CAN STATIC MAGMA DECCAN STATIC MAGMA DECOMPRESSION TRIGGER AN ERUPTION? EVIDENCE FROM KĪLAUEA VOLCANO, HAWAI`I

During June 17–19, 2007, the summit of Kīlauea Volcano, Hawai`i, deflated rapidly as magma drained from the subsurface to feed an intrusion and eruption on the volcano’s east rift zone. Coincident with the deflation, summit SO₂ emissions rose by a factor of four before decaying to background levels over several weeks, releasing 1560 tonnes of excess SO₂ over that period. The SO₂ release was unexpected, since heightened volcanic SO₂ emissions are generally associated with ascending magma. We propose that volatile exsolution was triggered by static decompression caused by magma withdrawal from a reservoir beneath Kīlauea’s summit caldera. Models of InSAR data spanning the June 2007 intrusion/eruption suggest a source depth of 1.2–1.8 km and a pressure drop of 0.5–3 MPa. Exsolution of the observed excess SO₂ could be triggered if the reservoir volume were 0.2–1.2 km³—a reasonable range for magma storage at 1–2 km depth beneath Kīlauea. A similar pattern of deflation and increased SO₂ emissions occurred in late 2007 and early 2008, immediately prior to and accompanying the March 19, 2009, explosive eruption at Kīlauea’s summit. That event marked the start of the first summit eruption at Kīlauea since 1982 and the first explosive activity there since 1924. We propose that the 2008 summit eruption of Kīlauea was triggered, at least in part, by static decompression of Kīlauea’s primary summit magma storage area, where magma withdrawal had been occurring since the formation of a new vent on the volcano’s east rift zone on July 21, 2007. The decompression led to volatile exsolution, dilation of pathways to the surface during volatile escape, and eventually magma ascent through the opened cracks and other preexisting void space; thus, no inflationary deformation preceded the explosion. Our observations and analysis suggest that static decompression of magma is a viable trigger for volatile emission, and possibly eruption, at Kīlauea and other volcanoes around the world, and argue for increased awareness of pressure fluctuations in magma reservoirs, especially at basaltic volcanoes. Hazards associated with magma decompression range from low, long-term, and distributed (e.g., heightened downwind SO₂ concentrations) to extreme, short-term, and localized (e.g., volcanic explosions).