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Plate Motions

Jeffrey A. Nunn
,
Louisiana State University
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This page first made public: May 16, 2008

Summary

Students are asked to removed the last 40 Ma of ocean crust from an isochron map and then put the plates back together. This exercise helps students see that plates and plate boundaries canNOT be stationary through geologic time

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Context

Audience

Introduction to Physical Geology. Mostly non-geology majors.

Skills and concepts that students must have mastered

Students must understand the basics of plate motion, the 3 types of plate boundaries and the type of plate motion that occurs at that boundary, and what an isochron represents.

How the activity is situated in the course

It is a stand-alone exercise at the beginning of the course to make sure that students understand plate tectonics so that it can be integrated into other parts of the course.

Goals

Content/concepts goals for this activity

Type of motion at plate boundaries. Plate reconstructions and plate motions. Conservation of area as a primary constraint on plate reconstructions

Higher order thinking skills goals for this activity

Students must use the pattern of isochrons to interpret how the plate boundary along the west coast of N. America has changed over geologic time. They must also formulate a hypothesis (at least graphically) as to plate motions over the last 40 Ma.

Other skills goals for this activity

How to interpret a map. Most students need work at getting ALL the information out of a map or a graph or a chart.

Description of the activity/assignment

To prepare for this exercise students read the Chapter on plate tectonics in their text book. In class, they are given a color isochron map of the sea floor. They are given 4 tasks:
  1. Answer basic questions about the timing and rate of opening of the N. and S. Atlantic;
  2. Determine what has happened to the oceanic crust that is created on the eastern side of the East Pacific Rise;
  3. Determine what type of plate boundary existed on the western edge of the N. America plate before the San Andreas Fault and when this transition occurred; and
  4. Reconstruct the motion of the plates over the last 40 Ma assuming that the surface area of the Earth has not changed.

Determining whether students have met the goals

Answers to parts 1 and 2 above are primarily factual so the answers are either correct or not. Parts 3 and 4 involve an interpretation with supporting evidence. So I want to see that they understanding can correctly infer from the isochrons that there used to be a subduction zone off of western N. America. Finally, in part 4, they have to match isochrons and conserve area. We discuss this in class afterwards as many students do not conserve area in their reconstructions

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Supporting references/URLs

Web version of assignment

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