Eileen Herrstrom

University of Illinois at Urbana-Champaign

Undergraduate class on introductory physical geology or quantitative reasoning for non-majors

Must have general knowledge about paleomagnetism and seafloor spreading (magnetic reversals, magnetic anomalies) and know how to do basic tasks in Microsoft Excel (enter data, enter formulas, fill down)

This is a laboratory activity that follows lectures on continental drift, paleomagnetism, and seafloor spreading and is the third laboratory exercise of the course.

Understand and explain a bathymetric map of the Atlantic Ocean, reconstruct how parts of the Pangaea supercontinent fit together, and measure how two pieces of the supercontinent moved apart

Describe and compare magnetic anomaly maps from the northern Atlantic Ocean and northeastern Pacific Ocean, interpret magnetic anomaly profiles in terms of age using the geomagnetic reversal time scale, and calculate the rate of seafloor spreading at two mid-ocean ridges

Compare and contrast the map of sediment thickness and the bathymetric map, graph the ages of sediments and distances at different sites to determine their relationship, and calculate the rate of seafloor spreading based on paleontological ages

As far back as 500 years ago, mapmakers drawing the coastlines of South America and Africa noted that the two continents looked as if they might have fit together in the past to form a larger continent. Alfred Wegener incorporated this idea into his hypothesis of continental drift. In spite of the wealth of detail that he assembled, most geologists rejected Wegener's idea of drifting continents, mainly because they found his explanation of exactly how continents could move around the face of the globe to be unconvincing. During the 1940s and 1950s, however, new instruments allowed scientists to map the ocean floors in detail, which finally provided enough evidence to convince the geological community that Alfred Wegener's basic hypothesis of continents in motion was indeed correct.

In Part I, students indentify continental shelves, abyssal plains, and the mid-ocean ridge from a bathymetric map of the South Atlantic Ocean, examine the fits of the coastlines and continental shelves of Africa and South America, and calculate a seafloor spreading rate based on the length of a fracture zone.

Part II involves comparing magnetic anomaly maps from the Reykjanes and Juan de Fuca Ridges, interpreting magnetic profiles across the Mid-Atlantic Ridge and the East Pacific Rise, and calculating rates of seafloor spreading for these two ridges based on paleomagnetism.

In Part III, students compare and contrast maps of bathymetry and sediment thickness and enter data and formulas into Excel to determine the rate of seafloor spreading based on paleontological ages.

As originally designed for a traditional face-to-face course and later used in the online version, this activity is assessed by the answers to the questions. It is also possible to have students submit their completed tracings and spreadsheets, although this option works best in a small class.

Ludman, A., and S. Marshak, 2010, Laboratory Manual for Introductory Geology, Chapter 2, pp. 26-36.

Cook, J., and V. Jones, 2014, Nannofossils Reveal Seafloor Spreading Truth: National Science Teachers Association: Online resource – Accessed 15 June 2019 https://ngss.nsta.org/Resource.aspx?ResourceID=753

Heirtzler, J.R., X. Le Pichon, and J.G. Baron, 1966, Magnetic anomalies over the Reykjanes Ridge: Deep Sea Research, v. 13, pp. 427-433: Online resource – Accessed 15 June 2019 https://www.geological-digressions.com/striped-oceans-and-drifting-continents/

Domeier, M.M., R. Van der Voo, R.N. Tomezzoli, T.H. Torsvik, E. Tohver, B.W.H. Hendrix, H. Vizan, and A.R. Dominguez, 2009, The Pangea Problem: Insights from New Permo-Triassic Paleomagnetic Data from Gondwana: AGU Fall Meeting Abstracts: Online resource – Accessed 15 June 2019 https://www.researchgate.net/publication/253282658

Kious, W.J., and R.I. Tilling, 2016, This Dynamic Earth: the Story of Plate Tectonics: US Geological Survey, ISBN 0-16-048220-8: Online resource – Accessed 15 June 2019 https://pubs.usgs.gov/gip/dynamic/developing.html

Meldahl, K., 2011, Magnetic Reversals and Seafloor Spreading: Online resource – Accessed 15 June 2019 https://www.youtube.com/watch?v=BCzCmldiaWQ

Seafloor Spreading Activity, 2013, National Oceanic and Atmospheric Administration: Online resource – Accessed 15 June 2019 https://oceanexplorer.noaa.gov/edu/learning/2_midocean_ridges/activities/seafloor_spreading.html#none