As the magma rises, many small earthquakes occur. The temperature in the surrounding area also begins to increase, and clouds of gas start to pour from the vent. Scientists use several devices to predict when such a volcano will erupt. They use an instrument called a tiltmeter to measure the expansion of a volcano. A device called a seismograph helps detect earthquakes.
Thermometers check temperature increases in the area, and gas detectors measure the amount of gas.
Benefits of volcanoes Volcanoes are among the most destructive natural forces on the earth. Since the 1400's, they have killed almost 200,000 people. But volcanoes also produce benefits. For example, many volcanic materials have important industrial and chemical uses. Rock formed from lava is commonly used in building roads.