Go With the Flow

In modern 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 thirty of 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”). 

 Source: Image created by Kristiana Lapo

Source: Image created by Kristiana Lapo

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 (https://opentextbc.ca/geology/chapter/16-1-glacial-periods-in-earths-history/)

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:

 Source: Washington State University ( http://rocky.ess.washington.edu/areas/Puget_Lobe/ )

Source: Washington State University (http://rocky.ess.washington.edu/areas/Puget_Lobe/)

The big hills that run north-south through Seattle are glacial features called drumlins:

 Source: Figure 8 from        ADDIN ZOTERO_ITEM CSL_CITATION
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of Seattle and the Seattle area,
Washington","container-title":"Reviews in Engineering
Geology","page":"1-36","volume":"20","source":"reg.gsapubs.org","abstract":"The
city of Seattle, Washington State, lies within the Puget Sound Lowland, an
elongate structural and topographic basin between the Cascade Range and Olympic
Mountains. The area has been impacted by repeated glaciation in the past 2.4
m.y. and crustal deformation related to the Cascadia subduction zone. The
present landscape largely results from those repeated cycles of glacial
scouring and deposition and tectonic activity, subsequently modified by
landsliding, stream erosion and deposition, and human activity. The last
glacier to override the area, the Vashon-age glacier of the Fraser glaciation,
reached the Seattle area ca. 14,500 14C yr B.P. (17,400 cal yr B.P.) and had
retreated from the area by ca. 13,650 14C yr B.P. (16,400 cal yr B.P.).\nThe
Seattle area sits atop a complex and incomplete succession of glacial and
nonglacial deposits that extends below sea level and overlies an irregular
bedrock surface. These subsurface materials show spatial lithologic
variability, are truncated by many unconformities, and are deformed by gentle
folds and faults. Sediments that predate the last glacial–interglacial cycle are
exposed where erosion has sliced into the upland, notably along the shorelines
of Puget Sound and Lake Washington, along the Duwamish River valley, and along
Holocene streams.\nThe city of Seattle straddles the Seattle uplift, the
Seattle fault zone, and the Seattle basin, three major bedrock structures that
reflect north-south crustal shortening in the Puget Lowland. Tertiary bedrock
is exposed in isolated locations in south Seattle on the Seattle uplift, and
then it drops to 550 m below ground under the north half of the city in the
Seattle basin. The 6-km-wide Seattle fault zone runs west to east across the
south part of the city. A young strand of the Seattle fault last moved
∼1100 yr ago. Seattle has also been shaken by subduction-zone earthquakes
on the Cascadia subduction zone and deep earthquakes within the subducting
plate. Certain postglacial deposits in Seattle are prone to liquefaction from
earthquakes of sufficient size and duration.\nThe landforms and near-surface
deposits that cover much of the Seattle area record a brief period in the
geologic history of the region. Upland till plains in many areas are cut by
recessional meltwater channels and modern river channels. Till plains display
north-south drumlins with long axes oriented in the ice-flow direction.
Glacially overridden deposits underlie the drumlins and most of the uplands,
whereas loosely consolidated postglacial deposits fill deep valleys and
recessional meltwater channels. Ice-contact deposits are found in isolated
locations across the uplands and along the margins of the uplands, and outwash
deposits line upland recessional channels. Soft organic-rich deposits fill
former lakes and bogs.\nA preliminary geologic map of Seattle was published in
1962 that is only now being replaced by a detailed geologic map. The new map
utilizes a data set of 35,000 geotechnical boreholes, geomorphic analyses of
light detection and ranging (LIDAR), new field mapping, excavation
observations, geochronology, and integration with other geologic and geophysical
information. Findings of the new mapping and recent research include
recognition of Possession- and Whidbey-age deposits in Seattle, recognition
that ∼50% of the large drumlins are cored with pre-Vashon deposits and
50% with Vashon deposits, and that numerous unconformities are present in the
subsurface. Paleotopographic surfaces display 500 m (1600 feet) of relief. The
surficial deposits of Seattle can be grouped into the following categories to
exemplify the distribution of geologic materials across the city: postglacial
deposits 16%, late glacial deposits 12%, Vashon glacial deposits 60%,
pre-Vashon deposits 9%, and bedrock 3%. Of these, 49% are considered
fine-grained deposits, 19% are considered intermediate or interbedded deposits,
and 32% are considered coarse-grained deposits. These percentages include only
the primary geologic units and not the overlying fill and colluvial
deposits.","DOI":"10.1130/2008.4020(01)","ISSN":"0080-2018,","journalAbbreviation":"Reviews
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    [ Troost and Booth , 2008]

Source: Figure 8 from [Troost and Booth, 2008]

On a smaller scale, glaciers can carve out rocks, leaving cool features like these, called striations:

Glaciers can also carry huge chucks of rock which are left behind when the glacier melts, called erratics:

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 (http://www.wta.org/go-hiking/hikes/hoh-river-blue-glacier) will afford you a more up-close-and-personal view of Blue Glacier.

 Blue Glacier, Mount Olympus  Source: Aaron Linville (https://en.wikipedia.org/wiki/Blue_Glacier#/media/File:Mount_Olympus_Blue_Glacier_from_Lateral_Moraine_Panorama.jpg)

Blue Glacier, Mount Olympus

Source: Aaron Linville (https://en.wikipedia.org/wiki/Blue_Glacier#/media/File:Mount_Olympus_Blue_Glacier_from_Lateral_Moraine_Panorama.jpg)

On Mount Rainier, glacier views are more accessible. The Nisqually Vista Loop (http://www.wta.org/go-hiking/hikes/nisqually-vista-loop) is 1.2 miles total and provides a spectacular view of Mount Rainier and Nisqually Glacier. The Emmons Moraine Trial (http://www.wta.org/go-hiking/hikes/emmons-glacier-view) off of the Glacier Basin Trail (http://www.wta.org/go-hiking/hikes/glacier-basin) 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 Creek Suspension Bridge – Emerald Ridge Loop (http://www.wta.org/go-hiking/hikes/emerald-ridge) 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

Emmons Glacier from the Emmons Moraine Trail, Mount Rainier. Photo Credit: Kristiana Lapo

Want to know more about glaciers? Check out my blog feministglaciology.com, where I talk about glacier and climate change, and the intersection of science and social issues.