Discovering Plate Tectonics

This activity involves students in “discovering” plate tectonics, and considering its historical relationship to continental drift and the technological advances in observing Earth which were critical to the emergence of plate tectonics. (Ohio competencies 1, 4, 9)

This activity expands upon Project MIST -> Earth’s Lithospheric Structure: Earthquakes, Volcanoes, and Plate Tectonics where students plot earthquake and volcano occurrence to construct plate boundaries, and a full description and background information on plotting is available on that website.

A. Continental Drift

1. Have students describe and plot geological data which was available early in the 20th century. Use world topographic maps which do not include ocean topography. Ideally they would show the edge of the continental shelf and mountain belts.

a. description:

  1. note continental shapes using edges of continental shelves;
  2. compare shapes on opposite sides of oceans; note pattern of mountain belts on opposite sides of oceans

b. plot:

  1. fossil distributions for Paleozoic Era (similar fossil groups for southern hemisphere continents plus India versus northern hemisphere continents) and for Mesozoic Era (dinosaur groups similar in now wide-spread areas) and for Cenozoic Era (great differences in mammal groups on different continents);
  2. late Paleozoic Era continental glaciations in southern hemisphere continents and India

2. observations suggestive of continental mobility, but not widely accepted because no plausible mechanism proposed

Data Sources

Maps showing locations of fossil groups and continental glaciers can be found in current Physical and Historical Geology texts. Websites which are useful include:

B. Plate Tectonics

  1. students plot geophysical data, which required technological advances of the 20th century
    1. earthquake epicenters.
    2. active volcanism data.
      • http://volcanoes.usgs.gov/: list of currently erupting volcanoes with links to photographs and histories of eruptions.
      • http://www.si.edu/: select Geology and Mineral Sciences and then select Global Volcanism Program. This site is maintained by the Smithsonian Institution.
    1. ocean floor data: can discuss using published maps, or maps from web
  2. recognition that far more evidence for continental mobility comes from the ocean crust than from the continental crust
    • sea-floor spreading model
  3. plotting of tectonic data
    • from Project MIST

Materials

large color world map for display, preferably showing topography of ocean floor and continents; latitude/longitude necessary; preferably laminated (for example: World Physical Map, 1:30,000,000, from American Map, ISBN 0-8416-8996-2; The World Physical Map, 1:32,000,000, from Kummerly & Frey, ISBN 3-329-04063-3); washable markers; computer(s) with Internet link: minimally, need access to one; for each student: ruler, pencil (3H), eraser; tracing paper, plain white or graph (metric or inches divided in tenths) paper; world map, including topography if possible, and latitude/longitude, on one sheet, less than desk size (for example, Rand McNally Outline Maps of World, 11"x15"; has outline map on one side/relief on other; 50 map package $11)

Introduction

Students will each gather data on earthquake and volcanic eruption occurrence worldwide from sites on the Internet. They will plot the earthquake epicenters and active volcanicity locations on world maps. This project can use current events, gathered daily over several weeks or months, and/or make use of archived data available on the web sites. Activities deriving from the tectonic data maps will allow each student to build towards the development of the basic plate model, which includes lithospheric structure, plate boundary types and geological characteristics of the plate boundaries.

Student Project

1. Data collection, recording, and plotting

Choice: Students use archived data for a defined period of time, or project can take place over several weeks/months with periodic collection of data. I prefer the latter, so that students experience that science takes place over time with accumulation of observations, and they can think about what the developing data set represents and puzzle out interpretations, rather than presenting science as a set of known facts.

  • Internet connection and web addresses
  • Earthquakes: http://geology.usgs.gov/
    • choice of data: all recorded quakes versus some defined minimum magnitude, e.g., 2.5.
    • record data on Earthquake Log (see handout)
    • plot epicenters (lat./long.) on world map
  • Volcanoes: http://volcanoes.usgs.gov/
    • choice of data: all reported activity, or some defined minimal activity level record data on Volcano Log (see handout)
    • plot volcano locations (lat./long.) on world map

Teacher: The display world map can be used to plot selected events; use different color marker for volcanoes and earthquakes; also can indicate focal depth of earthquakes using different colors: e.g., shallow: surface to 100 km depth; intermediate: 100 - 200 km; deep: >200 km.

