2. 2019 Collection
3. Plate Tectonics and the Seafloor
4. Instructor's Guide

Summary

This exercise introduces students to the connection between seafloor features and plate tectonic processes. It invites and encourages students to make their own observations, first by general identification of seafloor features, followed by application of plate tectonic principles to relate those features to the type of plate tectonic environment that generated them. Students will then use earthquake data to explore and interpret processes occurring within each tectonic environment. Lastly, students tie all of their observations together to propose a hypothesis about geologic and oceanographic hazards that may be present.

The lab exercise is set up in steps that progress from simpler concepts to more difficult ones. It is up to you how far to go with your students.

Enduring Understandings

The most important understanding that students are to gain from this exercise is that features, earthquakes, and hazard sources on the seafloor have a direct connection to the movement and interaction of Earth's tectonic plates.

Learning Goals and Assessments

After engaging with this Data Exploration a student will be able to:

• Apply their prior-learned background to a reality-based scenario using improved data skills.
• Interpret time-series and geographic earthquake data.
• Explain the relationship between seafloor features, earthquakes, and plate tectonics.
• Determine plate tectonic settings by connecting real-world relationships to background knowledge.
• Articulate some of the challenges that scientists face when data does not fit simple models.
• Identify a reasonable hypothesis for the potential natural hazards in the studied area.

Context for Use

This exercise can be used for both introductory and upper level oceanography (or geology) courses. As described above, it is set up as a series of steps in which students can progress as far as you want to guide them. However, it is presumed that students who use this exercise have, at a minimum, been introduced to the basic concepts of plate tectonics, including plate boundaries and interactions.

Teaching Notes

In addition to a basic understanding of plate tectonic theory, students should know the basic, physical layers of the earth and their characteristics (lithosphere, asthenosphere, mesosphere, outer core, inner core). It might also be helpful to familiarize students with the basic concept of earthquakes and their causes (see the Invitation section below). If you are going to have the students work through all of the steps or activities, then it would be helpful to the students to also know of the connection between earthquakes and tsunamis, and to be familiar with the concept of a natural hazard. They should also know what it means to form a hypothesis.

To help students understand the connections between the seafloor, natural hazards, and plate tectonic processes as presented in this exercise...

Subject / Topics

• Earthquake(s): epicenter(s), energy build-up and release
• Seafloor features: seamounts, mid-ocean ridge, transform zone, deep sea trench
• Plate Tectonics: plate boundaries, plate movement, subduction
• Natural hazards: earthquakes, tsunamis

These exercises should be used to give students their own "hands-on" experience in understanding plate tectonic processes — applying what they have learned previously. It requires them to apply map-reading, data interpretation, and synthesis skills. The terms and topics listed above should mean something more to the students by the time that they finish this exercise.

Grade Level

These exercises are for use in undergraduate Introduction to Oceanography courses, for both marine science majors, as well as non-science majors. They could also be used for advanced high school students, such as those in AP science courses. Students who do these exercises are expected to have already been introduced to basic plate tectonic concepts.

Data Scope

The data in these exercises consist mostly of seismic, geographic, and time-series data. Students are given the opportunity to:

• Use professionally-collected data.
• Visualize and interpret oceanographic and geographic relationships.
• Interact with data in a user-friendly way.

Quantitative Skills

The quantitative skills in these exercises are related to the interpretation of a time-series plot and map reading.

• Seafloor features are featured on a digital, physiographic map. Students are asked to identify these bathymetric features, including their locations and relationships. Students will need to understand map orientation and approximate scale. They will also need to recognize how elevation represented on the map.
• Epicenters are presented geographically in both location (in 3-dimensional space) and scale. Students will need to recognize that the diameter of the epicenter symbol represents magnitude scale and that the color represents depth within the earth.
• Epicenters are also plotted as a time-series graph. Students will be need to recognize that the y-axis represents magnitude and that the x-axis represents time. Color, again, represents depth.
• Students should be able to make connections between location, depth, and magnitude.

Expansion ideas

For more advanced students, an emphasis on earthquake patterning in time vs space is recommended. Discussions could revolve around:

• Having the students calculate earthquake recurrence intervals for various ranges of earthquakes.
• Focusing on larger clusters of earthquakes and the features that they may be associated with.
• Why does the subduction of the plate beneath Japan form a much deeper trench than the subduction beneath the Pacific Northwest? Students can calculate distance (and age) between the subduction zones and the mid-ocean ridge.

Additional resources for instructors

• An article about the underwater earthquake video
• A National Geographic Society introduction to the 2011 Tohoku, Japan earthquake
• USGS resources related to the 2011 Tohoku, Japan earthquake
• The State of Washington's Department of Natural Resources page on tsunami hazards
• The IRIS Earthquake Browser (an interactive, earthquake map of the world)

Science Explanation

The Theory of Plate Tectonics is the grand, unifying theory of earth science. The basic idea is that the outermost, physical layer of the earth, the lithosphere, is brittle and thus broken into sections called tectonic plates, much like the thin shell of an egg is brittle and can be broken (see figure).

These plates float on top of a more fluid, but still solid, layer called the asthenosphere. The plates of the lithosphere ride on top of the asthenosphere and move due to convection within the earth (see figure).

These plates move relative to each other, sometimes colliding (forming mountains and explosive volcanoes), sometimes pulling apart (forming rift valleys and mid-ocean ridges), and sometimes sliding past each other (forming strike/slip faults and transform zones between mid-ocean ridge segments) (see figure).

These interactions are directly related to many of the features that we find on the seafloor, such as volcanic seamounts, mid-ocean ridges, transform and fracture zones, and deep sea trenches (see figure).

These interactions between tectonic plates lead to earthquakes as tremendous amounts of energy are built-up and then released, much like energy is built up within a pencil as it is bent. Eventually the strength of the wood will be overcome by the energy and the pencil will break (see figure).

By examining the size and location earthquakes, we learn something about the behavior of tectonic plates. For example, the deepest earthquakes occur in convergent settings, where one plate subducts beneath another (see figure).

If, as part of the earthquake, the seafloor is disturbed, either by direction movement of the seafloor, or by the generation of a underwater landslide, the energy from this disturbance can be transferred to ocean itself, forming a tsunami (see figures). Thus, areas of active plate tectonic interactions within the ocean present a significant natural disaster hazard.

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