CHARLIE Anderson has been in the crater of Mount St Helens more times than anyone else 鈥� 140 at the last count. And so the 62-year-old from Federal Way, Washington, was one of the few people who weren鈥檛 surprised when geologists announced that in the time since its 1980 eruption, North America鈥檚 most famous volcano has grown a full-fledged glacier in its crater.
Anderson is one of a small cadre of adventurers who call themselves glaciospeleologists (glacier-cave explorers). He made his name by helping to map out the 13 kilometres of passageway that once tunnelled beneath the Paradise Park glaciers on nearby Mount Rainier, the highest peak in the Cascade Range, which runs from California to British Columbia and includes Mount St Helens.
But with glaciers on the retreat virtually everywhere, Anderson was running low on places to explore. The last of the Paradise Park ice caves melted away in 1991. And so Anderson is understandably thrilled that Mount St Helens has given his dangerous sport a new lease of life.
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Anderson is not alone. Professional glaciologists are, if anything, even more pleased. As global warming sets in they鈥檝e been facing up to the fact that their discipline is literally disappearing. The new glacier on Mount St Helens presents them with a unique opportunity to watch a glacier as it forms, says Neil Humphrey, professor of geology and geophysics at the University of Wyoming in Laramie. 鈥淢elting,鈥� laments Humphrey, 鈥渋s something you can see virtually anyplace.鈥�
The only other time modern geologists could have watched a glacier grow from scratch was after the 1912 eruption of Mount Katmai in Alaska, which left a deep, north-facing crater perfect for glacier formation. But Mount Katmai lies in the Aleutian Range, 450 kilometres south-west of Anchorage. In the early 20th century, it was too remote for detailed study. Not much seems to be known except that by the 1950s, a pair of glaciers had formed below the crater, with the larger one extending about a kilometre downslope. The early stages of these glaciers鈥� formation went unobserved by anything but bears and birds.
This time geologists don鈥檛 intend to miss a thing, and by lavishing attention on Mount St Helens they hope to learn much about the ways in which glaciers form, grow, and evolve.
The watchword, though, is 鈥渉ope鈥�. With Mount St Helens rumbling ominously back into life and geologists saying that another major eruption is inevitable, the question is, how long have they got?
When Mount St Helens blew its top in 1980, the brunt of the explosion went sideways, through the northern ramparts of the 2950-metre peak. When the ash settled, 57 people were dead, hundreds of square kilometres of terrain had been flayed into a lifeless moonscape, and the mountain was 400 metres shorter. It also sported a new U-shaped crater, 700 metres deep, beneath a north-facing horseshoe of steep walls that blocked out much of the midday sun (see picture).
As far back as 1982, Anderson predicted that a glacier would grow beneath those shady slopes. But in its early years, the crater was rocked by eruptions that gradually created a 270-metre-high lava dome in its centre. Heat from the eruptions melted each winter鈥檚 snow.
鈥淭he volcano is rumbling ominously. How long have geologists got?鈥�
Then the eruptions stopped. In the first quiet year, 1986, a small patch of ice appeared in the south-west corner of the crater, says Steve Schilling, a geologist at the Cascades Volcano Observatory in Vancouver, Washington, and lead author of an article introducing the glacier to the outside world (Quaternary Review, vol 61, p 325). Soon after that, volcanologists found they had to dig their equipment out of the snow each year as the snow bank expanded.
By 1996, a glacier-like feature, complete with several crevasses, covered 0.1 square kilometres of the crater. There was also a large crack called a bergschrund between its head and the cliffs, at the spot where the snow and ice were beginning to slump downhill.
Crevasses and bergschrunds are classic signs that ice is beginning to move, marking the transition from snowfield to glacier. In a snowfield, the snow and ice simply sit there 鈥� not particularly interesting unless you happen to be a skier. In a glacier, the snow has become so deep that the bottom layers compact into a high-pressure form of ice that flows ever so slowly down the mountain, cracking open to form crevasses and bergschrunds as it moves.
