A Contrast Of Two Volcanoes, Mayon And Pinatubo

EG Corpuz¹, E Laguerta¹, CG Newhall^2,1

¹PHIVOLCS, Quezon City and Legazpi City
² USGS (emeritus), Sto. Domingo, Albay

Mayon and Pinatubo volcanoes offer an instructive contrast in styles of degassing and eruptive behavior. Most of the time, Mayon magmas are degassing freely, producing ~500 t/d SO₂ during quiet times and ~1000-2000 t/d SO₂ during more active but still non-eruptive times. During a typical eruption day, Mayon produces between 10⁴ and 10⁵ t of SO₂. From a rough estimation of overall gas budget, it appears that Mayon magmas release about 90-95% of their gas passively, between eruptions. As a result, there is never any great accumulation of gas. We infer that gas is released by more-or-less continuous convection of magma within the conduit, in which rising gas-rich magma develops a permeable foam at the top, releases its gas, and dense degassed blebs sink back down the conduit, displacing more gas-rich magma up the conduit. Variable glassy to crystalline groundmasses of bombs provide evidence for differential degassing of magma in Mayon’s conduit. Even the largest explosive eruptions of Mayon, such as 1814, seem to reflect fresh influx of gassy magma rather than long accumulation of gas. In contrast, the more viscous dacite magma of Pinatubo seems to plug up its conduit between eruptions and store most of the gas that enters the base of its reservoir between eruptions. Day-to-day passive emission of SO₂ is practically unmeasurable, whereas during the large 1991 eruption about 2 x 10⁷ t of SO₂ were released in one day, representing ~ 80-90% of the gas that we infer to have been supplied in the preceding five centuries. This very large accumulation of gas drove the very large eruption.

This contrast in degassing behavior also manifests itself in seismic and deformation unrest at the two volcanoes. At Mayon, because conduit is open, incoming magma produces very little seismicity and deformation. This makes forecasting of Mayon eruptions difficult. Measuring SO₂ flux and subtle and/or near-summit deformation may be the best tools for forecasting at Mayon. Also, following eruptions of Mayon, more-or-less continuous recharge of magma means that seismicity and deformation after eruptions are rather similar to those before eruptions. It has been in near steady-state unrest, especially since 1999. At Pinatubo, where the conduit had been plugged for centuries, magma intruding toward the surface in 1991 produced ample seismicity (and probably deformation, though we captured very little deformation data). This makes forecasting of eruptions like that of Pinatubo more tractable. Since 1992, Pinatubo has been quiet, perhaps plugged once again.

We gratefully acknowledge opportunities and ideas provided by our late friend and colleague Ray Punongbayan.