Matanuska Glacier Over Time
Continuous Movement and Changes of a Glacier
Possibly, about 22,000 years ago, the Matanuska Glacier reached its peak in size. The terminus once reached the north end of Anchorage. The Knik Glacier met the Matanuska and formed a lobe-like shape during this period. Geological evidence indicates that these two glaciers retreated in two separate phases.
Glacial ablation may have occurred through stagnation of the Matanuska. In other words, the glacier was melting faster than it could form. After the second, faster melt phase, about 13,700 years ago, the glacier retreated. Down wasting is the thinning of glacier ice due to melt, similar to the size it is today.
Once these two segments began, the world’s glacial systems went into dramatic retreat. Since that last period, the Matanuska Glacier has not changed as dramatically. When you drive down the Glenn Highway into the Mat-Su Valley, there are many indicators of past glaciation.
As you wind down the highway, you can see etched shelving in the sedimentary rock. And, spot glacial striations across nearby igneous rock. The Matanuska River sits at the bottom of the valley, braiding through highly erosive sediment. The riverbanks illustrate how glacial melt continues to shape the valley.
If you were to take a raft ride down the river, you would be able to look up to the giant sheer cliffs. These cliffs are evidence of ice that was previously taller by hundreds of feet. Ice once so dense and strong, pushing grooves into the valley, and taking off whole walls of mountainsides by force.
As the river winds from Caribou Creek, raging rapids sit at the base of a sharp facet of rock. This rock is a remnant of cooled volcanic rock. Lion’s Head is a volcanic plug that, at one point, was in the glacier’s moving pathway. The aftermath of the glaciers’ great force is the blunt side wall of Lion’s Head.
The Matanuska Now
The Matanuska Glacier, as we know it today, is approximately 27 miles long and four miles wide. This slow-moving river of ice spills between the peaks of Mt. Marcus Baker and Mt. Wickersham, carving away a section of the Chugach Mountains. Snow accumulates at the base of these peaks, turning into a granular intermediate substance (Névé) between snow and ice.
During the freezing and melting stages of the glacier, the basal layer of ice (the bottommost layer) slowly grinds into the valley floor and walls. With the help of growing ice weight and gravity, the glacier slowly fills the land. The terrain changes as the ice carves through its weight and force.
The glacier is down wasting at about one foot per year. During the melting seasons, the glacier can increase in speed by hydroplaning on a layer of water that forms under the ground ice. As ice melts, water follows the most available and least resistant pathway. Water tunnels into weaker ice layers, creating moulins.
Water moves through these weak layers, pouring from glacial openings called vents or from the end (toe) of the glacier into the Matanuska River. These melting changes also influence the opening of crevasses. Crevasse is the French term for the large cracks in the ice that often appear blue.
As the glacier moves and the ice continues to break down the rocky material, it eventually forms moraine. As the glacier moves down the landscape, the ice encounters obstructions or cliff-like drops along the valley floor.
These unseen features below the ice are revealed by what is happening in the surface layer we can observe: an icefall. As this river of ice hits the obstruction, it slowly peels over itself, creating the illusion of a waterfall standing still in time. The cross-sections of crevasses then form giant fins called seracs, another French term. The best way to spot these is to take a quick flight and explore with a guide.
During the melt season, you will find a large glacial lake at the bottom of the icefall at the end of the glacier. The glacial melt and rises feed this lake, then drop and stagnate as temperatures change—eventually meltwater and confluences with the Matanuska River. The meltwater is constantly stirred by glacial silt and rock, appearing in different shades of grey. The shade of grey depends on how much water is present, how fast, and how much sediment is currently present.
Over the Years
For decades, people have been exploring this roadside glacier and stacking up their stories. First-hand accounts from glacier guides and locals tell the tale of decades past and how the glacier is changing in our lifetimes. The pictures show the number of feet of surface ice melting per year. This down wasting is reducing the glacier’s height, increasing the size of the glacial lake, and leaving a larger expanse of rocky moraine at the glacier’s toe.
Where beaming white ice once lined the pathway to the fall, it is now rocky moraine and dirt cones. Climbers previously scaled giant walls adjacent to the icefall. These walls are now much smaller mounds of ice. The glacial lake has grown in size as water flows over the thinning ice underneath. Sometimes, water currents cut large chunks of ice free to float on as icebergs.
Glacial Changes
Globally, there is extensive documentation of current glacial recession events. Many times we see videos of giant glacial cavings of tidal ice, and we may ask ourselves, ‘How is the Matanuska only losing a foot of ice per year?’ Seems like a stark contrast to the massive losses we see on the Alaskan coastlines.
Glaciers undergo recession by melting faster than they can form. Tidal glaciers are not only facing melting rates, but also rising sea levels that accelerate and increase the volume of recession. In conclusion, valley and alpine glaciers are receding or down wasting at a much slower rate.
