EVIDENCE FOR PLATE TECTONICS

CONTINENTAL FIT The first mention of continental fit goes as far back as 1620 when Francis Bacon matched the rough outlines of South America and Africa. Alfred Wegener built on these ideas and found other countries aligned as well. He came up with the idea of Pangea and continental drift in However, his ideas where rejected as he was not an expert in the field. Since then, his ideas of continental drift have become widely accepted as new evidence supports the claim. One of the main problems with the idea of continental fit was that the land didn’t match up exactly but the continental shelves did. The continents don’t match exactly as erosion takes place destroying the land closest to the sea. Geological, Biological and Climatological evidence also supports this theory.

GEOLOGICAL EVIDENCE Rock Similarities – in South-east Brazil and South Africa rocks have been found that have the same age, type and show the same signs of their formation but they are on opposite sides of the Atlantic Ocean. Mountain Chains – mountain chains, likes similar rocks, can be matched up in places like Eastern USA and Northern Europe when placed together through Pangea.

BIOLOGICAL EVIDENCE Fossil Distributions – fossils of similar plants and animals can be found in many countries that are not situated near each other today but are when Pangea is assembled. There are two main examples for this one being the fossils of the mesosaurus that is only found in South America and Southern Africa and the second are the fossils of gymnosperm found across the southern continents of Pangea providing strong evidence that the continents where all joined. Marsupials – marsupials is only a small piece of evidence on its own but provides strong evidence for Pangea when added with others. Marsupial species, like Kangaroo, are only found in Australia today which suggests the land mass was one of the first to break away from Pangea before predators wiped out marsupials in Australia like they had done on the mainland.

CLIMATOLOGICAL EVIDENCE Coal Deposits – coal deposits found in the UK, North America and Antarctica are all of similar age and formed under tropical conditions. However, these countries are no longer near the equator meaning they must have drifted away at some point in time. India’s Migration – India has several pieces of evidence for glaciation similar to that in Antarctica. This suggests India broke away from the South Pole and is making its way north 3.7cm a year. This has caused it to crash into the Eurasian plate creating the Himalayas mountain range.

SEA FLOOR SPREADING The Mid-Atlantic Ridge was discovered in 1948 as chain of sea mounts on a constructive plate margin. After dating the ocean floor scientists found that the rocks closest to the ridge and the sea mounts where the youngest while the rocks closest to the UK and the USA where the oldest. As it is located on a constructive plate boundary, magma is rising through cracks in the ocean floor and creating volcanoes, as magma continues to erupt and cool on the sides of these sea mounts, they build up and create new land. If convection currents haven’t diverged the newly created land away from the margin, sea mounts may breach the sea level and forms islands such as Iceland that is situated on the Mid- Atlantic Ridge and serves as evidence for this theory.

SEA FLOOR SPREADING Sea floor spreading can also be supported by paleomagnetism in the rocks either side of the Mid-Atlantic Ridge. Paleomagnetic striping is where the rocks either side of the ridge contain iron-rich minerals from the core and align with the magnetic field around the earth created by the core giving each strip a north or south alignment. As the polarity changes every 400,000 years the rock is laid out in alternating strips along the sea floor creating this striping effect that is symmetrical either side of the Mid-Atlantic Ridge as the rock gets diverged equally either side of the ridge when new magma is brought to the surface to cool and create more land.

SUBDUCTION The idea of sea floor spreading could not be accepted fully as if the Atlantic Ocean was expanding the Earth should be too but it wasn’t. Therefore, research was undertaken to find an explanation to this theory. What they found where destructive plate margins. These are margins where two plates meet and if a continental meets an oceanic plate subduction of the oceanic plate occurs as it is more dense as the convection currents converge pulling it down into the mantle. This was found around the Pacific Plate under the Pacific Ocean. Volcanoes where also found where the excess magma created by the melting of the oceanic plate in the Benioff Zone rises through the crust to form volcanoes. These volcanoes are often referred to as The Pacific Ring of Fire. The subducting of the Pacific Plate means that this plate is getting smaller as it is destroyed. This destroyed amount of land is given back to the Earth’s crust by constructive plate boundaries like the Mid-Atlantic Ridge keeping the Earth in harmony.