are you drifting? no, it's the continents!
Continental drift
CONTINENTAL DRIFT IN ACTION
Continental drift is the movement of the Earth’s continents relative to each other by appearing to drift apart/together across the ocean bed. It is the theory that continental land masses have altered overtime. Francis Bacon was the first to notice a Jigsaw fit between continents in the seventeenth century; however this theory is largely attributed to the German meteorologist, Alfred Wegener, who in 1912 wrote a paper offering a range of evidence in support of this theory. He was trying to suggest that all continents were once joined in a superior land mass called Pangaea.
Alfred Wegener, being a meteorologist, had proposed tidal forces and pole flight force as main driving mechanisms for continental drift. However, these forces were considered far too small to cause such large scale move of enormous land masses. Therefore, Wegener converted to convection currents as the main driving force in the last edition of his book in 1929. The plate tectonic theory explains the idea that the lithosphere (The outer, rigid layer of the earth) is divided into 8 major plates and several minor tectonic plates that move across the earth’s surface caused by convection currents within the Earth’s mantle. Convection currents are formed when the rock in the asthenosphere is heated by the outer core and turned molten. This molten rock is less dense and lighter meaning that it will rise through the surrounding material up the mantle; after this molten rock cools and solidifies (When it reaches colder regions of the lithosphere) it becomes more dense and heavier so it sinks back down through the mantle. It moves the plates by creating a pulling/pulling force on the plates that sit on the mantle; it creates this moving motion of the plates.
Evidence for Continental drift:
Wegener, Bacon and predecessors came up with many pieces of evidence to support this theory of continental drift; here are a few pieces of evidence that were produced:
· Rock types and geological structures are similar on two sides of the Atlantic, e.g. Appalachian mountains in North America and Caledonia mountains in Scotland both have the same sequences of igneous and sedimentary rocks
· The way in which continents such as South America and Africa appear to fit together like pieces in a jigsaw suggesting that they may have once been joined together in the supercontinent of Pangaea
· Fossils of a small fern, Glossopteris, are found widely across all the southern continents
· Coal and oil reserves found in Antarctica suggest that this area was once in a different climate because it requires the decomposition of biological matter which needs heat for the decomposition to work
· Glacial striations in Brazil and West Africa along with glacial deposits in India, South America and Vaal Valley in South Africa indicate that these areas once had very similar climatic conditions
· Fossils of Triassic Period reptiles have been found in areas that are now far apart. Examples include Cynognathus
in South America and Africa, and Lysosaurus in India, Africa and Antarctica
· Fossils such as Mesosaurus (a small freshwater reptile) are found in both Brazil and West Africa. This is interesting because if the Mesosaurus has always been a freshwater reptile it would not have had many ways in crossing the Atlantic between South America and Africa
Acceptance of the theory of continental drift:
The theory of continental drift was not accepted for many years. One problem was that a plausible driving force was missing. And it did not help that Wegener was not a geologist. Other geologists also believed that the evidence
that Wegener had provided was not sufficient.
The British geologist Arthur Holmes supported the theory of continental drift at a time when it was deeply unfashionable. He proposed that the Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface. His Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.
David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could I prove there was a force that could move continents, then he might think about it. The idea was moonshine, I was informed."
Alfred Wegener, being a meteorologist, had proposed tidal forces and pole flight force as main driving mechanisms for continental drift. However, these forces were considered far too small to cause such large scale move of enormous land masses. Therefore, Wegener converted to convection currents as the main driving force in the last edition of his book in 1929. The plate tectonic theory explains the idea that the lithosphere (The outer, rigid layer of the earth) is divided into 8 major plates and several minor tectonic plates that move across the earth’s surface caused by convection currents within the Earth’s mantle. Convection currents are formed when the rock in the asthenosphere is heated by the outer core and turned molten. This molten rock is less dense and lighter meaning that it will rise through the surrounding material up the mantle; after this molten rock cools and solidifies (When it reaches colder regions of the lithosphere) it becomes more dense and heavier so it sinks back down through the mantle. It moves the plates by creating a pulling/pulling force on the plates that sit on the mantle; it creates this moving motion of the plates.
Evidence for Continental drift:
Wegener, Bacon and predecessors came up with many pieces of evidence to support this theory of continental drift; here are a few pieces of evidence that were produced:
· Rock types and geological structures are similar on two sides of the Atlantic, e.g. Appalachian mountains in North America and Caledonia mountains in Scotland both have the same sequences of igneous and sedimentary rocks
· The way in which continents such as South America and Africa appear to fit together like pieces in a jigsaw suggesting that they may have once been joined together in the supercontinent of Pangaea
· Fossils of a small fern, Glossopteris, are found widely across all the southern continents
· Coal and oil reserves found in Antarctica suggest that this area was once in a different climate because it requires the decomposition of biological matter which needs heat for the decomposition to work
· Glacial striations in Brazil and West Africa along with glacial deposits in India, South America and Vaal Valley in South Africa indicate that these areas once had very similar climatic conditions
· Fossils of Triassic Period reptiles have been found in areas that are now far apart. Examples include Cynognathus
in South America and Africa, and Lysosaurus in India, Africa and Antarctica
· Fossils such as Mesosaurus (a small freshwater reptile) are found in both Brazil and West Africa. This is interesting because if the Mesosaurus has always been a freshwater reptile it would not have had many ways in crossing the Atlantic between South America and Africa
Acceptance of the theory of continental drift:
The theory of continental drift was not accepted for many years. One problem was that a plausible driving force was missing. And it did not help that Wegener was not a geologist. Other geologists also believed that the evidence
that Wegener had provided was not sufficient.
The British geologist Arthur Holmes supported the theory of continental drift at a time when it was deeply unfashionable. He proposed that the Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface. His Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.
David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could I prove there was a force that could move continents, then he might think about it. The idea was moonshine, I was informed."
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