The Earth and its atmosphere-GCSE CHEMISTRY

The Earth and its atmosphere
The composition of the atmosphereThe atmosphere is made up of the following gases, Nitrogen 78%, Oxygen 21%, Argon 1%, Carbon dioxide 0.03%, water - variable
Earth's early atmosphere and volcanoesThe primary atmosphere of the Earth was hydrogen and helium. These light gases were slowly lost. They were replaced by a secondary atmosphere produced by the action of volcanoes.
Composition of the Earth's early atmosphereThe Earth's secondary atmosphere was made up of some left over hydrogen, carbon dioxide, water vapour, nitrogen, carbon monoxide, sulphur dioxide ammonia and methane.Task C6.10 Match these formulae to secondary atmosphere gases: CH4, SO2, CO2, H2, NH3, CO, N2, H2O.
Origin of the oceansAs the Earth cooled to below 100oC oceans were formed when water vapour condensed and formed liquid water. Oceans are reservoirs for carbon dioxide because they can store the gas when it dissolves in them. The new oceans dissolved a great deal of the carbon dioxide in the atmosphere.

The oceans still play a part in keeping carbon dioxide levels constant. If there is a lot of carbon dioxide in the air then more can dissolve. If there is less carbon dioxide in the air then some comes out of solution back into the air.

The release of oxygen into the atmosphereAs the temperature of the Earth cooled simple green plants evolved in the oceans to use the carbon dioxide in the environment. These green plants steadily removed carbon dioxide and produced oxygen by photosynthesis. Oxygen levels in the atmosphere slowly increased.

The carbon cycleThe carbon cycle helps to keep the atmospheric composition constant by adding carbon dioxide to the atmosphere and also taking carbon dioxide away from the atmosphere. Carbon dioxide is taken away from the atmosphere or out of the cycle by photosynthesis, dissolving in water and by chemical reactions, for example with rock. It is brought into the atmosphere or into the cycle by respiration, combustion, volcanic activity and decay.

Formation of igneous rocksIgneous rocks are formed when magma pushes up into the crust and cools. It is made up of crystals It does not contain any fossils. Any living thing falling into the molten rock, from which it is made would be burnt and leave no trace. Igneous rock forms as magma cools slowly under the surface e.g. Granite. Magma reaching the surface through a volcano cools quickly e.g. basalt.

Crystal size and igneous rockIgneous rocks which cool slowly have large crystals e.g. granite but rock forming quickly has smaller crystals e.g. balsalt.

The formation of sedimentary rockThis rock is formed in shallow seas. After long periods of time sediment layers pile up and the lower ones come under great pressure.
This pressure pushes the water out of the layers or sediments and salt crystallizes and sticks the particles together to form sedimentary rocks. This process is called lithification. Living things falling into the sediments leave an impression as the rock forms (a fossil). Fossils show that a rock was made from sediments.