We humans may wish for many things, but gravity is a horse we can’t wish away. And this horse drives what is happening at Kilauea.
So far, the new stage of the old Kilauea eruption that started about one week ago has given us a very small subsidence caldera, a large earthquake, drained two lava lakes, and given us a rifting fissure eruption at the Leilani Estates. And the force of gravity is behind it all, because in the end all things must come down.
There is also a great mystery that needs to be solved. And that is were all the disappearing magma have gone. After all, roughly 0.3 cubic kilometres of magma have intruded into the East Rift, and only about 0.0001 cubic kilometre has erupted.
The last thing has so far saved most of the homes of the residents of Leilani Estate, but it might not be such good news in the long run.
But first things first, we must go back to the beginnings of the current iteration of Kilauea to understand what is happening.
Subsidence calderas at Kilauea is believed to have come and gone since the volcano entered it’s shield building stage.
The current caldera formed about 1500 years ago and was followed by a prolonged era of explosive eruptions since the caldera floor had fallen below 615 meters below the caldera rim. That specific number is rather interesting, because that is where the watertable is located.
About 1 100 years ago the caldera stopped erupting explosively and large lava flows started to happen at the same time as the caldera floor started to rapidly infill. Sometime between then and 1790 the Halema’uma’u crater formed inside the caldera. The actual pit crater of Halema’uma’u is in the centre of a mini-shield volcano. The name means ‘house of the ʻāmaʻu fern’. A bit of a misnomer today since all the ferns are blasted to smithereens by now.
Inside the Halema’uma’u Crater we have the current iteration of the lava lake (there has been previous ones), it is called Overlook Vent. It formed during a series of explosions back in 2008.
The current eruption
The current eruption of Kilauea started on the 3rd of January 1983 out on the East rift zone at Puu O’o, it has mainly produced lava flows, flank eruptions, a lava lake, and other assorted mischief, but none of the activity has been really big. As mentioned above in 2008 the Overlook Vent formed at Halema’uma’u giving the current eruptive phase two different loci of activity.
In 2016 things started to change in regards of seismic activity and tilt. And after a while the lava lake inside the Overlook Vent overflowed unto the floor of the Halema’uma’u Crater.
The seismic activity continued in a waning and waxing pattern from then on until it intensified in the beginning of 2018 and concentrated out further along the Eastern Rift Zone after Puu O’o. And here is where gravity sets in.
Enter the gravity
At no place on Earth has gravity had such visible and measurable effects as at Hawaii. As the volcanoes grew they forced the seafloor and the crust below down. So much so that Mauna Kea near Kilauea is the highest mountain on Earth if you count from the bottom of the ocean floor to the top, and Mauna Loa and Kilauea are not that far behind.
The weight of these gargantuan mountains has pushed down the crust 17 kilometres. There is though a nastier side to gravity.
If you have a solid triangle pointed upwards the maximum down force (towards the Earth centre) will be at the upper point, that down force will be forwarded down the flanks of the triangle pushing the bottom corners outwards.
After a while structural weaknesses will form in the triangle and the top will crack and the sides will start to shear off. This shearing off happens far easier at a volcano that has alternating layers of lava, ash and soil intermixed.
If one side is braced against something it will not shear off. In Kilauea’s case it is braced against the even larger Mauna Loa. Only problem is that Mauna Loa is so large that it is itself going through the same “triangle-process” as Kilauea, and it is so big that it’s slow flank collapse is pushing the entirety of Kilauea 2.5 centimetres towards the ocean each year.
An interesting part is that during a slide of the side of the triangle, the top will move far less than the bottom corner of the triangle on the slumping side.
There was quite a bit of speculation about what created those earthquakes out on the East Rift Zone. Was it magma intruding? Or was it purely tectonic activity? In a way it was a question about which came first, the hen or the egg. But, in this case we do know that it was the egg.
