Once, they thought the Earth was flat, and now they know how mountains move.
The term ‘plate tectonics’ describes the process whereby lithospheric plates move laterally on the asthenosphere (Skinner and Porter 2000). Basically, chunks of the Earth’s crust move about- constantly.
The Earth’s crust is divided into 6 large plates and many smaller plates; the edges of these plates are called ‘faults’ or ‘plate boundaries’. Convection currents in the mantle, caused by heat within the Earth drives movement of these pates. Although the Earth’s plates move very slowly (centimetres per year) the theory can be used to explain many of the large scale features of the planet.
For many years, scientists have been trying to explain features such as valleys, mountains and most notably, the position of the continents. In the mid-1500s, Flemish cartographer, Abraham Ortelius first proposed the theory that the continents had somehow been ‘torn away’ from one another. Other scientists noted this phenomenon. In 1795, James Hutton, commonly named ‘The Father of Geology’, began to explore the formation of rock and proposed that the Earth was a molten ball. Unbeknown to him, and other scientists at the time, the idea was in fact a key element to explaining movement of the continents.
In 1912, German scientist Alfred Wegner proposed his theory of Continental Drift. In is arguable that Wegner was most forceful to propose his theory at the time. Wegener drew upon a number of evidences in order to support his claim. Firstly, he too noted the shape of the continents, that they appear as though they have been torn apart and could fit back together like a jigsaw. To support this, Wegner drew upon other solid evidence- the geology of the rocks (identical in South America and Africa); fossilised plants and animals (Mesosaurus discovered in South America and Africa, and Trilobites in North America and Europe); and last, but most compelling, ‘wrong latitude evidence’. If continents had not moved, how could there be evidence of an ancient ice sheet in Africa and coal reserves in Britain? However, Wegner’s theory was discredited: scientists could come up with no plausible mechanism for how continents could have drifted across the sea floor.
Henry Hess was a Scottish geologist who first proposed the idea that the Earth was a molten ball, and that new crust is created from inside. He got his ideas from a glass factory, when he observed that glass of the same chemical composition formed a completely different structure when subjected to different temperatures and different rates of cooling. He proposed his ideas to the national geologic survey who laughed at him. He tried convincing them by showing them areas of rock which had melted into one- but could not prove it.
British Geologist Arthur Holmes was arguably the first man to piece all evidence together and propose the theory that large slabs of crust move across the Earth’s surface due to convection on the mantle, and published his findings in his 1944 book ‘The Principles of Physical Geology’. However at the time it was very much a hypothesis. It is said that David Attenborough once asked his professor if they were going to cover the part of plate tectonics. His teacher replied, ‘When it is true, we will cover it’.
In the 1950s ‘Apparent polar wandering’ led scientists to reluctantly admit that plates had moved (Skinner and Porter 2000) When molten rock cools, magnetic minerals within the rock align themselves with the Earth’s magnetic polarity. As the magnetic field could not be different in the same part of the world, scientists had to conclude, that somehow, movement of crust had taken place.
Soon after, while mapping the ocean floor, Harry Hess and scientists discovered something conclusive: the Mid Atlantic Ridge. Upon investigation they realised that new rock was being released from this ridge, and becomes progressively older the further away from the ridge. They also noticed polarity stripes were apparent on the rock as new crust had formed over many millions of years, each time the magnetic minerals aligning themselves with the current, normal or reversed polarity of the Earth.
The spreading centre that Hess and his colleagues discovered is one of three main plate boundaries named today: a Divergent Boundary. This is where convection currents I the mantle move plates apart creating new crust at the spreading centre.
Scientists now understand that convergent boundaries exist, which off-set the spreading. There are two types of convergent boundary: continental-oceanic and continental-continental. At continental-oceanic boundaries, subduction zones exist where crust sinks into the mantle as plates collide. The second type are called collision zones. This tectonic uplift explains continental mountain chains, and many feature of the Earth’s topography such as the Himalayas.
The final movement is where two plates move parallel to one another. These are called transform boundaries, or ‘conservative’ boundaries, and no creation or loss of crust occurs. The San Andreas fault in California is a famous Transform Boundary.
Plates, on average move 7 centimetres per year. This can be measured using satellites and hot spots. Hot spots are areas of the Earth’s mantle which possess long lived volcanism. Because these points are fixed, ‘chains’ of volcanos can be observed on land as plates move.
Convection in the mantle caused by heat from radiation is so far the only explanation of how Earth’s plates move. It is now accepted today that in fact continents don’t move- but entire plates of continental and oceanic crust. The one question scientists still face is: what exactly drives the movement direction? Currents theories include ridge push, slab pull, and gravity slide; or a combination of all three. Scientists have come a long way in exploring the Earth’s bimodal topography through plate tectonics however there is still more to be understood. The possibility remains that they are wrong about all of it.
Geogger 