Cracking and Crumbling: Exploring Mechanisms of Dike Emplacement

Phillip Resor
,
Wesleyan University
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Summary

Students explore the shape of basaltic dikes intruded into shale near Shiprock, New Mexico using published maps and a simple mechanical model. Students get practice in making quantitative measurements, evaluating models, and generating alternative hypotheses.

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Context

Audience

I plan to use the activity in an undergraduate structural geology course for majors. The activity was originally developed by David Pollard for a graduate-level fracture mechanics class. The activity can be scaled to different levels by adjusting the level at which the mechanical model is explored.

Skills and concepts that students must have mastered

Before beginning the activity students should have mastered concepts of stress, strain, constitutive laws (specifically elasticity), and rock failure. The activity can be integrated into a discussion of rock strength and fracture.

How the activity is situated in the course

The activity is designed as a stand alone activity that may be completed in one or two problem sets.

Goals

Content/concepts goals for this activity

Strength of rocks, fracture mechanics, application of mechanical models to real-world problems, mechanisms of dike emplacement.

Higher order thinking skills goals for this activity

Critical evaluation of models and formulation of hypotheses.

Other skills goals for this activity

Collection and evaluation of quantitative data from geologic mapping.

Description of the activity/assignment

This case study is based on exercises developed by Dave Pollard and uses images and mapping from Delaney and Pollard (1981). Students explore mechanisms of dike emplacement by measuring dike thicknesses along two segments of a dike intruded into shale near Ship Rock, New Mexico and mapped by Paul Delaney. Students then compare the observed dike profiles to a mechanical model—the solution for a crack filled with magma under uniform pressure. Using the equation for opening of the crack students estimate the driving stress (difference between the magma pressure and remote principal stress acting perpendicular to the dike plane) using several different estimates of the rock stiffness parameters. Students are then asked to observe how well their best-fit model fits the two dike profiles and to investigate the map further and hypothesize why the observed widths may deviate from the simple model. Deviations from the mechanical model can lead to discussions on mechanical interaction as well as brecciation and stoping.

Determining whether students have met the goals

Student performance can be assessed by grading a written report with graphs and supporting discussion.

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