by Elena Bird
A big part of an Earth scientist’s job is field work. Being able to see and feel the features you are studying gives scientists a clearer idea of the characteristics of the features and their relationships to surrounding earth systems. Right now, I am taking part in a 10-week field work program with Dartmouth’s Earth Sciences department. We call it the Stretch. We began the Stretch in Banff, Canada on September 3rd. Twenty-one Earth Science students met up and piled into three vans, which have essentially become our home for the past five weeks.
We spent our first week studying the Athabasca Glacier in Jasper National Park. Every day we headed out onto the ice to measure flow rates and regression rates of the glacier using a variety of methods. In the afternoon, we returned to our hostel, where we played in the river, worked on assignments, and sang songs around the campfire.
My days have continued on this way for the past seven weeks-- all day in the field then returning to different camps to finish assignments and enjoy all the beautiful places we’ve traveled through. Since leaving Banff we have mapped sedimentary rocks in Bighorn Basin, searched for gold deposits in the Absaroka range, analyzed water samples from Yellowstone hot springs, traced faults in the Grand Tetons, learned the story of the arches in Arches National Park, and calculated erosion rates of Zion Canyon. No amount of power point slides, lectures, and photos in class could make me feel as comfortable as I’m beginning to feel now at identifying rocks, or looking at a landscape and spotting hints of a vast geological story.
Spending time on the Athabasca glacier provided me with a sliver of insight into what working on the ice of Antarctica will be like – though it still seems a bit unimaginable to me! I learned about the ways in which a glacier flows and the different techniques used to measure this flow. While the flow of a glacier and an ice sheet are different, they share some mechanics that can help me better understand the West Antarctic Ice Sheet during our VeLveT Ice field campaign.
Calculating the flow of the Athabasca Glacier required first determining the thickness of the ice. We used radio echo sounding and GPS surveys to measure this. Radio echo sounding is used on glaciers and ice sheets. It involves sending a pulse of electromagnetic energy down through the ice and receiving an echo reflected off the bedrock below. The time it takes for the pulse to be transmitted and then received is used to calculate the distance traveled from the ice surface to the bedrock and back to the surface. We used this thickness and an ice flow equation developed by John Glen in the 1950’s to approximate the velocity of the Athabasca glacier’s ice flow. On our last day on the ice, we mapped moraines (piles of glacially eroded sediments, known as till, that are pushed into mounds at the tip of a glacier) and dated them using vegetation and dating done by scientists before us to approximate regression rates of the glacier.
The study of the ways in which ice responds to stress is called rheology. Ice has an interesting rheology because it has a range of responses to common stresses. Most people have seen a brittle response to high strain rates. Picture ice shattering when you drop a cube on the floor, or the brittle fracture of a frozen pond when thin ice is broken through. Less commonly seen is a viscous response to stress. Low strain on ice can cause ice to flow slowly or behave plastically.
Analyzing ice flow near the WAIS Divide is one of the primary goals of the VeLveT Ice field campaign. Ice sheet flow does have similarities to glacier flow, but while ice sheets can be miles deep, and as extensive as a continent, glaciers are typically no more than hundreds of feet deep and several miles long, causing changes in stress, and in turn, mechanics. I will talk more in depth about how impurities and crystallography of ice layers can affect ice flow in another post later on!