Geotectonic Plate Boundaries in Three Dimensions

I have used this activity in a General Education-level introduction to Earth Science. It may also be appropriate in high school courses.

CONCEPTS

Before this activity, students should have an understanding of the following terms and concepts: lithospheric plate, plate boundaries, subduction, earthquake hypocenter and epicenter, subduction-related volcanism, rift valley, continental shelf, passive margins, and differences in the relative thickness and density of lithospheric plates. They also should understand the meaning of the terms map view, profile view, and cross section.

SKILLS

Before beginning this activity, the students should be able to do the following:

* Sketch cross sections illustrating the different types of geotectonic plate boundaries.

* For a given location on Earth, use plate boundary maps to identify whether the site is located at a plate boundary, and if so, of what type.

* For any given plate boundary, use maps of lithospheric thickness to determine whether the plates involved may be considered as continental or oceanic.

The activity is situated as a "put it all together" exercise after students have learned about the three types of plate boundaries and the geologic features that occur at each type.

After completing this activity, students will be able to do the following:

SKILLS

* Sketch a cross section illustrating the configuration of plates for any given plate boundary or passive margin.

* On that cross section, illustrate geotectonic features that would exist at that boundary (e.g. volcanic activity, mountain ranges, islands, trench, earthquakes of different focal depths).

* For the same boundary, on a three-dimensional model, sketch a map in the correct relation to the cross section.

* For the same boundary, sketch a cross section of the boundary as viewed from the opposite side.

* Illustrate multiple plates, boundaries, and geotectonic features as they would fit within the context of an extensive cross section, essentially, a line extending around the entire planet.

CONCEPTS

This activity attempts to explicitly develop students' spatial thinking as it relates to three-dimensional representations of plate boundaries. After completing the activity, students should understand the relationship between the cross-sectional and map views of geotectonic features occurring at plate boundaries of different types. It also explicitly invites students to consider the bigger picture of not just the isolated boundary, but the entire Earth system of geotectonic forces and movements.

OVERVIEW

I use this activity in a class of 24 students who work in teams of two. I stretch a string across a wall map of the Earth in such a way that it crosses 12 different plate boundaries or passive margins. Each team is assigned one location. They use maps to determine the type of boundary or margin, and then they use markers, poster paper, and a cardboard box to create a three-dimensional model of the boundary. Their task is to draw a map of the boundary on top of the box, and then cross sections on either side, illustrating the correct cross sectional view from each perspective. They add features such as trenches, volcanoes, rift valleys, and earthquakes of different depths. Once all the teams' box-models are complete, the boxes are placed end to end to illustrate an "around the world" view of plates and boundaries.

MATERIALS NEEDED

Large map illustrating the ocean floor (e.g. the Tharp-Heezen "World Ocean Floor" map). I use the physical wall map, although a projected map might work as well.

String and a method to attach it to the map in a zig-zag path (magnets, thumbtacks, rubber cement, etc.), or a means of drawing a line on a projected map.

One set of 8.5"x 11" printed world maps for for each team. I put these in page protectors and re-use them each semester. The maps should illustrate the following; see the references section for web links:

* Plate boundaries (I provide students with three different plate boundary maps so they have several sources of information.)

* Lithospheric thickness (Students use this to determine if the plates at their assigned location should be considered as continental or oceanic lithosphere.)

* Volcanic activity

* Depth of earthquake hypocenters

Cardboard boxes of the same size and shape; one per team. I use the boxes that reams of printer paper comes in, as they are plentiful around the university.

Flip chart poster paper, one sheet per team (a common size is 25" x 30").

Sticky tape, a few pieces per team. Each team wraps the cardboard box in the poster paper and uses a few pieces to hold the paper in place.

Thick permanent markers, a few colors for each team.

A one-page handout with instructions in two parts:

* A "pre-study" on which students work out what they intend to draw before they begin drawing it.

* Steps students follow to complete their box-model drawing.

Handout in .pdf format Handout in pdf format (Acrobat (PDF) 123kB Dec22 17)

Handout in .docx format Handout in .docx format (Microsoft Word 2007 (.docx) 29kB Dec22 17)

I conduct this activity start to finish in a class period lasting 100 minutes. It could be broken into smaller sections for shorter time periods. The boxes and drawings would then need to be stored from one class period to the next. The final part of the activity, in which all twelve boxes are placed end to end, requires that tables be pushed together in a long line. If space is limited, alternatively, this lining up could be done on the floor or in a hallway.

Before class begins, I arrange the string on the wall map so that it crosses twelves boundaries or passive margins and ends in the same location where it starts. An image of the line I use appears here.

