As magma makes its way to the surface, the pressure lowers, causing it to degas: Gas bubbles form and want to occupy more space at the magma continues to rise. They also often want to rise more quickly than the magma. All of this creates forces in the magma body, which can change how it erupts. Different types of gas leave the magma at different times or depths. Thus, detecting the amount and types of gas gives us vital clues about the inside of the volcano we cannot see.
When the amount of gas exhaled by a volcano is particularly high, we can sometimes detect them with our noses, even miles away from the volcano. In the days before and during the Soufrière Hills eruption, Montserratians could frequently smell the “rotten egg” odour of hydrogen sulfide exhaled by the volcano.
Volcanic gases are not only important indicators of what happens beneath a volcano, some of them are also toxic and corrosive! Downwind of the volcanic plume on Montserrat, vegetation changed and metal corroded more rapidly due to the aggressive volcanic gases.