Play-Doh model of a plunging syncline

Provenance: Carol Ormand Ph.D., Carleton College
Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.
Working in small groups, students build Play-Doh models of 3 folds (one upright, one vertical, one plunging). They slice each of their models to create 3D block models, sketch block diagrams of each fold, and sketch structure contour maps.

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Students use the height and radius of Olympus Mons to estimate its volume. They then propose a method to estimate the volume of lava that has erupted over from the Hawaiian hotspot over time. I then show them a graph of the cumulative volcanic volume as a function of distance from Kilauea (from Clague and Dalrymple). They compare these volumes and also consider the possibility that some of the lava erupted from the Hawaiian hotspot has been subducted.

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Students gesture the orientations of cross-bedded sandstones, and in particular the relationship between a single cross bed and the bed sets. They do this for photos of undeformed and deformed cross-bedding.

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Students use gestures to re-create the motion of fault blocks adjacent to restraining bends and releasing bends. They then answer a few questions about a map view of the San Andreas Fault and two of its bends.

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Students watch a video of the instructor sketching two geologic block diagrams (of flat stratigraphy and of an upright anticline), then practice sketching additional geologic block diagrams.

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Students identify and draw slices through an ice cream cone, a pyramid, and a beverage six-pack.

Students identify and draw slices through cylinders and partial cylinders, and use gestures to visualize slicing planes. This practice with visualizing slices through idealized geometric shapes is preparation for visualizing slices through geological features.

Students construct a topographic profile through ONE of the following:

Mt. St. Helens, before the 1980 eruption
Mt. St. Helens, after the 1980 eruption
Crater Lake

They then compare those topo profiles to each other, to a profile through one of the Yellowstone calderas, and to a profile through Mauna Loa. Finally, they write a brief summary of the topography of mafic volcanism (Hawaii), intermediate volcanism (Mt. St. Helens, Crater Lake), & felsic volcanism (Yellowstone).

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Students use a small mirror to explore the meaning of mirror symmetry, and then use their hands to gesture mirror planes for a group of familiar objects. They also explore the rotational symmetry of a group of familiar objects, and then use their hands to gesture the rotational axes and rotation. Finally, they use gestures to show mirror and rotational symmetry of wooden crystal models.

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Students use gesture to convey information about mineral cleavage and the relationship between crystal structures and cleavage planes.

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Students identify individual polyhedra in a variety of diagrams and answer questions about shared oxygens in diagrams of common silicate structures.

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In preparation for this exercise, students have studied 2D strain, become familiar with strain ellipses, and have plotted 1+e2 vs. 1+e1 for progressive pure shear and simple shear deformations. They have measured 2D strain using a variety of standard lab methods. And they have read about 3D strain and the strain ellipsoid.

During class, I have each student make a play-doh cube and mark circles on at least three of the (mutually perpendicular) sides. Then I have each student deform their cube (maintaining an overall rectangular prism shape). I request that they make a different shape than their neighbors' as they deform their play-doh. I ask them to describe what happens to the inscribed circles, and therefore what would be happening to an imaginary sphere within their cube.

Next I introduce the idea of a Flinn Plot, as an abstract but elegant means of conveying 3D strain ellipsoid shapes. I describe the axes, point out that the origin is at (1,1), and plot an example, using my own play-doh parallelipiped, deformed like theirs. Each student then calculates (1+e1)/(1+e2) and (1+e2)/(1+e3) for their parallelipiped, and plots their strain ellipsoid on a Flinn Plot on the board. As a class, we examine each deformed block of play-doh and compare it to its corresponding point on the Flinn Plot. I ask the class to generalize about the deformed shapes above the "plane strain" line versus those below the "plane strain" line. Each student thus practices measuring and calculating 3D strain, and plotting that strain on a Flinn Plot. And they have the opportunity to relate some concrete strain shapes to the abstract Flinn Plot.

I follow this activity up by having students measure 3D strain in a rock sample and plotting their results on a Flinn Plot. Then we go on to discuss the element of time, and also the behaviors of various strain markers during deformation.

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