6.1 structure of the earth
The earth is almost spherical with a radius of about 6400 km. at the surface is a fin, solid crust, the thickness of the crust varies 5-70km. It is thinnest under the ocean plates
· The mantle is under the crust, it is about 3000 km thick and reaches half way into the earth. The mantle is solid but can flow very slowly.
· The core is very dense and made of metals mostly nickel and iron, the outer core is liquid and the inner core is solid. This model of the earth was made up by seismic evidence from earthquakes.
Scientists once thought that the earth was shrinking and that as it continued to cool the crust wrinkled.
6.2 the restless earth
Scientist now believe that mountains form at boundaries between tectonic plates.
Tectonic plates are large parts of the lithosphere the earth's crust and upper mantle, tectonic plates move a few centimetres a year.
Natural radioactive processes in the earth cause convection currents in the earth this moves the plates.
At the plate boundaries huge forces push up the crust to form mountains and cause earthquakes and volcanoes.
6.3 the Earth's atmosphere in the past
· The earth was formed 4.5 billion years ago.
· In the first billion years the crust was covered with volcanoes that released carbon dioxide, nitrgoen and water vapour.
· As the earth cooled most of the water vapour condesed to from the oceans, so the early atmosphere was mainly carbon dioxide and nitrogen, and there may have been small amounts of ammonia and methan there as well.
· In the following two billion years, algae and plants evolved, they used carbon dioxide for photosynthesis to produce and this released oxygen into the atmosphere. As the number of plants increased the amount of carbon dioxide decreased and the amount of oxygen increased.
6.4 gases in the atmosphere
For the last 200 milion years the proportions of gases hasn’t really changed, the atmosphere is almost 4/5 nitrogen, 1/5 oxygen and 1% other gases.
The other gases are all noble gases, carbon dioxide and water vapour.
Most of the carbon dioxide in the early atmosphere ended up in sedimentary rocks or as fossil fuels
Noble gases are the least reactive gases in the periodic table. They are used for light bulbs, advertising and balloons.
6.5 the carbon cycle
Carbon dioxide moves in and out of the atmosphere and natural processes have kept this in balance for millions of years,
Plants take in carbon dioxide to make food
· Carbon dioxide is released back into the atmosphere when plants and animals respire or decompose.
· Some of the carbon is used to make animal shells and in the past these have formed sedimentary rocks,
· Volcanoes produce carbon dioxide by decomposing carbonate rocks that have moved deep into the ground.
· Carbon in plants and animals went into fossil fuels
· When we burn fossil fuels we release carbon that has been absorbed millions of years ago as the level of industry has gone up the amount of carbon in the atmosphere has gone up.
· As the amount of carbon dioxide increases more of it dissolves in the oceans.When carbon is realeased into the atmosphere