Each volcano is an expression of a magma architectural construction, a great sculpture of chambers, pipes and sills, as intricate as an ant colony, or rather like the roots of a plant. This is all hidden away from our view, under kilometres or tens of kilometres of rock that makes it impossible for us to know what lies down there, or perhaps not? To me a volcano is a puzzle of many aspects, geochemistry, earthquakes, structure and eruption behaviour, among others. A puzzle can only be completed by taking all the pieces and putting them together. Not an easy task. I will attempt to assemble the puzzle, or at least part of it, of my favourite volcano. Kilauea. And it all starts with a caldera collapse in 2018.
As I write this, three years ago the summit of Kilauea volcano in Hawaii was collapsing into a caldera. Its shallow magma chamber emptied laterally through fissures in its rift. It might seem like this was a while ago and is now over, but is it? The collapse set off a series of cascading consequences that are still rippling through the molten structures of Hawaii Island. The deflations of Mauna Loa, a swarm of tremor and earthquakes under the town of Pahala, the recent eruption of Kilauea, all might be connected and it has just barely started, I will argue how this all happened and how all the events would be related.
We should first know a little about the connections of Kilauea with Mauna Loa and Pahala.
Kilauea and Mauna Loa
These two volcanoes have quite an interesting relationship. When one volcano is more active the other is less so. In particular each takes over for a period of 100-250 years, during which it gets most of the magma supply in Hawaii, then this activity goes to the other. They share the same magma supply. A far as I know this has been going non-stop for at least 2000 years. On a future article I will talk more about this aspect.
Kilauea was dominant throughout the 18th century and until 1840. 1840-50 was a transition decade. Mauna Loa was dominant from 1850 to 1950. And Kilauea from 1960 to present. We are currently in an era of Kilauea and there is still more than forty years to go.
This relationship is best seen in the supply and the chemistry of the magmas. While Mauna Loa was dominant it erupted every ~3-4 years at an average rate of 1.2 m3/second. The total supply would also include dyke intrusions and slow spreading which could increase the total number by a factor of two or so. After the 1950 eruption its eruption frequency plummeted spectacularly. Dormancy periods have been 25, 9 and 37+ years. The output was reduced to 0.11 m3/second, a difference of a whole order of magnitude!
Kilauea mirrors Mauna Loa but in the inverse way. When Kilauea erupted continuously in the 1840-1920 period it did so at a rate of 1 m3/second and there were practically no large dyke intrusions. In contrast when Kilauea erupted continuously after 1960, during the Mauna Ulu and Pu’u’o’o eruptions, it did so at a rate of 4.1 m3/second, four times greater than in the subdued state. Ever since 1960 Kilauea has been rapidly inflating, spreading, and erupting. It has produced over 6 km3 in this time. The most productive volcano in the world. Overall Kilauea seems to erupt more, but it is clearly impacted negatively when Mauna Loa is more active.
Both volcanoes must feed from the same magma reservoir at depth, and through some mechanism the supply to this reservoir goes to either one or the other. But where? And how? All will be answered, but will have to wait for future articles, lets first take a look at the Pahala Swarm.
Kilauea and Pahala
A swarm of earthquakes has been raging under the town of Pahala since 2019. Actually the swarm has always been there, but its earthquake rates have skyrocketed starting in January 2019 and rising to unprecedented levels. The earthquakes are 30-40 km deep. And there is the tremor. The area just offshore Pahala is an extraordinary source of volcanic tremors. Almost all deep tremors of Hawaii originate from this location and are thought to be caused by magma rising up from the depths. Pahala would be a magma conduit then. The swarm has two parts the southern, deep, offshore swarm of of tremors, and the northern, shallower swarm of fracturing earthquakes. These two distinct parts are important when looking at the post-2018 series of events.
The Pahala Swarm is thought to be the magma feeder to Kilauea. How could we know this? When looking at a map the connection shows up quite clear, the Southwest Rift of Kilauea, which runs straight from its summit to the swarm, providing a useful connection. However the Pahala earthquakes are very deep, 30 km, does the rift really extend down to here?
Hawaii has a peculiar tectonic system powered by the magma supply. The rifts and the decollement faults. A rift is like a fracture system that is opening up and filling with magma, when simplifying things down a lot. The rift pushes away the flank of the volcano to open up space for the magma. The flank creeps on top of a nearly horizontal fault that is slightly upslope, a reverse fault, this fault is know as the basal fault, the basal detachment, or basal decollement. Why basal? Because it runs through the base of the giant lava pile that is Hawaii. Here the volcano rests on top of the depressed ocean floor 8-10 km below sea level.
