All volcanoes are different but in general they release gases such as H2O, CO2, SO2, and a little nitrogen. Notice that these molecules are not the Nitrogen and Oxygen which are the majority of gases in the air today, however. In fact, oxygen, which is what humans and other animals require to live, was not a part of the Earth's earliest atmosphere! Oxygen came into the atmosphere later.
Because this atmosphere is different from the primitive atmosphere of
the Earth (what might have formed with the Earth out of the cloud shown on
page 1 of this tour), scientists call it a secondary
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