Volcanoes spring predictable surprises. Ever since Puʻu ʻŌʻō began to inflate it was clear that a new break-out was brewing. The 61-G flow had been in decline for many months: the spectacular ocean entry had ceased some time ago, and the flows had withdrawn to within a few miles of the origin. That is typical for a reducing flow rate: the lava can only travel until it solidifies, and the lower the rate the sooner it becomes stuck. A lava tube can extend the flow but cannot save it.
Why did the flow rate reduce? Either the input into the magma chamber had declined, or the outlet had become restricted. Why did the inflation suddenly start? Either the magma input went up, or the outlet had become even more restricted. HVO has reported that gas emissions did not increase during the inflation which argues against increasing input. However, there had been an increase in deep earthquakes in the region, from about August 2017. In either case, the system began to back up, and the lava started to accumulate in Kilauea itself. Its lava pond began to rise, and finally overflow on the floor of the crater. Inflation at Kilauea came long after the 61-G reduction, showing that the main problem was an obstruction downstream.
The plot shows the extension of the Puʻu ʻŌʻō crater. It began expanding mid-March, stabilized (mostly) early April but a massive acceleration started mid-April. (This was also the time Kilauea started to respond.) Eventually it reached 20 centimeters, far in excess of anything seen in recent years. At the same time the crater floor was coming up. It was only a matter of time before something would give.
A deflation at Kilauea lasting several days stabilised the system. However, this reversed on April 29 and the lava pond began to rise. The failure began at 3pm local time on April 30 (1 am UT, May 1) with two brief, tiny tremors at Puʻu ʻŌʻō, picked up by the seismographs. Just after 3:30 the tremor restarted and now became continuous. The first earthquakes were picked up. Seismological hell now broke out, with tremor continuing for an hour, after which it slowly subsided. Midnight found things quiet, but now a new, fainter burst of tremor appeared, lasting six hours. Already at 4pm HVO noted that the crater floor was collapsing, indicating the magma had found a way out. But where?
Now things got really interesting, for the earthquakes began to migrate east, which I had certainly not expected. The seismographs saw the activity coming and passing, amidst increasingly shallow swarms of earthquakes. Clearly the old rift zone to Puna was being re-activated. A second burst of earthquakes occurred at the coast, reaching M4.2, but these had no tremor associated with them. The steep edifice apparently was adjusting to the changing pressure. The earthquakes reached highway 130 at Kalapana and stopped for a few hours, but afterward resumed their march towards Puna. As of writing, they have reached near the Leilani estates. The plot below shows the various seismographs, more or less in order along the rift zone, for a 48-hour period. The start of the event is very clear, as is the progression as the activity moves east. KAED is the coastal location and does not show tremor other than that from the initial burst: the migration from the crater to PHOD on the eastern edge took about 20 hours.
Clearly, the magma chamber had caused a dike to form. The progression is very similar to that of the Bardarbunga eruption, except things go faster. So far, the dike is some 15 kilometers long and extending at 1 km per hour.
The logical conclusion of this event will be that the magma will reach the surface somewhere along the eastern rift. This is not very convenient as it brings lava into proximity to people. And even the famous black sand beach (which in my memory doubled up as a nudist beach) is now at risk. The 61G flow was cut-off by the collapse and will cease soon.
The events are an imperfect repeat of the 1960 eruption which devastated parts of Puna. You can read about it on the HVO web site. The current dike seems to run closer to the coast which should help.
At the moment, the tilt on Puʻu ʻŌʻō is decreasing further, suggesting magma is draining out. Whether it has reached the surface yet is unclear: the weather is not conducive to investigations. Kilauea has not yet responded to the events, which may be surprising giving its good magma connection to Puʻu ʻŌʻō. But in the 1960 eruption, it took four days after the start of the eruption before Kilauea itself began to deflate, so it is probably just a matter of time.
There is one further interesting aspect to the events. The changes in the Kilauea system coincided with a stop in the inflation of Mauna Loa. Their magma chambers are not connected, but do the two systems affect each other? It will be interesting to find out. In the mean time, we wait and see.
Albert, 2 May 2018