Lab 4 – Sea Floor Changes in a Volcanically Active Setting
Instructor Guide

This lab is meant to give students an opportunity to apply and extend their introductory knowledge about plate tectonics to make sense of what goes on inside a magma chamber before, during, and after an eruption. 

Approximate time involved: approximately two hours total.

Learning outcomes

  • LO1. Students will develop skills working with data that include: distinguishing between raw and processed data; identifying relevant data to answer questions; reading multi-axes graphs.
  • LO2. Students will understand how depth data are calculated from pressure data.
  • LO3. Students will describe the possible relationship between earthquakes and seafloor topography changes.
  • LO4. Students will determine what earthquake & bathymetry data tell us about processes within the volcano.
Learning outcome Activity 1 Activity 2 Activity 3
Outcome 1 introduced guided practice guided practice
Outcome 2 introduced
Outcome 3 guided practice independent practice
Outcome 4 guided practice guided practice

Materials needed: none

What students should know before this activity

  • Data knowledge: basic graph reading abilities
  • Content knowledge: plate tectonics, volcanic processes, sea floor features

What instructors should know before this activity

Optional pre-lab activities:

Prior to this lab, it would be worthwhile to introduce background about why volcanoes erupt, what happens during an eruption, and how calderas form.  It is recommended that students are guided to understand the basics about processes at plate boundaries through some sort of engaging exercise in addition to any lecture provided on this topic.

Pre/post-lab assessment questions – none

Teaching notes

Intro page:

  1. Review where all the devices are located around Axial Seamount. What kind of data will be collected from them?
    • The bulleted lists in Figure 1 provide quite a bit of information for students to answer this question. The purpose of some instruments might not be obvious to students but the point of this question is for students to get a feel for how intensively studied this seamount is. There are over 20 instruments at the site and is considered the “most advanced volcanic observatory in the worlds’ oceans” ( Note that the North arrow in Fig. 1b is easy to miss and located in the lower right corner.  Also, the colorations for this figure were enhanced a bit to highlight the features.  Use the map in Lab 4.1 for a more accurate representation of the topography of the seafloor.  The location of lava fields in the caldera can be seen in the map at  .
  2. Define the key terms vent field and caldera. Students can click on the link to view definitions:
    • vent field – Concentrated occurrences of fissures in the seafloor that emit heated mineral-rich fluids and sometimes form black and white smokers where, commonly occurring in volcanically active areas on the seafloor.
    • caldera – Depression in the center of an active volcano.
  1. Review the information about “broadband” and “short period” seismometers from links supplied beneath the photos and note the differences between the two. Why do you think scientists need each?
    • The intent with the background provided to students in this exercise is to point out that one seismometer type would detect waves that originated from longer distances and the other only for local earthquakes.  The technical details are not something they would be expected to make sense of unless a student happens to have additional relevant background.  While discussing with students, provide some additional details that show the complexities of the instrumentation and science in detecting earthquakes just as an FYI.

Activity 1:

Explore the location where data in this exercise were collected.  Zoom in and out of the map to develop familiarity with the highlighted feature, as well as nearby features.  

  • Students should be able to identify the seamounts in the Cobb-Eickelberg Seamount Chain and note that the shape of Axial Seamount is a bit different from others in the area: larger, less symmetrically shaped; larger caldera.   

Axial Seamount and is one of several in a chain of seamounts of the Cobb-Eickelberg Seamount Chain that extends from Axial to the northwest. The top of Axial sits at a depth of approximately 1400 meters and its base sits at 2500 meters depth making the seamount about 1100 meters tall

