By monitoring the seismic and geological activity, the USGS can warn people ahead of time about impending danger. These volcanologists measure the size of an eruption in two ways: the eruption magnitude (by the volume or mass of magma erupted) and eruption intensity (by the rate of magma erupted).[27] There are also various forms of satellites and imagery, such as satellite InSAR imagery, that monitor the activity that isn't exposed to the naked eye.[28]