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.

Anna Seybold couldn’t believe it ‘We saw the dust cloud first, then it appeared to liquefy’. Photo: Mark Watson

Earlier in the day, mountaineer Anna Seybold had left Plateau Hut with two others for a ‘warm-up’ climb of the East Ridge of Mt Dixon, with plans to climb Aoraki the following day. They were down climbing a couloir on their return when they heard what Anna described as ‘deep thunder’. Unable to see anything, it took the trio a moment to confirm they weren’t at risk.

A moment later Anna’s climbing partner Jono called: “Anna, watch this!”

“I really couldn’t believe it,” says Anna. “It took us a while to fully realise the scale of the avalanche. We saw the dust cloud first, then it appeared to liquefy. For a moment we wondered if it would hit the hut.”

They continued their descent and returned to Plateau Hut, not deterred from their plans to climb Aoraki the following day.

“We knew it would be at least an extra hour to cross the debris, but we never really thought about cancelling the climb.”

It wasn’t a particularly restful evening though, she says. “[The] event made us very sensitive to every further noise, resulting in a poor sleep, before we got up to climb at 12am.”

Many readers will recall the colossal 1991 rock avalanche that lowered Aoraki/Mt Cook’s summit by 20m. By comparison that event deposited more than 50 million cubic metres of rock, snow and ice (the initial rock collapse was estimated to be about 12 (+/-2.5) million cubic metres of rock) onto the Tasman Glacier 2700m below, and seven kilometres distant. This debris flow missed Plateau Hut by 300m, but was about 100m below it.

While events such as the Aoraki avalanche may happen once in about 100 years, similar smaller events can occur as frequently as every 20-30 years – or less. Geologists have identified more than 100 previous (historic and prehistoric) rock avalanches in the Southern Alps. The very process that builds the mountains, coupled with erosion, causes their collapse and many an alpine valley is littered with the debris of ancient events.

Including the Aoraki collapse and the 2013 event on the south-east ridge of Mt Haast, there have been at least 14 significant rock avalanches from mountains on or near the Main Divide in the past 32 years, but it’s during the past decade that there has been a greater trend for these spontaneous bedrock failures. These typically occur on slopes steeper than 45-degrees and the common factors contributing to these gravitational failures are low overall strength of the rock mass (the ridges and buttresses of massifs themselves) due to glacial erosion, valley incision (glacial and riverine valley erosion), and freeze-thaw processes which shatter the rock. One of the largest of these avalanches took place on Mt Adams in 1999 when 10 million cubic metres fell and there was an alarming flurry in 2008 when Vampire Peak, Douglas Peak, Mt Spenser and Mt Halcombe all suffered similar, smaller, collapses.

In the case of the Mt Haast ridge collapse, GNS senior geologist Graham Hancox describes the main reason for the failure as stemming from the very weak, shattered nature of the rock mass in that section of the ridge, mainly due to the presence of a wide fault zone. “Over time, the dilated and crumbly rock mass (mixed argillite and jointed sandstone) has lost strength, and finally reached the point where it could no longer support itself,” he says.

Hancox attributes some of the strength deterioration and possibly the timing of the rock avalanche from Mt Haast to heavy rain that occurred during early January, but says the reasons for rock avalanches depend mainly on local conditions: slopes oversteepened or undercut by erosion; weak, shattered rock and unfavorably-oriented defects within the bedrock (the rock layering itself and fault zones) dipping down-slope. “These are generally the common factors at most recent and prehistoric rock avalanche sites,” he says.

A mountaineer himself, Hancox notes that when he was climbing in the Mt Cook area in the 1960s and 70s as a recently graduated geologist, he was unaware of large spontaneous rock falls or rock avalanches occurring in the Southern Alps without an obvious triggering event such as an earthquake. “Now such events seem to be quite common,” he says “and my feeling is that this can possibly be attributed to glacial and snow level retreat, combined with continued warming following the Little Ice Age (~18th and 19th century in NZ).

Descending the summit icecap of Aoraki/Mt Cook with the Haast avalanche scar far below. Photo: Mark Watson

“The frequency of recent events is rather typical of what we have seen since the early-1980s. But their frequency seems to be much higher than it was in the 1950s to 70s.”

Regarding the spate of avalanches in 2008, Simon Cox, also a GNS geologist, wrote in the 2008 NZ Alpine Journal, “Summer and autumn 2008 were notably warm, so there has been much speculation as to whether or not ice retreat and warm weather and a lack of freezing (or freeze-thaw) may have been a causal factor. There are many difficulties in such an assessment … not the least that the record of historic events in the area is too small.”

Hancox believes glacial recession and global warming is probably the main cause of these ridge collapses, although says “based on photos from 1968 and the present day we don’t think that glacial or snow level retreat contributed to any great extent to the recent failure on Mt Haast”.

The sites of possible future landslides aren’t unknown to geologists and to some degree they can predict where future bedrock failures might occur.

“Generally we look for sites where there have been previous collapses and the oversteepened areas adjacent to failure scarps,” says Hancox. “Areas of weak, crumbly and ‘rotten’ rock, such as on the Haast-Dixon ridge north-west of the saddle, and the east face of the summit ridge of Aoraki/Mt Cook are good examples.”

Climbers too can pick out potential slope failure sites. Alex Palman’s climbing guide for Mt Cook refers to a ‘rotten rock ridge’ when describing the Dixon-Haast ridge route on Mt Haast. Such areas are most vulnerable to significant collapse during very strong earthquakes. Even small earthquakes can trigger minor rock falls, but large earthquakes can trigger hundreds and thousands of them. Indeed, GNS have noted that shaking caused by the rock avalanche on Mt Haast appears to have triggered two moderate rock falls on the north side of Mt Haast and another failure was started on the end of the south-west ridge of Mt Dixon (during the initial failure). “This developed into a larger rock fall five days later,” Hancox says.

“One large event in the area was enough to change the equilibrium of slopes that are only just able to stand up, and are close to failure.”

Park users might wonder whether DOC facilities are at any greater risk due to the apparent increase of rock avalanche events. Hancox doesn’t think so: “Generally, in assessing the risk of such slope failures to DOC facilities, [exposure] has been taken into account by considering the secondary effects, such as rock falls and avalanches, of large earthquakes such as an Alpine Fault rupture and severe rain storms.”

And the avalanches themselves? Given current climate warming trends it seems we are likely to see an increase in these events. In the meantime, tread lightly on the earth and hope that the next collapse doesn’t take place on the mountain you’re climbing.

Spontaneous rock avalanches in the Southern Alps since 1981

1981 Franz Josef Glacier (Cape Defiance): 1,000,000m3

1991 Aoraki/Mt Cook: 10-15,000,000m3

1992 Mt Fletcher: 1,000,000m3

1995 Murchison Glacier: 100,000-1,000,000m3

1996 Mt Thomson: 100,000-1,000,000m3

1999 Mt Adams: 10-15,000,000m3

2004 Mt Beatrice: 10,000-100,000m3

2006 John Inglis (Joe River): 1-5,000,000m3

2007 Young River: 11,000,000m3

2008 Vampire Peak: 100,000-1,000,000m3

2008 Douglas Peak: 10,000-100,000m3

2008 Mt Spenser: 10,000-100,000m3

2008 Mt Halcombe: 10,000-100,000m3

2011 Franz Josef Glacier: 100,000-1,000,000m3

2013 Ball Ridge: 100,000m3

2013 Mt Haast/Mt Dixon: 2,000,000m3

– Source: Graham Hancox, GNS