Reprinted from here.
Magma within Kīlauea’s summit vent never goes flat
|On May 14, a brief bit of clear weather shows the Halema`uma`u plume rising and blowing with the wind.|
Two weeks ago, readers of this column learned about the genesis of brown plumes and sudden gas release from Kīlauea’s summit vent. This week we will continue that exploration, looking farther into Kīlauea’s magma plumbing system to explain why the summit vent has become a long-lived feature of the volcano.
As residents of the Big Island can attest, vog has been a substantial irritant since the formation of Kīlauea’s summit eruptive vent in early 2008. Can we expect this release of volcanic gas to go away anytime soon? The short answer is no, but that requires a bit of explanation.
As magma rises to shallow levels beneath the ground surface, pressure on the magma drops, and gas is released—similar to opening a can of soda (dropping pressure), allowing the dissolved carbon dioxide to bubble out. The magma will go flat once all of the gas is released, suggesting that Kīlauea’s summit should eventually stop releasing gas.
The persistence of volcanic gas emissions from Kīlauea’s summit is evidence that the supply of gas-rich magma is being replenished. To understand why, we’ll need to understand the principle of convection—in other words, how a lava lamp works.
In a lava lamp, heat added at the bottom warms the colored blobs within the lamp, causing them to become less dense and rise to the top. Since the top of the lamp is away from the heat source, the colored blobs gradually cool, become denser, and sink. The cycle repeats itself until the lamp is turned off.
A similar process is probably occurring beneath Kīlauea’s summit but is driven by gas release instead of by heat. Magma within the summit vent is like an open soda, where dissolved gases gradually come out of solution. As the magma goes flat, its density increases. Eventually, the dense, flat magma will sink and be replaced by less-dense, gas-rich magma—a lava lamp in action! As a result of this process, the summit plume of gas and ash is constantly renewed.
Besides the unusual persistence of the summit plume, there is other evidence that convection is occurring within Kīlauea’s shallow magma system.
If you read the Kīlauea daily activity updates, posted on the Hawaiian Volcano Observatory (HVO) Website each morning, or are a regular reader of this column, you’ve probably heard of “DI” events. “DI” is shorthand for “deflation-inflation.” These events occur as the summit suddenly begins to deflate and then, after about 12–48 hours, just as suddenly begins to inflate and returns to normal.
Prior to 2008, the average number of DI events occurring in any given year was about 10. In 2008, however, there were 47 DI events and, in 2009, there have already been 15 thus far.
DI events may be a physical manifestation of convection within Kīlauea’s shallow magma plumbing system. DI deflation could be caused by downward flow of dense, “flat” magma, with DI inflation representing the rise of fresh, gas-rich magma. During the time between DI events, fresh magma would gradually degas and become denser. The increase in the frequency of DI events in 2008 probably reflects the fact that, due to the start of the summit eruption, the magma rose to very shallow depths, allowing for much more efficient and rapid degassing.
Vent collapses, like the one that generated the spectacular brown plume in early May, are sometimes associated with DI events. Assuming that DI deflation signifies the downward flow of dense magma, collapses might be expected, due to removal of magmatic support from the eruptive vent.
Although magma convection is suspected at several volcanoes worldwide, the process is difficult to document, because there is no way to see directly into a magma chamber. At Kīlauea, however, the outstanding level of geophysical and geochemical monitoring has provides good evidence of convection just beneath the volcano’s surface.
Scientists at HVO will continue to study DI events, gas emissions, brown plumes, and other signals from the summit vent in hopes of learning more about Kīlauea’s magma plumbing system. There is no doubt that this lava lamp will be going for a long time to come, so stay tuned to this column and the daily activity updates for the latest information!
Kīlauea Activity Update
The Waikupanaha and Kupapa`u ocean entries remain active and are topped by robust laze plumes. Frequent small collapses have prevented either entry from building a large delta. There have been no lava breakouts from anywhere along the tube system reported in the last week.
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. Glow, gas-rushing noises, and the emission of juvenile ash during the past week suggest that a small lava lake is still present below the floor of Halema`uma`u Crater.
No earthquakes beneath Hawai`i Island were reported felt this past week.
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 activity summary; email questions to askHVO@usgs.gov. Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.