A crusted lava pond was present (at a depth of about 200 yards below the crater floor) within the Halemaumau vent cavity July 4, 2009. Continuous spattering at the northeast margin of the pond illuminated the vent cavity and produced glow that could be seen from the Jaggar Museum Overlook. (Photo courtesy of Hawaiian Volcano Observatory)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

If you have followed the HVO daily updates or tuned in to the Webcams over the past few months, you have probably seen that activity in the vent of Halemaumau Crater — at Kilauea’s summit — has been highly variable. There have been large and small collapses, rising and falling lava levels, spattering, and even a small explosion that deposited ejecta around the Halemaumau Overlook.

Given all these recent changes, have we learned anything about the volcanic system that allows us to forecast any upcoming activity?

The answer, it turns out, is ‘yes.’

First, however, we need some background.  The vent opened in Halemaumau Crater in March 2008 and has enlarged to its present width of about 150 yards.

Just below the vent rim is a large, cavernous area, which we call the “vent cavity,” that is about 220 yards deep.  At the bottom of the cavity is what we refer to as the “floor,” and it is at or near this level that we see the top of the lava column.

Watching the activity over the past year-and-a-half, and particularly over the past few months, we have observed several fascinating patterns in vent behavior.

Observation #1: The lava level often changes due to pressure cycles in the summit magma system.  Each cycle, which lasts about a day or two, begins with a period of deflation, followed by inflation.  As many of you know, these cycles are called “DI events,” which happen commonly at the summit, up to a few DI events per week.

During deflation, we have observed that the lava level in the vent can drop.  During inflation, the lava level can rise.  So during DI events, we have seen cycles where the lava level drops, then rises again.

Observation #2: The lava level can control the stability of the floor of the vent cavity.  When the lava is at its typical moderate level, it tends to crust over and form a hardened surface on the floor of the vent cavity, often with several spattering vents poking through.

When the lava level drops, however, it removes support from the crusted-over floor of the vent cavity.  What happens next should not be surprising: the vent floor collapses in dramatic fashion, producing a robust brown plume of rock dust and creating a seismic event.

Observation #3: Rapid filling and draining cycles (which may last from a few minutes to several hours) normally appear when the lava is at a high level, such as during an inflation phase.

These three behaviors tie together to produce a now-familiar pattern that we have seen at least half-a-dozen times.

First, a DI event begins with deflation, and the lava level begins to drop.  At some time during the deflation phase, the floor of the vent cavity collapses, producing a brown plume and deepening the vent cavity considerably.

Hours later, when inflation begins, the lava rises back to its previous level.  In some cases, if inflation is significant, it rises slightly higher than its previous level.  When the lava is at this high stand, rapid filling and draining cycles ensue.

Within a few days or less, the inflation ends, the lava returns to its typical level, and it crusts over.  We are left with a crusted-over floor of the vent cavity, often with a few spattering vents.

Preliminary observations suggest that only the largest DI events produce this pattern of behavior, but more observations are needed to confirm this.

So what use is this knowledge?  First, when we see what appears to be a large DI event starting, we know there is a higher-than-normal likelihood that a collapse (and brown plume event) may occur.  Second, when we see that significant inflation is occurring, we can anticipate with good certainty that the lava pond will rise up and begin to undergo filling and draining cycles.

Most activity in the vent is impossible to forecast.

For instance, it is still not possible to know when an explosive event will occur, or, more importantly, when the current eruption will end.  Through close monitoring, we can keep watching for more patterns that may help us forecast other behaviors, keeping in mind that everything is ultimately at the whim of Pele.