Lab 3 – Plate Tectonics and the Seafloor
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.
Much like the shell of the egg in this image, the outer layer of the earth, or lithosphere, is a brittle layer that is broken into sections. These sections are called tectonic plates (image copyright: Benjamin R. Jordan, used with permission; Earth images after Google Earth).
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.
The lithosphere is the outermost layer of the earth. It sits on another, softer (but still solid) layer, called the asthenosphere. Although solid, the rocks within the earth still move in slow convection cycles. This movement is directly connected to the movement of the lithosphere’s tectonic plates (image copyright: Benjamin R. Jordan, used with permission).
These plates move relative to each other, sometimes colliding (forming deep-sea trenches, 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).
Tectonic plates interact in three main ways: divergence, convergence, and transform motion (image copyright: Benjamin R. Jordan, used with permission; right image after USGS).
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.
Mid-ocean ridges are locations where two tectonic plates are moving apart (image copyright: Benjamin R. Jordan, used with permission; Map imagery from the Global Multi-Resolution Topography (GMRT) Synthesis, funded by the National Science Foundation (NSF).
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.
Much like bending a pencil will eventually lead to it breaking, Tectonic forces within the lithosphere will lead to the breaking of rock, forming an earthquake (image copyright: Benjamin R. Jordan, used with permission).
By examining the size and location of 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.
Because earthquakes occur within the brittle lithosphere, the depth and location of an earthquake tell us something about the geometry of a subduction zone (image copyright: Benjamin R. Jordan, used with permission; left figures after USGS).
If, as part of the earthquake, the seafloor is disturbed, either by direct movement of the seafloor or by the generation of an underwater landslide, the energy from this disturbance can be transferred to the ocean itself, forming a tsunami. Thus, areas of active plate interactions within the ocean present a significant natural disaster hazard.
If the surface of the seafloor is displaced, its motion will transfer energy into the overlying water, forming a tsunami wave (image copyright: Benjamin R. Jordan, used with permission).
A tsunami wave moves radially out from its source, much like the ripples from a rock tossed into a pond. A tsunami usually consists of multiple waves, not just one. In fact, the largest wave of a tsunami is often NOT the first one that reaches land (image copyright: Benjamin R. Jordan, used with permission; bottom right image after USGS; upper right image after NOAA/PMEL/Center for Tsunami Research).
Once a tsunami wave reaches shore, it can arrive as a massive surge of water, several meters high. The bottom, center image denotes the height of the tsunami wave that is shown in the other two images. This wave hit Hilo, Hawaii in 1946 (image copyright: Benjamin R. Jordan, used with permission; top left image courtesy of NOAA; lower right image courtesy of USGS).
This data exploration is designed to give students the opportunity to use bathymetry and earthquake data to explore the relationship between plate interactions and topographic features on the seafloor. Students should come away from this having used real data to make interpretations for themselves about what earthquakes tell us about the seafloor, as well as the potential hazards that are associated with these processes.
The exercise consists of three activities that will lead students from simple observations to plate tectonic interpretations to hypothesizing about potential hazards.
In this lab activity, students will first observe and label features on the seafloor. After that, they will overlay earthquake location data and relate that data to the features they observed and labeled. Using the relationship between the topography and earthquake data, students will determine the various plate boundaries and types. The students should be able to independently identify the divergent boundaries (mid-ocean ridge) and transform zone and identify the subduction zone, which does not have a distinctive trench, making it a challenge to identify the process of subduction. Students, with your guidance, should be able to determine the subduction zone based on the depth of the earthquakes. Lastly, in thinking about the subduction zone, the comparatively limited number of earthquakes, and what happened in the Tohoku region of Japan in 2011, students will hypothesize about the earthquake and tsunami hazards that potentially exist in the area discussed in this exercise (spoiler: they are significant).
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.
Approximate time involved: Activity 1 – 15-20 minutes, Activity 2 – 20 minutes, Activity 3 – 25 minutes.
- LO1. Students will successfully apply their learned background to a real scenario using improved data skills.
