Sensitive instruments detect changes in temperature, chemical composition of emissions, Earth movements, magnetic fields, gravity, and other physical properties of the volcano. The intensity, frequency, and location of earthquakes provide important clues to volcanic activity, particularly impending eruptions. Movements of magma typically produce numerous tremors, sometimes exceeding 1,000 per day. An almost continuous tremor generally accompanies a lava outpouring. Researchers also monitor variations in the chemistry and petrology of the lavas and the chemistry of emitted gases.
Volcanoes erupt in a wide variety of ways.
Even a single volcano may go through several eruption phases in one active period. Eruptions are classified according to the geochemical composition and viscosity of the lavas, nature of the flows or ash release, and associated phenomena. Magmatic eruptions are the most common, but the most violent arise from steam explosions when the fiery magma reaches surface water, ice, or groundwater. Pelean eruptions, named after the 1902 eruption of Mount Pel e on the Caribbean island of Martinique, are characterized by incandescent flows of rock and pumice fragments. Eruptions of intermediate force are typified by Plinian eruptions, named after Pliny the Elder, who witnessed the volcanic destruction of Pompeii and Herculaneum.