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Erebus Field Campaign

Erebus Geology

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Photo courtesy of Clive Oppenheimer

Mount Erebus is the southernmost active volcano in the world (at 77 degrees South latitude) and rises to 3,794m above sea level with a crater diameter of 500 - 600m and a relatively flat floor 120m below the crater rim. A smaller inner crater of ~250m diameter also occurs (Sweeney et al, 2008).  A persistent lava lake is present within this inner crater, which is constantly convecting and passively degassing and appears to have been doing so since observations began in early 1970s. The lava at Erebus is phonolitic in composition with large phenocrysts of anorthoclase hosted in a glassy groundmass of olivine, augite, opaques and occasional nepheline (Giggenbach et al, 1973).  The volcano is located within the Western Antarctic Rift System over relatively thin crust thought to be the result of upwelling asthenospheric mantle caused by the Erebus plume. Like Etna, Mount Erebus is a polygenetic stratovolcano composed of both effusive lava products and pyroclastics (tephriphonolite) from explosive events. An earlier volcanic edifice (Proto Erebus) composed of basanite is thought to have occured. Age dating using K-Ar from the anorthoclase indicate the lavas from Erebus are between 0.15 - 1.3 million years old (Esser et al, 2004).

The volcanic activity at Erebus is dominated by Strombolian and minor ash eruptions. Generally since observations in 1973, Erebus experienced up to 6 small Strombolian events per day with a brief 4 month period in 1984 when larger more frequent events generated bombs up to 10m in diameter and ejected them to over 2km from the vent. Some of these Strombolian type events observed in 1973 ejected volcanic lava bombs up to 50m in height above the vent. The lava lake present in the inner crater often experiences "quiet eruptions" (Giggenbach et al, 1973), which involves the growth of a large lava bubble on the lake surface which bursts causing splattering of the surrounding area. Few large explosive eruptions are recorded at Erebus with description of a larger event on 4th February, 1973 which lasted up to 45 seconds and generated a small ash cloud reaching heights of up to 150 - 200m above the vent. Giggenbach et al (1973) attribute this event to a vent clearing process.

Degassing at Erebus

Photo courtesy of Clive Oppenheimer

The lava lake at Mount Erebus is seen to be an 'open window' to the magma reservoir near the summit of the volcano and is the source of a persistent gas plume. The alkalic nature of the volcano means that the gas plume is CO₂ - rich and melt inclusions from minerals within the basanite contain between 4000 - 7300ppm CO₂ (Wardell et al, 2004). Assessment of plume CO₂  and CO concentration is achieved using portable infrared LI-COR gas analyzer attached to an aircraft or using spectrometers placed at the crater rim (to determine direct degassing from the lava lake) or at around 2100m elevation to take a transect of the plume after it has cleared the vent. The results of a 3 year study (1997, 1999 and 2001) of the gas plume from Erebus indicate that the CO₂ emission rates are relatively constant with an average of 1930 tonnes per day (Wardell et al, 2004). Suprisingly the last measurement taken in 2001 was during a time of increased activity but appears to show little variation in the amount of CO₂ released. The same field campaign yielded a CO : CO₂ ratio of 0.12.

The gas plume at Erebus is also rich in other volatile species measured using open-path Fourier transform infrared spectroscopy (FTIR) aimed directly at the lava lakes within the crater (Oppenheimer and Kyle, 2008). The composition of the plume is as follows:

Fluxes of gas species from FTIR, December 2004