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the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) are expelled from the cylinder. As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.
The gases associated with a volcanic eruption are initially dissolved in magma, which is hot molten rock found in the deeper layers of earth. During a volcanic eruption these gases are released into the atmosphere. The most abundant gas expelled during a volcanic eruption is water vapor.ther Volcanic Gases. There are other gases released during volcanic eruptions, but these gases are often emitted in trace amounts. They may include hydrogen sulfide and hydrochloric acid. If you ever found yourself inside a chemistry lab, you're familiar with these two chemical compounds.