| Today on R/V Kilo Moana: CTD casts and redeploying BPR moorings that were recovered yesterday. Weather is preventing use of underwater vehicles. |
There are several reasons Axial Seamount has been studied extensively. It is fairly close to U.S. ports, it erupts frequently (3 times since 1998), it has hydrothermal vents (an important place to study chemical/ocean interactions- which helps with climate models), has interesting fauna (including high temperature chemo synthesizing microbes), and helps us better understand volcanic processes on the seafloor. Most of the world’s volcanoes are erupting under the ocean.
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| Relative sizes of Axial and Kilauea.Vertical exaggeration of 10. |
Kilauea and Axial have many similarities. When you see images and video from Hawaii of the glowing lava flow forming a black crust, you are seeing an eruption similar to the ones that have been recorded at Axial. Many of the techniques used in Hawaii to understand and forecast eruptions are similar to those being used at Axial, only the instruments are very different due to the fact that Axial monitoring of Axial occurs in the deep ocean.
On this research expedition, we are focusing on studying the inflation and deflation of Axial Seamount caused by magma moving into the magma chamber below the summit caldera. On Kilauea, this is studied by placing tilt meters and GPS units at various locations around the volcano. At Axial, we have an array of instruments specially designed to operate at the bottom of the ocean, where they have to be waterproof and and be able to withstand the high pressure at depth. Our tilt meters are similar to those used in Hawaii, but they are just put in pressure cases. GPS does not work underwater, so to measure vertical movements of the seafloor we use Bottom Pressure Recorders (BPRs) that effectively measure how much ocean is above the seafloor. If the seafloor moves up, there is a little less ocean above the instrument, and so a little less pressure; or if the seafloor moves down, there is a little more. We convert the pressure to depth to see how much the seafloor is moving up or down. We use the following pressure-recording instruments, each in a slightly different way, but all together they allow us to develop a rich understanding of the magmatic workings within Axial Seamount.
1. Cabled BPRs (part of the Ocean Observatory Initiative’s cabled observatory at Axial Seamount), which are powered from the cable and measure continuously.
2. Moored BPRs, are battery powered and are dropped from the ship at specific locations and record continuously until they are released and picked up in a year or two.
3. Mini-BPRs, which spend a year or more on seafloor benchmarks and record continuously.
4. An MPR (mobile pressure recorder) is carried around to all the benchmarks by the Jason ROV to precisely determine the relative depths of all the benchmarks and tie all the measurements together.
5. The MBARI AUV, with which we precisely re-survey the bathymetry of Axial caldera and its surroundings to measure changes in depth between surveys.
We can measure the height of the seafloor to a precision of about 1 centimeter (~0.4 inches) using the pressure sensors. The AUV re-surveys can measure depth changes larger than ~20 cm (~8 inches), so they are less precise but the measurements can be made over a much larger area. Fortunately, the vertical movements of the seafloor at Axial Seamount are very large from one eruption to the next – up to 2.5 to 3.5 meters (8-11 feet!), so both of these methods work well at Axial for measuring its ups and downs. To see real-time data from the Cabled BPRs at Axial go to:
http://www.pmel.noaa.gov/eoi/rsn/
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| Ocean Bottom Pressure Recorder (BPR). |
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| Kilauea GPS and tiltmeter instrument. |
More about the science of how these work together in a later post…
