Volcanic activity is monitored with many different instruments.
The perhaps simplest to understand is GPS. The way scientists use GPS is different from that of everyday life. It can detect minuscule movements of a few centimeters. On volcanoes, any upward movement on the surface detected by GPS indicates that something is pushing from underneath.
Even more sensitive are tiltmeters, which are in essence the same as bubble levels that people use to hang pictures on a wall. Any change in the tilt on a volcano slope indicates that the volcano is “breathing,” again because of magma moving below.
A very important tool is watching for seismic activity.
Volcanoes like Hawaii’s are monitored with a large network of seismographs. Any movement of magma below will cause tremors that are picked up by the seismometers. A few weeks before the eruption of Mauna Loa, scientists noticed that the tremors came from ever shallower depths, indicating that magma was rising and an eruption might be imminent. This allowed scientists to warn the public.
Other ways that volcanic activity is monitored includes chemical analysis of gases coming out through fumaroles — holes or cracks through which volcanic gases escape. If the composition changes or activity increases, that’s a pretty clear indication that the volcano is changing.
Gabi Laski is a professor in geophysics at the Institute of Geophysics and Planetary Physics where her research includes evolution and seismic imaging of the Hawaiian mantle plume. Laske receives funding from the National Science Foundation.