BigIslandVideoNews just put up an edited video of Saturday’s USGS press conference. My hero Don Swanson is there. It’s INCREDIBLY informative on what they think is happening at the summit and why they think it’s a repeat of the “much smaller eruptions” of 1924 and not larger ones.
What I want to do is present a perspective about the current explosive activity that’s taking place at the summit of Kilauea. And in order to do that, I want to make an explosive statement. And that is that Kilauea is an explosive volcano.
Now, it’s taken us a long time to realize that. But it turns out that recent research [starting with his pioneering fieldwork and research!] shows that, for the past 2500 years, which is about as far back as we can see, Kilauea has been in a dominantly explosive period more than half of the time.
Right here at this golf clubhouse [Volcano Golf Course and Country Club, since park’s closed], we are resting on more than a foot of volcanic ash deposited between 2200 years ago and 1000 years ago. On top of that foot there are several inches of ash that were erupted between about 1500 and 1800. Those were large eruptions. And that series of eruptions lasted a long time. But they demonstrate the explosive character of Kilauea.
More recently, in May of 1924, we had a series of much smaller eruptions, so small that you can’t find their deposits here or anywhere outside of the caldera. They killed a person, because a person got too close.
The current activity, we think, is directly analogous to what happened in 1924: small eruptions, preceded by lava lake withdrawal out of sight, by many felt earthquakes in the summit area, and then by interaction of red-hot rocks with— probably with groundwater, and perhaps with magmatic gas. We’ll get into those issues later during the discussion period.
But I really wanted to put this [current] activity in perspective. Because this didn’t come out of the blue. We’ve had a lot of explosions at Kilauea in the past. The reason that we hadn’t realized that until recently is that for the almost 200 years that westerners have been in Hawai’i and have kept written records, there were only the 1924 explosions that took place. And so we thought Kilauea was normally a docile volcano that erupted lava flows that can do horrendous damage, as Carolyn will describe, but that were not explosive.
We now know that that is not the case, and that we’re unfortunately living through a proof of that statement.
Kyle Anderson (California Volcano Observatory Geophysicist):
So the summit of Kilauea Volcano continues to deflate, caused by the withdrawal of magma to the volcano’s rift zone. Associated with that process, magma left the Overlook Vent. It’s completely withdrawn now.
#Kilauea caldera changes imaged by @AgenziaSpaziale #CosmoSkymed radar satellites. Enlargement of summit eruptive vent between May 5 and 17; some ash accumulation SW of vent from emissions prior to May 16. Similar to what took place in 1924. pic.twitter.com/moiirxv7Lq
— USGS Volcanoes🌋 (@USGSVolcanoes) May 18, 2018
That has destabilized the vent and caused a series of rockfalls. These rockfalls lead to both small and sometimes larger explosive eruptions at the summit, Some of these have put ash clouds to more than 20,000 feet. The longterm background deflation rate has been fairly steady since a magnitude 6.9 earthquake.
However, the minor explosions and some of the larger explosions we’ve had are associated with brief, actual pressure increases in the reservoir. And that has complicated the picture a little bit. It actually relieves somewhat the stress on faults in the caldera, and has reduced the background seismicity rate. But the overall picture is that the subsidence is continuing, caused by withdrawal of magma to the volcano’s rift zone.
Q: Would you please clarify for us with the idea of the lava and the water table and the fact that they mix creates heat. Those kind of steam eruptions, will those only come from up in the caldera, or from Pu’u O’o? [YES! I emailed that question but they’ve been a little busy!] Do they happen elsewhere? Can you just break that down and explain it to us novices?]
The water table is everywhere, so it is possible to get interaction of groundwater with hot rocks or with magma anywhere— anywhere where there’s magma. And so we know there have been rather powerful explosions in lower Puna in the past. And we suspect— this is in prehistoric times— but we suspect that these were driven by water, which, of course…you’re close to sea level down there, so there is a lot of water present.
Prior to 1969, it was possible to see large blocks that had been ejectced from a crater on the East Rift Zone named Alai[??] Crater. And the inference was that there had been a phreatic eruption, an eruption involving groundwater, there at Alai Crater, which is pretty near where Mauna Ulu is today.
So, in answer to your question, I think it’s possible, anywhere along the Rift Zone, given the right circumstances, to have a groundwater-powered eruption.
Q: Could you also tell me if the one that you saw at midnight just recently, this latest eruption of ash, what caused that— the 30,000 foot ash plume, could you just break that down?
There was a lot of discussion about what caused the really high column. And we don’t really know now.I think that we’re anticipating that both steam from heated groundwater was involved and perhaps the involvement of a pressurized gas that had come out of the magma system. But this is a topic of ongoing research.
Kyle Anderson (answering a question this edit didn’t include):
And just to emphasize that that dropping, the subsidence of the caldera floor, was not a rapid event associated with an earthquake. As magma is evacuated from the summit reservoir, the pressure decreases, and that causes the floor to subside at a gradual rate. So over the last— some time, since that magnitude 6.9 earthquake, that’s happened at a relatively steady rate, so it’s been a gradual subsidence. And that now, at a maximum point in the caldera, has exceeded probably about three feet.
As far as we can tell, that will probably continue, as long as magma is evacuating the summit reservoir more quickly that it enters from the magma supply. I
I said earlier very briefly that these explosive eruptions are associated with brief pressurizations of the summit reservoir. That’s an interesting process we’re working to understand. Effectively, it gives us a little extra pressure in the reservoir, which has effectively slowed the rate of subsidence over the last several days. But background, going forward, we expect subsidence to continue at a fairly steady clip.
Q: [fairly unintelligible question about how closely this is following the 1924 eruption] and “what have we got in store?”
So, I’ll address the deformation part of that. The big, geological perspective I’ll pass over to Don. The deformation we have observed in the caldera is well short of what occurred in 1924, as you stated. So we’re nowhere near, as far as that goes. But I’ll turn it over to Don for the big picture, especially for 1790.
I think the question of whether or not we’re approaching a 1790 eruption keeps coming up, and we see absolutely no evidence for that. The situation in 1790 was that there had been almost 300 years of explosive activity. The entire caldera, or a large portion of it, was probably down to about the water table, and there was a lot bigger chance of involvement with a wide conduit and groundwater than there is at present. Also, in 1790, there were multiple vents on the caldera floor, some to the north of where Halema’uma’u is, and one perhaps to the south of there. And we just see no evidence of any multiple vent locations today.
So I think that the analogy between the 1924 event/explosions and the one today is… virtually perfect. I may live to regret that phrase, but I really think that everything that was observed in 1924, we have observed today. And so the analogy is really good.
Q: [Have there been] any big boulders or rocks that have come out the summit yet?
Well, yes, there have been. We have not been able to get close to know the maximum size. But I personally have seen one block that was about 60cm in diameter. And it actually broke on impact, and fitting it back together, about 60cm, which is just short of 2 feet or so. And this way way on the outskirts of where I think the ballistics would have fallen. So I would not be surprised if the largest ballistics were several times that.