“Volcanic and seismic events are major pieces of evidence towards proving that the plate tectonic theory is valid” Discuss the extent to which you agree with this statement (40). The theory of plate tectonics explains the structure and motion of the Earth’s lithosphere. The theory states that the Earth’s crust is split into large sections called tectonic plates, and these move relative to one another creating boundaries at which the plates converge, diverge or move past each other.
These plates are either continental or oceanic and are powered by convection currents, which is the circular movement of magma that comes from within the mantle. These currents are powered by the core, which heats the magma, causing it to rise, cool and fall back down. This circular motion causes the plates, which float on the mantle, to move. In 1912, Alfred Wegener, a German meteorologist, was the first man to state that the continents were once joined in a super continent called Pangaea, conversely he couldn’t explain why and what happened to cause the plates to move apart.
He based his theory on the extraordinary fit of the South American and African continent coastlines. Notably the eastern edge of South America and the western edge of Africa showed very similar geological features suggesting that at some point in the Earth’s history the landmasses were joined together. Another indicator that the continents were once distributed differently was geological evidence of glaciations in India – it is unlikely that glaciers could ever reach such low latitudes, but this problem can easily be explained by the theory of continental drift.
Fossil distribution also provided some of the earliest evidence for plate tectonics. Interestingly, plant and animal fossils were found on the matching coastlines of South America and Africa. These are now widely separated by the Atlantic Ocean, therefore he reasoned that is physically impossible for most of these organisms to have swum or have been transported across the wide oceans. From this he suggested that the plates were once connected. Unfortunately, at the time of Wegner’s work many of the geophysical tools that are used today did not exist making the theory much more difficult to support.
Since the original theory in 1912, newer evidence has appeared which supports the theory. In 1962 Hess studied the age of rocks around the mid-Atlantic ridge. He discovered that the newest rocks were closest to the ridge and the oldest were towards the USA and Caribbean. He therefore reasoned that the earth’s crust was expanding along the oceanic ridges, so it must be shrinking elsewhere. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. The old oceanic crust was consumed in the trenches causing new magma to rise and erupt along the spreading ridges to form a new crust.
He explained why the earth doesn’t get bigger with sea floor spreading and why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are so much younger than continental ones. Paleomagnetism is one of the strongest pieces of evidence for plate tectonic theory and was developed to convince scientists of the theories validity. Basalt lava forms volcanoes when it rises to the surface, cools and then forms land. When new crust is formed certain minerals align themselves with the Earth’s magnetic field.
Fascinatingly, new technology has shown that the magnetic field of the Earth is known to reverse every few hundred thousand years. So this implies new material is constantly being produced and the sea floor is spreading. However, as the Earth is not changing in size, material is being destroyed in other areas which are called subduction zones, and the position of the Earth’s crust must be constantly changing. Seismic activity can also tell us a great deal about plate tectonics. Plotting the locations of large earthquakes allows us to see where they most frequently occur.
Earthquakes are caused by the movement of tectonic plates. The plates move past each other which causes friction causing pressure to build up resulting in sudden jolts. This only happens on plate boundaries meaning areas that lie on them experience the most intense earthquakes. The fact that earthquakes appear in connected lines as opposed to clusters is good evidence for the theory of global plate tectonics. Volcanic events are also useful as they provide good evidence for the theory of plate tectonics.
Similarly to earthquakes, plotting the locations of active volcanoes on a map of the world will also be useful as it will show a similar pattern. The three main places where volcanoes are created are at subduction zones, constructive plate boundaries and in hotspots, with around 75% of the world’s volcanoes being located on the ‘Pacific Ring of Fire’. The area where two plates converge is called a subduction zone, here one plate is pushed underneath the other due to differing in density. This results in magma rising up to form volcanoes or volcanic island arcs.
At constructive plate boundaries, new material is created by magma rising through the crack. The fact that volcanoes in certain areas have different types of eruption provides good evidence for plate tectonic theory. All the above reasons suggest that the theory is valid. However, there are some anomalies which go against the theory. Not all volcanoes occur on plate boundaries, some occur on hotspots which are areas where the mantle is particularly hot, causing it to rise and create volcanoes on the crust above.
This suggests that plate tectonics theory may not be valid as volcanoes can exist without the theory being correct. Hawaii is a notable example of an active hotspot – the islands are volcanic yet lie in the middle of the Pacific plate. It is part of a chain of extinct volcanic islands in the Pacific Ocean which decrease in both age and size as you move north-west. Since the hotspot is created by the underlying mantle this suggests that the crust is moving over this hotspot meaning the volcanoes eventually become extinct and eroded away.
Since Wegener’s first theory, there is now a great deal of evidence to support the theory of plate tectonics. Seismic and volcanic events can provide lots of evidence to support the theory, although much of it requires tools which were not available when the theory was first developed such as accurate methods of mapping earthquakes. This made it difficult to prove the theory of plate tectonics as there were no hard facts. Although others may disagree, there are large amounts of evidence to prove the theory and so in my opinion the theory is a valid method.