Australia: The Land Where Time Began

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Antarctica – Mobile Magma Beneath the Ice

West Antarctic Ice Sheet (WAIS) has had active volcanoes beneath it for millions of years, now there is evidence of recent activity. In 2010 and 2011 swarms of small earthquakes were detected that suggests there is movement of magma in the crust beneath the ice.

As the climate warms and the floating Ice Shelves that surround Antarctica that  buttress the land-based glaciers are melting at their base by ocean water that is warmer (Cook et al., 2013). Even on land the ice can melt from below, however, where a high heat flow through the crust is caused by subglacial volcanic activity (Behrendt, 2013; Kaus, 2013; Blankenship et al., 1993; Corr & Vaughn, 2008). According to Lough et al. (Lough et al., 2013) observations of hundreds of small seismic events in the crust beneath the WAIS infer there is current movement of magma in a volcanic system beneath the ice, and this may transfer heat to the rock-ice interface and thereby affect the flow of ice.

Volcanic activity in the Late Cainozoic that is associated with the West Antarctic Rift System (WARS) (Behrendt et al., 1991) extended over a wide area of West Antarctica, including beneath the WAIS that flows through it. Generally, the volcanic activity appears to have migrated to the south, along fractures that are oriented north-south, away from the Marie Byrd Dome (Behrendt, Cooper & LeMasurier, 1992). At a few other places in the WARS active volcanism has also been reported (Blankenship et al., 1993; Corr & Vaughn, 2008), and evidence has been provided by aeromagnetic surveys for a number of centres of volcanism beneath the WAIS, though it has not been clear if magmatic activity is continuing at present.

Seismic data that had been recorded by a deployment of 137 seismic stations in Marie Byrd Land, a highland region in West Antarctica, was analysed by Lough et al. (Lough et al., 2013). There were 2 swarms of earthquakes that were about 1 year apart, in 2010 and 2011, that were identified. Hundreds of small seismic events with a magnitude of between 0.8 and 2.1 comprised the swarms. The quakes occurred close to the boundary between the crust and the mantle beneath Marie Byrd Land, at depths of about 25-40 km, which is much deeper than normal crustal earthquakes. Deep earthquakes of long period that have been associated with volcanoes worldwide typically have these characteristics as well as the frequencies that were observed (Nichols et al., 2013; Okubo & Wolfe, 2008; Power et al., 2004; White, 1996). Therefore lough et al. interpreted the observed seismic activity as an indication that magma was moving within an active subglacial magmatic system, though it was not clear whether the swarm activity observed presages an imminent eruption.

The point of origin of the earthquake swarms was beneath a subglacial mountain complex that had an elevation of about 1,000 m above the surrounding low-lying areas. A 400 nT magnetic anomaly was shown by aeromagnetic data at the high point, which suggests that rocks in the region are highly magnetised. Magnetic anomalies from rocks with a shallow source often indicate a volcanic origin, so the subglacial mountain complex was interpreted by Lough et al. as a volcanic edifice. A prominent elliptical ash layer with an area of 20 x 25 km was identified by them in the ice above the subglacial peak, about 400-1,400 m below the surface of the ice. Given modern accumulation rates of ice of about 12.5 cm/year, Lough et al. estimate that the ash layer formed about 8,000 years ago and was probably sourced from Mount Waesche that is nearby.

Measurements of the thickness of the ice in West Antarctica is provided by radar ice-sounding data, though gaps in the data – the aerogeophysical data lines are spaced about 15 km apart - which leads to uncertainties. Deep earthquakes of long periods can occur up to 5 km from active volcanic vents (Nichols et al., 2013; Okubo & Wolfe, 2008; Power et al., 2004; White, 1996) and at this distance from the earthquakes’ source the ice is about 1,100 m thick. It was shown by Lough et al. that it took an exceptionally large eruption to breach the surface in Marie Byrd Land to vent to the surface.

About 55 km south of Mount Sidley in the Executive Committee Range, which is to the south of the area of volcanic activity (LeMasurier et al., 1990) in the Holocene, is the location of the earthquake swarms, magnetic anomaly and ash layer. The observations, therefore, fit with the idea that volcanic activity in this region is migrating southwards, along the Executive Committee Range (LeMasurier & Rex, 1989), which indicates that it is potentially linked to a mantle plume beneath a large area (Behrendt, Cooper & LeMasurier, 1992). It was suggested by Lough et al. that if the volcanic activity continues to migrate southwards, the thicker ice in the deeper interior of the WAIS could begin to be affected, as has been reported from magnetic anomalies (Behrendt, 2013). The bed of the ice sheet could potentially be lubricated, thereby helping the overlying ice to flow, and facilitate the evolution of ice streams. Acceleration of the loss of ice mass in West Antarctica could result from all these processes.

A magmatic system beneath the WAIS was shown by Lough et al. to be currently active. Heat flow from an active magma system could melt the ice sheet base and affect the ice regime in the future, though only an exceptionally large volcanic eruption could penetrate the thick ice.

Sources & Further reading

  1. Behrendt, J. C. (2013). "Volcanology: Mobile magma under Antarctic ice." Nature Geosci 6(12): 990-991.


Author: M. H. Monroe
Last updated 17/06/2018
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