Approximately 4.55 billion years ago, the Earth was just a ball of molten material. Since then, parts of the Earth have cooled forming the solid crust-mantle. This process has been occurring for roughly about 3.8 billion years. The mantle is about 2900 km. thick, which lies above a layer of molten magma that still exists today. The immense heat from the m

One of the most prominent signs that molten material resides below the crust and mantle is the display of volcanoes. Magma seeks out weak spots on the crust where it could seep out. Volcanoes are mostly present at fault lines especially at the ocean ridges where new magma is constantly being poured out. This accounts for about 81% of all magma that escapes to the surface. The other 19% rises at certain points rather than along fissures. On of the most famous examples of volcanic activity is The Ring of Fire, located around the Pacific Plate. There, a continuous 'ring' of volcanoes exists. 'Island arcs' are formed there by many volcanoes developing islands in the form of a curve. The longest island arc is the Aleutian Islands stretching more than 3000 miles from Alaska to Asia. One explanation for this arc is that the Pacific plate is rotating very slowly. The westward-moving plate moves away from the source of volcanic activity making the volcanoes arise in an arc due to the r!

ovement. Transform plates are caused by fracture zones. When a rift opens from the upwelling of magma it causes a crack in the crust. As new magma rises to the surface, the crack increases caused by the pressure, resulting in a horizontal faulting. The fractured plate pieces travel in the same direction as the original plate was traveling -- away from the ocean ridge.

During the early 1900's, a theory of a 'super-continent' was developed by Alfred Wegener. He was ridiculed for his ideas that continental drift produced the present positions of the continents from a single 'super-continent' called Pangea. This theory is widely accepted today, however. There was abundant evidence for Wegener to believe in the existance of Pangea. The shape of the continents could be pieced together like a giant jigsaw puzzle suggesting that the continents were once 'glued' together. The fossils found on the continents were not distinct to that particular land, but were also found in lands that were separated by thousands of kilometers of water. Fossils indicated that identical species existed in different continents. Geological structures also demonstrated that the continents were, in fact, one giant land mass; old mountain ranges from one continent matched with those from another (i.e., South America and Africa).

agma (approximately 2700(C) causes convection in the mantle (Figure 1). Convection is caused by non-uniform temperature in a fluid and density differences. This continuous convection is the cause of plate movement. Each complete cycle, called a convection cell, drives the plate in the direction of the cell. How does a 'solid' mantle move? The mantle may be solid but, as with most solids, it will deform if long-term stress is applied; "...like Silly Putty which seeps into the rug when left unattended, mantle material flows when subjected to small long-term stresses."1

forms the high alpine mountains such as the Himalayas which were caused by India crashing into Asia.

are produced by this compressional force. Like normal faults, these faults also cause vertical movements where one side is pushed upwards vertically on an inclined plane. These faults produce high vertical 'upward-fault' structures called horst.

If the mantle is always being convected back down into the depths of the Earth, then why doesn't the continents disappear in the deep ocean trenches as well? The crust contains two different crusts: the granite continental crust and the basaltic oceanic crust. Only the basaltic crust is thrust back into the Earth while the granite crust floats on top of it. This is due to the difference in densities. The granite crust is less dense (2.7 g/cm3) and thicker than the basaltic crust (2.8 g/cm3) making it seem as if the land is actually floating, instead of one big solid mass that extends down to the Earth. Using Broecker analogy:

ty of the ocean floor spreading can then be determined. It takes about 50 to 150 million years for the crust to trave

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Argon-40 Calcium-40, , Silly Putty, Using Broecker, Nevada Range, Germany Reverse, South Asia, Brunhes Epoch, Alfred Wegener, Alaska Asia, magnetic field, ocean floor, continental drift, floor spreading, sea floor, south america africa, ocean trenches, land masses, magma flows, earth's magnetic, hot magma, sea floor spreading, deep ocean trenches, magnetic field rock, ocean floor spreading,
Approximate Word count = 2665
Approximate Pages = 11 (250 words per page double spaced)