2. Written description of map pattern

After sufficient data have been plotted, students write descriptions of any earthquake and volcano patterns which emerge on their maps. Curvilinear bands (= possible plate boundaries) will emerge as data accumulate; there will also be “rogue” occurrences in the interior of plates, for earthquakes because there are weaknesses within the lithosphere that quake, although less frequently than at boundaries. For volcanoes, there are mantle “hotspots” which cause volcanoes at the surface unrelated to plate structure). Students should identify possible plate boundaries based on their plotted data. Students can prepare a plate boundary overlay of the map: place a piece of tracing paper over the map. Trace map edges or corners so overlay can be easily repositioned. Draw locations of possible plate boundaries as defined by plotted data.

3. Interpretation of plate boundary types

Each student chooses a possible plate boundary, from their map, to interpret.

Construct a cross-section (= vertical section) of the possible boundary, using earthquake depth data and epicenter location, and any volcanic occurrences in the boundary vicinity.

Cross-Section Construction

  1. Use graph paper if available, otherwise plain white paper.
  2. Choose a convenient, enlarged with respect to map, scale: horizontal scale = vertical scale.
  3. Choose the line of section. This will be perpendicular to the selected plate boundary.
  4. Plot latitude/longitude points along top edge of cross-section.
  5. Mark depths down the edge of the cross-section.
  6. Plot depths of earthquakes which occur along the map-view boundary ( “move” the epicenters of all the quakes on this boundary to the line of section by projecting them perpendicularly “down” or “up” to the line of section.
  7. Plot any volcanoes which occur near this boundary, again, by “moving” them to the line of section.

Write a description of the cross-section pattern of earthquakes and volcanoes.

Answering the following questions should aid in your interpretation:

  1. Do the earthquake foci define a vertical or a sloping boundary?
  2. What is the maximum foci depth? Is this consistent with a “normal” 100 km thick lithosphere, or does it suggest the occurrence of brittle material at greater than “normal” depth?
  3. Are any volcanoes associated with this boundary? If not, what does that indicate about boundary processes? If so, where are the volcanoes with respect to the earthquake locations? eg. coincident; all to one side? What styles of eruption are associated with this boundary? What do the occurrence of such eruption styles and locations suggest about boundary processes?
  4. What type of motion is indicated for this plate boundary, based on these observations?
    1. divergent: plates move away from each other; shallow quakes define vertical zone; basaltic, effusive volcanoes;
    2. transform: plates move parallel to each other but in opposite directions; shallow quakes define vertical zone; no volcanoes;
    3. convergent:
      1. subduction zone: ocean plate moves under continental or oceanic plate; quakes (all depths) define sloping boundary; andesitic to rhyolitic, often violent, eruptions;
      2. collision of two continents: shallow to medium depth quakes in diffuse zone; no volcs.

Teacher: To summarize the class ask each student (or groups of students who worked on same boundary) to describe their results. Plot the results on the display map, using different line symbols or colors for the different boundary types. Have the class discuss the relationship of topography to plate boundary types. If you have one, show the class a world tectonic map or pull one up from the internet (available on these earthquake and volcano sites). Compare the boundaries the students discovered to the boundaries delineated on the published tectonic map.

Earthquake Log

Date Latitude Longitude Depth Magnitude Comments
           
           
           
           

Volcanicity Log

Date Latitude Longitude Name Comments Eruption Style
           
           
           
           

References

Havholm, Karen Gene. 1998,. An activity to introduce the geoscience perspective: Journal of Geoscience Education. Vol. 46: 2, 137-140.

Leong, Cheryl. 1994. The Domino Effect in: The Best of BAESI, The Bay Area Earth Science Institute, San Jose State University, p. 119-126.

Liukkonen, Barbara. 1993. How Much Water is There? in: On the Rocks, Society for Sedimentary Geology, p. 71-72.

Martindate, Gary. 1994. Hand-Held Magma Chamber in: The Best of BAESI, The Bay Area Earth Science Institute, San Jose State University, p. 186-194.

Project MIST. 1999. Ohio Wesleyan University. Project Primary. 1997. Ohio Wesleyan University.

Triplehorn, Don. 1994. On the Back of an Envelope: Journal of Geological Education, Vol. 42, p. 164.