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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.
Magma contains dissolved gases that are released into the atmosphere during eruptions. Gases are also released from magma that either remains below ground or is rising toward the surface. Gases can escape into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems.