What causes an eruption to pause? A pause is caused by an interruption in the supply of magma to the eruption site or vent. Now, how do we know the eruption has paused? Well, that’s a deceptively simple question with a surprisingly complex answer in this situation.
Molten lava is glowing red and generally easy to spot erupting from the vent, especially at night. And, likewise, generally easy to tell when it’s stopped. However, in the case of the lava lake, the vent supplying the lava eventually crusted over and lava was no longer visible.
The best way to tell if lava is still filling the crater (without being able to see the lava) is to see if the level of lava in the lake is continuing to rise. The crater forms a natural measuring cup, except that it’s a very strange shape and it has no markings to use to measure volume. We measure the distance to the surface of the lava lake using laser rangefinder instruments and then apply some algebra, trigonometry, and a computer to find the depth, volume, and lava supply rate of the lava lake.
The amount of sulfur dioxide gas (SO2) being released from the vent is directly related to the amount of lava being erupted. Both erupted lava volume, approximately 8 million cubic meters or 10 million cubic yards per day, and SO2 gas release, 40,000 tonnes per day, were extremely high at the beginning of the eruption. These exponentially decreased over the course of the eruption to about 70,000 cubic meters or 90,000 cubic yards of lava per day and less than 1000 tonnes of SO2 per day by early- to mid-April.
The eruption rates and SO2 gas emissions declined precipitously after April 16th, coincident with the level of lava in the lake reaching the level of the vent. This relationship suggests that the colder, degassed lava within the lake essentially drowned the vent.
This period also saw a decrease in seismic tremor related to magma movement and degassing within the dike feeding the vent. In addition, a weak pattern of inflation associated with increased earthquakes beneath Kilauea caldera suggest that magma has shifted to accumulating at depth instead of rising to the surface.
Finally, the top of the lava lake crusted over as the lava supply dwindled and stopped, with the last surface activity seen on May 23rd. So, without being able to directly see if new lava was entering the lake, USGS Hawaiian Volcano Observatory (HVO) scientists used lava lake depth measurements to infer that the lava supply stopped and that the eruption “paused.”
Many eruptions undergo brief pauses then resume activity within the same vents. There were numerous pauses followed by resumption of activity during the Pu‘u ‘O‘o eruption and many have also been recorded in the past including during the Maunaulu, Kilauea Iki, and 1955 lower East Rift Zone eruptions.
So, how do we know if the 2020-21 Halema‘uma‘u eruption is paused or pau? Statistically, most eruptions that resume, do so within three months after pausing. For now, we can only watch and wait to see if activity returns to this same vent system ending the pause. Or alternatively, if the eruption is truly pau, we may be entering a period of quiescence prior to Kilauea’s next eruption.
Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.