This lecture introduce on Gas Monitoring in Volcano. Commonly gas output from a volcano increases or changes composition before an eruption. As magma rises to the surface it releases (ex-solves) much of its gas content. This can be measured. When magma rises towards the surface the decrease in pressure causes it to lose some of its gas content. As gas is released from the magma it often vents at the surface, leaking out of small cracks in the ground or from the large volcanic vent.