Erratics in Antarctica
A team from the University of Leeds and Aberystwyth University has returned from the Antarctica with exciting new information on the behavior of the giant Antarctic Ice Sheet. The Antarctic Ice Sheet is of exceptional interest to geoscientists due to its size and location, which mean that it reacts quickly and dynamically to climate change. The team of four found that the ice sheet had expanded and then retreated across neighboring James Ross Island several times over the last 25,000 years. The findings are crucial for understanding the thickness and extent of the ice sheet through time, and so its past and future contributions to sea level rise.
The group spent seven weeks in a field camp on the Ulu Peninsula - which forms part of James Ross Island - mapping in detail an area of 600 square kilometers. They examined the distribution of so-called 'erratic' rocks on the Peninsula, a term used to refer to stones and boulders that have been moved from their original site by glaciers and ice sheets. The team found hundreds of large granite boulders scattered all across the Peninsula despite the islands' volcanic make-up, indicating that they were ripped up by the Antarctic Ice Sheet and moved to their new location.
A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometers. Erratics can range in size from pebbles to large boulders such as Big Rock (17,000 tons) in Alberta.
Erratics are often formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes: abrasion/scouring, plucking, ice thrusting and glacially-induced spalling. Glaciers crack pieces of bedrock off in the process of plucking, producing the larger erratics. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. Glacially-induced spalling occurs when ice lens formation with the rocks below the glacier spall off layers of rock, providing smaller debris which is ground into the glacial basal material to become till.
"Geologically, the Antarctic Peninsula is completely different to James Ross Island - so we know that wherever we find these erratic rocks we can be fairly sure that an ice sheet from the Antarctic Peninsula brought them onto the island" said Dr Jonathan Carrivick, from the University of Leeds. "It is then a relatively easy task to match the exact composition of the rocks to those on the Antarctic Peninsula. In doing so, we can trace the historic movement of the ice sheet across the Prince Gustav Channel".
The burning question is when was the Antarctic Ice Sheet big enough to have brought all these granite erratic boulders onto James Ross Island? With this answer there can be some attempt at correlation to global geological warming and cooling trends.
To try and answer this, the team collected rock samples from the surface of more than 50 granite boulders. These samples will be used to date precisely the former expansion of the Antarctic Ice Sheet, using a laboratory technique called cosmogenic exposure age dating.
Cosmogenic analysis is a relatively new technique. Because cosmogenic isotopes have long half-lives (anywhere from thousands to millions of years), scientists find them useful for geologic dating. Cosmogenic isotopes are produced at or near the surface of the Earth, and thus are commonly applied to problems of measuring ages and rates of geomorphic and sedimentary events and processes.
Cosmogenic isotopes (or cosmogenic nuclides) are rare isotopes created when a high-energy cosmic ray interacts with the nucleus of an in situ atom. These isotopes are produced within earth materials such as rocks or soil, in Earth's atmosphere, and in extraterrestrial items such as meteorites. By measuring cosmogenic isotopes, scientists are able to gain insight into a range of geological and astronomical processes. There are both radioactive and stable cosmogenic isotopes.
For further information: http://www.leeds.ac.uk/news/article/1622/antarctic_ice_sheets_erratic_behaviour