The folded moraine of Tokositna Glacier

Peter Moore
,
Iowa State University
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Summary

Using satellite imagery of the Tokositna Glacier (AK, USA), students digitize a folded medial moraine and make simple computations to infer the progressive development of strain in the glacier.

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Context

Audience

Intended for undergraduate geoscience courses in structural geology, glaciology or glacial geology.

Skills and concepts that students must have mastered

Students should understand the processes of glacier motion and mass balance at a basic level. They should also be able to conceptualize coaxial strain in 2 and 3-dimensions, and should understand how strain is represented in a Flinn diagram.

How the activity is situated in the course

A stand-alone exercise.

Goals

Content/concepts goals for this activity

Develop familiarity with visual interpretation of geological strain in 2 and 3 dimensions; terminology for fold morphology and kinematics; modes and spatial distribution of glacier motion and mass balance.

Higher order thinking skills goals for this activity

Crossing disciplines between glaciology and structural geology; Making sense of real but imperfect data by making simplifying assumptions, then evaluating those assumptions.

Other skills goals for this activity

Use of a simple GIS package for digitization and practical measurement of large-scale geological features.

Description of the activity/assignment

Students are asked to estimate the total strain history of the Tokositna glacier tongue using the deformed medial moraine as a strain marker. Tokositna glacier is a surge-type glacier in central Alaska that last surged in late spring and summer 2001. During decades-long quiescent periods (the time between surges), motion in the 25-km long snout is minimal compared with motion during surge episodes. During surges, motion is accommodated mostly by sliding at the ice-bed interface, but the sliding rate is not uniform in time or space, resulting in deformation of the tongue. The glacier is also affected by surges of some of its minor tributaries, and the folds we see in the tongue and terminus region may be initiated by these tributary surges. Once initiated, however, fold geometry changes due to progressive strain in the tongue of the main glacier. The student's initial task is to use a deformed medial moraine as an indicator of the strain experienced by the ice as it moves through the lower glacier. This activity allows structural geology students (using a space-for-time substitution) to see the effects progressive development of strain in a fold train exhibiting a longitudinal gradient of total finite strain. Alternatively, it allows glaciology/glacial geology students to use some of the basic principles of structural geology to understand the long-term flow kinematics in a valley glacier tongue.

Determining whether students have met the goals

Once they become comfortable with digitizing and measurements, the data collection and processing is simple. Plotting and written questions following the analysis require interpretation of the results and evaluation of the results. This requires a relatively deep understanding of the development of strain in time and space, as well as an ability to scrutinize the assumptions that allow them to do so. The attached instructors guide provides some guidance for assessing student success.

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