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 air we breathe today is made mostly of gases called nitrogen and oxygen, but also hydrogen, helium (like what is inside a helium balloon), water vapor (steam), carbon dioxide, sulphur dioxide, methane, ammonia, and many other gases including smog from human activities. Scientists think that the atmosphere we breathe today is nothing like the atmosphere the Earth started with, which might have been made of only of H and He. Thus scientists call the atmosphere of today a secondary atmosphere
, because it came along second and replaced the earlier, first atmosphere.
Scientists think that the secondary atmosphere came from the many volcanic eruptions which took place in Earth's early history, and which continue today. The formation of the early ocean was also important to the forming of the secondary atmosphere.
We will talk more about the changes in Earth's atmosphere later.
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