Gas Monitoring at Long Valley and the Surrounding Region

Volcanic gases produce the driving force that causes most volcanic eruptions. Deep in the earth, they are dissolved in magma, but as pressure decreases when magma rises to the surface, gases separate from the liquid. Because gas is less dense than magma, it may rise more quickly and be detected at the surface of the earth. Increased gas output or appearance of new gas vents at a volcano can be some of the first signs that magma is moving nearer to the surface.

The most commonly analyzed volcanic gases are carbon dioxide (CO₂) and sulfur dioxide (SO₂). By monitoring the relative abundances of these gas types, scientists may be able to infer magma movement through the volcanic system. Gas monitoring can either be conducted close to the source in ground-based campaigns or from a distance using remote sensing.

Between 1988 and 1989 a large amount of CO₂ was detected near the edge of Long Valley Caldera at Mammoth Mountain; the event also coincided with an earthquake swarm. The gas discharge has been responsible for a large kill-off of vegetation in the area.