Summary
Highlights
Geophysicists and volcanologists find large eruptions exciting but face the critical task of predicting when and how a volcano will erupt to ensure public safety. Despite advancements since Mount St. Helens, accurate forecasting remains a significant challenge, as evidenced by the complexities at Pinatubo.
In spring 1991, Mount Pinatubo, dormant for centuries, began showing signs of activity with explosions driving tribespeople from their homes. Initial assessments underestimated the threat, but continued seismic activity and rising sulfur dioxide levels (from 500 to 5,000 tons a day) soon indicated rising magma and a high probability of an eruption.
Scientists, including Dave Harlow and Rick Hoblit, analyzed past eruption deposits and radiocarbon-dated charcoal, revealing that Pinatubo, despite infrequent eruptions, tended to erupt violently. They considered various eruption types, including lava flows, ashfall, mudflows (lahars), lateral blasts, and pyroclastic flows, all potential outcomes for Pinatubo.
Volcanologists at the Pinatubo Volcano Observatory (PVO) grappled with erratic seismic data and the immense responsibility of their predictions. The alert level was raised to two, initiating a limited evacuation. The military and local authorities faced difficult decisions, balancing public safety with the disruption of potentially false alarms.
The volcano's activity fluctuated, creating a 'rollercoaster' of uncertainty. A sighting of new magma led to a level three alert, but a misidentification caused temporary doubts. However, continuous, strong earthquakes pushed the alert to level four, signaling an eruption within 48 hours. General Studer, observing the volcano firsthand, ordered a full evacuation of Clark Air Base.
After initial eruptions, Pinatubo delivered its 'main course' on June 15th, with continuous, massive explosions sending ash 100,000 feet into the air. This catastrophic eruption coincided with Typhoon Union, intensifying the chaos as heavy rain mixed with ash, pumice, and pyroclastic flows. Scientists at PVO described the events as 'biblical' as they lost contact with their instruments and were forced to evacuate.
The eruption and typhoon caused widespread devastation, burying towns in ash and leading to the collapse of roofs. While the prediction saved thousands of lives, almost 500 people died. Pinatubo's eruption, the largest in 80 years, released 5-8 cubic kilometers of material, 10 times that of Mount St. Helens. The global atmospheric impact temporarily reduced average global temperatures.
The eruption left massive deposits that, when remobilized by rain, formed destructive lahars, continuing to cause widespread flooding and property damage for years. Over 650,000 people lost their jobs, and 50,000 families lost their homes. Clark Air Base was abandoned, and although Subic Bay was initially salvaged, the US military eventually withdrew from the Philippines.
Mount Pinatubo continued to have smaller eruptions and lahars for years after the main event. For the scientists, the successful prediction that saved countless lives was a monumental achievement, yet it also left them with the challenge of what could possibly compare to such an intense and impactful experience.