- LO2. Students will identify seafloor features and their relationships using earthquake data.
- LO3. Students will successfully determine plate tectonic settings by connecting real-world relationships to background knowledge.
- LO4. Students will be able to articulate some of the challenges that scientists face when data does not fit simple models.
- LO5. Students will successfully identify a reasonable hypothesis for the potential natural hazards in the studied area.
|Learning outcome||Activity 1||Activity 2||Activity 3|
|Outcome 1||introduced / guided practice||guided practice||guided practice|
|Outcome 2||independent practice|
|Outcome 3||guided/independent practice||independent practice|
|Outcome 4||guided practice||guided practice|
|Outcome 5||guided practice|
Materials needed: computer
What students should know before this activity
- Students should have some basic map reading skills and be familiar with using widgets. Widget use could be introduced as part of the lab exercise.
- Students should be familiar with general plate tectonic processes as well as the types of plate boundaries and the specific geologic features associated with those boundaries (i.e. mid-ocean ridge = divergent boundary, explosive volcanoes and/or deep sea trench = convergent boundary, etc.).
- Students should know the basic cause and process of earthquake formation and the relationship between tsunami formation and earthquakes.
What instructors should know before this activity
- As the instructor you should understand and be familiar with the basics of plate tectonics, how seafloor features are related to tectonic processes, and the relationship between earthquakes and plate tectonics.
- You should also understand the cause of earthquakes and how earthquakes are able to generate tsunamis.
- Related to the students, it is important to give time to the students to explore the widgets on their own before guiding them. After introducing them to the widget, let them experiment a bit.
Optional pre-lab activities:
- Prior to the start of the exercise, ask students to briefly discuss if they know about plate tectonics. Review with them the three basic types of plate boundaries (convergent, divergent, and transform) and what are the typical features associated with each type of boundary.
- Show students a global map of plate boundaries. Discuss the impacts that plate movement and interactions can have on human populations and the natural environment.
- You can introduce the relationship between plate movement and earthquake formation by demonstrating the breaking of a wooden pencil (the hexagonal kind, not the round kind). As you slowly bend the pencil, talk about how energy is built up within the pencil (and lithosphere) as force is applied. Have the students listen quietly as you slowly bend the pencil. There should be some small cracking sounds before the pencil completely breaks. Ask the students what they think is happening to cause the sounds (correct answer: bonds breaking within the pencil). Finally, bend the pencil enough to break it completely. Explain to the students this break is a “pencil quake” and that earthquakes occur in the same way, as plate tectonic forces store energy in the earth’s lithosphere until the rocks break, causing an earthquake.
- Block models can be used to demonstrate plate boundary interactions and earthquake formation.
- YouTube videos describing plate tectonics and plate boundary processes can also be shown.
Pre/post-lab assessment questions
- What is the potential for such an event occurring in the area that they have explored?
- What plate tectonic similarities and differences exist between Japan and the Pacific Northwest?
- Describe what the earthquake data indicates about the buildup of tectonic energy.
- Why do we care about collecting the type of data used in this exercise?
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, these are some things to introduce them to:
- Using Google Earth, show the students the area around Japan, including the Japanese Trench, and then remind the students of the events related to the 2011 Tohoku Earthquake and Tsunami. If not already seen, show the students footage of the tsunami (Recommended Video: https://www.youtube.com/watch?v=3618dZoiaPE).
- Explain to the students that this exercise will introduce to them the type of information that can help them to determine if a similar event has the potential to occur at other locations.
- Ask the students what they would need to know in order to determine if a similar hazard exists at another location. Have them write down these ideas. (ANSWER: Three very important things are: volcanoes, earthquakes, and a trench – be aware that the Pacific Northwest has the volcanoes and earthquakes, but not the trench due to the fact that the Juan de Fuca Plate is subducting at a very shallow angle. It is important to help the students to recognize subduction without the obvious presence of a trench – see below).
- Prior to the start of the exercise, ask students to briefly discuss with a partner, or in a small group, what they know about plate tectonics. After giving them a short time do this, lead a whole group review of the three basic types of plate boundaries (convergent, divergent, and transform) and the typical features associated with each type of boundary.