In the eight years since crevasses first appeared, the Mount St Helens glacier has grown spectacularly. In places it is 240 metres deep. Steep crevassed sections called icefalls are forming as it squeezes around the sides of the lava dome, shaping the entire glacier into a crescent. Peter Frenzen, chief scientist at Mount St Helens National Volcanic Monument, compares it to a dress-shirt collar wrapping around the dome. It鈥檚 known colloquially as the Crescent glacier.
By 2001, the glacier had grown to cover a square kilometre, and that may be just the beginning. Most glaciers comprise an upper-altitude 鈥渁ccumulation zone鈥�, where snow piles up each winter, and an 鈥渁blation鈥� or melting zone low enough down the mountain for the summer sun to get to work on the ice. As the glacier flows downhill, ice moves from the accumulation zone to the ablation zone and eventually melts.
Whether the glacier grows or shrinks depends on the balance between the two zones. When a lot of snow is piling up in the accumulation zone, the glacier advances. When the climate warms or winters become drier, the accumulation zone shrinks and the ablation zone enlarges. Ice continues to flow downhill but the glacier appears to retreat as its lower end melts away. Thanks to global warming, most glaciers today are in retreat.
The Mount St Helens glacier, however, is so young that it has yet to flow beyond its accumulation zone. This means that over the whole glacier, snow is accumulating more rapidly than it can melt.
And the accumulation rate is astronomical, says Schilling鈥檚 colleague Joseph Walder. Typically, a glacier is doing well to add a metre of new ice per year to its accumulation zone. At Mount St Helens, the figure is between 10 and 20 times that.
Schilling puts its spectacular growth down to a combination of four factors. The first is simple: the area has a very high snowfall. Nobody knows exactly how much, but a monitoring station at Mount Rainier, 75 kilometres away, records 17 metres of heavy, wet snow each winter. Other factors are the shady north-facing amphitheatre created by the crater walls, and the extra snow avalanching down onto the glacier from those same steep walls.
The fourth factor, Schilling believes, is the rock and dust falling from the crater walls each summer. Mountaineers have described Cascade volcanoes as 鈥渄irty snow cones鈥� whose crumbly rocks are welded together by ice. When the ice melts, the mountains fall apart. From a mountaineering perspective, this means that ascents of major peaks, such as Rainier (at 4350 metres), are generally begun at night, so climbers are on the summit shortly after dawn, before the mountain begins to pelt them with rocks.
鈥淣obody is suicidal enough to climb up the walls of the crater鈥�
When this happens on Mount St Helens, rocks fall into the crater, whose inside walls nobody is suicidal enough to climb. But anyone who has ever entered the crater via the northern entrance or sat on its rim has noticed that the mountain is literally falling in on itself. On a hot summer day, the clatter of falling rocks is continuous. Kathy Cashman, a geologist at the University of Oregon in Eugene, notes that you can tell the time of day from the direction of the falling rocks. In the morning, they鈥檙e falling from the sun-warmed western walls, but as the sun tracks across the sky and shadows move, the rockfalls shift around the crater rim like a sundial. As a result, each summer, a fair amount of the previous winter鈥檚 snow gets buried under rock and dust.
You might assume that rock-covered snow would melt more quickly because the dark rock absorbs heat. But studies on Oregon鈥檚 Mount Hood have found that as little as 2 centimetres of volcanic rock or dust is enough to create an insulating layer that preserves snow rather than melts it. Even solitary rocks have this effect, Schilling notes. By summer鈥檚 end, they finish up sitting on pedestals, elevated above the more rapidly melting snow around them.
The Mount St Helens glacier is unique because it is the only one whose formation has been extensively studied from the beginning, allowing scientists to observe not only the speed at which it is forming, but also to watch how it flows over the underlying terrain and to calculate the year-to-year increase in its volume.
