Are the Southern Alps coming down around our ears? Climber and photographer Mark Watson, one of three climbers on the slopes of Aoraki/Mt Cook during the massive rock avalanche off Mt Haast, looks at the causes of alpine landslides
Despite the brutal afternoon heat, Matt, Jamie and I made fast progress as we threaded our way through the crevasses of the Linda Glacier. Sitting in a trench between the country’s two highest mountains, Aoraki/Mt Cook and Mt Tasman, the glacier is the arterial route most climbers take to the summit of Aoraki. It’s the most common descent, too. We were on our way to a bivouac site at the base of the upper Bowie Ridge – one of the few spots on the Linda that’s completely safe from avalanches – with the intention of climbing the North Ridge the next day. From time to time I glanced upwards, trying to judge if the heavy ice-riming the route had received in the previous storm would prevent us even setting foot on the climb. Each in our own world and under a lacquer of sweat we barely flinched when we heard the sound of a large avalanche towards the northern end of the Grand Plateau, out of sight around the corner. Ice avalanches are common in our mountains and park visitors soon become accustomed to their distant rumbles. This one sounded like a biggie, but nothing out of the ordinary. Fifteen minutes later I turned to look down the Linda Glacier towards the Grand Plateau and noticed large dust clouds being whisked upwards by the afternoon wind. “Must have had a bit of rock in it,” I commented to Jamie. “Yeah, maybe bigger than we thought, eh?” he replied. It’s not uncommon for ice avalanches to pick up rock debris, or be caused by rock fall in the first place. We attributed the dust to this, and accepted it as a reminder that our alpine zones are unstable places – especially during the heat of a summer afternoon. We toiled higher and after another hour finally reached the site where we’d bivvy for the night. First on the rope, Matt plodded up the soft snow to the ridge crest. There, he stopped dead in his tracks; staring down towards the Grand Plateau and issued a string of expletives. Jamie was next: “Holy s***!” Anxious to see what the fuss was about, I ran up the last few steps and then stopped dead in disbelief when I saw the carnage that had unfolded beneath us. A gigantic landslide had peeled off the precipitous ice-covered ridge connecting Mts Dixon and Haast and cascaded 900 vertical metres and almost 3km horizontally down the mountainside and onto the central névé of the Grand Plateau where it had spewed into a roughly 500m-wide chaos of snow, ice and rock debris. From its western side emerged our tracks. Had we left Plateau Hut a few hours later we would have been pulverised beneath the debris, which lay in tottering piles up to five metres high. Thanking our lucky stars, we settled into our pre-climb routine of melting snow, hydrating and resting for the following day while the sky began to buzz with helicopters. The avalanche occurred as a spontaneous collapse from the south-east ridge of Mt Haast (3114m), releasing an estimated one million cubic metres of rock. When the rock debris hit the névé and began to slide towards the centre of the Plateau it entrained a large volume of snow and ice. This, combined with ‘bulking’, which takes place when a rock mass disintegrates into angular gravel, resulted in the slide’s volume almost doubling to about two million cubic metres. While we were making our way up the Linda, there were more than half a dozen climbers at Plateau Hut and three on the slopes of Mt Dixon watching in amazement as the river of rock and snow flowed to within 200m of the hut. After surging over the foot of Syme Ridge, the average speed of the avalanche was estimated to have been about 130-150km/h. The climbers on Mt Dixon were reported to have described the landslide as sounding like ‘a 747’ flying past nearby. [caption id="attachment_6986" align="aligncenter" width="1280"]
