Theory of plate tectonics

In 1912, German meteorologist Alfred Wegner came up with the theory of continental drift. This theory states that all of Earth's continents and land masses were once joined together in one landmass called Pangea. Then, 200 million years ago, the continents split apart into two smaller land masses called Laurasia and Gondwanaland. Over millions of years, the continents slowly moved away from each other to form the continents as we know them today.

Wegner realised that if you look at the world map, the land masses look as though they could fit together like a jigsaw. For example, the east coast of South America and the west coast of Africa, or Ireland and the UK. He realised that certain countries had identical folding patterns to other countries, meaning that during the orogeny that they were created, they must have been one single landmass. This can be seen in the fold mountains in Ireland and North America.

Wegner also identified identical plant and animal fossils in countries that are far apart from each other. These animals and plants would have needed a certain climate to survive and so the land masses must have once been connected. Some people at the time believed that perhaps land bridges had connected the land masses, but Wegner realised that animals could not have travelled these great distances. Despite this proof, Alfred Wegner could not explain how these land masses had moved.

In 1928, Arthur Holmes came up with the theory of convection currents which explained how plates move. He said that magma in the mantle is heated by the core. When heated, it rises upwards towards the crust where it is cooled, moves laterally and sinks back down again.

The sideways movement of magma pulls the plates along with it, causing them to either pull apart, collide or slide past each other. As this explained how land masses move, it offered proof for Wegner's theory of continental drift.

Maurice Ewing discovered a series of undersea volcanoes in the Atlantic Ocean which became known as the Mid-Atlantic Ridge. He discovered rocks became thinner closer to the ridge. This suggested that plates were pulling apart here, with the rocks becoming thinner as they were stretched.

He noticed the mid-ocean ridges contained active volcanoes. He suggested that magma from the mantle rises through the crust and fills the space between the separating plates, forming the volcanoes.

Harry Hess, a naval officer, came up with the theory of seafloor spreading. He realised that new seafloor was being created at mid-ocean ridges and that this was widening the ocean floor, causing the Atlantic Ocean to expand, i.e. seafloor spreading.

Hess proves his theory by examining rock samples from the ocean floor and proving they get progressively older the further they are from the ridge. Younger rocks are found at the ridges, suggesting new land is being created there.

Seafloor spreading explains continental drift. The Eurasian and North American plates were once joined together but seafloor spreading pushed them apart over millions of years and the Atlantic Ocean formed in between.