Háskóli Íslands

Chris Newhall and colleagues from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the US Geological Survey (USGS)

 

 

The largest eruption on Earth in the last 100 years: Precursors, processes, and muddy aftermath at Mt. Pinatubo

The largest eruption on Earth in last 100 years, of Mt. Pinatubo, Philippines, began with a tiny steam-blast explosion on April 2, 1991, growth of a lava dome from June 7-12, 1991, and a series of sixteen moderate size explosive events from June 12-June 15.  A climactic eruption occurred on June 15, 1991, maintained full strength for 3.5 hours, and produced about 5 km3 of magma.  The products, pumice and ash with a bulk volume of about 10 km3, were spread on the slopes of the volcano and across the South China Sea.  A new 2.5-km-diameter caldera was formed at the summit of the volcano, which later filled with a lake. For the first time, modern monitoring captured the pre-, syn-, and post-eruption signatures of a sulfur-rich, plinian, caldera-forming eruption. Deposits of past eruptions told that a giant eruption was possible; however, no one knew exactly what precursors to expect.  A number of new concepts arose, and some others, previously suspected, were confirmed by study of this eruption, including: (i)  M 7.8 Luzon earthquake of July 16, 1990, 100 km NE of Pinatubo, apparently started a chain of events that led to Pinatubo’s large eruption in June 1991.  A plausible mechanism is that movement of crust during the earthquake increased compression in the Pinatubo area by ~1 bar and squeezed basalt magma up from depth. (ii) A modest volume of basalt intruded a ~100 km3 body of residual, viscous, crystal-rich dacite magma 8-12 km beneath the volcano.  Mixing of the two began as early as 3 months before magma was erupted. (iii) Many of the early precursory earthquakes were 5 km northwest of Pinatubo, rather than directly beneath the volcano, along pre-existing regional faults. (iv) Pinatubo’s geologic past, rather than unique precursors, was until the last 24 hours the main reason to suspect that a large eruption was pending. (v) There may have been a significant expulsion of acidic groundwater from Pinatubo, because water samples collected on April 8-10, 1.5 km downstream from a known thermal area, had pH 2.45 vs. a baseline pH of 7.6-8.0. (vi) Pinatubo’s eruption was unusually powerful because the volcano had been dormant for ~500 years and had apparently trapped incoming carbon dioxide, water, sulfur dioxide, and other magmatic gases so effectively that these volatiles saturated the melt and then formed a discrete phase of volatile bubbles. The climactic eruption seems to have simultaneously fed high-energy pyroclastic surges, pyroclastic flows, and a high, tephra fall –producing eruption column.   Areas at risk from pyroclastic flows were successfully forecast, one month before pyroclastic flows completely devastated an area of about 400 km2 around the volcano.. The date of eruption was also successfully forecast, just days in advance.  About 85,000 people were evacuated from the most dangerous areas before the climactic eruption, and at least 10,000 of these people were saved from certain death by such evacuations.  Ash from the June 15 eruption, including a large component of fine-grained co-ignimbrite ash, was encountered by 13 flights, and many other flights were grounded by the ash. Lahars began during the eruptive activity and continued for roughly a decade thereafter.
 

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