Tracking Tectonic Plates: An Introductory-Level Exercise in Testing Plate Tectonics by Two Independent Methods

Students come to this activity familiar with the basic assumptions of plate tectonics. Using a Google Earth platform showing commonly accepted lithospheric plate boundaries as well as locations of GPS stations, students are assigned a plate boundary to study and form a hypothesis about motions expected across their boundary. They then set about testing their hypotheses by plotting motion vectors for appropriate stations using two independent methods. METHOD 1: LONG-TERM “MODEL” RATES OF PLATE MOTION To understand this method, students work though exercises in which they determine long-term average plate motions using age data from mid-ocean ridges and the Hawaiian Island/Emperor Seamount chain. They learn that this and other data form the basis of plate motion models easily accessed on the Internet via “Plate Motion Calculators” (e.g. http://sps.unavco.org/crustal_motion/dxdt/model/), which enable the determination of “model” rates of plate tectonic motion for any point on Earth, averaged over millions of years. METHOD 2: GPS MEASUREMENTS INTERPRETED IN TERMS OF PLATE MOTION After being introduced to the principles of high-precision GPS, students are directed to the JPL website (http://sideshow.jpl.nasa.gov/mbh/series.html) and learn how to interpret GPS measurements for over 900 stations as near real-time rates of plate motion. RESULTS Students find that in general, plate tectonic theory holds up. However, they also discover sophisticated detail -- over the time-span of GPS measurements, plates move at rates different than the long term model averages and thus rates must not be constant; some plate boundaries are more diffuse than others; short-term rates for stations on the same plate are not always identical, indicating that internal deformation of plates does occur. Student evaluations of the activity demonstrate that they feel engaged and empowered as they work with authentic data, and gain a sophisticated understanding of a fundamental theory as well as the process of doing science.