action of the crust of the Pacific plate being pulled under the North American plate exposes it to intense heat which causes the crust to melt. Some of the melted crust finds its way back to the earth’s surface in the form of volcanoes. In fact, the volcanic action is so dramatic and persistent along the northern edge of the Pacific plate that an arc of volcanic islands has formed. This string of volcanoes is called the Aleutian arc and it extends over a length of 1,550 miles from western Alaska to the Kamchatka Peninsula in Russia. Volcanic activity is frequent in the Aleutian arc, which unfortunately has created economic repercussions. There are times when it is not possible for air traffic to follow prescribed flight paths; the volcanic ash and smoke produced by the volcanoes interferes with planes in the Asia-North America-Europe flight paths. Interestingly, 40 of the 76 volcanoes in the Aleutian Island chain have erupted during the last 250 years.
The Kuril Islands have been formed by the same forces as the Aleutian Island. The Kuril Islands stretch from the Siberian Kamchatka Peninsula to the Hokkaido a Japanese island to the south. Here too, for this island group, a lithoscpheric slab is being subducted under another slab. The edges of this slab melts when it has reached a depth with sufficiently high degrees of temperature, and the melted material from the slab rises and leaks into the earth’s crust to form a series of volcanoes. The volcanic island arcs take shape on the opposing edge of the subducted slap, and a trench is always associated with the subducting slab.
The Rift Valley, Africa
The earth is covered with a rigid outer layer called the lithosphere which consists of rock plates that float on the denser fluid layer beneath them. As heat from the interior of the earth causes deep movements to occur, the rock plates move slowly — about two inches per year — across the surface of the dense fluid layer. When the movement of the plates is away from one another — a divergent boundary — a rift is formed that widens over time. New plates are created when a rift splits apart older plates. The Rift Valley in Africa is typical of a rift which is an elongated basin that is bounded by the opposing and steeply dipping faults. The process is exceptionally well illustrated in East Africa, such that the newly forming plate has been named the Somalian plate.
An upswelling in the crust can form a dome that splits, typically into a Y-shaped fracture. The Great Rift Valley is part of a triple rift that is occurring between Saudi Arabia and Africa. The two arms of the Y-shaped fracture are in the Red Sea and the Gulf of Aden, while the stem of the Y is the Great Rift Valley. A deepening and widening rift will permit the surrounding waters to enter the fracture. Eventually a new sea or gulf will form on the land side as the Somalian plate separates from the Nubian plate of Africa.
Because most of the rift activity occurs on the ocean floor, the only two places on earth where the results of two major tectonic plates moving apart can be seen are the Great Rift Valley of eastern Africa and Pingvellir in the Reykjaneshryggur-Langjokull rift system of Iceland in the North Atlantic. The Pingvellir area shows the crustal rifting and volcanism characteristic of sea-floor spreading below.
The Alps, Europe
The development of The European Alps was not a straightforward occurrence, if only because so many different forces were in play over many millions of years. The Alps extend from southern Europe into Asia Minor, northern India, Russia, and China. The Alpine/Himalayan System consists of sediments deformed by convergent margin collisions between continents. The northward movement of the African plate caused collision with the Eurasian plate. This activity partly closed the Tethys Sea and created the Mediterranean Sea. Much of the structure of The Alps consists now of piles of allochthonous (original position) thrust sheets or nappes that were driven against the older landmasses or carried up onto them. A nappe is a large segment of rock that has been thrust far away from its original position by the thrust faulting of plate collisions between continents. The folds of the nappes bend back over themselves, like what is seen in the Jura Mountains of Switzerland — the open Jura-type decollement folds — and are sheared or break into large recumbent pieces. A thrust fault is particular kind of low dip-angle fault that forms during the compression and deformation of a region. The actual tectonic transport of the sheets involved both compression and gravity gliding — some of which was submarine — off of the raised tectonic welt. Also, there is evidence — in the form of the presence of nappes and decollement — of shortening of the crust through subduction. Further folding and deformation occurred by lateral compression (orogeny), beginning in the Late Mesozoic and ending in the Miocene. The High Alps exhibit a ruggedness that is due to the uplift that continues into the present, and also by erosion of Pleistocene glaciers.
The Himalayas is another example of mountains formed by a continent to continent collision. India is embedded in the Indo-Australian plate and it pushes relentlessly into the Tibetan region of the southern edge of the Eurasian plate. The towering sharp peaks of the Himalayas are still thrusting up by the action of the two plates, which has exhibited bending, twisting, and folding at the collision zone.
References
Aleutian Islands, OceanDots.com (2011). Retrieved http://www.oceandots.com/pacific/aleutian/
Island Arc Formation, Windows to the Universe, National Earth Science Teachers Association. (2011). Retrieved http://www.windows2universe.org/earth/interior/island_formation.html
Mosar, J. (1999). Present-day and future tectonic underplating in the western Swiss Alps: reconciliation of basement-wrench-faulting and decollement folding of the Jura and Molasse basin in the Alpine foreland, Earth and Planetary Science Letters, 173, 143 — 155. Retrieved https://www.unifr.ch/geoscience/geology/research_sites/
Mosar_Research/PDFfiles/alps-presentfuture.pdf
The Aleutian Islands, The North American Tapestry of Time and Terrain, NationalAtlas.gov (2011). Retrieved http://www.nationalatlas.gov/articles/geology/features/aleutians.html
The Great Rift Valley will lead to the Somalian plate, PlateTechnonics.com. (2011). Retrieved
http://www.platetectonics.com/oceanfloors/somali.asp
Wood, J. And Guth, A. (2011). East Africa’s Great Rift Valley: A Complex Rift System. Geology.com. Michigan Technological University. Retrieved http://geology.com/articles / east-africa-rift.shtml
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