The Earth is unique among the worlds of the solar system. It has a thick atmosphere, 21% of which is the corrosive and chemically active gas oxygen. Much of the planet has just the right combination of temperature and atmospheric pressure to allow liquid water to exist in the open. Some 70% of it's surface is covered with water which regulates temperature variation by carrying heat away from the tropics and releasing it near the poles. It's strong magnetic field and thick atmosphere protects the surface from dangerous radiation from space.

This unique combination of conditions makes possible a numerous and diverse variety of lifeforms

Structure

In the centre is an Iron or Nickel/Iron core . Its temperature is about 7000 degrees. The core has two parts, the solid inner core and the liquid outer core. In spite of the high temperature the inner core is solid because of the immense pressure of the layers of material above it.

Next is the mantle made of Silicon, Magnesium, Oxygen with some Iron, Calcium and Aluminium. Like the outer core it is liquid.

The surface layer of the Earth is the crust made of silicates, that is silicon, oxygen and small amounts of other minerals. In stable continental regions the crust is about 40Km thick. In active mountain building regions it can be up to 70Km thick and under oceans about 12Km.

The crust consists of large plates called tectonic plates. The plates are moving, driven by currents in the mantle. Plates can collide forcing one of the plates under the other forming a subduction zone. The downward moving plate is forced into the mantle where it melts. The plates can also slide along each other. Usually the motion is erratic with periods where no movement takes place but stresses and pressures build up in the rock. When the pressure becomes large enough to fracture the rock, the plate slips in a sudden violent movement creating an earthquake.

The continents are attached to the plates so the movement of the plates causes the continents to move over very long periods of time. 200 million years ago all the continents were joined together into a supercontinent called Pangea. At about 180 million years ago Pangea broke up into a number of land masses one of which was Gondwana from which Australia, Antarctica, South America, Africa and India came.

Atmosphere

The atmosphere consists of gases around the earth in a number of layers. The thickness of the air becomes less with height fading out over 800km although no sharp boundary exists.

The troposphere reaches up to 7Km -18Km and is the region in which weather takes place. Air is heated by contact with the ground and rises. Under certain conditions water vapour will condense out of the rising air creating clouds. Temperature decreases with height.

The stratosphere reaches from the top of the troposphere to about 50km. In the lower part the temperature is constant with height. At about 25km there is a concentration of ozone (ozone layer). The ozone absorbs ultra-violet radiation from the sun, shielding the surface of the earth from its dangerous effects. In absorbing ultra-violet the ozone warms up in turn warming up the air in this part of the stratosphere. This causes the temperature of the air in the middle and upper layers of the stratosphere to increase.

Next is the Mesosphere up to 80km or so. This is the part of the atmosphere where most meteors occur.

Above that is the Thermosphere reaching to 650km. The atmosphere is extremely thin at this level, in fact it would be more rarefied than what would be considered as a good vacuum on the surface. The aurora occurs at this level.

The outermost layer is the Exosphere. It extends to about 800km but this varies with solar activity. The majority of artificial satellites orbit in this zone.

Life on Earth.

Life has not only colonised the Earth but has changed the very environment to suit itself. The atmosphere of the early Earth was not the atmosphere we have now but one made of Hydrogen, Ammonia, Carbon Dioxide, Methane and many other nasties. The early forms of life were anaerobic, that is they did not require oxygen. In fact oxygen was a waste product and a poison to many of these species. At first the oxygen combined with dissolved iron in the ocean forming iron ore but eventually the oxygen level in the atmosphere began to increase which placed the anaerobic bacteria at a disadvantage. New forms of life arose which were able to use the poisonous and corrosive oxygen, these are called aerobic. Oxygen enables a much more efficient metabolism giving the oxygen based life form substantial advantage. Over time anaerobic lifeforms were forced into places where little or no oxygen existed such as the bottom of swamps. Some diseases such as Tetanus and some types of food poisoning are caused by anaerobic bacteria. Eventually aerobic bacteria came to dominate and about 1000 million years ago multi-celled life arose. From this arose the diversified forms of life we have today.

Vital statistics for Earth click here >>>

© Matthew Wallace 2006