Our planet is a not-quite-regular sphere, layered like an onion. In the centre, its inner core consists of solid iron. Around this lies first the outer core, of molten iron, and then the mantle, made up of molten rock called magma. Floating on top of the mantle is a thin crust made of solid rock. We live on the surface of the crust. Although humans have been to the Moon, no one has gone deeper below the surface than 4 km, the depth of the deepest mine.
The Earth has one more layer, a gaseous skin. This is the atmosphere, more than three-quarters of which is nitrogen and one-fifth oxygen, essential to most forms of life. There are small amounts of other gases, but of these carbon dioxide and methane – the so-called greenhouse gases – have a crucial bearing on life on Earth (see here ), as does the presence of water vapour, an essential component in all weather systems. The density of the atmosphere grows thinner with altitude and gradually fades into space.
Just as the gases in the atmosphere are constantly in motion, so too are the rocky plates that make up the crust. Scientists used to assume that the continents and seas had always been in the same positions. Then in 1915 a German meteorologist called Alfred Wegener suggested that rather than being static, the continents had drifted over time. He had observed that the rocks and fossils along the east coast of South America were similar to those on the west coast of Africa, and that certain extinct plants were found not only in these two locations, but also in Madagascar, India and Australia.
Over the years, more and more evidence came to light to support Wegener’s theory of continental drift. It became clear that this process had had a crucial impact on the distribution and dispersal of different groups of plants and animals around the world. Geologists now agree that two enormous continents, Laurasia in the north and Gondwanaland in the south, came together about 300 million years ago to form an even bigger supercontinent, Pangaea. This in turn began to break up about 200 to 180 million years ago, first back into the original two continents, and then eventually to form the various separate continents of today.
But it was not until the 1960s that scientists identified the mechanism by which continental drift occurs, and named it plate tectonics. The crust of the Earth is made up of plates which float on top of the liquid mantle, and so are able to move.
Volcanic winter
It is along the world’s active plate boundaries that most earthquakes and most volcanic eruptions occur – events that can have a devastating impact on life on Earth, including mass extinctions (see here ). Within recorded history, the largest volcanic eruption was that of Mount Tambora in Indonesia in 1815. It blasted so much ash into the Earth’s atmosphere that for many months much of the Sun’s light was blocked out, and 1816 became known as ‘the year without a summer’. Crops failed and livestock died, resulting in widespread famine in Europe and North America.
