What First Caused People to Consider That the Continents Were Once One Large Landmass?
Plate Tectonics: The Rocky History of an Idea
Close examination of a earth oft results in the observation that nigh of the continents seem to fit together like a puzzle: the west African coastline seems to snuggle nicely into the east coast of Due south America and the Caribbean sea; and a similar fit appears across the Pacific. The fit is even more hit when the submerged continental shelves are compared rather than the coastlines.
In 1912 Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a unmarried protocontinent which he called Pangaea (meaning "all lands"), and over time they have drifted apart into their current distribution. He believed that Pangaea was intact until the late Carboniferous period, about 300 1000000 years ago, when it began to pause up and drift apart. However, Wegener's hypothesis lacked a geological machinery to explicate how the continents could drift across the earths surface as he proposed.
Searching for evidence to further develop his theory of continental migrate, Wegener came across a paleontological newspaper suggesting that a land bridge had in one case continued Africa with Brazil. This proposed country span was an endeavor to explain the well known paleontological observation that the same fossilized plants and animals from the same fourth dimension period were found in South America and Africa. The same was truthful for fossils found in Europe and North America, and Madagascar and Bharat. Many of these organisms could not have traveled across the vast oceans that currently exist. Wegener's drift theory seemed more than plausible than state bridges connecting all of the continents. But that in itself was not plenty to support his idea. Some other observation favoring continental drift was the presence of evidence for continental glaciation in the Pensylvanian period. Striae left by the scraping of glaciers over the country surface indicated that Africa and South America had been close together at the time of this ancient water ice age. The same scraping patterns tin can be found along the coasts of South America and South Africa.
Wegener's drift hypothesis also provided an alternate explanation for the formation of mountains (orogenesis). The theory existence discussed during his time was the "Contraction theory" which suggested that the planet was once a molten ball and in the process of cooling the surface croaky and folded upward on itself. The big problem with this idea was that all mountain ranges should be approximately the aforementioned age, and this was known non to be truthful. Wegener'south explanation was that equally the continents moved, the leading edge of the continent would come across resistance and thus compress and fold upwards forming mountains near the leading edges of the drifting continents. The Sierra Nevada mountains on the Pacific coast of North America and the Andes on the declension of South America were cited. Wegener also suggested that India drifted northward into the asian continent thus forming the Himalayas.
Wegener somewhen proposed a mechanism for continental migrate that focused on his assertion that the rotation of the earth created a centrifugal strength towards the equator. He believed that Pangaea originated virtually the southward pole and that the centrifugal forcefulness of the planet caused the protocontinent to break apart and the resultant continents to drift towards the equator. He chosen this the "pole-fleeing strength". This idea was speedily rejected by the scientific community primarily because the bodily forces generated by the rotation of the earth were calculated to be insufficient to move continents. Wegener also tried to explicate the westward migrate of the Americas past invoking the gravitational forces of the sun and the moon, this idea was also quickly rejected. Wegener'due south inability to provide an adequate caption of the forces responsible for continental drift and the prevailing belief that the world was solid and immovable resulted in the scientific dismissal of his theories.
In 1929, about the time Wegener'due south ideas began to be dismissed, Arthur Holmes elaborated on 1 of Wegener's many hypotheses; the idea that the mantle undergoes thermal convection. This idea is based on the fact that as a substance is heated its density decreases and rises to the surface until it is cooled and sinks over again. This repeated heating and cooling results in a electric current which may exist plenty to crusade continents to move. Arthur Holmes suggested that this thermal convection was similar a conveyor belt and that the upwelling pressure could break apart a continent and so strength the broken continent in contrary directions carried by the convection currents. This idea received very little attending at the fourth dimension.
Not until the 1960's did Holmes' idea receive any attention. Greater agreement of the body of water floor and the discoveries of features like mid-oceanic ridges, geomagnetic anomalies parallel to the mid-oceanic ridges, and the association of island arcs and oceanic trenches occurring together and virtually the continental margins, suggested convection might indeed be at work. These discoveries and more led Harry Hess (1962) and R.Deitz (1961) to publish similar hypotheses based on drapery convection currents, now known equally "sea floor spreading". This idea was basically the same as that proposed by Holmes over xxx years earlier, merely at present there was much more prove to further develop and support the idea. To learn more about the current theories which describe the mechanisms backside continental drift get to the "Plate Tectonics: The Mechanism" page.
To learn more about plate tectonics, read the books I used as references listed below:
- The New View of the Earth by Seiya Uyeda, 1978 by W.H. Freeman and Co.
- The Earth's Dynamic Systems past W. Kenneth Hamblin, 1975 past Burgess Publishing Co.
- Global Tectonics past Philip Kearey & Frederick J. Vine, 1996 past Blackwell Sciences Ltd.
- Physical Geology by Carla Due west. Montgomery, 1987 by Wm. C. Brown Publishers.
Source: https://ucmp.berkeley.edu/geology/techist.html
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