Volcanoes are mostly unpredictable. There are, however, some means of detection and prediction, both natural and scientific.
Close observance of the area around a volcano can provide important signals. Clouds of steam or smoke may arise from the volcano’s crater. The ground may rumble. There may be minor shudders or earthquakes.
The increased temperature of nearby streams, springs, and even surface rocks may give a clue. Rock slides or avalanches from the volcano’s cone or slopes provide evidence of possible eruption. However, all these things may happen without further volcanic activity.
Scientists have developed various instruments to measure predictors of volcanic eruption. The ground may tilt slightly before an eruption; a tiltmeter measures this activity. Seismographs, commonly used in recording earthquakes, can also be used to gauge the rumblings that sometimes come before an eruption.
Rising magma distorts Earth’s natural electric currents and magnetic field. A restivity meter and a magnetometer note these changes. Thermometers register changing temperatures in the surrounding environment, which may precede volcanic eruptions.
Despite all of these ways to monitor volcanoes, eruptions and explosions are difficult to predict. For example, in 1980, scientists thought that Mount Saint Helens—a dormant volcano in Washington State—was preparing for a mild eruption.
They were taken by surprise. On the morning of May 18, the volcano blew its top, spewing clouds, lava, ash, and rocks; causing avalanches, mudslides, and floods; and registering moderate earthquakes between 3 and 5 on the Richter scale.
The Richter scale measures the magnitude of earthquakes on a scale of 0 to 9.0. More than 60 people died and about 200 were left homeless. The damage and destruction covered more than 200 square miles (518 square kilometers).