Lab 4.2 – Don’t volcanoes erupt UP?

Fundamental concept: Describe the possible relationship between earthquakes and seafloor topography changes and determine what these data tell us about processes within a volcano
Estimated time to complete: 45 minutes
Data skills preparation: Lab 2.1 – Time series, Lab 2.2 – Bathymetric charts
Materials needed: none

Axial Seamount is the most active seamount in the Pacific Ocean.  It was discovered in the 1970s and eruptions were detected in 1998 (the first time an underwater volcanic eruption was detected), 2011, and 2015 via a variety of monitoring projects.  The 2015 eruption was detected by the Axial Seamount OOI Array and some of the data is included in this exercise. The April 2015 eruption occurred both within the caldera and in a rift zone that extends across the north flank of Axial Seamount; the extent of the lava flows are shown in the following image (Figure 4.2.1).

seafloor bathymetry image showing placement of lava by volcanic eruptsion

Figure 4.2.1. A NOAA/PMEL image created in part from a research cruise conducted in August of 2015 after the April 2015 eruption was detected from data included in this lab. Note that the 2015 eruption occurred in both the caldera and in a fissure/rift zone on the northside of the caldera. The lava flows did not occur in the exact same locations as the 2011 and 1998 lava flows. From

Earthquake activity often increases leading up to a volcanic eruption everywhere they occur as pressure within the underlying magma chambers builds when magma accumulates filling the chamber and applying pressure to the rocks about it (Step 1 in Figure 4.2.2).  During the eruption, the chamber empties to some degree as magma is extruded into the surface of a shield volcano into the caldera or at sites along rift zones extending away from the caldera on the flanks of a volcano (Step 2).  Addition of lava could raise the depth of the seafloor.  After an eruption, the caldera floor might collapse to depths lower than the it was before the eruption if the chamber beneath it empties enough (Step 3).  When this occurs, there should be evidence in both data and seafloor observations.

Figure 4.2.2. Schematic of basic changes to a magma chamber during and after an eruption.

The datasets in this activity include the calculated water depth studied in activity 4.1, as well as timing, depth and magnitude of earthquake occurrences for several months in 2015.  The goal of this exercise is to use the data to draw conclusions about magma movement under the seafloor.  To do so, identifying and describing patterns in the data is an important step for scientists to explain a phenomenon.  This process requires articulation of general and very specific details of patterns discovered.  And it requires the use of specific quantitative information from the data.

Examine the Earthquake data included in graphs 2 and 3 below, keeping all data viewable.

Earthquake occurrences were recorded from data about earthquake magnitude and earthquake depth. Note that Earthquake depth in the second figure plots depth down into the lithosphere on the y-axis; you might be more familiar with greater values being plotted above the x-axis.  This approach is done to help readers visualize the location of earthquakes at different depths beneath the earth’s surface.

Interpretation Questions:

  1. Describe the patterns in each and compare patterns in both graphs and to the water depth plot.

If you are unsure what to consider to answer Question 1, follow these prompts:

  • Generally, when do earthquakes occur more and less?
  • Are there any patterns as to when deeper or shallower earthquakes occur?
  • Are there any patterns to when lower or higher magnitude earthquakes occur?
  • More specifically, how have earthquake depths and magnitudes varied over time? What is the range in depths and magnitude? Does the range change over time?

Keep in mind that because identifying patterns in data is an important step for scientists to explain a phenomenon, recognizing when patterns occur AND when they don’t occur is important in the scientific process. So, if you don’t see a pattern for any of the prompts, it could be that there isn’t one! But be sure to look carefully before you conclude that no pattern is present.


  1. Considering the combination of seafloor depth and earthquake data, what are some possible causes for events that occurred on or about April 24, 2015?  Consider changes in seafloor depth as well as the difference between the number of earthquakes before and after the April 24 event?  Think about what might be occurring beneath the surface of the volcano.  Record your ideas and revisit them for next steps.

Application Questions:

Before you draw final conclusions about the cause of the April 24, 2015 event, view the following graph (4.2.3) that shows a combination of data collected at Axial Seamount since 1998 from numerous studies.  Note that the 2015 event you’ve studied is included in the graph along with similar changes to the seafloor at two other times since 1998.

changes in seafloor depth in the Axial Seamount central caldera showing three events in which the depth dropped downward by 2-3 meters

Figure 4.2.3. Changes in depth of the Axial Seamount central caldera between October 1997 and December 2016. The y-axis displays offset depth in meters compared to the deepest observed depth (0 meters offset depth). For example, an offset depth of 2.0 meters means that the seafloor rose up 2 meters, or was 2 meters shallower than the deepest depth. Adapted from Figure 2 in Nooner & Chadwick (2016).

  1. What happened to seafloor depth prior to the April 2015 event in this longer time scale graph? (Describe patterns in the data over time)

Discuss with your classmates and instructor in class or in a discussion board how the movement of magma in the subsurface within the volcano might influence the behavior of the earthquakes before, during, and after the 2015 event? Also, how might the movement of the magma influence the depth of the seafloor?  Is the 2015 change in seafloor depth simply a result of caldera collapse?  If you are struggling still to picture what takes place beneath the surface as magma accumulates in a magma chamber before an eruption, and after it is extruded, watch this video or changes to Mount Etna over time; or this series of images about what occurs in a magma chamber and caldera formation; or this site about the August 2015 research cruise to Axial Seamount and how research goals were met.

  1. After discussing, write your own explanation for the trends and patterns in the sea floor depth data for the April 24, 2015 event.  Be sure to include quantitative evidence from the data set as well as important science concepts introduced in this lab and/or your course material.