And it has been extensively and repeatedly photographed since the eruption. One set of photos shows the contour of the crater floor before the ice began accumulating. Later ones show the contour of the mountain-plus-glacier as it existed before the recent eruptions. By comparing digital versions of these before-and-after photos, Schilling鈥檚 team has been able to calculate the quantities of both ice and rock. According to this calculation, the glacier is an average of 120 metres thick, with a total volume of 120 million cubic metres, of which 40 million cubic metres is rock.
Even without the benefit of digital photos, it is obvious that a lot of rock has fallen into the crater. Shortly after the 1980 blast, the crater walls were sheer, as if they had been cleaved with a giant hatchet. Today, their base is buried in an enormous skirt of rubble called talus, while the upper walls are creased with deep gullies. Last year, Schilling鈥檚 team had trouble locating an experimental station on the east rim. Eventually, they found pieces of it dangling into the crater. The last time the geologists had visited that station, it had been standing 2 metres back from the rim.
Typical glaciers are comprised of less than 1 per cent rock, says Walder. The Mount St Helens glacier contains 33 per cent. This means that the new glacier straddles the boundary between an ordinary glacier and a little-understood geological feature called a rock glacier. For now, the Mount St Helens glacier contains too much ice to be a true rock glacier, but Humphrey suspects that its ultimate fate 鈥� if it survives 鈥� will be to move towards that status. If it does, scientists will have the opportunity to learn some important things about these enigmatic features.
Rock glaciers are typically half rock and half ice. Common in the Alps, they look a lot like ordinary talus slopes but creep downhill at between 1 and 15 centimetres a year. This is a lot slower than conventional glaciers, which can easily cover 15 centimetres in a day.
Geologists have known about rock glaciers since the late 19th century but it was only six or seven years ago that scientists realised that their super-slow movement means that they must contain some very old ice 鈥� older than any other glacial ice found at temperate latitudes. This ice, Humphrey says, provides a new source of ancient climate records.
Because all that insulating rock allows rock glaciers to exist as far south as New Mexico, the year-by-year climate data stored in their ice layers may provide the longest detailed record of palaeoclimate information for these areas. The more we learn from the Mount St Helens glacier, the better we may be able to understand the climate information in the ancient ice of other glaciers.
For Humphrey, one of the most interesting aspects of watching the glacier advance will be observing the way in which it forms rock piles, or moraines, as it moves forwards.
Classically, glaciologists think of glaciers as 鈥減ushing up鈥� moraines at their snouts, like snow ploughs clearing the road. But that comes from studying moraines near glaciers that have repeatedly advanced and retreated. Humphrey calls that a 鈥渃ontaminated record鈥�, in which successive surges have furrowed the ground into a scrambled confusion of overlapping moraines.
At Mount St Helens, on the other hand, glaciologists have the chance to watch the annual progress of a glacier moving across a virgin bed of easily eroded rock.
Humphrey is excited by what has already been seen, because instead of pushing up moraines, this glacier appears, at least in part, to be overriding the underlying rubble. 鈥淭hat could change everything about our knowledge of moraines,鈥� Humphrey says. 鈥淚 think sometimes glaciers push up moraines and sometimes they don鈥檛.鈥� He believes this is related to the way meltwater pools up beneath the glacier or percolates into underlying rock, and thinks Mount St Helens offers a unique opportunity to examine these interactions.
Because so many factors are involved, nobody wants to hazard much of a guess about the future of the glacier 鈥� assuming, of course, that it is not obliterated by a new eruption. As 快猫短视频 went to press the volcano was relatively quiet and the glacier intact, but the Cascades Volcano Observatory, was warning that activity could escalate suddenly and without warning.
Could the glacier eventually engulf the lava dome? Maybe. And because the insulating layer of rock will continue to move with the ice, protecting it from melting as it reaches warmer and less shaded zones, Walder won鈥檛 be surprised if the glacier ultimately extends a long way out the crater. How far it will go, no one yet knows. The only thing that is certain is that whatever the Mount St Helens glacier does, geologists will be watching from as close a vantage point as they dare.