  1. What are these measurements converted to feet? miles? What equation can you set up to convert any measurement of a dimension from km to miles and the reverse?
    • Students can practice unit conversions to put size in context of unit they might be more familiar with.  They might need help setting up an equation that can be used to convert any measurement.  And instructors might compare the size of Axial Seamount to a structure students might be familiar.
  2. From your zoom in and out, describe how Axial Seamount compares to other seamounts in the chain with regards to shape, size (using the ruler provided), , depth, height, ruggedness, etc.
    • Students put into words differences and similarities they notice, as well as location on sea floor relative to other feature they might know about (e.g. Juan de Fuca Ridge).
  3. What are the y and x axes and what does the graph show? What are the units for the axes?  
  4. Explain why you think pressure is graphed with depth.
    • Gather students ideas for consideration in next steps.
  5. Pressure is what is normally measured and depth calculated from it…what calculation can you run that would enable you to convert pressure to depth?
    • This is likely to be difficult for some students…help them think thru that the ratio of between depth and pressure is important and can be calculated from a point on the graph, then used as the means to convert any pressure reading.
    • The Bigger picture: The volume of water at each depth in the water column exerts pressure downwards; the deeper the water column, the greater the pressure. Pressure increases about 15 psi (pound-force per square inch) for every 10 meters (33 feet) of depth. Thus a pressure of 2250 psi equates to 1500 meters of depth [2250 psi ÷ (15 psi/10m) = 2250 psi ÷ 1.5 psi/m = 1500 m depth].
  6. Look at only pressure in the graph. What do you think might cause the variations in pressure you notice in the plot?
    • Consider students’ ideas and then provide additional details to fill in gaps.
  1. What challenges might they present for oceanographers when they want to study average water depth at this site
    • When multiple processes occur at the same time, their impact on a particular feature (pressure in this case) can cause the data to be complicated and “messy”.  Scientists need to process messy data somehow when possible.
  2. Describe the patterns in the depth data. Be sure you zoom in and out of the data to notice small and large changes. Include details for range of changes.
    Apply your learning in previous steps to propose what do you think may have caused the changes you described?

    • Discuss with students to help them articulate both general descriptions and some specifics.
  3. What do you think may have caused the changes you described?
    • Discuss with students to reveal some of their preliminary thinking.
  1. Adjust the widget to zoom into the day before and after the major change in depth.  How much depth change occurred and how long did it take for the change to take place?
  2. Describe the pattern(s) you see in the depth plot; be as thorough as you can and consider what the graphed line reveals occurred over time.
    • This is an exercise in being really thorough in describing trends and patterns.  Help students include use of specific data.

Activity 2

  1. Describe the patterns in each and compare patterns in both graphs and to the water depth plot.
    • This is a next step exercise to thoroughly describe the data. Students should describe trends and patterns in each individual set and across data sets. They may start by noting general patterns; encourage them to include both general and very specific patterns.
  2. Considering the combination of seafloor depth and earthquake data, what do you think happened at Axial Seamount on April 24th, 2015?
    • Discuss to collect students’ ideas
  3. What do you think might explain the difference between the number of earthquakes before and after the eruption?
    • Discuss to collect students’ ideas; encourage their use of relevant science concepts previously addressed.  Don’t underestimate their ability to be creative and propose some of a correct response (addressed in next question).
  4. How might the movement of the magma in the subsurface within the volcano influence the behavior of the earthquakes before, during, and after the eruption? How might the movement of the magma influence the bathymetry?
    • Ask students to propose an explanation, then with their ideas, discuss to help them understand an appropriate explanation. 
  5. After discussing in class, write your own explanation for the trends and patterns in the data.
    • Students should refer to sufficient and appropriate patterns in the data as well as their understanding of relevant specific science content provided by the instructor regarding earthquake processes, magma extrusion, and volcano/caldera formation. In assessing students’ work and providing feedback, address both these aspects of a complete scientific explanation as well as the overall coherency of the explanation.
  6. Adjust the widget to show pressure data for the time span that includes only March 15 through March 22. Describe the pattern(s) you see in each of the graphs.
    • Help students provide as detailed a set of descriptions as possible.
  7. What do you think might explain the patterns you’ve noted?  What background from this lab might be useful in trying to explain them?
    • This is a bit tricky to answer and in fact scientists are trying to figure this out!  However, if students consider ALL possible data sets provided in Lab 4, they may propose what scientists have hypothesized!

Activity 3

  1. Re-view the underwater eruption video. Record the volcanic activity you observe.
    • Help students to be as thorough as possible in recording details for activities they can observe through the entire video.
  2. Hypothesize, with explanations, what you think could be occurring below the seafloor based on what you learned from activities 1 and 2.
    • Discuss ideas with students and add details to fill in the gaps.  Students will individually write their own complete explanation next.

