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...
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…
