Lab 4 – Sea Floor Changes in a Volcanically Active Setting

Plate motions result in many active locations on the earth’s surface.  This lab will give learners the opportunity to learn about a particularly active specific seafloor setting.  Review the background on this page, answer the questions included below to help you learn about the setting, then proceed to Lab 4.1.

In 2006, researchers documented and witnessed for the first time a deep ocean volcanic eruption. Before completing this lab, view the video of the event near Samoa (Discovery Channel & NOAA, 2009).

Remarkably, it is the first time a deep sea eruption has been recorded on video, even though eruptions occur much more frequently in the ocean than on land (note the excitement in Dr. Resing’s voice!) (Rubin et al. 2012). Such events are not only common, they are important. In addition, the production of seafloor rock and associated hydrothermal vents support unique communities of organisms. Learning about ocean floor eruptions can help scientists understand processes that drive plate tectonics and the interior of the earth, provide critical data on how to protect unique ecosystems, and insights into the causes of tsunamis and land-based destructive eruptions. In this exercise, students will study data that reveal some of the changes to the seafloor that occur as a result of a volcanic eruption documented at Axial Seamount in the northeast Pacific Ocean in 2015. The information comes from several data sets, and later by observations of new lava deposits. Disclaimer: the data sets included in this exercise are “messy” with gaps, outliers, multiple patterns, and complexities. Real data can make interpretations challenging for scientists.

Prior knowledge needed: To complete the following exercises, you should have some background about earthquakes, plate tectonics and sources of magma.

Activities in this Lab

Learning outcomes

  • LO1. Demonstrate skills working with data that include: distinguishing between raw and processed data; identifying relevant data to answer questions; reading multi-axes graphs. (Activity 4.1)
  • LO2. Explain how water depth data are calculated from pressure data (Lab 4.1).
  • LO3. Describe the possible relationship between earthquakes and seafloor topography changes. (Lab 4.2, 4.3)
  • LO4. Determine what earthquake & bathymetry data tell us about processes within a volcano. (Lab 4.2, 4.3)

Background information

  • Key terms: magma, eruption, extrusion, earthquake, lithosphere, plate margin type, subduction, convergence, divergence
  • OOI Array: Cabled sea floor Axial Seamount Array
  • Sensors: cabled ocean bottom seismometers; pressure sensors

Axial Seamount is an active volcano located about 480 km (300 miles) west of the Washington/Oregon border in about 1400 m (4600 ft) of water. Scientists from multiple institutions have established instruments in multiple locations around Axial Seamount to study processes in the overlying ocean and the underlying lithosphere. The array of instruments set up on and nearby the seamount are illustrated in the following diagram.

Figure 1b. Diagram of Axial Seamount (

Figure 1a.  Diagram of the instrument locations within the Axial Seamount Array with Axial Seamount highlighted in small circle and expanded in larger circle. (


Figure 1b. Diagram of Axial Caldera (

Figure 1b. Diagram of sensors placed in and around Axial Caldera (  North arrow located in lower right corner.  Data for this lab were collected at the Central Caldera site in this diagram. See adjustable map in Lab 4.1 for more information about the topography at this site.

Quick Check: What is a “vent field”?  “Caldera”? 

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. In Figure 1, review where all the devices are located around Axial seamount. How many stations are set up, where are they relative to the caldera, and what kind of data will be collected from them?  

Among all the devices are two types that are important for this lab.

Pressure measurements (in pounds per square inch – psi) reveal details about water depth and variations in currents. It is measured electronically with sensors that can detect pressure variations resulting from change in water depth and water flow. Figure 2 shows two of these sensors on Axial Seamount.

Figure 2. Pressure sensors on the seafloor located on Axial Seamount (

Figure 2. Pressure sensors on the seafloor located on Axial Seamount (

Seismometers are used to document earthquake activity. The energy released during earthquakes takes the form of mechanical energy (the movement of rocks along a fault) and seismic waves that are transmitted through the rocks. Seismic waves behave differently as they pass through rock depending on their properties, including frequency and period. Wave frequency is a measure of the number of wave crests that pass a point in 1 sec (unit is Herz-Hz), so 10 Hz equals 10 wave crests in 1 second. Wave period is the inverse of frequency, the time it takes for one wave to pass a point, measured in seconds. Seismic waves with higher frequencies (shorter periods) get absorbed into the rocks it passes through more effectively than lower frequency (longer period) waves and thus do not travel as far as lower frequency waves. Seismometers are used to document earthquakes by detecting ground movements and seismic waves. Different types of seismometers are used to detect different frequency earthquakes.

Broadband seismometer information (Figure 3a).                              Short period seismometer information (Figure 3b).

2. 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?