Density and composition

Earth has a slightly larger mass, diameter and average density than Venus. Because of this, the strength of gravity on Earth is slightly more than that of Venus. A 75 kg person on Earth weighs 735 N. Earth is also the densest planet in the solar system because it has a large nickel-iron core.

Scientists know a lot about the interior of Earth from their study of earthquake (seismic) waves and volcanoes. Earthquakes release enormous amounts of energy that travel through Earth and along its surface as waves or vibrations.

The interior of Earth is divided into three main layers with distinct chemical and seismic properties: the core, mantle and crust. The core has an inner and outer layer. The inner core makes up only 1.7 per cent of Earth's total mass and is solid iron and nickel. It is extremely hot (about 5000°C) but remains a solid because of the pressure from surrounding layers. At the centre the pressure is about four million times greater than at the surface. Surrounding the inner core is a liquid outer core that contributes about 30 per cent of Earth's mass. The outer core has a temperature of about 4100°C. Convection currents in the liquid outer core are thought to produce electrical currents that generate Earth's magnetic field.

The outer core is surrounded by the mantle, which contains 67 per cent of Earth's mass. The mantle is 2820 km thick and contains iron and magnesium silicate minerals and oxygen that are kept solid by high pressures. Nearer the surface of the mantle there are some molten regions that are liquid enough to flow. Sometimes this molten material rises to the surface and forms volcanoes and lava flows.

Surrounding the mantle is a thin outer layer or crust about 40-70 km thick that accounts for about 0.4 per cent of Earth's mass. It is made of granitic and basaltic rocks, which contain silicon dioxide (quartz) and other silicates like feldspar. The surface of the crust is covered by oceans (70%) and continental land (30%).

Table 6.3 Chemical composition of the Earth

Element

Proportion (%)

Iron

0 0

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