snip:
“It is relatively easy to monitor the temperatures of these gases, and an anomalous increase in temperature might be a sign that magma has moved closer to the surface. Monitoring the composition of the gases is more difficult to do, and changes in the composition are way more difficult to interpret. Many times just visual changes to fumarole areas are indications of impending activity. If the area of active degassing gets larger, if the plants nearby die suddenly, if the color of any lakes or ponds nearby changes…”
http://volcano.oregonstate.edu/what-are-signs-volcano-about-erupt
