CONTINENTAL DRIFT - ahmad ghani B.S geology part3, UOP

AHMAD GHANIB.S (HONS) GEOLOGYPART - 3 : 

AHMAD GHANIB.S (HONS) GEOLOGYPART - 3 PRESENTING… PLATE TECTONICS

DEVELOPMENT OF A THEORY : 

DEVELOPMENT OF A THEORY The Earth is a dynamic plate with an internal power source that continually shifts and reshapes the planet's surface. We know that the entire surface of the Earth is mobile and that this mobility accounts for many of the geological processes that shape our planet. Like a giant eggshell, the outer rocky layer of the Earth is broken into shifting plates which can cause entire continent to move, and whole oceans to open or close, as they jostle and compete for position. This dynamic view of the Earth forms the basis of a comprehensive theory known as "Plate Tectonics".

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Yet up to the recent twentieth century , the continents and ocean basins were considered fixed and permanent features of the Earth's surface and when the understanding of moving continents were first proposed by a "German meteorologist Alfred Wegener" as continental drift, it was met with ridicule from the scientific community of the time. But now we know that the Earth's surface is broken into a mosaic of shifting plates whose movement over the course of geologic time has dramatically changed the geography of our planet, and that is the basis of the "Plate Tectonics" theory, which has revolutionized our notion of the forces that shape our planet.

CONTINENTAL DRIFT: : 

CONTINENTAL DRIFT: German meteorologist Alfred Wegener proposed the hypothesis of “Continental Drift”. He proposed that once the continents were a single landmass or supercontinent during the Late Carboniferous Period, some 300 mya. He called this supercontinent “Pangea (or Pangaea) meaning “all lands”. According to which the fit of the coastlines of Europe, Africa and Americas was not coincidence. Rather the match was so striking because the continents of Europe, Africa and Americas were once joined together and had subsequently broken up and moved apart over very large distances.

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Wegener illustrated it by his own maps.

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THE JIGSAW FIT OF CONTINENTS: So what was Wegener’s evidence for continental drift? Of course, the jigsaw fit of continents. But it failed because the coastlines were known to be ephemeral features of Earth’s surface that could be changed by a single storm. So how could their shape have any meaning after millions of years of coastal erosion? But the best fit between the Atlantic continents are shown by computer analysis of their geometry to lie not at their coastlines, but along lines drawn halfway down their continental slopes at an ocean depth of 1000m (500 fathoms).

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Because the continental slopes are the fundamental features of the Earth’s surface than the coastlines. But this was not mapped in Wegener’s day, so he had no way of knowing the shape of the continental slopes. But at those days Wegener proposed evidences of features like ancient mountain belts, major fault lines and certain distinctive rock formations in support of his idea of continental drift, each of which was brought into continuity when the continents were reassembled.

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Along the Atlantic seaboard of N-America and Europe, several ancient mountain ranges like those of the Appalachian, Caledonian and Hercynian belts are abruptly truncated or cut off by coastlines.

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These mountains were known to have been formed in two main stages during the Mid-Paleozoic (350-400 mya) and Late Paleozoic (270-300 mya), before the breakup of Wegener’s single landmass during the Early Mesozoic (160-230 mya). The 2nd evidence was that the rocks of identical age and geology to those of the Late Paleozoic stage of Appalachians also occur in the Hercynian belt. When the continents were reassembled into Wegener’s single landmass, those ancient mountain ranges are brought together to form two continuous mountain belts. In Wegener’s view these were once continuous, but severed by the opening of the Atlantic Ocean.

The mystery of ancient climates: : 

The mystery of ancient climates: Other matching patterns lay in the rocks themselves and provided one of Wegener’s most compelling lines of argument. On many of the southern continents, very distinctive glacial deposits had been found that were formed during the Late Carboniferous, some 300 mya. Their widespread distribution across large areas of S-America, S-Africa, India and Australia suggested that these continents had experienced a massive continental glaciations during the Late Paleozoic.

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Detailed field measurements on the flow directions of the ice, as recorded in the grooves and scratch marks carved in the bedrock by the glaciers, which proved that these continents were once together.

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Therefore, according to Wegener, the Late Carboniferous continental glaciations did not take place at tropics. Rather the southern continents at that time were assembled near the South Pole. Long after the ice age was over, continental drift dispersed these continents and moved the glacial deposits towards regions closer to the Equator.

The puzzling fossil record: : 

The puzzling fossil record: Wegener also found evidences for continental drift in the distribution of certain fossils. For example, identical species of the Permian reptile “Mesosaurus” has been found on both sides of south Atlantic. Although it was a marine reptile and was not thought to be a strong swimmer and, like the modern alligator, would not have been able to swim across an entire ocean.

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Similarly, fossils of reptile “Cynognathus” has been found in S-America and Africa. While those of Triassic reptile “Lystrosaurus” had been found in Antarctica, Africa and India, which are separated today by major oceans. Wegener also found similar evidences in the distribution of an unusual, apparently stunted assemblage of fossil fern known as “Glossopteris”. This unique flora of Late Paleozoic had been found in S-America, S-Africa, India, Australia and Antarctica, as well as in the Madagascar. Reference: book= EARTH SCIENCE TODAY.

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QUESTIONS? THANKS..