Because Kilauea is the volcano getting most of the magma now, then it is in front of Kilauea’s main rift, the East Rift, where we see these processes in action. The East Rift is continuously spreading very slowly, pushing against the south flank which swells upward and contracts, being squeezed by the lateral pressure. This can be seen in action if one visits the data of the GPS stations that the Hawaiian Volcano Observatory is monitoring on the south flank. When the basal fault can hold no more it breaks. The fault can rupture in earthquakes reaching M 7.9, some of the most powerful volcano-tectonic earthquakes in the Planet.
With all the mightiness of the basal fault a second, more modest system is often forgotten. Which is this unfortunate fault? The Mantle Fault Zone. It is called this way because it runs at a depth of 30 kilometres, in the lithospheric mantle. It happens to be a nearly horizontal fault similar in many ways to its shallower, larger relative, it can produce earthquakes reaching up to M 5.2, but most importantly has a very particular slip direction, 137º clockwise from the north, exactly perpendicular to Kilauea’s southwest rift, and positioned is such a way that it would open up the line from Pahala to Kilauea’s summit, and at the perfect depth. To put it simple, the southwest rift extends down to the Pahala Swarm and could provide an effective pathway towards Kilauea. More evidence for a Pahala-Kilauea connection is seen in the events following 2018.
I should also say that the magma chambers of Kilauea are most of them aligned with the southwest rift. This shows when there are deep sources or deflation or inflation, one of the most common sources of deformation that shows up is an egg-shaped pattern that is centred just southwest of Kilauea’s summit, is aligned with the southwest rift, and has the long end pointing towards Pahala. The main caldera of Kilauea formed in 1500-1790, has its outer scarps rectangle shaped with its long sides running parallel to the strike of the southwest rift. Currently the main axis of the Southwest Rift runs more or less coincident with the southeast margin of Kilauea Caldera, making a nearly perfect N 47º E direction line, that is perpendicular to the Mantle Fault Zone slip, and connects the most primitive magma eruptions of Kilauea, at Kilauea Iki, and Keanakako’i Craters, and the December 1974 dyke which marks the main rifting axis of the rift zone. This is seemingly a little detached from what must have been its original position.
There is yet another structure that links Pahala to the summit of Kilauea. The Kaoiki Pali. I think this pali, which means cliff, is none other than the rim of an large ancient caldera of Kilauea. Why? you may think. Because it hasn’t ruptured in over 9000 years. Fault-related scarps like those linked to the slumping or to the rifts rupture frequently over and over. Kaoiki Pali also wraps around the summit of Kilauea like other caldera faults do.
The formation of this structure happened at a very particular time. Kaoiki breaks the Pahala Ash layers that formed until 11,000 years ago, but is covered by 9000 years old flows of Mauna Loa. It formed at 11,000-9000 years. This marks the most important transition in the recent volcanism at Hawaii, where Kilauea ended a lengthy period of powerful explosive eruptions known as the Pahala Ash, and resumed summit overflowing. Mauna Loa went from a period of low activity to a period of very high activity, maybe even almost uninterrupted dominance lasting a few thousands of years. It was a big change. Kaoiki Pali could have formed if the whole complex of magma chambers and sills placed along the Southwest Rift of Kilauea collapsed. Which I think is the best, although perhaps not only, explanation.
After the 2018 eruption
Back to the big eruption. In 2018 Kilauea erupted 1.2 km3 of lava and its summit caldera underwent a nested collapse. An event on this scale had not shaken the island since at least the 1868 eruption of Mauna Loa, if not since 1790. If there was any eruption that could disturb the hellish furnaces deep under the mountains of Hawaii it would be now. What do I mean by this?
An eruption drops the pressure of a magma chamber by extracting magma from it, which may increase the resupply from the magma source to restore the pressure. It just happens that 5 months after the 2018 eruption ended the seismicity of the Pahala Swarm started ramping up towards record levels. The seismicity of the previously suspected feeder to Kilauea Volcano. So presumably it did react to Kilauea’s pressure drop.
First came the tremors. As I’ve said the Pahala Swarm has two parts, the tremor swarm at ~40 kilometres deep, and the fracturing swarm at ~35-30 km. The Pahala tremors tend to always start with a sudden jolt and then a series of later spikes, these can be located much more easily, like they were earthquakes, than other types of tremor.
The tremor came much stronger than usual in 2019. There were 4 pulses. The first pulse had its main phase in January 11-January 23. The second pulse peaked March 14-April 2. The third May 12-June 3. The last pulse came more dispersed in a series of peaks and could be considered more like a bunch of small pulses lasting from June 26 to July 27.
After the 4 big pulses the tremor activity dropped to the typical low levels until mid-2020 or so when a second series of strong tremor pulses started. I will focus only on the first series though. I do know however that there was at least one pulse in the second series, in January 2021, that does rival the 4 big pulses of 2019.