- Show students a global map of plate boundaries and have them discuss with a partner, or in a small group, what they think are the impacts that plate movement and interactions can have on human populations and the natural environment. Lead a whole group discussion afterwards, of their ideas and add information and clarification as needed.
- After showing the following video to the students, you might do the following:
- Video: https://www.newsflare.com/video/118479/other/57-magnitude-earthquake-underwater (if that page doesn’t work, here is a YouTube version: https://www.youtube.com/watch?v=V-KA_3nAi94). This is a video of the impacts of an earthquake underwater. The video was taken by Jan Paul Rodriguez in the Mabini (Batangas Province), Philippines in early April 2017. The magnitude of the earthquake was 5.7.
- Ask the students to describe their observations. What might have caused the motion of disturbance of the water? (Answer: an earthquake)
- What might have caused the earthquake? (Answer: movement along a fault and breaking of rock).
- Ask them to discuss the impacts the motion of the seafloor might have on the shape of the seafloor, marine organisms, and/or coastal areas. What predictions can you make about the impacts from an even larger earthquake?
- You can introduce the relationship between plate movement and earthquake formation by demonstrating the breaking of a wooden pencil (the hexagonal kind, not the round kind). As you slowly bend the pencil, talk about how energy is built up within the pencil (and lithosphere) as force is applied. Have the students listen quietly as you slowly bend the pencil. There should be some small cracking sounds before the pencil completely breaks. Ask the students what they think is happening to cause the sounds (correct answer: bonds breaking within the pencil). Finally, bend the pencil enough to break it completely. Explain to the students this break is a “pencil quake” and that earthquakes occur in the same way, as plate tectonic forces store energy in the earth’s lithosphere until the rocks break, causing an earthquake. This exercise can be done at any time during the exercise – wherever you think it would be most appropriate.
- 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.
- Real-world application: https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/Tsunamis.
- Compare the area of this exercise with other areas of the world using Google Earth. Ask the students if they know of or have experienced earthquakes for a given location. What was the experience like? Where was it? How was the area similar to the one in the data exploration?
- Show the students the area around Japan, including the Japanese Trench, and then remind the students of the events related to the 2011 Tohoku Earthquake and Tsunami. If not already seen, show the students footage of the tsunami (Recommended Video: https://www.youtube.com/watch?v=3618dZoiaPE).
- Ask the students, what is the potential for such an event occurring in the area that they have explored? What plate tectonic similarities exist between Japan and the Pacific Northwest? What does the earthquake data indicate about the build-up of tectonic energy (refer back to the breaking pencil demonstration)?
- Ask students whether residents in the Pacific Northwest should be worried? What additional data might be helpful to clarify the potential threat (data such as occurrence interval (link 1 and link 2) vs the length of the record from the USGS and OOI)? Why does it matter?
- PowerPoint slides with background information: none
- Handouts/worksheets of the graphs and questions: students will turn in their answers for grading.
- Screen captures (especially for online courses) for accessing online resources–to supply to the students if external resources used (e.g. Google Earth): none
- A National Geographic Society introduction to the 2011 Tohoku, Japan earthquake: https://www.nationalgeographic.org/thisday/mar11/tohoku-earthquake-and-tsunami/
- USGS resources related to the 2011 Tohoku, Japan earthquake: https://earthquake.usgs.gov/earthquakes/eventpage/official20110311054624120_30/executive
- The State of Washington’s Department of Natural Resources page on tsunami hazards: https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/Tsunamis
- The IRIS Earthquake Browser (an interactive, earthquake map of the world): http://ds.iris.edu/ieb/index.html?format=text&nodata=404&starttime=1970-01-01&endtime=2025-01-01&minmag=0&maxmag=10&mindepth=0&maxdepth=900&orderby=time-desc&src=usgs&limit=1000&maxlat=50.96&minlat=-50.96&maxlon=117.33&minlon=-117.33&zm=3&mt=ter