This is an image of Mt. St. Helens, in Washington, USA.
Click on image for full size
Image from: USGS, courtesy of Volcano World
The activity of all the volcanoes helped the Earth form an atmosphere.
All volcanoes are different but in general they release gases such as water, carbon dioxide, sulphur dioxide, 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.
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