The fracturing swarm exploded into existence in August 2019. I remember being really impressed by this strong activity and yet this was nothing compared to what was to come. The rock fracturing of this type is thought to be volcano-tectonic, caused by an increase in magma pressure which shatters the rock around the magma. The Pahala Swarm started almost above the location of tremors. If the tremors were related to magma flow, then the fracturing may have been due to this same magma pulse pushing against the rocks on its way up.
The swarming initially exploded near Palima Point, and then propagated 10 km northeast, towards Kilauea, until it reached the opposite end of the Pahala Swarm, where I suppose the magma must have dived into the southwest rift of Kilauea. This happened around the time of peak earthquake activity on February 2020, when HVO was locating 80 earthquakes per day here. Unprecedented for Pahala. The levels then gradually dropped through the remainder of 2020. In 2021 earthquakes rates have started rising again and have nearly reached the levels of February 2020. The first rise August 2019-February 2020 seems to correlate with the first series of strong tremors, the second rise in 2021 seems to correlate with the second series of tremors.
I was eagerly waiting the magma pulse to hit Kilauea. Perhaps a bit too impatient. It took more than a year for the pulse to move just 10 km up and 10 km sideways from the tremor area to the closest end of the swarm to Kilauea, it was obviously going to be a long wait! But finally something happened, not in the way or the place I would have expected however.
Mauna Loa got weird. Weird means that the volcano started deflating quite markedly with no apparent reason to do so. This was around October 20, 2020. Hawaiian volcanoes, and volcanoes in general, deflate when magma is removed from their magma chambers to feed intrusions or eruptions. There was certainly no eruption. I didn’t notice any intrusion either, and I think I would have noticed, or at least HVO would have. If magma wasn’t removed by the conventional ways, where did it go?
The next thing did happen where I was expecting the way I was expecting. On the morning of December 2 a sudden surge of magma hit the summit of Kilauea. It was quite dramatic. I had been following Kilauea since the 2018 eruption and not much had happened. But all of a sudden there was the fastest inflation I had seen, within a matter of hours the summit “broke”, a dike rose towards the surface but failed to erupt. Kilauea kept inflating, on December 20 it snapped again this time the dike intersected the crater of Halema’uma’u and started an eruption.
History then repeated again, Mauna Loa started deflating abruptly on March 23, 2021. Then on June 1 Kilauea set off in another episode of rapid inflation. It was time to write down the hypothesis.
Timing of events
What do we know? First, Mauna Loa and Kilauea share the same supply and must be connected at some depth somewhere. Second, a large amount of magma rose through the Pahala Swarm in a series of 4 pulses in 2019. Third, Mauna Loa and Kilauea each have been hit by two rare magma waves, producing inverse effects on them.
Here is what I think. There was a magma chamber which reacted to the 2018 eruption, this chamber is where Mauna Loa and Kilauea are connected. A series of magma pulses rose, each pulse sent two waves, a positive wave towards Pahala and then Kilauea, and a negative wave towards Mauna Loa. Why negative? With each pulse the magma chamber pressure would have dropped pressure, remember that pressure drops when it feeds an eruption or an intrusion. This is an intrusion. The pressure drops would have manifested eventually at the summit of Mauna Loa, which is apparently closer to the source because it was hit by the changes earlier. Basically it would have lost magma due to the 2018 draining of Kilauea needing urgent resupply.
Presumably each volcano has been hit by the first and second pulses of tremor that took place in 2019. There is one problem though, the distance between them seems to have changed. The first and the second came 62 days apart in the tremor region, 154 days apart as they hit Mauna Loa, and at 181 days as they hit Kilauea. This would suggest the second pulse is travelling slower than the first. It is possible that the speed at which the waves move could be slowing down. The depressurization wave created by Kilauea was the fastest to reach Pahala in 8 months, the first pulse took 23 months to travel the same route up, the second 25 months.
This makes predicting the arrival of the third pulse complicated. Assuming that the interval between the second and the third changes by the same factor as the first interval, the arrival of the wave can be estimated, though how accurate it may it be remains to be seen. It may arrive at Mauna Loa on August 17, and at Kilauea on November 20. The third could be the strongest if the number of tremors are a good indicator.
The 2018 eruption of Kilauea keeps giving new surprises 3 years later. It seems that it set off a series of waves that are rippling right now through the magma system under Hawaii, and might continue to do so for years to come. There could be a lot to learn from this. The third pulse could provide the final confirmation that Kilauea and Mauna Loa are connected through a magma chamber that feeds into Pahala, and it may also provide data regarding the properties of this connection.
Kilauea is likely to keep erupting from Halema’uma’u Crater episodically and fill up the caldera or part of it. Seeing how the pulses interact with the filling of the caldera will be most interesting.
I plan to continue this article with some others about Kilauea too. See you in the next one!.