Glacial Exploring at Aoraki Mount Cook National Park
Mt. Cook Village, nestled at the end of the road leading north past Lake Pukaki, is enough out of the way that we were tempted to skip it. Fortunately, we kept this amazing place in our plans. Our roughly 24 hours here offered great views and much learning.
Our good fortune of great weather continued as we headed to The Tasman Lake, at the base of (surprise!) the Tasman Glacier, New Zealand’s largest. The glacier once extended well down this valley. When it retreated, it left a 1,000′ deep moraine deposit that often looks as flat as a lake surface.
The porous collected rock supports a glacial stream that is itself so dense with suspended rock dust that it stays on the surface. But the abundant rainfall that hits this valley goes right through.
A mile-long path leads across the valley floor to Tasman Glacier Terminal Lake.
Careful walkers avoid the matagauri bushes that frequent the area. Maouri once used the long, strong needles to create tattoos.
A glacial advance about 800 years ago pushed up piles of rock and then left more residue when the glacier receded again.
We crested a hill …
and got our first view of Tasman Lake, white because of suspended rock dust. Fortunately the glacier had “calved” about 10 days earlier, so there were numerous icebergs to explore. Several had drifted to the end of the lake furthest from the glacier.
Linus (a Swede who pronounces his name “lee-nus”), directed us to a boat small enough to get us within touching distance of several icebergs.
Icebergs go through a variety of stages.
Ice that has not been exposed to the elements is very dense and looks blue. With exposure, the gases inside the ice escape, separating the ice into individual crystals that appear white.
Melting ice exposes rock debris, giving the iceberg a grey cast.
As the above-water area melts, more dark areas become exposed and absorb more heat. Eventually, however, an insulating layer of trapped debris forms to slow the process. Older icebergs can look very dirty and often carry large, exposed rocks.
At this stage, the berg becomes less stable, because the melting reduces the above-surface mass. Icebergs “rebalance” to the natural ratio of 10% above the surface and 90% below. You don’t want to be too close the a flipping iceberg. This one looks to be in the process of rebalancing.
We were able to touch several icebergs to experience the differences between the various stages, and we broke off a few pieces for a closer look. Clear crystals refracted light like a diamond, but when melted in the mouth still revealed some grit (no doubt rock dust).
Icebergs also melt where the ice contacts water. Because the top few inches is warmer than deeper water, the surface layer lapping at the iceberg melts it faster. The resultant “thermal notches” create overhangs of various shapes.
When an overhang gets too big, it can collapse of its own weight.
Different melting patterns create a wide variety of interesting sculptures.
The Tasman Glacier runs down a valley, terminating at the lake. The blue vertical stripes in this photo show clean areas of the glacier’s end.
A glacier that terminates in a lake melts where exposed to water. “Calving” happens either of two ways. Most common are “block calvings” in which a chunk of the exposed above-water face, often undercut by thermal action from the water, falls off. The loss of above-surface ice creates an ice shelf sticking out from the glacier below the surface.
The presence of the shelf (or, more accurately, the absence of weight above) increases stress on the ice below the shelf that is trying to float. When the strain gets too great, “basil calfing” occurs, and a piece of shelf breaks off and rises to the surface, producing building-size icebergs. The risk of basal calfing surprising people from under the water keeps tour boats away from the front edge of the glacier.
It was a glorious day for enjoying the lake, the icebergs, the glacier and the mountains.