In southern Argentina, the Perito Moreno glacier fluctuates so freely that it indirectly submerges land areas. When it will strike next, nobody knows.
Spread out across a plain in Patagonia, as if with a butter knife, the Perito Moreno glacier is an intriguing subject for tourists as well as glacial geologists. For tourists because of its undeniable beauty, characteristic shape and accessibility; for scientists because of its free-ranging movement and its effect on the surrounding geology.
The glacier, covering 259 square kilometres (100 square miles), lies in the Los Glaciares National Park; a UNESCO-listed protection area located in south-western Argentina, near the border of Chile. Here is also the Patagonian ice field; the largest glacial area outside of Antarctica, with a size of 14,000 square kilometres (5,400 square miles).
There are several spectacular glaciers here, but the Perito Moreno is different. Out of the park’s 250 glaciers, it is among three that are not shrinking. It also moves forward, with interesting consequences. Incidentally, a stream of the nearby Lake Argentino runs straight in front of it, with a peninsula lying on the other side. The stream feeds a southern arm of the lake, called Brazo Rico. When the glacier edges a few hundred metres forward, it blocks the river stream, leaving Brazo Rico closed off.
When such a dam is formed, the water level starts to rise. On occasions it has built up to 10 metres higher than the rest of Lake Argentino – some have even reported 30 metres. This, inevitably, floods nearby land areas. However, it does not last forever: the water pressure eventually becomes too great for the glacier to sustain. At some point, the dam breaks the ice that blocks the stream, and a flood of water gushes into the lake. It is a spectacular sight. With its front broken off, the glacier slowly starts rebuilding and the process is eventually repeated.
This phenomenon means that while the glacier is not shrinking, it cannot really grow either. Once it has moved forward, it is always cut back. “It would like to be in that sort of position, but it can’t quite achieve that because the ice keeps breaking,” says Andrew Hein, a glacial geologist at the University of Edinburgh. “Some glaciers fluctuate for a variety of reasons, but this one is quite stationary at the moment.”
Geologists have found it difficult to say when the next break-off will take place. In the past, it has occurred with irregular frequency. It first happened in 1917. Halfway through the century it occurred quite often, but after 1988 there was a 15-year pause until the next rupture in 2003. Since then it has happened in 2004, 2006, 2008 and in March 2012.
Why is the glacier not retreating like all the others? According to Helmut Rott, professor at the University of Innsbruck and co-author of the study Mass fluxes and dynamics of Moreno Glacier, the movement is due to the glacier’s unusual location. Each glacier has a part where the snow builds, and another where it melts. In the centre, says Rott, is the so-called ‘equilibrium line’, where the level of snow and ice is reasonably stable. “On the Moreno glacier, this is in a very steep part,” says Rott, referring to altitude. “And so there is little difference whether it gets a bit warmer, because the accumulation area will change very little.”
This makes sense. The Perito Moreno comes out of the Andes Southern Patagonian ice field, at a height of 2,100 metres, and goes all the way down to Lake Argentino, at 180 metres. (It has a length of 30 kilometres (18 miles).) In the steep mountainside, the temperatures vary little.
But that does not mean the glacier is immune to global warming. At the moment, the height of the glacier’s front is roughly 30 metres. This means that the ‘calving’ – the term for when ice breaks off its front – has little effect on its overall size. However, the glacier gets deeper upstream. If the glacier retreats further, the front will become taller, and the calving will cause more ice to depart. “If it retreats by 300- or 400 metres, it would get into deep water,” says Rott. “I think significant calving would start, and it would retreat significantly, not only on the front, but also towards the back.”
And so with good reason, the Perito Moreno is being watched carefully by glacial geologists. “Further upstream the bed of the glacier is below sea level,” says Rott. “It decreases from the front, and the glacier gets much thicker. Eight kilometres (four miles) upwards, we measured it to be 700 metres deep.” Let us hope it never gets to that level.
Photo: Galina Barskaya, Joshua Raif [both via Shutterstock.com].