QUIZ 17a. The Earth's Atmosphere

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(a) The Earth’s atmosphere was originally very from what it is today.

(b) It has been much the for the last 200 million years and provides the conditions needed for life on Earth.

(c) Recently have produced further changes.

(d) For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today - about () nitrogen and about () oxygen small proportions of various other gases, including , vapour and gases.

(e) The gases are in of the table.

(f) They are all chemically gases and are used in and discharge tubes e.g. lights.

(g) The gas is much less than air and is used in balloons.

(h) During the first billion years of the Earth’s existence there was intense activity.

(i) This activity released the gases that formed the atmosphere and vapour that condensed to form the oceans.

(j) Some theories suggest that during this period, the Earth’s atmosphere was mainly carbon dioxide and there would have been little or no oxygen gas (like the atmospheres of ('red planet')and today). There may also have been water vapour and small proportions of (CH4) and (NH3).

(k) Plants produced the that is now in the atmosphere.

(l) Most of the from the carbon dioxide in the air gradually became locked up in as carbonates (e.g. ) and fossil fuels (e.g. ).

(m) Nowadays the release of by burning fossil fuels increases the level of carbon dioxide in the atmosphere considered to be causing .