Deep beneath the surface, gasses are dissolved in magma, but as magma rises toward the surface the pressure decreases and 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.
Similarly, water can also transport material from depth up to the surface where it can be studied by scientists. Groundwater circulates deep within the Earth's crust in volcanic regions, where it can be heated by magma to over 200 °C (around 400 °F). This causes it to rise along fractures, bringing dissolved material up toward the surface. By studying the chemical makeup of this thermal water, scientists can gain a better picture of the conditions deep within a volcano, a region they cannot observe directly.
Gas released from a volcano relates directly to the type, amount, and depth of magma beneath the surface, while thermal waters provide an indirect look at the subsurface. An increase in gas output, the appearance of new vents, or a change in the chemical makeup of the gas and water can be some of the first aboveground signs of heightened volcanic activity.
The USGS Volcano Hazards Program investigates and monitors gas and water at many U.S. volcanoes to better understand the types of processes occurring within a volcanic system. Temperature, chemistry, and water levels of heated and non-heated groundwaters are measured, and the chemical components and emission rates of discharging gases are tracked. Volcanic gas and water are sampled and measured both when volcanoes are erupting and when they are quiet. The research results acquired during non-eruptive times provide the background level of measurements. The first signs of impending volcanic unrest are often signaled by shifts away from the background.