The Leilani Eruption
The weight and seaward motion of the oceanside part of Kilauea had increased the strain on the Eastern Rift Zone sufficiently to start tectonic activity. This in turn created voids, and since nature abhor voids it was rapidly filled with magma from the nearby supply at Puu O’o.
As the magma drained into the dyke an under-pressured magma intrusion formed. Nonetheless, the added weight of the intruding magma was sufficient to trip the system above the threshold and an intense M7.1 earthquake occurred.
During the earthquake the top part on the seaward side of the Eastern Rift Zone moved half a metre outward, but the bottom part galumphed an astonishing 2.5 meters. Before this it was assumed that the next volcano over, the Loihi Seamount, would work as a stopping brace hindering slumps like this. Instead the side of Kilauea shoved Loihi sideways.
The earthquake caused movement that further under-pressured the dyke intrusion, and this explains why so little lava has come out compared to what has been sucked into the expanding cavity of the dyke.
So, here is a ground rule of fluid dynamics. A highly pressurized dyke will squirt out more lava when it erupts, than an under-pressurized dyke.
This might seem like a good thing to the people living around the Leilani Estate where miniature vents have cropped out. Because if there had been more pressure most, if not all, of the area would have rapidly been inundated by the first vent to open.
As it is now the pressure is not high enough to keep the vents open more than a few hours, and the amounts that flow out are miniscule.
That being said, even a miniscule amount of very hot lava entering your house is not a good thing. I am not at all making light of the plight of the Leilani residents.
But, in the end, if more lava had poured out it would have been better. Because the rapidly intruding magma is once again increasing the tectonic strain, further increasing the risk for another larger earthquake.
Now, before any ambulatory English tabloid reads this and think that I am writing that the entire side will fall off tomorrow, then you are wrong as usual. What I am is that the risk is increasing that the side might slump yet another few meters.
Back to the narrative. As the lava was sucked out from Puu O’o, both the small lava lake there and the main vent emptied out, then the shallow magma chamber was vacated and Puu O’o became a very small subsidence caldera in a few minor puffs of ash as the roof caved in.
After that the lava started to be sucked out of the lava lake in Overlook Vent at Halema’uma’u. Now we are ready to discuss what will happen, now that we know what has happened.
In the near future
In the beginning I wrote that the watertable is 615 metres below the highest point of the caldera wall. But to really understand that reference we have to go back to 1924. That year another lava lake at Halema’uma’u Crater was sucked away before earthquakes started at Puna. Same thing, but reverse order of events.
As the lava level hit the 615 metres mark the water interacted explosively with the lava and phreatoplinian detonations followed. The largest detonation created a column that was 7000 meters high.
In the end the vent that held the lava lake was destroyed completely.
In this case it means that phreatic to phreatoplinian detonations will start somewhere between today Friday and Tuesday latest, unless the lava level starts to rise again. The most likely moment for onset is on Sunday at 10.14 CET.
Prior to that explosions caused by rockfall from the cooling vent walls are probable, these rockfalls are expected to continue during the phreatoplinian phase. The current Overlook Vent is likely to be completely destroyed during this phase.
If the lava continues to withdraw to greater depth, subsidence inside Halema’uma’u Crater will occur, or even subsidence of the general caldera. The subsidence will most likely not occur through blocking of the roof, instead it will be as piston driven subsidence.
If this happens lava flows are likely to increase anywhere from western Leilani up to, and including, Puna. There is also the potential for another large earthquake, depending on how much of the pressure that is alleviated by out-pouring of lava.
But, in the end this is likely to lead to the end of the Kilauea eruption, at least for a short while. The changes in the volcano is just to big. Question is more for how long the eruption will continue, and how many months the interlude will be.
Whatever you do, do not think that breaking the law and going down into the caldera is a clever idea. If there is a phreatoplinian eruption you will be dead if you are on the caldera floor. Heed the warnings of the local authorities at all time. Staying away completely from the area around the caldera is best. Remember that all of Hawai’i Volcanic National Park is closed for general population. Being pancaked by a bus sized stone that has been lofted, is over-rated.