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: about 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
Outcome 2 Introduced
Outcome 3 Guided practice Applied
Outcome 4 Guided practice Applied

Materials needed


Lab 4 Student Answer Form

What students should know before this activity

  • Data skills: basic graph reading abilities, including time series graphs (introduced in Lab 2.1) and bathymetric charts (introduced in Lab 2.2)
  • Content knowledge: plate tectonics, volcanic processes, sea floor features. In addition to course readings and lecture, the activities in Lab 3 help to develop some of these topics.

What instructors should know before this activity

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.

Additional scientific background information (all accessed 5/25/2021):

Teaching notes

Intro page:

A video of an underwater eruption near Samoa provides a hook and starts students thinking about magma emerging through the seafloor. This video is revisited in Activity 4.3, after students examine data series from the focus of this lab, the Axial Seamount cabled array.

Background information below the video reminds students of key terms and introduces the necessary background on the geologic setting of Axial Seamount and the cabled array. More background on the OOI array and sensors can be found in Lab 1.

Question 1 prompts students to use the background information to review where all the devices are located around Axial Seamount and the kinds of data collected from them. The bulleted lists in Figure 4.0.2b 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

The bottom of the page contains links to information about “broadband” and “short period” seismometers. This may be unnecessary detail for many introductory courses, but may be of interest to some courses or more advanced students. It will be important in the activities in this lab that students understand the relationship between pressure measured at the seafloor by sensors and the depth of the water calculated from those pressure measurements.


Activity 4.1:

The first portion of this exercise asks students to 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.  

Question 1: Encourage students to spend a good amount of time exploring the bathymetric map and write a detailed description of their observations. 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.   

Questions 2-3 ask students to practice unit conversions to put size in context of units they might be more familiar with, as well as within the metric system that is more commonly used in earth science.  They might need help setting up an equation that can be used to convert any measurement. The quick-check questions allow students to test their conversion method before answering the graded questions. And instructors might compare the size of Axial Seamount to a structure students might know, such as a tall building near you. Consider adding additional practice with conversions if time allows.

With the seafloor geology spatial context in mind, the second part of this activity explores time series pressure and depth data from the Axial Seamount central caldera. Students can zoom and pan using the controls at the bottom of the graph. Quick-check questions below the time series point out that this is a dual y-axis graph.

Question 4 investigates the relationship between seafloor pressure and water depth. This is likely to be difficult for some students…help them think through 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].
  • An example calculation that students can use as a guide is provided under the “Click for a hint” button. Instruct students to select a different date and time to demonstrate their understanding of the calculation.

Questions 5-6 ask students to speculate on the cyclical trends in the pressure data, which instructors may choose to cover as a class discussion. The “raw” data shows an unrelated phenomenon that turns out to be an excellent example of mixed semidiurnal tides but because tides typically come sometime after geology topics in a typical oceanography class, this step and question is meant merely to observe the “raw” data and review some interesting information that they are not expected to explain at this point. A key point (also covered in Lab 2.1) is that the depth time series was averaged to remove the tide cycles and highlight the changes in seafloor depth that scientists were most interested in: A drastic change during a single event in April.

Questions 7-10 refer to the depth data only. Students should deselect measured water pressure on the widget. The coding for this interactive widget has a bug that sometimes causes the y-axis to flip when certain zooming actions are implemented. To reset the y-axis, click the “All Data” button.

  • These questions are meant to help guide students to develop their data patterns description skills; the authors have noticed that it is a skill needing some guidance for most students particularly in an intro class.  Without extensive experience, students may not understand that thorough data descriptions are needed first in order to draw conclusions, and some descriptions are then needed as evidence to support conclusions in a complete explanation.
  • The last question of this activity is a step that asks students to refine some of their ideas without formalizing an answer yet. These concepts will be further developed in the next activity. If possible, discuss with students to reveal some of their preliminary thinking and allow them to consider other students’ ideas.

Activity 4.2

The primary goals of this activity are to guide students through connecting the event apparent in the seafloor depth time series to volcanic activity (subsurface magma movement and earthquakes). The provided background material can be presented in a lecture or required reading.

  • As in the previous activity, encourage students through careful observation to note both general and very specific patterns in the three displayed datasets.
  • If possible, discuss to collect students’ ideas as they progress through the activity. Encourage their use of relevant science concepts previously addressed and don’t underestimate students’ ability to be creative and propose some of a correct response. This can be accomplished in the classroom or on an asynchronous discussion board. Such discussion will be especially valuable between Questions 3 and 4.
  • Question 3 looks back to earlier eruption events in 1998 and 2011, in addition to the 2015 event students have already examined.
    • The data in this activity was provided by William Chadwick at NOAA PMEL, and was adapted from Figure 2 in: Nooner, S. L., and W. W. Chadwick, Jr. (2016), Inflation-predictable behavior and co-diking-eruption event deformation at Axial Seamount, Science, 354(6318), 1399-1403, doi:10.1126/science.aah4666.
    • Bring up the idea of volcanic eruption prediction. The high density of instruments and long history of monitoring this seamount is greatly advancing the science of underwater volcanism. Students may be interested to think about how this compares to land-based volcano hazards.
    • NOAA PMEL has an Axial Seamount eruption forecast blog that discusses ongoing data collection and what it means for the probable timing of the next eruption.
  • The last question (Q4) is the formal step where students can be evaluated on their ability to support a conclusion with a thorough explanation that includes appropriate and sufficient evidence from their data descriptions and their understanding of important relevant science concepts.

Extension Exercise for 4.2 (revisit after studying tides!)

  1. 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.
    • The earthquake data are very noisy but patterns are present. If this step is done interactively with students, they likely would benefit from some guidance like prompts included in Question 1 of Activity 4.2.
  1. What do you think might explain the patterns you’ve noted?  What background from this lab might be useful in trying to explain them?  Take a look at Wilcock et al. (2016) for their ideas about what might be happening. 
    • 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 4.3

This activity revisits the underwater eruption video from Samoa and provides students with a chance to apply their observation skills and reinforce the conceptual model of subsurface magma chambers and eruptions from Activity 4.2.

  • Help students to be as thorough as possible in recording details for activities they can observe through the entire video.
  • Encourage students to hypothesize and expand on what could be occurring below the seafloor based on their answers in Activities 4.1 and 4.2.
  • Discuss ideas with students and add details to fill in the gaps before students individually write their own complete explanation.
  • The final question (#4) asks students to generate additional questions. Guide students to consider questions that would extend their learning beyond the activities in Lab 4.  Emphasize that developing research questions is a key part of the scientific method and allow for creativity. Depending on time available, students’ questions could be discussed particularly to clear up misunderstandings or gaps in understandings.  If time allows, students could attempt to research answers, or such work could be assigned as homework, etc.

Additional Resources

Related OOI interactive data widgets used for different activities (some might be useful to send students for additional visualization of the 2015 eruption event):

For side scan sonar images to consider:

For open source introductory texts:


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.