The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet.
The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today.
The Earth is almost a sphere. These are its main layers, starting with the outermost:
- crust - relatively thin and rocky
- mantle - has the properties of a solid, but can flow very slowly
- outer core - made from liquid nickel and iron
- inner core - made from solid nickel and iron
Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.
The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.
The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.
Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.
It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in places known for having earthquakes.
The theory of plate tectonics and continental drift were proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before his time it was believed that the planet's features, such as mountains, were caused by the crust shrinking as the Earth cooled after it was formed.
It took more than 50 years for Wegener’s theory to be accepted. This was because it was difficult to work out what the mechanism was that could make whole continents move, and it was not until the 1960s that enough evidence was discovered to support the theory fully.
So what was the evidence for Wegener's theory?
- Plate tectonics explained why earthquakes and volcanoes were concentrated in specific places - around the boundaries of moving plates.
- The match in shape between the east coast of South America and the west coast of Africa suggests both were once part of a single continent. There are similar patterns of rocks and similar fossils on both sides of the Atlantic - including the fossil remains of land animals that would have been unable to swim across an ocean.
You need to know the proportions of the main gases in the atmosphere.