9 Oct 2015

Kiwi scientists calculate glacier erosion rate

1:57 pm on 9 October 2015

Scientists from New Zealand have been part of a team to establish a formula for the rate of glacial erosion, which will help indicate future erosion and the potential effects of climate change.

Sunrise from Franz Josef Glacier.

Sunrise from Franz Josef Glacier. Photo: AFP

The work was carried out on Franz Josef Glacier and also involved scientists from Switzerland, France and the United States.

The work is the cover story of this week's issue of the prestigious journal, Science.

The paper provides a mathematical formula to describe erosion by an alpine glacier, which shows the rate of glacial erosion is proportional to the square of the glacier's speed.

Dr Simon Cox

Dr Simon Cox Photo: Supplied

Dunedin-based GNS geologist and principal scientist Simon Cox said this meant fast-moving glaciers erode much more rock than slow moving glaciers.

"As the glaciers warm up, they speed up, or if they get more precipitation on the western side of the mountains, they speed up which means that they're going to be eroding their holes and eroding those valleys out much faster than they were.

"The actual erosion rates will be greater and the amount of sediment coming out in the rivers is greater."

Dr Cox said he wouldn't expect to see that much change during his life time.

He said the work was a way that scientists could look at the history of glaciers, assess erosion rates and look at their change and their contributions to the way the mountains and the landscape has evolved in the last 10,000 to 20,000 years.

It will also help to predict the shapes and the contributions glaciers make to the landscape on long time frames.

Dr Cox said the formula applied for generally thinking about glaciers on a whole mountain range scale.

Local effects in and around the glaciers because of more melting and the shape of the glacier would cause greater change on an individual glacier, he said.

Franz Josef glacier.

Franz Josef glacier. Photo: Supplied

"It's not something that you will be able to pick up and use on a day-to-day basis - it's more for understanding global impacts of glaciation and how it might change long-term."

Dr Cox said New Zealand's glaciers were temperate glaciers which had quite a lot of water under them, and they were steep.

He said the group working on Franz Josef put together two pieces of important work, one of which was to track the velocity of the ice using satellite images.

"Using differences between images, we could get how fast the glacier was moving and it's up to three metres per day in some places."

The second part was to collect regular samples of glacial water, the melt-water coming off the ice, and then track those particles back to the rocks that they originated from.

"We used a method called Raman spectroscopy, so we're using the carbonaceous material, the fossil carbon out of the rock, and we shine a laser on to it and get a pattern which we can use to track back to the original source rocks.

It was a combination of these two methods that then enabled us to build a model of each grain of material coming out of the melt water, where has that come from, and then we can say, 'there's this much eroding in this area of the glacier, and in this other area there's not much eroding'."

He said Franz Josef was a very fast glacier, whereas one like the Tasman Glacier was much slower.

Dr Cox said there are thousands of glaciers in New Zealand and while they all varied in speed, on the whole they were fast-moving.

"We're at the fast end of things and the polar glaciers are at the slow end of things, and that's part of the reason it's always been hard to work out these sort of erosion laws because there's such a variety of different glaciers."

He said how big a finding the formula was was yet to be seen, but there were important parts of global models for trying to understand the impact of climate change.

"We need to understand the erosion - to put into context the way the land will respond as you take the ice off: it's going to respond differently, there's a certain amount of erosion, there's a certain amount of uplift, certain amount of heat.

"So they're very complicated equations and scenarios that people are trying to understand and this is quite a big cog in understanding that."

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