The Evidence In Support Of The Theory Of Continental Drift
- Pages: 5
- Word count: 1160
- Category: Earth
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The continental drift theory was first proposed by a German called Alfred Wenger in 1915. He suggested that the earth’s continents were once joined together, but gradually moved apart over millions of years. This once ‘supercontinent’ was known as Pangea – which translates to ‘’All Earth’’. Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents. However, Wenger did not have the evidence to explain how these continents could move. Over the course of this essay I will review some of the evidence that support continental drift, such as fossil records and mountain ranges. One of the strongest pieces of evidence is Continental break up/fit, this is the idea that the continents were once joined together. This theory became apparent when Alfred Wenger identified that the edge of tectonic plates, continental shelf, seeming to fit together in a jig-saw fashion.
For example, if you take the eastern edge of South America and the Western side of Africa they fit together almost perfectly, too perfect for it to be a coincidence. Note that it is the edge of the tectonic plates themselves that fit together and not the forever changing individual shorelines of the countries. Continental fit is the case with all the continents around the world. Therefore continental break up is a strong piece of evidence to support the theory of continental drift. Geology of rock formations and mountain ranges on separate continents were also suggested by Wenger as evidence of continental drift. Areas of South America and Africa have rocks of the same age, around 550 million years old, and composition. Mountain ranges tell a similar story, For example the Cairngorm mountain range in countries such as Scotland, Norway & Finland are very similar to the mountain range on the east coast of North America Called the Rockies. Both these mountain ranges show consistently similar minerals, bedding planes and sedimentary layers, beyond which could be a coincidence.
We already know that if you match up the continents they fit together so for both continents to have such similar rock types must mean that they formed under the same conditions, this could only be possible if the continents were once joined. As well as geological evidence to back up the continental drift theory, Wenger also used biological evidence to back him up, this was done with the aid of fossil records. By fitting the land masses up together Wenger was able to match up the distribution of some fossils, e.g. Mesosaurus (all reptiles) and Glossopteris (a plant). Remains of Mesosaurus, a freshwater crocodile-like reptile that lived during the early Permian, are found solely in Southern Africa and Eastern South America. As a freshwater species it would have been physically impossible for the Mesosaurus to swim the current distance between the two continents, therefore this can only suggest that the two continents were once joined when the organisms were alive, hundreds of millions of years ago. Similar to the fossil theory, living species are also a key piece of evidence.
The same living organism can be found on different continents, for example, earthworms of the family Megascolecidae are found in the Gondwana areas of continental break up (the more southerly of the two supercontinents). Found in New Zealand, parts of Asia and North America it’s unlikely that some of them migrated across the oceans, or evolved in different locations. Another piece of biological evidence is fossil forests. Wenger used the giant fern Glossopteris as one example. The fossil remains have been found in Southern Africa, South America, Australia, India and coal deposits in Antarctica. As Glossopteris seeds are very heavy Wenger argued that they could not possibly be blown such a distance by wind. To add to this the fern flourishes in cold conditions, the current climate in Southern Africa, South America, Australia and India in which the fern is found in are all within the tropics, are not the right conditions for it go grow. This lead Wenger to believe that there once existed a huge forest many times the size of the current amazon, made up of entire Glossopteris which spanned much of southern Pangea. Climatology is the study of the climate and changes in weather conditions over time.
There’s evidence that the past climates of some continents were similar, despite being thousands of miles apart now. This suggests that they were once located together and in a different place on the earth to where they are now. Similar glacial deposits are found in Antarctica, Africa, South America, India and Australia. By fitting these places together you can match up the distribution of the deposits, If the continents haven’t moved, then this would suggest an ice sheet extended from the south pole to the equator at this time, which suggests that they were joined together millions of years ago and located close to the South Pole. Large coal deposits that were formed in tropical conditions have been found in North America and parts of Europe. This suggests these regions were once closer to the equator than they are now. So there is lots of evidence to suggest that the continents were in fact once together but still are still yet to explain the mechanism that drives the movement and separation of them.
Wenger proposed that the continents slid over the over the ocean floor, but geologist argued this could not happen. It wasn’t until the 1950’s that a creditable mechanism was proposed – Paleomagnetism & sea floor spreading. Paleomagnetism is the study of the history of the Earth magnetic field, it has provided the evidence for the process of sea floor spreading. Once every 200,000 years or so the earth’s magnetic field reverses polarity. Magnetic minerals in the crust created in normal polarity are perfectly aligned in the opposite direction to those in crust created in periods of reversed polarity. This creates a series of alternating magnetic strips along the sea floor. The stripes show that the crust is older the further away from the mid ocean ridge, this means the plates are moving apart.
Sea floor spreading was an idea proposed by Harry Hess in the early 1960’s. Sea floor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Magma rises up the gap created and then cools to from new crust. Seafloor spreading helps explain continental drift in the theory of plate tectonics. So to conclude, there are at least 6 different pieces of evidence that support the theory of continental drift. One of the strongest pieces of evidence is the jig-saw like fit between all of the continents. Those alongside any biological evidence, e.g. fossil records, are very strong ideas to support the theory of continental drift.