 Plate tectonics and continental drift. The theory of plate tectonics has enabled us to explain how the continents drift and why there are mountains, earthquakes and volcanoes and so much more. From our video, The Structure of the Earth, you will have learnt that the Earth's thin solid crust lies over the mantle of hot rocks, which, though solid, are able to flow like ice in a glacier. This movement is caused by convection currents. The cooler mantle near the surface is more dense so sinks towards the iron core, driving hotter, less dense mantle towards the surface. The Earth's crust rides on these slow-moving convection currents. Over the whole Earth, there are several convection currents which divide the Earth into a number of plates. It is at the boundaries of these great plates that interesting things happen. Notice that the shape of the plates is not the same as the shape of the continents. They are simply carried about floating on the top of the plates because they are made of lighter rocks than the ocean crust. At the boundary of two plates and where the convection current is moving upwards, like this, the plates move apart and new magma, which is molten rock, erupts to fill the gap, forming new oceanic crusts made of basalt and igneous rock. And this is called a divergent boundary. 100 million years ago, when dinosaurs roamed the Earth, Europe and America were joined and there was no Atlantic Ocean. These convection currents gradually pulled the two continents apart. First, a rift valley formed which got flooded with seawater and the new Atlantic Ocean was born. Even today, the Atlantic is spreading about as fast as your fingernails grow and down the middle are underwater volcanoes where strange life forms grow amongst the so-called black smokers. If the ocean is spreading in some places, it must be being consumed in others and this is where two plates are moving towards each other, the so-called convergent boundaries. Continental rocks get eroded and form a sediment on the ocean floor. As the ocean crust is carried down, called subduction, the sediments get carried down or subducted and melt, but being less dense than the basalt rock of the ocean crust get pushed up, forming volcanoes along the mounted chain. Sometimes, the plates simply slide along side each other as they do in the famous San Andreas Fault in California. The relative movements in the so-called transform boundary cause major earthquakes. It will also mean that Los Angeles will come alongside San Francisco in about 20 million years' time. Sometimes, a convection current in the mantle is just a single upwelling causing what is called a hotspot. As the plate moves over this hotspot, a series of islands form as we see in Hawaii. Gradually, the older islands get eroded and disappear beneath the ocean. Oceanic crust is never more than a few hundred million years old because it is constantly being created at mid-oceanic ridges and consumed or subducted at convergent boundaries. However, continental crust has simply been carried around, broken apart and brought together ever since the formation of the Earth four and a half thousand million years ago. And so, to the big picture, watch now as we look back in Earth history 250 million years ago when all the continents were bundled together in what is called Pangaea. Plate movements broke this up. Here we are 65 million years ago when the dinosaurs vanished. Watch now as the Atlantic Ocean opens up and India crashes into Asia the world we see today.