For 600 years, Mount Pinatubo appeared to be sleeping. Towering 1745 meters (5725 feet) over the Philippine island of Luzon, the seemingly dormant volcano showed no signs of an impending catastrophic eruption. Then, in early April 1991, Pinatubo stirred, sending puffs of steam into the air. Soon after this first event, vulcanologists began measuring vibrations inside the mountain using seismometers, the same instruments used to measure earthquakes.

Seismometers record vibrations in the Earth. When rocks crack or slip, or when magma applies pressure to the inside of a volcano's magma channel, the surrounding rock vibrates, much like a tuning fork vibrates when it is struck. Seismometers record both the intensity of the vibration, measured by the height of the line on the seismogram, and the frequency, measured by the distance between the peaks and valleys on the graph. Scientists use the information they collect from seismic events to understand volcanic activity inside a volcano and to make predictions about impending eruptions.

By May 1991, seismic data, coupled with increased sulfur dioxide released from the volcano, suggested to scientists that new magma was filling the volcano's magma chambers. Pinatubo's rapidly growing lava dome foretold an imminent large-scale eruption. The combined evidence suggested to the vulcanologists that the volcano's magma chamber held a gas-charged, potentially explosive type of magma called andesite magma. Based on the evidence, all the necessary preconditions appeared to be in place to create a catastrophic eruption.

Part of the hazard assessment of Pinatubo's threat included field surveys to determine the size and extent of the volcano's previous eruptions. Scientists found volcanic debris from earlier eruptions cast up to 16 kilometers (10 miles) from the volcano. As a result, in early June officials evacuated 50,000 people from within this 16-kilometer (10-mile) perimeter.

When the volcano erupted on June 15th, the explosion was devastating. Still, thanks to accurate predictions and emergency preparations, only 500 people died, compared to the tens of thousands who might have lost their lives if they hadn't been evacuated in time.

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