Friday, December 19, 2008
Tuckerman Ravine one of the better known glacial cirques on Mt. Washington, NH.
Can you imagine that? I have a hard time picturing a vast continental ice sheet more than a mile thick covering the White Mountains, but, of course, all the evidence says at least four continental-sized glaciers covered this region over an enormous span of time. And, to make things even a bit more more complicated, during the thousands of years between these continental glaciers there were smaller, alpine glaciers on the higher peaks of the Presidential Range and possibly on Mt. Moosilauke as well. According to accepted theory small, alpine glaciers that existed more than 100,000 years ago began carving out the cirques. Tuckerman Ravine, in the photos above, is the more famous of the glacial cirques in the White Mountains and the eastern US. Part of it's fame is that it looks like a glacial cirque is suppose to look, the characteristic 'bowl' shape and the enormous volume of snow that fills the bowl every year and lingers until the mid-summer months. It's relatively easy to picture a glacier spilling out of the Ravine and down over the Little Headwall in the foreground of the top picture and 'flowing' down the mountain.
The high snowfall winters of 1968-69 (the total snow accumulation of the 1968-1969 winter is still the record), and 1995-96, and 1999-2000 with near record snow accumulation, each filled Tucks almost to the point where snow lingered around the seasons. Unfortunately (for those of us who would like to have a glacier on Mt. Washington), it would take many record breaking winters back to back to produce glacial ice. Tuckerman is one of four or five of the cirques in the Presidentials that for the past 80 years, or so, are enjoyed each spring by thousands of skiers and snow boarders who hike up and ski every inch of the terrain even the vertical and near vertical chutes and drops. If you click on the bottom picture to magnify it you will see a thin black line of skiers hiking up in the very center of the photo. They're heading up over the 'Lip', a nearly vertical area of the face, and, from there, some will continue to the upper snowfields.
The enormous continental glaciers arching across the top third of the northern hemisphere of Earth for those millions of year were formed by countless billions of cubic feet, or cubic miles, of snow. With their point of origin being in the topmost arctic regions the continental glaciers were energized by the downward pressure of the snow, or by gravity in other words. The snow, as it accumulated, sublimated into ice under the pressure and settled to the lower two thirds of the glacier adding enormous weight but also a plasticity. This ice layer is compared to silly putty in texture and tensile strength/flexibility. With the enormous volume and weight the glacier came to life and began oozing outward and down towards the equator. Glaciers move by continuous pressure at the “top” of the glacier in the snow accumulation zone. The front of the glacier is being pushed by gravity from behind and being “pulled” outwards by gravity to a lesser extent.At the front of the glacier, the terminus, the process of ablation is a limiting factor. This process includes all the factors that decrease the forward movement and volume of the glacier including melting and evaporation, calving (when the glacier ends in a body of water), sublimation, run off and downwasting which is the thinning of the glacier.