Lab 4 – Sea Floor Changes in a Volcanically Active Setting
Lithospheric 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. Eruptions at any volcano occur after magma accumulates in a magma chamber beneath the lithosphere surface. It is ejected or extruded onto the earth’s surface through a central caldera, vents , or fissures (“rift zones”) once the pressure of accumulating magma and gasses beneath the surface cannot be contained by overlying rock.
Before completing this lab, view the video of the event near Samoa (Discovery Channel & NOAA, 2009). The eruption occurred at a “hot spot” volcano, features you likely learned about in your oceanography class. Volcanic eruptions occur when the buildup of accumulating magma and gasses beneath the surface cause enough pressure for overlying rocks to fail, fracture, or explode outward.
While eruptions on land have been observed for many years, remarkably, the 2009 video above shows the first time a deep sea eruption has been recorded, 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. 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 volcanic eruptions. In addition, the production of seafloor rock and associated hydrothermal vents support unique communities of organisms. 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 by OOI scientists. 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 seafloor features, earthquakes, plate tectonics, volcano formation and sources of magma. For some Introductory Geology text resources: Physical Geology by S. Earle (2019); An Introduction to Geology by C. Johnson et. al. (2017).
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 (4.1)
- LO2. Explain how water depth data are calculated from pressure data (4.1)
- LO3. Describe the possible relationship between earthquakes and seafloor topography changes. (4.2, 4.3)
- LO4. Determine what earthquake & bathymetry data tell us about processes within a volcano (4.2, 4.3)
Background information
- Key terms: magma, eruption, extrusion, shield volcano, earthquake, lithosphere, plate boundary type, divergence
- OOI Array: Cabled sea floor Axial Seamount Array
- Sensors: cabled ocean bottom seismometers; pressure sensors
Axial Seamount is an active shield volcano located about 480 km (300 miles) west of the Washington/Oregon border in about 1400 m (4600 ft) of water. Shield volcanoes are built from mostly low viscosity lava flows that produce low sloping sides and occur in the ocean away from continental crust. Some are submerged like Axial Seamount but some are the largest volcanoes on earth and thus form islands above the sea surface, like Mauna Loa in the Hawaiian Islands. Shield volcanoes grow from layers of lava accumulated on the sides (“flanks”) of the volcano from both the central crater (“caldera”) at the summit, and along fissures or fractures (“rift zones”) that extend away from the calderas (Figure 4.0.1).
Figure 4.0.1. Magma chamber and conduits through which magma is released to the surface in the main crater and flank rift zones.
Scientists from multiple institutions in the OOI research program have established instruments working in numerous locations around Axial Seamount (See Figures 4.0.2a & b) to study processes in the overlying ocean and the underlying lithosphere. Axial Seamount occurs where a hot spot and divergent plate boundary coincidentally occur so it is a more complicated site than the eruption near Samoa shown in the introductory video. However, magma at both types of setting is very similar so the eruption processes are also similar.
Watch this video to learn more about deployment of some of the instruments, the goals of the projects and scientists involved.
The array of instruments set up on the seamount used to collect the data in this Lab are illustrated in the following diagrams
Figure 4.0.2a. Diagram of the instrument locations within the Axial Seamount Array with Axial Seamount highlighted in small circle and expanded in larger circle. From https://oceanobservatories.org/array/cabled-axial-seamount-array/
Figure 4.0.2b. Diagram of sensors placed in and around Axial Caldera (https://interactiveoceans.washington.edu/research-sites/axial-caldera/). 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”?
- In Figure 4.0.2, 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.
Water 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 variations in pressure of the overlying water column resulting from change in water depth and water flow. Figure 3 shows two of these sensors on Axial Seamount.
Figure 4.0.3. Pressure sensors on the seafloor located on Axial Seamount (https://interactiveoceans.washington.edu/instruments/pressure-at-the-seafloor/)
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. 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 4.0.4a). Short period seismometer information (Figure 4.0.4b).
Activities in this Lab
- Lab 4.1 – How do they measure water depth?
- Lab 4.2 – Don’t volcanoes erupt up?
- Lab 4.3 – Applying your knowledge to an actual eruption
