#US Geological Survey Hawaiian Volcano Obversvatory

What are symptoms leading up to an eruption? The main player in monitoring volcanoes is seismicity—the frequency and magnitude of earthquakes.  When magma enters the volcanic edifice, it accumulates and makes space for itself by compressing many tiny void spaces.  Continued filling by magma creates pressure on the walls of reservoirs, causing slip, faults, or cracking in the surrounding brittle rock.  All these motions result in the generation of earthquakes.