Glaciovolcanic Studies: a Pivotal Role in Characterising Past ice Sheets
[*John Smellie*] (British Antarctic Survey, High Cross, Madingley Road, Cambridge
CB3 0ET, UK; ph: +44 1223 221418; fax: +44 1223 362616; email: jlsm@bas.ac.uk)
Glaciovolcanic sequences can be used as proxies for a uniquely wide range of palaeo-ice
parameters. They are thus potentially highly useful archives of palaeoenvironmental information,
particularly for pre-Quaternary periods, and the parameters derived can be incorporated by
climate and ice sheet modellers in the same way as other environmental proxies. However,
despite their unique potential, there are still few published environmental studies using
glaciovolcanic methodology. On Earth, at least nine different types of terrestrial subglacial
volcanic successions can be identified using landform characteristics, lithofacies and sequence
architecture. They include mafic tuyas, felsic tuyas (2 types), pillow mounds/ridges, pillow
sheets, tephra mounds/ridges, felsic lobes/domes, and subglacial sheet-like sequences (2 types).
When tectonic influences are removed (e.g. eruptions from fissures), the different landforms are
relatively well distinguished on morphometric diagrams using height versus width (i.e. aspect
ratio). The potential for each landform and/or succession type for yielding useful environmental
information is variable and is poorly known for several. Conversely, because of the abundance of
terrestrial basaltic volcanism, the only mature palaeoenvironmental studies published so far are
those that have focused on products of mafic magmas. This is illustrated here by using the results
of a five-year investigation of the Antarctic Ice Sheet using information from basaltic sequences
in northern Antarctic Peninsula. It is the most detailed palaeoenvironmental glaciovolcanic study
conducted to date.
The James Ross Island Volcanic Group is a large (c. 6000 km2) basaltic volcanic field situated in
northern Antarctic Peninsula and dominated by the long-lived (> 6.25 m.y.) Mount Haddington
stratovolcano. It is in a pivotal position (northerly latitude, low elevation) to record the dynamics
of the adjacent Antarctic Peninsula Ice Sheet. At least 50 eruptive phases have been identified and
Mt Haddington is now the most intensively isotopically dated volcanic centre in Antarctica.
Eruptions were mainly of large-volume (individually tens of km3) lava-fed deltas, which gave the
volcano its low-profile shield-like form. Most of the deltas show well preserved and beautifully
displayed structural features diagnostic of eruption in association with erosive, wet-based ice, but
a few deltas and several tuff cones were also probably constructed in the sea during interglacial
periods, particularly during the early Pliocene. More than 90% of eruptions took place in
association with a glacial cover. However, because of high errors associated with dating young Kpoor basalts, it is currently impossible to determine at what stage(s) in any glacial cycle eruptions
took place. The lack of a precise dating method is now a major factor limiting further
environmental research in these and similar sequences worldwide. However, the overwhelming
evidence for a former ice cover affecting most eruptions suggests that both glacial and interglacial
conditions are being preserved. Thus, the so-called interglacial periods are best regarded as icepoor rather than ice-free, and may have been similar in appearance to today. The thickness of the
glacial cover was typically just 200-300 m, with maximum thicknesses of about 700 m achieved
rarely. The ice sheet would thus have had a low profile, with a large dome centred on Mount
Haddington dominating the local morphodynamics. There is no evidence for a giant ice sheet at
any stage during the eruptive period.
These results are the first evidence for the morphology, thickness and thermal regime of the
Antarctic Peninsula Ice Sheet for pre-Quaternary periods. The study illustrates the utility and
importance of the new glaciovolcanic methodology as an important palaeoenvironmental proxy.
It is uniquely capable of yielding more quantifiable parameters of past ice sheets than any other
methodology currently used.
ORAL
Abstract for VI-2, session on “Using glaciovolcanic rocks to constrain former ice conditions
and palaeoclimate”, Vancouver, 19-22 June 2007