foundation system of burj al arab

Continental Drift

Definition

It is believed that there was a single great continent mass in the beginning which broke up into smaller units with the passage of time. The broken continental blocks started separating out and drifting apart over underlying elastic and viscous substratum under the influence of gravitational forces.


Nature of Continents and Sea Floor
The continents are the slabs of sial (crust) underlain by the basaltic layer. The ocean floors are essentially the basaltic. However, there are thin patches of sial present along the sea floors. Also the outer boundaries of continental blocks of each group are marked by orogenic belts (mountain ranges).


Background of this concept

The geological studies on different continents indicate that the continent have been experiencing changing climates in the geological past. Places having warm climates now were at one time cold region so. This raises the question that, was it the climate which was changing or the continents shifted their positions. The presence of similar plant and animal fossils and the identical geological structures on these widely separate continents also needs explanation that whether the in-between block have sunk or the continents were initially united and separated later on. The mountain building process also demand explanation for the horizontal compression and is related to the continental drift. The outer margins of the continents have mountain ranges marking their boundaries.
Wegener’s hypothesis of continental drift:
Wegener observed that the shorelines of some continents looked like they could fit into the shorelines of others, almost like a Jigsaw puzzle. Wegener also realized that there were geological elements common to both shorelines. Rocks of around 200 millions year of age (200 Ma) were similar on both sides and they contained identical land-based beasties (e.g. dinosaurs). He hypothesized that in the past (200 Ma), Africa and South America were united together. Not only that, he was able to put most of the continents together into a supercontinent, he called Pangaea. He published a map shown in fig-1 in 1912 and his idea soon became known as Continental Drift.


Hypothesis
He believed in the existence of single great continental mass, a compressed combination of Laurasia and Gondwanaland, to which he gave the name Pangaea. Where northern group of continents comprises Laurasia and southern group comprises Gondwanaland. This great continental mass was surrounded by a vast ocean called Panthalassa.

With the passage of time this single continental block, Pangaea, broke into pieces which started drifting apart, taking the present set-up. Along with continental drift, he also believed in the wandering of the poles. According to him the poles have been migrating from place to place.


Drifting Directions

According to Wegener the drift was equator-ward and west-ward. He ascribed the equator-ward movement of continents to the gravitational attraction caused by the earth’s equatorial bulge; and west-ward movement to the force of attraction caused by the sun and the moon. The equator-ward drift is stated to be responsible for the formation of the Alpine-Himalayan mountain ranges; and the west-ward drift responsible for the formation of Rockies and Andes mountain ranges. The gaps left in between the continents made the oceans.

Wegner’s hypothesis of continental drift very nicely explains the past climate variations , similarities in the geological structures and fossil content of the separated parts of the earth’s crust (continents) and mountain building.

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