February 5, 2013
Note: Dr. Sergio Rocchi, an associate professor at the University of Pisa in Italy, presented a talk, “Intravolcanic sills, lava flows, and lava-fed deltas (Victoria Land, Antarctica): Paleoenvironmental Significance” at the LASI V workshop in Port Elizabeth, South Africa in October 2012. The article below is based on this talk and also an interview with Dr. Rocchi. Over a few weeks, I am highlighting some of the research that was presented at the LASI V workshop. This is the third post in that series.
When volcanoes erupt underneath and in the vicinity of glaciers and ice sheets, a unique geological record is created that provides information about both the volcanism and the snow and ice which interacted with the lava as it was being erupted. “Glaciovolcanism” is the term used to describe the interaction of lava with ice, snow (in all its forms, such as “firn” or compacted snow), and meltwater.
Glaciovolcanism includes study of modern examples in places such as Iceland and Antarctica as well as study of ancient examples. For the ancient examples, the ice and snow have generally long since melted away as a result of changing climate over the ages. Furthermore, the sediments and sedimentary rocks—tills and moraines and diamicts— associated with the glaciers and ice sheets have also often long since eroded away. However, volcanic rocks which interacted with the ice and snow are harder and slower to erode, and they often remain long after ice and sediment have disappeared.
Glaciovolcanic rocks can provide much valuable information about ancient glaciers and ice sheets. For example, study of glaciovolcanic rocks can help geologists identify if ice was present and, if so, can help geologists learn about the thickness of the ice, the elevation where the ice was present, the temperature conditions at the base of the ice, and the structure of the ice. A limitation is that volcanic eruptions do not occur continuously. Depending on the circumstances, they may occur at intervals of 10s—or even of 100s or 1000s— of years. Also, over time even hard glaciovolcanic rocks can erode away. Nevertheless, study of glaciovolcanic rocks is a powerful tool for reconstructing past ice cover and conditions, which in turn provides much information about past climate that can complement other paleoclimate studies—for example, study of sedimentary and coral records. Volcanic rocks are also fairly easy to date using isotopic techniques, so they can provide clear age constraints to help with paleoclimatic reconstructions.
Dr. Sergio Rocchi is a volcanologist who has studied glaciovolcanic rocks in Antarctica along with his colleague Dr. John Smellie and other co-workers. Dr. Rocchi explains, “Volcanic eruptions in subglacial environments generate some glacial volcanic lithofacies [units of rocks with certain characteristics] which can tell us the thickness and also the type of ice that was present at the time of the eruption. Additionally, the volcanic rocks can be dated by isotopic means, so the combination of the age and thickness of the ice can be a very useful source of paleoenvironmental information.”
As an example, Dr. Rocchi and his co-workers have studied Late Miocene glaciovolcanic rocks of Victoria Land, Antarctica. There, hyaloclastite-rich glaciovolcanic rocks, including some “lava-fed deltas” (features which form when lava enters water either in a marine/lacustrine or a glacial meltwater environment), have enabled reconstruction of Late Miocene glacial cover over Victoria Land. The glaciovolcanic rocks indicate that at this time Victoria Land was covered by a thin (<300 m thick) cover of ice. This ice sheet is much thinner than that predicted by some modeling studies and implies a more complex climatic transition in the Miocene than previously thought.
While most work regarding glaciovolcanism to date has been carried out in Antarctica, study of glaciovolcanism can also be done in many other places. “Similar work can obviously been done wherever there are or were volcanoes and ice,” says Dr. Rocchi. “The main places where volcano-ice interaction can be studied are Antarctica, Iceland, and British Columbia in the northern Cascades. A future project for which we are raising funding is making a comparison of the glaciovolcanic record in Antarctica with that in Iceland.”
In the future, study of glaciovolcanism will no doubt continue to help geologists and climate scientists unravel the history and nature of past glaciers and ice sheets. Combined with other paleoclimate records, study of glaciovolcanism will help scientists to better understand how Earth’s climate used to be and how climate changes over time. This information is invaluable in a time when humans are experiencing the effects of anthropogenic climate change and when scientists need as much information as possible in order to evaluate what may happen to Earth’s climate in the coming years.
Smellie, J., Wilch, T., and Rocchi, S., 2013. ‘A‘ā lava-fed deltas: A new reference tool in paleoenvironmental studies. Geology. (to be published in the April issue).
Smellie, J., Rocchi, S., and Armienti, P. 2011. Late Miocene volcanic sequences in northern Victoria Land, Antarctica: products of glaciovolcanic eruptions under different thermal regimes. Bulletin of Volcanology, Vol. 73: 1-25.
Smellie, J., Rocchi, S., Gemelli, M., Di Vincenzo, G., and Armienti, P. 2011. A thin predominantly cold-based Late Miocene East Antarctic ice sheet inferred from glaciovolcanic sequences in northern Victoria Land, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 307: 129-149.
Smellie, J., Johnson, J., McIntosh, W., Esser, R., Gudmundsson, M., Hambrey, M., van Wyk de Vries, B. 2008. Six million years of glacial history recorded in volcanic lithofacies of the James Ross Island Volcanic Group, Antarctica Peninsula. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 260: 122-148.