Deformation at the open-vent Erebus volcano, Antarctica, from more than 20 years of GNSS observations

Abstract

Erebus volcano on Ross Island in Antarctica is an iconic open-vent volcano that has hosted a convecting anorthoclase-rich phonolite lava lake for at least 50 years. Recent magnetotelluric observations have imaged a conduit system originating at least 60km below Erebus. This terminates in a seismically-imaged shallow magma reservoir about 500m NW of the crater with an upper surface at about 500m depth. A narrow and inclined terminal conduit system connects the shallow reservoir to the lava lake. Bomb-ejecting Strombolian eruptions from the lava lake and sporadically active adjacent vents are common. Larger eruptive activity with locally substantially elevated hazard has also been observed, including exceptionally energetic Strombolian activity in 1984 and an Inner Crater phreatic explosion in 1993. Despite sustained degassing and frequent eruptions, geochemical data show the composition of the lava has remained stable for the last 17ka, consistent with the long-lived transcrustal magmatic system underlying the lake.Global Navigation Satellite System (GNSS) data collection on Ross Island began in the late 1990s with campaign observations. In the early 2000s, additional benchmarks were added closer to the summit of Erebus and continuous GNSS (cGNSS) sites were co-located with a seismic network. Today, seven cGNSS sites operate on the summit plateau and flanks of the volcano, and a network of eight campaign benchmarks has been surveyed episodically.We present the first comprehensive geodetic data analysis and modeling results integrating these more than two decades of data collected at Erebus. We resolve long-term subsidence of Ross Island, which a simple viscoelastic model links to the long-term growth of Erebus over the last 20ka. The data also show multi-year cycles of inflation and deflation in the summit area, consistent with activity in the shallow summit magmatic system. These small amplitude (several mm/yr) transient dynamics suggest multi-year pulses of pressurization and depressurization, or geometric changes within the shallow magmatic system that we can reproduce with analytical source models. The most recent inflation pulse lasted from November 2020 until March 2022 when several stations moved radially away from the shallow magmatic system and upwards at 10–15mm/yr. Based on prior cycles, this deformation may result in increased eruptive activity suggesting that continued and enhanced surveillance of Erebus is warranted. These observations contrast with the long-term general stability of the lava lake, but reflect Inner Crater dynamics, which can include changes in lava lake elevation and associated topographic changes of over 20m on multi-year time scales. Our results emphasize the value of long-term and campaign-aided high-accuracy GNSS observations at open-vent volcanoes. This is especially true for volcanoes like Erebus which are remote and may only be accessible for a few months a year, and that deform at amplitudes and periods that may be difficult to resolve with satellite-based radar.