April 2, 2009
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
The soda can analogy for Kīlauea's current summit eruption
The current eruption at Kīlauea's summit is unique in the recorded history of the volcano. Never in the past 200 years has there been a long-lived eruptive vent that emitted mostly gas, with only a small amount of spatter and ash, while—at the same time—a second persistent eruption occurred elsewhere on the volcano.
Why did such a unique eruptive vent form at Kīlauea's summit? How can such a strange eruption be sustained for such a long period of time (13 months and counting)?
The Hawaiian Volcano Observatory (HVO) is in a prime position to observe the eruption and has a battery of instruments trained on the summit vent, including temperature sensors, GPS stations, tiltmeters, seismometers, microphones, cameras, and gas sensors. Perhaps not surprisingly, the summit eruption is more complicated and mysterious than previous eruptions observed at Kīlauea.
Most Kīlauea eruptions are preceded by swarms of earthquakes and ground uplift as magma rises to the surface. In 2007 and early 2008, however, Kīlauea's summit was deflating (a sign that magma was draining from the summit), and relatively few earthquakes were recorded. At the same time, rising levels of seismic tremor and gas emissions suggested that magma was ascending to shallow levels beneath the surface.
These data are contradictory, and it wasn't until March 19, 2008, when an explosion occurred at the summit as the new eruptive vent formed, that the outcome of the activity became clear.
With all the data collected before the start of the summit eruption and since, we can address the "why" and "how" questions by reconstructing the conditions in the subsurface magma chamber before the eruption.
Between 2003 and 2007, Kīlauea's summit inflated as pressure increased within the summit magma reservoir. After a new eruptive vent formed on the east rift zone in mid-2007, magma began draining from the summit to feed the new vent, and pressure began to drop in the magma chamber.
This pressure drop may be the primary cause of the current summit eruption. To understand why, we'll need to open a can of soda—not just any soda, but one that has been shaken. Opening the can releases built-up pressure, allowing carbon dioxide dissolved in the soda to come out. The result? You end up wearing your drink!
Opening the shaken can is similar to what happened at Kīlauea in mid-2007. Formation of the new east rift zone eruptive vent tapped magma stored in the summit, causing pressure within the magma system to drop. This pressure drop was gradual—a bit like slowly opening the can of soda—which is why there was not large explosion. Still, the pressure drop caused gas to be released from the magma, resulting in increased gas emissions and seismic tremor by early 2008. Gas build-up created a path between the magma chamber and the surface and eventually resulted in the March 19 explosion.
"But," you ask, "doesn't soda eventually go flat?" Of course, and if that had occurred at Kīlauea, the summit vent would have shut off a long time ago. Imagine, though, that there is a giant tank of soda connected to the can, and whenever the soda in the can starts to go flat, it is replaced by fresh soda from below. This is essentially what is occurring right now at Kīlauea's summit. As magma rises and goes flat by losing gas, it is replaced by fresh, gas-rich magma from below. This cycle of convection is manifested by subtle changes in deformation and seismic tremor observed at the summit.
These answers to the "how" and "why" of Kīlauea's current summit eruption are new ideas, and still need to be rigorously tested. Nevertheless, the idea offers a novel explanation for the contradictory precursors of the activity—and it's as simple as the can of soda that's sitting in your refrigerator!
The Waikupanaha and Kupapa`u ocean entries remain active. Surface flows over the past week have broadened the Kupapa`u delta, which now extends along the coastline for about 1 km (0.6 miles). A deflation/inflation (DI) event at Kīlauea's summit (ongoing as of Thursday, April 2) has begun to disrupt the supply of lava through the TEB tube system.
At Kīlauea's summit, the vent within Halema`uma`u Crater continues to emit elevated amounts of sulfur dioxide gas, resulting in high concentrations of sulfur dioxide downwind. The active lava pond in the vent, visible only with a thermal camera, remains more than 90 m (100 yds) below the vent rim.
Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea summary; email questions to askHVO@usgs.gov.
Updated: April 13, 2009 (pnf)