Plate Tectonic Theory | AQA GCSE Geography | Tectonic Hazards 2

Plate tectonic theory is the scientific theory that attempts to explain why the Earth’s crust acts the way it does and produces the landforms we can see on the Earth’s surface.

Plate tectonic theory & Alfred Wegener

Plate tectonic grew out of a theory that was first developed in the early 20th century by the meteorologist Alfred Wegener.

In 1912 he noticed that the coastlines of the east coast of South America and the west coast of Africa appeared to fit together like jigsaw pieces.

Further examination of the globe revealed that all of the Earth's continents fit together somehow and Wegener proposed an idea that all of the continents had at one time been connected in a single supercontinent called Pangaea.

He believed that the continents gradually began to drift apart around 300 million years ago. This was his theory that became known as continental drift.

The main issue with his theory was that he had no explanation as to why the continents moved like this. Further investigations into this provided him with fossil evidence linking continents together and he proposed that the spin of the Earth had split the continents and dragged them away from their central location; however, this was dismissed.

In 1929 Arthur Holmes, a British geologist, came up with the theory of convection currents and he said that as a substance is heated its density decreases and it rises until it cools sufficiently to sink again. According to Holmes it was this heating and cooling cycle within the Earth's mantle that caused the continents to move. This theory gained very little attention at the time.

Palaeomagnetism

In the 1950s Wegener and Holmes’ theories were taken more seriously and studies of palaeomagnetism began which involved studying the rocks formed by underwater volcanic eruptions in relation to the Earth’s magnetic field. When basaltic lava cools on the sea floor, individual minerals separate - especially iron - and these minerals then align themselves on the sea floor in the direction of the magnetic pole. New technologies allowed these rocks to be dated and their pattern of movement mapped between their origin and sampling. The maps suggested the migration of seafloor rocks.

More recently there has been a discovery that the Earth’s magnetic field reverses periodically and it is possible to see an identical pattern between rock formations on either side of the mid Atlantic ridge.

The final confirmation was that of sea floor spreading. In 1962 Harry Hess dated the rocks of the Atlantic sea bed from the Mid-Atlantic Ridge outwards to the coast of North America. He discovered that the newest rocks were at the centre near Iceland, and the oldest at the coast. This suggested the earth’s surface was splitting and expanding in certain places.

However, given that the whole earth was known to be of a stable size and not inflating like a balloon, an area of the Earth where plate destruction was taking place had to be found to balance crustal expansion with crustal destruction. This evidence was discovered along the edges of the Pacific Ocean where a destructive plate boundary lies between the Pacific, Philippine, North American and Australian plates.

Finally, by the mid-1960s a coherent theory of plate tectonics was accepted that accounted for different crustal rock types, orientations, continental shapes and an evidenced process that drove the movements.