A cutaway diagram of the Earth's interior, showing the paths taken by P-waves (black) and S-waves (white). P-waves travel through...Read More >>A cutaway diagram of the Earth's interior, showing the paths taken by P-waves (black) and S-waves (white). P-waves travel through both the mantle and the core. S-waves are slower and cannot travel through the liquid outer core. By noting times the waves take to travel from quakes to various seismometers (earthquake detectors) around the world, scientists have been able to work out the exact depth of the boundaries between the crust and the mantle and the mantle and core. They can also tell whether the rocks deep inside the Earth are solid or liquid. On the outside, the Earth seems hard and solid. But if you could make a journey down a deep hole almost 6400 kilometres (4000 miles) to the centre of the planet, you would notice several changes as you descend. It becomes warm, then hot. The average increase in temperature is about 3°C for every 100 metres (8°F for every 500 feet) of depth. You pass through the various layers of rocky material, from the hard crust on the outside through the hot, thick, molten rocks of the mantle to the liquid outer core. When you reach the inner core there is no rock at all: it is almost solid metal. Studying shock waves from earthquakes has enabled scientists to build an accurate picture of the interior layers of the Earth.
Layers inside the Earth
The Earth with a section cut away so we can see its internal layers.The four main layers of the Earth are the crust, mantle, outer core and inner core. At the base of the crust is a boundary called the Moho (Mohorovicic discontinuity). This separates the crust from the mantle and the temperature here is about 1500°C (2700°F). The mantle is about 2900 kilometres (1800 miles) thick. The next layer is the outer core which is about 2200 kilometres (1400 miles) thick. At the centre is the inner core, a solid ball of iron with a radius of about 2500 kilometres (1550 miles).
At temperatures of around 1500°C (2700°F), rock in the upper mantle is molten. Called magma, it flows like hot tarmac.
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