Hands-on physical models that help students learn about crustal deformation and strain Abstract

abstract

  • A group of geoscientists affiliated with UNAVCO has developed a curricular module in which students use GPS velocities from three non-colinear sites to compute the mean instantaneous strain in the crust between them. This module, "Curricular module for introductory geophysics or structural geology courses to quantify crustal strain using EarthScope PBO GPS velocities," has been tested in undergraduate physical geology, structural geology and geophysics courses, with good results. In preparation for work on this module, we find it useful for students to investigate strain by working with simple analog models. Early testing of the module revealed that the physical models helped students to solidify their abstract knowledge and connect it with the physical world. Simple physical models made of inexpensive materials are used to build student intuition about strain. These models are not intended to represent properly scaled physical models of crustal deformation, but rather are analogs that display useful similarity with some aspect of crustal deformation. A selection of these hands-on demonstrations will be available during the poster sessions at the Science Workshop • A bungee cord that can be stretched across a board from a fixed initial length to a fixed final length is used to illustrate the physical meaning of 1-dimensional extension (elongation) and stretch of an elastic material. • A rubber ball is used to illustrate an elastic solid. • Silly Putty is used to illustrate a continuous material that has an elastic response to quickly applied-and-released stresses, but a viscous response to differential stress applied over a longer time period. A block or cylinder of Silly Putty is imprinted with a circle and allowed to flow under the influence of gravity so that the circle quickly becomes an ellipse. The fact that the ellipse becomes progressively less perfect in shape with greater strain can generate discussion about finite inhomogeneous strain. • Elastic fabric harvested from a compression T-shirt has a circle drawn on it, and then is manipulated by students to simulate elements of total strain: translation, positive and negative rotation, positive and negative dilation, and distortion in which the circle becomes an ellipse. This also generates discussion of homogeneous versus inhomogeneous strain when students find it challenging to turn the circle into a perfect ellipse. • Foam rubber rectangles with precuts illustrate mode I, II and III cracks, and provide a nice finger-sized model of elastico-plastic stick-slip behavior leading to shear displacement accompanied by an audible "pop" -- a tiny earthquake. • Classroom chalk or larger "sidewalk" chalk is deformed using a simple bench vise to make a mode I crack. • Dry models involving fine sand, powder (e.g., flour, dry sheetrock mud, dry clay), or some combination are used to illustrate normal, reverse, strike-slip and oblique faulting. • A plastic cup filled with dry powder is pinched slightly to produce sets of conjugate faults. The rim of the cup is a circle when at rest, and an ellipse when pinched to produce the fault sets. • A powder model illustrates deformation and relative motion of GPS sites due to inflation/deflation of a volcano. • A model that uses a uniform layer of powder on elastic fabric generates a system of tension cracks when the fabric is stretched. If a circle is imprinted on the powder before deformation, it becomes an ellipse even though the deformation in the powder "crust" is discontinuous. From this we infer that mean infinitesimal crustal strain measured between GPS sites separated by tens of kilometers might still be meaningful even when the intervening area contains active faults.

authors

publication date

  • 2014

presented at event