Scientists can determine the amount and types of gas in a rock, in the minerals within a rock, or in the gas inclusions in minerals or glass (Ihinger and others, 1994). The methods fall into four classes: bulk extraction, energetic particle bombardment, vibrational spectroscopic techniques, and phase equilibrium studies. These method are used in experimental studies and for rocks from all tectonic settings, historic eruptions, and large pre-historic eruptions. Some methods have been found to underestimate the observed amount of gas released during some historic eruption. Thus, the methods may give an estimate of the minimum amount of gas released. For example, Gerlach and McGee (1994) used melt inclusion data to estimate 0.08 Mt of SO2 emitted by the Mount St. Helens eruption. TOMS, COSPEC and ash leachate data provide an estimate of 2 Mt. Gerlach and McGee suggested a vapor phase carried most of the SO2.