Go with the flow
These days, when we think of glaciers, we think of alpine glaciers. They're relatively small, and they occur only in the mountains. By definition, a glacier must flow, so a stagnant piece of ice is just ice, not a glacier. In alpine areas, glaciers flow for a couple of reasons, and the slope of the mountain is only part of the reason.
Ice is a solid, but under enough stress, it acts "plastic," which means that under enough stress, it can deform, or flow, without fracturing or breaking. If you hit an ice cube with a hammer, it will break into a million tiny pieces, but with the right amount of stress, applied over time rather than suddenly, the ice will deform, instead of shatter. It's similar to silly putty; if you pull silly putty apart quickly, it will snap, but if you pull it slowly, the putty stretches. If you’re unfamiliar with silly putty, check out the first 30 or so seconds of this video to see how it stretches and breaks.
Alpine glaciers flow downhill partially because gravity pulls the material down the slope of the mountain but there is another, more important reason as well. More precipitation falls at the top of the mountain and the top of the mountain is colder. More snow means more ice, and cold temperatures make the ice stick around so the glacier grows (or “accumulates”) high up in the mountains. The lower part of the glacier gets a lot less precipitation and is warmer, so the lower part of the glacier is where the ice melts and the glacier shrinks (or “ablates”).
As the glacier grows high up in the mountains, the top part gets so heavy that it can't hold itself up, and the ice starts to deform and flow under its own weight. The slope of the mountain helps out a bit, but if there is enough ice, the same thing will happen even on flat land, which is exactly what happens to continental ice sheets (also called “continental glaciers”)
The Cordilleran ice sheet is the ice sheet that covered western Canada and parts of northern Washington (including Seattle and the Puget Sound) about 20,000 years ago. During this time, the western Canadian coast was so cold and there was so much precipitation that the ice grew and grew until it couldn't hold up its own weight and began to flow. The ice flows from where it was thick to where it was thin. For the Cordilleran Ice sheet, precipitation was greatest on the coast, so the ice sheet flowed away from the coast further inland, and to the south. The Puget Lobe of the ice sheet reached as far south as Olympia and was about 3500 feet thick (or about the height of five Space Needles).
|Source: Steven Earle|
Even when ice sheets or glaciers melted away long ago, you can still see signs that the ice was once there.
The Puget Sound was carved by the Puget Lobe of the Cordilleran Ice Sheet. The image on the left shows the extent of the Puget Lobe (grey) and the image on the right shows modern elevation:
|Source: Washington State University|
The big hills that run north-south through Seattle are glacial features called drumlins:
|Source: Figure 8 from [Troost and Booth, 2008]|
On a smaller scale, glaciers can carve out rocks, leaving cool features like these, called striations:
|Source: Walter Siegmund|
Glaciers can also carry huge chucks of rock which are left behind when the glacier melts, called erratics:
|Source: Robert H. Goun|
In addition to these awesome glacial features, there are still some glaciers in Washington state that you can actually go see! The majority of glaciers globally and in the Pacific Northwest are shrinking but it’s not too late to go see them!
In the Olympics Mountains, Blue Glacier can be seen from the vantage point at Hurricane Ridge, but it’s far away, so bring the binoculars! If you’re up for a backpacking trip, the Hoh River Trail will afford you a more up-close-and-personal view of Blue Glacier.
|Blue Glacier, Mount Olympus. Source: Aaron Linville )|
On Mount Rainier, glacier views are more accessible. The Nisqually Vista Loop is 1.2 miles total and provides a spectacular view of Mount Rainier and Nisqually Glacier. The Emmons Moraine Trial off of the Glacier Basin Trail is a relatively short trail that will take you to see Emmons Glacier, which is the biggest glacier (by surface area), and my favorite glacier on Rainier. For a slightly longer hike and similarly spectacular views, the Tahoma CreekSuspension Bridge – Emerald Ridge Loop is a tough but rewarding 14-mile round trip hike with views of Tahoma Glacier, but be careful and check trail reports before you go because the trail can be washed out.
|Emmons Glacier from the Emmons Moraine Trail, Mount Rainier. Photo Credit: Kristiana Lapo|
This post was co-posted on the March for Science - Seattle Blog. Check out March for Science for more writing about topics like materials science, healthcare, and cognitive dissonance and climate change.
Troost, K. G., and D. B. Booth (2008), Geology of Seattle and the Seattle area, Washington, Rev. Eng. Geol., 20, 1–36, doi:10.1130/2008.4020(01).