Hazard maps can be drawn (as in Montserrat) to show which areas are safest and which are most at risk.
Lava flows can be diverted by channels or explosives, dammed or sprayed with cool water.
People can be evacuated.
Seismometers can record the earthquakes that occur as the magma rises.
Tiltmeters can record changes in the shape of a volcano before an eruption.
Satellites can record changes in the temperature and shape of a volcano before an eruption.Â