Feature

Monitoring Icy Mountains

Researchers investigating an Antarctic mountain chain had to "look" through a kilometre of water and ice.

Keith Lyons

Even with 24 hours of Antarctic daylight to help them shed some light on the formation of one of the world's longest chain of mountains, Victoria University geophysicist Ron Hackney and colleagues Julie Quinn and Tony Haver could only see their subject with specially made geological equipment.

The Transantarctic Mountains extend 3,000 kilometres from the entrance to the Ross Sea towards the South Pole, but only the peaks of the mountain range, some reaching over 4,000 metres, are visible. The Ross Ice Shelf, which floats on 400 metres of water, hides valuable clues to the origin of the adjacent mountains. For many people chiselling out a forgotten bag of frozen peas at the bottom of the freezer is the closest they get to ice exploration, but for Hackney, looking into the ice sheet half-way between New Zealand's Scott Base and the South Pole is like finding a geological photograph millions of years old.

"The area is great for studying what has happened because there has been very little erosion by water. Rain buggers rocks by promoting chemical reactions which break down the rocks and destroy the structures which can give clues to what's happened. Antarctica is the driest continent on Earth and instead the Transantarctic Mountains have been scoured by glaciers which scrape away the debris in a cleaner action."

There are actually two plates which make up Antarctica. The Transantarctic Mountains mark the boundary where the West Antarctic plate has been pulling away from the East Antarctic plate over the last 60 million years, breaking and stretching the old crust. The mountains are regarded as the best example of what can happen when plates break and pull apart at the edge of continents.

With ice covering everything but the higher peaks of the Transantarctic Mountains, the researchers faced the problem of finding out what rock structures and shapes were underneath. They used ice-penetrating radar as well as measurements of the Earth's gravity and magnetism at points along a 250-kilometer traverse from mountains to the ice shelf.

Hackney says the radar showed the ice thinned further away from the mountains.

"Our gravity meter and magnetometer were able to detect small changes caused by bodies of rock under the ice. We found what we suspect are broken and bent rocks more than 150 kilometres from the mountains, and these deformed rocks indicate there has been some action down there, with bodies of rock being squashed or stretched. Each piece of data provides us with another clue about what has happened to the rocks under the surface."

The work carries on research by Victoria University's Tim Stern, who that found much of the rock under the Ross Ice Shelf was composed of flat-lying sediments. The new data should help improve the model of mountain uplift proposed by Stern and others.

Mountains are usually formed by plates colliding and crumpling together or sliding under each other, like some sort of highway head-on or rear-end smash in slow motion. However, the Transantarctic Mountains represent a third type of mountain formation when two plates pull apart, with the East Antarctic plate springing up as it loses material from on top by erosion and adjusts to a new level, while the West Antarctic plate sinks as part of the rift. The "floating" elevated mountains are similar to what happens when you take a heavy load out of a car boot and the car rises on its springs.

The new data will be combined with information from the west side of the Transantarctic Mountains gained by Hackney and others last summer on the polar plateau to refine the model of mountain uplift, which can be applied to similar mountains in South Africa and the Eastern Australian Highlands.

Geophysics technician and radio engineer Haver was pleased that the scientific equipment stood up to the freezing temperatures and travel over the ice.

"The sensitive measuring devices were kept in insulated and shock-proof boxes, and we used solar panels to recharge batteries as they drained quickly in temperatures averaging -70^oC."

The route taken by the researchers was not only a timewarp into Antarctica's tectonic history, it also crossed the footsteps of early explorer Robert Falcon Scott's ill-fated polar expedition. The last 80 years has seen the development of new technology, such as the satellite navigation Global Positioning Systems (GPS), which meant the party knew their exact location along a straight line, even in white-out conditions.

The GPS helped the three snow mobiles and four sleds travel across the terrain with ease. The deep field party had to be self-sufficient for the three weeks of the expedition, and had to spend an extra 10 days at latitude 82.5^oSouth because of logistical problems.

Honours student Julie Quinn says they were relieved when the Hercules finally landed on the ice to return them to Scott Base, but that the hardships were worth the data they found.

"Not much has been known previously about the area, and we are still not totally sure what is happening down there, but our remote sensing of the geology is providing us with better information to develop a model of the formation of this large mountain chain."

The researchers are further analysing their data, as part of a multi-national effort aimed at understanding the origin of the Transantarctic Mountains, piecing together the Antarctica jigsaw and learning more about the processes of heat and material transfer within the Earth.

Keith Lyons is a journalist from Christchurch