Mount Pinatubo's enormous eruption on 15 June 1991 and its muddy aftermath made the volcano's name--"to make grow"--seem singularly inappropriate. But 10 years after the eruption (1), growth has again overtaken destruction. New towns and industries are replacing those that were buried, and plants and animals are filling old and new ecological niches.
Volcanology has also grown. For the first time, modern instruments captured the signatures of a sulfur-rich, explosive, caldera-forming eruption before, during, and after the eruption (see the first figure). Some concepts were affirmed; others were shredded and swept away. Some puzzles were solved; others remain.
Short-term coupling between regional earthquakes and volcanism is enigmatic and often dismissed. Yet, after sleeping for 500 years, Pinatubo awoke just hours after a magnitude 7.8 earthquake along the Philippine fault, 100 km northeast of the volcano, in July 1990. During the interval between the earthquake and Pinatubo's 1991 eruption, data are too sparse to determine the exact relation between the two. The detection of long-period earthquakes 35 km beneath Pinatubo before the eruption and the nature of early-erupted rocks suggest fresh basaltic magma from Earth's mantle (2, 3). Fault slip subjected the crust beneath Pinatubo to static compression of 1 bar and could have squeezed preexisting magma upward (4).
Pinatubo produced between 15 and 20 megatons of S[O.sub.2], about 20 times more than could have been dissolved in the volume of the erupted magma (see the second figure) (5). It thus confirmed an earlier inference from El Chichon's eruption in 1982 that C[O.sub.2], [H.sub.2]O, S[O.sub.2], and other volatiles could accumulate in magma far in excess of saturation, forming a discrete bubble phase at least 5 to 10 km beneath Earth's surface, not just in the top few kilometers of the crust (6-8). At Pinatubo, 200 to 1000 megatons of these supercritical volatile bubbles accumulated in the uppermost part of a large (~100 [km.sup.3]), long-lived, Si[O.sub.2]-rich dacitic magma reservoir. Work since the Pinatubo eruption suggests that many, perhaps all, large explosive eruptions are of magma that contains a substantial bubble phase at depth.
In theory, basaltic and dacitic magma should not mix because of their different viscosities: Basaltic magma is fluid, whereas dacitic magma is highly viscous. But Pinatubo confirmed such mixing on a time scale of days to weeks (9, 10). A relatively small plume of basaltic magma rose into the large, crystal-rich dacitic magma reservoir that had resided for thousands of years about 5 to 15 km beneath Pinatubo. Cooling and crystal growth in the...