Ground Deformation - Tiltmeters and GPS measure the rate of swelling in the volcanoe, swelling suggests magma is moving towards the surface. Thermal monitors detect magma movement.
Siesmicity - Siesmographs measure any vibrations in the ground, earthquakes are the earliest warning of a volcanic eruption.
Chemical composition - When magma nears the surface, the pressure is lowered and gases escape. An increase in sulphur dioxide suggests magma is rising.
Aircraft and GPS measure heat,gas and…