Page 6 - Curriculum Visions Dynamic Book

Oxygen in the air
Air is a gas made up chiefly of nitrogen and oxygen. Oxygen (O2) makes up 21% of the atmosphere by volume and 23% by mass (weight). The weight
of oxygen molecules in the atmosphere helps
to create the air pressure on the Earth’s surface, on average one kilogram on every square centimetre.
Although ozone (O3) is only present in concentrations as little as 12 parts per million, it is present both in the upper atmosphere (see page 12) and
close to the ground (see page 42).
How the atmosphere gained its oxygen
The gases in the early atmosphere (over three
billion years ago) were produced during volcanic eruptions. The gases would have included water vapour and carbon dioxide, but there was no free oxygen. The first free oxygen must have been created from these compounds.
The oldest rocks certainly formed in an oxygen-poor atmosphere. When exposed to the air, these very old rocks oxidise (react with oxygen) rapidly.
Oxygen was probably produced at first by the effects of light energy on water. This would have released free hydrogen and oxygen. Yet this process could only have produced about 1% of the oxygen now in the atmosphere. The rest must have been produced in a different way.
The lack of oxygen in the early atmosphere also means there was no ozone in the upper atmosphere to shield life from the harmful effects of ultraviolet light, so this may be why the first life forms developed in the protected surface layers of the oceans, rather than on land.
Most scientists believe that the vast majority of the oxygen now in the atmosphere was produced in the last three billion years of the Earth’s history, when ocean plants began to release oxygen as part of the process called photosynthesis (see page 14). Oxygen must gradually have seeped out of the water and into the atmosphere. Eventually the build-up of oxygen in the air provided the conditions suited to life on the surface of the Earth.
 The accumulation of oxygen created the present mixture
of gases found in the modern atmosphere.
 The oxygen that could not
be absorbed in the ocean waters seeped into the atmosphere.
 The earliest life forms were plants in the oceans. They released oxygen as a waste product.
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