Planet Earth


Earth: Vital Statistics

What Makes the Earth an Unusual Planet

Overview of Earth Systems

The three main components of the Earth are the atmosphere, its gaseous envelope, the hydrosphere, the surface coating of water, and of course, the solid earth. All three are subdivided into subsystems. The atmosphere and hydrosphere get their energy mostly from the Sun, and the solid earth gets its energy from internal heat, some of which is produced by radioactive decay and some is left over from the formation of the earth. A tiny amount of energy also comes from gravitational interactions between the Earth, the Moon, and the Sun.

The atmosphere is driven by unequal solar heating: unequal heating of land and water, unequal heating between day and night, and unequal seasonal heating. The hydrosphere is partly driven by winds, which drive ocean currents, but the hydrosphere is also the earth's principal heat storage system. Evaporation of water from the hydrosphere, its transport by the atmosphere, its eventual condensation as rain or snow, and its eventual return to the oceans make up the hydrologic cycle.

The earth's internal heat causes hot material to rise and cool material to sink in the earth's interior, and this movement causes large slabs of the outer rigid crust of the earth to move around.

The hydrologic cycle modifies the surface of the earth. Water breaks rocks down chemically and mechanically, a process called weathering. Water flowing on the surface carries loose material with it, a process called erosion.

Plate tectonics drives geologic processes like mountain building, earthquakes, and volcanism. It also contributes to heating and melting of rocks.

Crustal movements uplift or lower the crust. Meanwhile weathering and erosion wear down mountains and transport the debris to lower areas. Between the two sets of processes, the earth's landscapes are created - and destroyed. Today's landscape is the modified remains of yesterday's landscapes, and landscapes of millions of years ago are lost beyond recovery.

The destruction of rocks by surface processes and their modification by subterranean heat and pressure results in a constant recycling of rocks called the rock cycle. Igneous rocks, the result of melting, are eventually exposed on the surface by uplift and erosion. They are broken down by weathering, and the debris transported and deposited to make sedimentary rocks. These in turn can be buried, changed by heat and pressure, and become metamorphic rocks. Some can even be heated to the point of melting and creating a new generation of igneous rocks.

Finally, extraterrestrial disturbances can disrupt earth systems. Large impacts create dramatic geologic effects near the impact, but also can cause global climatic and environmental effects. If a star within a few light years of the earth were to go supernova, the radiation effects on earth's life could be dramatic. Close approaches of other stars to the Sun can cause distant comets to drop into the inner solar system, possibly increasing the risk of impacts on Earth.

Earth's Subsystems

The diagram above shows the main components of the earth:

Solid Earth

The solid earth consists of concentric shells that differ chemically and mechanically. The middle shell, the mantle, can be defined differently depending on whether we use chemical or physical definitions.

The chemically defined shells are:

The mechanically defined shells are:


The domain of life, from several kilometers deep in the lithosphere to 10 km or so above the surface.


The zone of liquid water on the earth, dominated by the oceans but also including lakes and rivers and liquid underground water.


The zone of frozen water on the earth, including the Antarctic and Greenland ice caps, glaciers, and permanently frozen ground (permafrost). The cryosphere is often considered part of the hydrosphere.


Earth's Interior

Convection in the Earth

Convection is the transportation of heat by moving hot or cold material from place to place. Convection works because warm material is light and rises, while cool material is denser and sinks. As long as there is a temperature difference with depth, there will be a cycle of rising and sinking material. A lava lamp is a perfect illustration of convection - if geophysicists, astronomers, and other scientists who teach about convection had their way, lava lamps would never go out of style.

In reality, there is a continuous cycle. Material at the base of the mantle becomes hot and rises. As it rises, it expands and cools, and near the surface, heat leaks through the crust and escapes. Cooled mantle material begins sinking and descends to the bottom of the mantle, to be heated again.

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Created 30 July 2008, Last Update 14 December 2009

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