The seismic drum room was a maze of wires and cables; the daily drum roll of seismicity posted on the walls. With air assistance from the U. All efforts were focused on answering the questions — will Pinatubo erupt catastrophically, and when? Volcanologists are first to admit that forecasting what a volcano will do next is a challenge. In late May, the number of seismic events under the volcano fluctuated from day-to-day.
Trends in rate and character of seismicity, earthquake hypocenter locations, or other measured parameters were not conclusive in forecasting an eruption. A software program called RSAM real-time seismic amplitude measurement , developed in to keep an eye on Mount St.
Helens, helped scientists analyze seismic data to estimate the pent-up energy within Pinatubo that might indicate an imminent eruption. The map was based on the maximum known extent of each type of deposit from past eruptions and was intended to be a worst-case scenario.
The map proved to forecast closely the areas that would be devastated on June The Clark Air Base sprawled over nearly 10, acres with its western end nestled in the lush, gently rolling foothills of the Zambales Mountains—only 9 miles 14 km east of Mount Pinatubo. At the time, the population of Clark and nearby cities of Angeles, Sapangbato, Dau and Mabalacat numbered about , Senior base officials listened to daily briefings and put together plans to evacuate.
Everyone agreed that if there were an evacuation, people must be moved to an area where they would be safe—not statistically safe, but perfectly safe. Early June 10, in the face of a growing dome, increasing ash emission and worrisome seismicity, 15, nonessential personnel and dependents were evacuated by road from Clark to Subic Bay.
By then, almost all aircraft had been removed from Clark and local residents had evacuated. Additional explosions occurred overnight and the morning of June Seismic activity during this period became intense. The visual display of umbrella-shaped ash clouds convinced everyone that evacuations were the right thing to do. When even more highly gas-charged magma reached Pinatubo's surface June 15, the volcano exploded.
The ash cloud rose 28 miles 40 km into the air. A huge cloud of volcanic ash and gas rises above Mount Pinatubo, Philippines, on June 12, Three days later, the volcano exploded in the second-largest volcanic eruption on Earth in this century.
Timely forecasts of this eruption by scientists from the Philippine Institute of Volcanology and Seismology and the U. Geological Survey enabled people living near the volcano to evacuate to safer distances, saving at least 5, lives.
Precursors to the Eruptions On July 16, , a magnitude 7. At Mount Pinatubo, this major earthquake caused a landslide, some local earthquakes, and a short-lived increase in steam emissions from a preexisting geothermal area, but otherwise the volcano seemed to be continuing its year-old slumber undisturbed. In March and April , however, molten rock magma rising toward the surface from more than 20 miles 32 kilometers beneath Pinatubo triggered small earthquakes and caused powerful steam explosions that blasted three craters on the north flank of the volcano.
Thousands of small earthquakes occurred beneath Pinatubo through April, May, and early June, and many thousand tons of noxious sulfur dioxide gas were also emitted by the volcano. Following Mount Pinatubo's cataclysmic June 15, , eruption, thousands of roofs collapsed under the weight of ash made wet by heavy rains see example in photo above.
Ash deposits from the eruption have also been remobilized by monsoon and typhoon rains to form giant mudflows of volcanic materials lahars , which have caused more destruction than the eruption itself photo at right shows village buried by lahars. Photo above courtesy of Peter Baxter, University of Cambridge. Because it had lost most of the gas contained in it on the way to the surface like a bottle of soda pop gone flat , the magma oozed out to form a lava dome but did not cause an explosive eruption.
However, on June 12 Philippine Independence Day , millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption. But how could scientists convince the nearly 1 million people living around the volcano that they may need to evacuate? The stakes were high: Just 6 years earlier, Nevado del Ruiz in Colombia erupted and killed more than 23, people. The scheme had to be effective and easily digestible—enough so that they could convince tens of thousands of people living around the volcano, who spoke several different dialects and even different languages , to evacuate.
The team persevered, gathering local leaders of cities, towns, and small villages to explain the dangers and answer questions. Part of this educational campaign involved showing gruesome video footage from the Nevado del Ruiz tragedy that depicted destructive ash flows, volcanic mudflows, ashfalls, landslides, lava flows, and more.
Here scientists learned a powerful lesson in hazard mitigation. By early June, officials called for the evacuation of 25, people living in the area, including American service people at Clark Air Base and the U. Naval Station at Subic Bay. Never before had such a widespread evacuation attempt been made before a volcanic eruption. By the time the volcano erupted on 15 June, scientists and public officials had convinced more than 65, people to evacuate. In , scientists had to look up information in books, make photocopies, and fax information to each other, Ewert said.
This was a time before GPS and before data could be sent via satellite. Smartphones were science fiction. One of these rumors claimed that a 3-mile-long fissure had formed after the eruption and that the nearby city of Olongapo would soon be hit by a giant lateral blast. Magma can be classified into types that distinguish how much silica they contain and how viscous they are, among other characteristics. Silicic magma—made of dacite or rhyolite—is stickier and more viscous.
It holds more gas that when depressurized, erupts more explosively. Studies of lava deposits after Pinatubo exploded revealed something curious: minerals juxtaposed that would not normally coexist together had magma come from one source, Newhall explained. Thermal signatures—for example, crystals partially resorbing, chemical diffusion between crystals—suggested that magma was initially a mix of basalt and dacite prior to the eruption.
But by the end of the eruption, magma was fully dacite. Instead, it rose into the dacite and mixed with it. But how? First, when the fresh, water-rich, and considerably hotter basalt hit the cooler dacite reservoir, the basalt crystallized, Newhall explained.
That added even more volatiles. The resulting slurry was still less dense than its surroundings, so it kept rising and was the first erupted. Eventually, the dacite itself heated enough to rise to the surface and erupt. This magma mixing manifested as subtly rumbling quakes that at times lasted about a minute long, called deep long-period DLP earthquakes. Prior to the eruption, Pinatubo was a little known volcano and it had been dormant for years.
There were no known historic eruptions. Before the eruption in Pinatubo was m high ca. Pinatubo mostly noted for a failed geothermal development project. Eruptions of Mount Pinatubo Pinatubo has had at least 6 periods of activity with large explosive eruptions in its past 35, years prior to the eruption. The eruption in this context actually ranks as one of the smaller eruptions. An eruption, which occurred 35, years ago and probably created the caldera, was likely much bigger. Show interactive Map.
0コメント