This path covers convergent boundaries of various configurations, divergent boundaries, and passive margins. Transform boundaries are more difficult to incorporate into this framework and are ignored for this activity. Instructors who wish to model transform boundaries as part of this activity may find it advantageous to provide those teams with two boxes that can be offset along the boundary, as opposed to relying on drawings alone.

If there are more than 24 students in the class, teams of three can work together. I have worked with teams of four, in which each pair of students is responsible for one side of the box, and they work together on the top. With fewer than 24 students, given enough time, teams may be responsible for two boundaries (perhaps of two different types). Alternatively, the convergent and divergent boundaries, which are more complex, may be assigned to the students, while the instructor takes responsibility for drawing the three passive margins, which are the simplest drawings and take little time to complete.

I give an overview in a brief lecture, emphasizing the objectives of the activity (develop spatial thinking, place individual plate boundaries in global context). I show the students this photo

, which gives an example of a 3-D model of a passive margin with only the overall plate configuration shown. The purpose is to point out the drawing conventions that all teams should use to make the activity have more impact when the boxes are placed end to end.

Teams of two are randomly assigned or choose numbers corresponding to a numbered boundary on the map. Following the instructions on the handout, they first study the boundary and sketch practice drawings in map view and cross-sectional view. After these are checked by the instructor, the students re-create their drawing on the three-dimensional box-model. The handout instructs students to add geotectonic features in the correct locations. Some teams will do this quickly, but others need significant time to work out on which side of the boundary the volcanoes will form, or what direction a subducting plate will appear to plunge on the two opposite sides of the box.

When all teams have completed their work, the boxes are placed end to end in numerical order. (I usually put the teams that finish first in charge of moving classroom furniture and arranging the boxes, which frees me to help those not yet finished.) An image of this appears here

.

When the completed box drawings are placed end to end in numerical order, they illustrate a long cross section that extends essentially around the globe. If everything is done correctly, drawings on adjacent boxes should match each other. I go through the entire sequence from 1 to 12, calling out the type of boundary and the significant geologic features on each one. When I get to the end of the line, for dramatic effect, I pick up the twelfth box and run it to the beginning of the line, placing it adjacent to first box, which it should match up with.

After the activity ends, I examine each team's drawings to ascertain whether they have successfully done the following:

* Correctly identified the type of boundary at their assigned location.

* Correctly identified the type of plates (e.g. oceanic or continental) and shown their thicknesses appropriately.

* Shown the correct orientation of subducting and overriding plates at convergent boundaries (check both sides).

* Correctly indicated the location of geographic features such as trenches, volcanoes, volcanic islands, mid-ocean ridges, rift valleys, and mountain ranges.

* Correctly indicated the presence of earthquake epicenters on the map view (top of the box) and hypocenters in the cross-sectional view (the two sides of the box). I ask students to identify shallow, medium, and deep-focus earthquakes and place their hypocenters in the correct locations in both the cross sections and map view.

Plate boundaries maps: Most good textbooks will include a map of plate boundaries. Several maps can be found online, and each may have its own advantages and drawbacks.

This one is in the creative commons: https://upload.wikimedia.org/wikipedia/commons/b/b0/Map_plate_tectonics_world.gif , but it does not identify each boundary as to its type.

Likewise, this one from the USGS only shows the boundaries, not the relative movement: https://pubs.usgs.gov/gip/dynamic/slabs.html

This one shows the type of boundary, but is a simplified map and located at a "dot.com" site, which we usually steer students away from: http://geoedu.weebly.com/inside-earth/volcanoes-and-plate-tectonics

The National Park Service offers this simplified image: https://www.nature.nps.gov/geology/education/images/GRAPHICS/Lillie_2005_Plate_Tectonic_Map-01.jpg

Lithospheric thickness map: I prefer this map, by Peter Bird: http://peterbird.name/publications/2008_torque_balances/012_total_lithosphere-Earth5N.jpg .

This one from the National Geographic is simpler: https://media.nationalgeographic.org/assets/photos/307/722/e781369c-d3af-4066-ac2d-e990f552e545.jpg

Earthquake map: Several have been in publication for years. Here is an example by the Tectonics Observatory of the California Institute of Technology. One caution: its color coding is not consistent with that used by the current USGS National Earthquake Information Center, which students may find confusing. https://www.nsf.gov/news/mmg/media/images/global_seismicity_h.jpg

Volcanic Activity: Most volcano maps suffer from the major drawback of failing to indicate volcanic activity in the oceans. The instructor will have to compensate for this failing.

Encyclopedia Britannica offers this map: https://media1.britannica.com/eb-media/49/4949-004-6F24AB8D.gif