The Largest Gravity Changes Ever Recorded: Continuous Gravity Monitoring of the Onset of Kīlauea’s 2018 Eruption

Abstract:

Eruptive activity at Kīlauea Volcano, Hawaiʻi, in April–May 2018 occurred at sites that were well monitored by continuous gravity. Draining of the lava lake from the summit eruptive vent starting May 1, recorded by a gravimeter on the vent rim, was accompanied by a drop of ~1300 microgals over 9 days. At the rim of the Puʻu ʻŌʻō eruptive vent, 20 km down the East Rift Zone from the summit, a gravity decrease of ~200 microgals over 8 minutes, followed by an increase of ~350 microgals over the subsequent 9 minutes, accompanied the formation of an eruptive fissure on the flank of the cone on April 30. About 45 minutes later, a decrease of ~1500 microgals occurred over 2 hours as lava drained from the vent. These gravity changes are the largest ever recorded anywhere in the world.

The evacuation of the summit and Puʻu ʻŌʻō eruptive vents provided opportunities to image the vent geometries, which were used to model the mass changes at the two locales. At the summit, joint modeling of gravity, lava level, and vent geometry indicate a best-fitting density of 1700 kg/m³ for the lava within the vent. There is no record of lava level over time at Puʻu ʻŌʻō, but the gravity data combined with the vent geometry can be used to reconstruct that process, suggesting that the lava had a density of ~1900 kg/m³ and that in 2 hours a bulk volume of 11 x 10⁶ m³ drained from the cone. The pre-collapse gravity decrease and subsequent increase at Puʻu ʻŌʻō are more difficult to model given the lack of other constraining data. We hypothesize that the gravity fluctuation is due to the emplacement of an eruptive fissure on the west side of the cone immediately prior to the collapse. The gravity decrease represents the opening of a dry crack, and the gravity increase is the subsequent filling of that crack with magma that was denser than the spatter that makes up much of the cone.

These data highlight the importance of continuous gravity for monitoring volcanic activity. Not only do the data provide important constraints on lava density, they can also be used to estimate the rate and volume of lava accumulation or withdrawal and can detect transient eruptive fissures, even in the absence of other observations. Without such data, our knowledge of the processes occurring at Puʻu ʻŌʻō during the crucial opening hours of the eruptive sequence would be as cloudy as the weather during that period.