Assessment questions

  1. Following discussions, compose your own explanation for your hypothesis that includes evidence from Activities 1 & 2 and key scientific ideas.
    • Students should individually compose a complete explanation to be assessed for the presence of use of evidence and their understanding of relevant science concepts.
  2. What questions do you still have about what drives changes in the seismicity and/or bathymetry over time at Axial Seamount?
    • Depending on time available, students’ questions could be discussed particularly to clear up misunderstandings or gaps in understandings.  Guide students to consider questions that would extend their learning beyond Lab 4.  If time allows, students could attempt to research answers, or such work could be assigned as homework, etc.

Additional Resources

For side scan sonar images to consider:


Arnule et al., Axial 3D Seismic Expedition 2019,

Chadwell, W.W., Butterfield, D.A., Embley, R.W., Tunnicliffe, V., Huber, J.A., Nooner, S.L., Clague, D.A., Spotlight 1: Axial Seamount (PDF). Oceanography. 23 (1): 38-39. Retrieved 14 August 2020.

Chadwick, J., Keller, R., Kamenov, G., Yogodzinski, G., Lupton, J., 2014, The Cobb hot spot: HIMU‐DMM mixing and melting controlled by a progressively thinning lithospheric lid, Geochemistry, Geophysics, Geosystems 15 (8): 3107-3122.

Discovery News, 2009, Undersea Volcano Eruptions Caught On Video,

Gudmundsson, A., 2013, Magma-chamber geometry, fluid transport, local stresses, and rock behaviour during collapse-caldera formation.

Leifert, H, 2016, An undersea volcano yields its secrets, Earth,

MBARI, Submarine Volcanoes Group

Mittelstaedt, E., Fornari, D.J., Crone, T.J., Kinsey, J., Kelley, D., Elend, M., 2016, Diffuse venting at the ASHES hydrothermal field: Heat flux and tidally modulated flow variability derived from in situ time‐series measurements, Geochemistry, Geophysics, Geosystems 17(4): 1435-1453. [For additional consideration of tidally influenced EQ activity]

NOAA Axial Seamount information:

NOAA, 2009, Deep Ocean Volcanoes,

NSF, Underwater volcano’s fiery eruption captured in detail by seafloor observatory, News Release 16-152.

OOI staff. 2015. Axial Eruption Site Identified – Bathymetric Survey Data Available Online.

OOI staff. 2015. OOI Team First to See April 24, 2015 Eruption of Axial Seamount.

OOI staff, 2020. From Whale Songs to Volcanic Eruptions: OOI’s Cable Hears the Sounds of the Ocean.

Rubin, K.H., S.A. Soule, W.W. Chadwick Jr., D.J. Fornari, D.A. Clague, R.W. Embley, E.T. Baker, M.R. Perfit, D.W. Caress, and R.P. Dziak. 2012. Volcanic eruptions in the deep sea. Oceanography 25(1):142–157,

Smith, L., 2018, Community Tools available on the OOI website to examine the Axial Volcano;

Smithsonian Institution, Global Volcanism Program: Axial Seamount,

Wilcock, W.S.D, Tolstoy, M., Waldhauser, F., Garcia, C., Tan, Y.J., Bohnenstiehl, D.R., Caplan-Auerbach, J., Dziak, R.P., Arnulf, A.F. and Mann, M.E., 2016, Seismic constraints on caldera dynamics from the 2015 Axial Seamount eruption, Science 354 (6318): 1395-1399.  [doi: 10.1126/science.aah5563]

Wilcock, W.S.D, Dziak, R.P., Tolstoy, M., Chadwick Jr., W.W., Nooner, S.L, Bohnenstiehl, D.R., Caplan-Auerbach, J., Waldhauser, F., Arnulf, A.F., Baillard, C., Lau, T., J.H., Tan, Y.J., Garcia, C., Levy, S., and Mann, M.E., 2018, The recent volcanic history of Axial Seamount, Geophysical Insights into Past Eruption Dynamics with an Eye Toward Enhanced Observations of Future Eruptions, Oceanography Vol.31(1): 114-123.