Over The Horizon

Ancient Antarctic Environment Rocked by Volcanic Eruptions

A scientific study of rock cores drilled from the bottom of the western Ross Sea, in Antarctica, has unexpectedly recovered the first evidence of large volcanic eruptions that occurred around 25 million years ago. The evidence is contained in layers of volcanic debris that were erupted explosively into the atmosphere and then settled through the air and the ocean onto the seafloor.

The thickness and coarseness of the main debris layer indicates a large volume eruption that generated an ash cloud reaching 50-70km into the stratosphere. The discovery of these volcanic layers demonstrates a far more spectacular history of volcanic activity than was previously suspected for the region, but it is also useful in providing material for accurate dating.

For the past two Antarctic field seasons, an international team of scientists has been coring the sea floor off the Victoria Land coast near Cape Roberts. The Cape Roberts Project is designed to study the climatic and geologic history of Antarctica during the last 100 million years.

Drilling this year had reached a depth of approximately 110 metres below the seafloor when this unexpected evidence of volcanic activity was encountered. The layers of volcanic debris are encased within muddy sands, indicating a relatively quiet seafloor with occasional weak currents before and after the eruptions. The sands also contain scattered stones dropped from floating icebergs calved from glaciers in the nearby Transantarctic Mountains at this time.

This relatively quiet seafloor environment, however, was disrupted at least twice and possibly as many as four times, by large and rapid inputs of volcanic debris, mostly pumice. The debris was supplied by voluminous eruptions from a nearby source, but the exact location and characteristics of that source are still unknown. The thickest distinct layer of volcanic debris is 1.2 metres, which suggests an eruption as dramatic as that of Krakatau in 1883.

These layers contain volcanic pumice up to 1cm in size, which suggests that the volcano was located within 50-100 kilometers of the drilling site and erupted in a style reminiscent of Vesuvius.

The eruptions probably had a significant impact, not only on the Antarctic environment, but also globally. Modern examples, such as Mount Pinatubo (a much smaller event), cooled world climate by 0.5^oC for a year after its 1991 eruption.

The volcanic layers will be used to determine the age of their strata more accurately, because volcanic debris can be dated precisely using isotopic techniques. In addition, future coring in the area may encounter these volcanic layers in other drillholes, and they are distinctive levels that can be used to link together strata of the same age.