(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
Regular visitors to HVO’s daily update Web page might notice what seems like a minor change in one of Kilauea’s two ongoing eruptions. While sulfur dioxide (SO2) emissions from Kilauea’s summit continue at substantially elevated levels, the amount being released from the east rift has declined substantially in recent months.
How might this change be important? Both the summit eruption within Halemaumau and the east rift eruption at Puu Oo and the Thanksgiving Eve Breakout (TEB) vent appear to be going strong. New lava covers the coastal area near Kalapana. And Kilauea still emits more choking SO2 than the single dirtiest coal-fired powered power plant on the U.S. mainland.
To better understand what the decrease in east rift emissions means, it’s useful to remember the significant role volcanic gases play in eruptive processes. Gases are dissolved (trapped) in magma within the Earth’s mantle where pressures are great — hundreds of thousands of pounds per square inch.
As magma rises toward the surface, the weight of overlying rock lessens, as does the pressure on the melt. This decrease in pressure allows some of the dissolved gas to bubble out.
A relevant characteristic of gas is its tendency to expand when pressure is decreased. Anyone who has shaken a bottle of soda and quickly opened it has gotten a palpable understanding of this basic physicochemical property. This same basic gas-release principle drives volcanic eruptions worldwide. Expanding gases released from the liquid they’re trapped in push that liquid, whether it’s soda, beer, or molten rock toward locations where pressure is lower.
At Kilauea, magma rises toward the surface beneath Halemaumau. When the magma reaches the shallow summit reservoir about 1 km (0.6 mi) beneath the crater the associated pressure decrease allows some of the gas to exsolve, rise, and escape. From this reservoir, the partly degassed magma is transported to Puu Oo and beyond, where the rest of the pressure is relieved as magma reaches the surface, and lava is erupted.
For the 20 or so years prior to the start of the 2008 summit eruption, SO2 emissions from Halemaumau were modest, averaging about 200 tonnes/day. But as magma got closer to the surface in early 2008, summit emissions climbed. Since the Overlook Vent opened, Halemaumau has emitted about 800 tonnes of SO2 each day.
This four fold increase in gas release at Halemaumau means there’s much less gas remaining in the melt being transported to Puu Oo. SO2 emissions from Puu Oo remained high through early September 2008, averaging nearly 2,600 tonnes per day.
Beginning mid-September, there began a significant decline. For the next year, east rift SO2 emissions averaged roughly half this amount, about 1,200 tonnes per day.
The east rift effects associated with the continuing Halemaumau eruption do not appear to be limited to a simple decrease in rift emissions, though. Remembering that gases exsolving from magma help drive it to the surface, we reasonably expect that less gas available in the melt reaching Puu Oo would affect east rift activity.
Since the decrease in east rift emissions began, a corresponding magma flow disruption between the summit to the east rift zone has been indicated by an increase in the frequency of deflation/inflation (DI) events. And while surface flows on the pali and coastal plain have continued, the last developed tube system transporting lava directly into the ocean died nearly four months ago.
All these changes suggest a sputtering of the east rift activity. Literally and figuratively, the east rift eruption appears to be running low on gas, while the summit eruption continues strongly.
Thus far in 2010, Kilauea’s summit emissions remain nearly what they’ve been throughout the summit eruption, around 750 tonnes per day. Comparatively, east rift emissions, which used to dwarf those of the summit, now are roughly equivalent to them.
What this all might mean in the long term for these contemporaneous eruptions is anyone’s guess. Volcanologists are generally better at forecasting the beginning of an eruption than the end of one. We know that dissolved gas and a pressure difference are both needed to have an eruption, whether it’s soda from a bottle or lava from a volcano.
It’s possible that the east rift eruption might stall or even stop. Activity within Halemaumau, however, could continue for months to years. On the other hand, many factors are in play, and Pele continues to show us that while Kilauea is perhaps the best studied volcano on earth, it is not necessarily the best understood one.