How much gas is emitted from a volcano during a certain time period is directly related to the volume of magma that sits in the subsurface reservoir. Measuring the rate at which a volcano releases gas or degasses, typically reported in metric tons per days, allows scientists to get a glimpse of what is happening below the surface. Changes in gases like sulfur dioxide and carbon dioxide are important to monitor in active volcanic systems since they can be indicative of activity occurring in the volcano’s magma reservoir and hydrothermal system. Emission rates can be measured either from the ground or from an aircraft. Gas ejected high into the atmosphere during a volcanic eruption requires satellites to measure the emitted gas.
Looking south across Halema`uma`u Crater at the gas plume rising from the Overlook vent. From USGS, HVO