Teledemocracia y administración electrónica

A great deal of evidence has been gathered to support the fact that Wegener’s vast continent of Pangea began to break apart about 200 million years ago. The breakup of Pangea and the formation of the Atlantic Ocean basin apparently occurred over a span of nearly 150 million years, with the last phase.

It seems reasonable to assume that the slow movement of mantle material could initiate continental margins and the large number of hot spots located along ridge crests led some geologists to a different conclusion. They have proposed that hot sports initiate continental fragmentation.

Hot spots are generally characterized by large outpourings of basaltic lava for relatively long periods of time. Worldwide, as many as 120 isolated volcanic sites have been attribut4ed to hot spot activity. Since hotspots appear to remain

nearly stationary over extended time periods, they form volcanic trails, such as the Hawaiian chain, upon the moving oceanic plates above.

The Canadian geologist J Tuzo Wilson and his associated have suggested that when a thick segment of continental lithosphere remain stationary over a hot spot for an extended period, the conditions are right for continental rifting.

Initially, upwelling of material from below generates a dome, roughly 200 kilometres in diameter, within the overlying continental crust. As the dome enlarges, it fractures with a characteristics three-armed pattern. Rifting continues along tow of the arms, resulting in the development of a new ocean basin, while the third arm often fails to develop further. An example of such a three-armed rift system is believed to be represented by the Red Sea, the Gulf of Aden and the Afar lowlands. Here the arm extending from the Afar Lowlands into the interior of Africa is the failed arm. The two active arms have subsequently generated long, narrow seas.

Professor Wilson suggested that about 20 hot spots guided the fracturing of Pangea.

In 1966, Willson also proposed that several times in the geologic past the continents joined to form a supercontinent, which later broke apart. The term Willson cycle is now applied to the cyclic processes that are responsible for the rifting of continents to form ocean basins and the subsequent closing of ocean basins to produce supercontinents. In their modern form, Wilson cycles begin with a stationary supercontinent composed of thick continental crust. Because continental crust is a poor conductor of heat, it acts like a thick blanket retarding the outward flow of heat from the mantle. Thus, a buildup of heat causes the supercontinet to bulge upward and eventually break apart. Upwelling of hot material between the rifted continental fragments produces a new ocean floor. This activity also increases the rate of heat flow from the mantle. As the ocean basin grows in size, the sea floor cools and becomes denser. After perhaps 200 million years, the oldest part of the ocean floor becomes dense enough to sink into the

mantle. The subduction of the continents, which eventually collide to once again form a supercontinent. The time period of each complete cycle, from breakup to reassembly of the continents, is thought to take about 500 million years.

Although the Wilson cycle has not yet gained general acceptance, it does seem to explain the evolution of the continental margins surrounding the Atlantic.

In particular, the closing of the proto-Atlantic is believed to have caused a collision between North America and Africa and Europe that resulted in the formation of the Appalachian mountain belt. The breakup that followed this event is once again dispersing these continental fragments around the globe.

When an ocean closes and reopens, the zone of fragmentation may not occur at the same location as the suture where the landmasses were joined. During the closing of the proto-Atlantic about 400 million years ago, the suture formed along a mountainous belt extending from Alabama to the British Isles and Norway.

However, when the Atlantic began to reopen about 200 million years ago, the split occurred along a some what different trend. Thus the sizes the shape of continental fragments appears to change through time.

ISOSTASY

The state of balance which the earth’s crust tends to maintain or to return to by isostatic compensation if anything occurs to upset that balance. This is based on the principle of buoyancy first outlined by Archimedes. It is best illustrated by a high mountain chain, which rises above the surface of the earth, but has to be compensated by deep roots.

Isostasy is a condition of equilibrium between floating landmasses and the asthenosphere beneath them, maintained despite the forces that tend to change the landmasses all the time.

Historical Background:

• In 1840’s Sir George Everest conducted the first topographical survey in India.

• He surveyed the distance between two towns Kalianpur and Kaliana, located South of Himalaya.

• He surveyed by both triangulation as well as Astronomical methods

• But astronomical calculation placed these two cities closer by 150m

• JH Pratt estimated the mass of Himalayas and discovered that the error should have been 3 times than that of observed

• George Airy suggested lighter crustal rocks inside mountains, that extend far beneath them and these mountains would exert a smaller gravitational attraction than that Pratt has calculated.