On the surface, 300 miles offshore of Oregon, the R/V Atlantis is surrounded on all sides by deep blue water and rolling swells, with no land in sight. There is no visual cue from aboard the ship to indicate that this location is any different than Slope Base or Hydrate Ridge. But nearly a mile below us (1.5 kilometers), a massive seamount rises up from the ocean floor, with a crater at the top full of basalt formations (evidence of former lava flows), towering hydrothermal vents covered in chemosynthetic animals, and shimmering plumes of superheated water streaming out of the seafloor. This active underwater volcano, which erupted in 1998, 2011, and again in April 2015, is one of the main instrumented sites of the Regional Cabled Array, making Axial the most advanced volcanic observatory in the ocean.
Following the 17 hour transit from Slope Base to Axial Base (during which people prepared equipment, caught up on work, did laundry, and drew on Styrofoam cups to shrink – a deep-sea tradition), we dove at Axial Base to replace the third (and last) Deep Profiler (see previous blog). We then headed 2 hours northwest to get to the southeastern side of the caldera (volcanic crater), at a site called “International District,” so named (most likely) because of how the vents were identified by different groups of scientists from NOAA and the UW over the years: Escargot (originally shaped like a snail, until the head fell off), El Guapo, Hermosa, El Antiguo, El Abuelo, etc.
The International District is an area of rugged topography full of all different types of lava flows (lobate, pillow, sheet, hackly) depending on how quickly the lava was ejected during the eruption. It also hosts many different shapes and sizes of vent chimneys, both active and extinct (which happens as new fluid channels open and old ones are sealed by tectonic activity or mineral deposits). Chimneys form as bottom water is pulled into deep channels in the seafloor, absorbs heat and minerals from the liquid rock (magma) far below, and then comes rushing back out in superheated plumes (reaching over 600 degrees Fahrenheit) that deposit the dissolved minerals as they hit the cold ambient seawater, forming hollow tubes. The structures can eventually build to a massive size, like the tallest chimney at Axial which is over 50 feet in height! These chimneys (and other spots where hydrothermal water escapes more diffusely) host dense patches of blue colonial ciliate mats, tube and palm polychaete worms, limpets, squat lobsters, and abundant sea spiders (Pycnogonida).
The International District is the largest hydrothermal field in Axial Caldera, and located near other interesting geological features, so it was chosen as the site for some of the most technologically advanced instrumentation deployed by OOI. These include a mass spectrometer to measure the volatile chemistry of diffuse fluids, adaptive diffuse fluid (major and trace element chemistry) and microbial DNA samplers, and two instruments to measure high-temperature vent fluid and volatile chemistry (a temperature-resistivity sensor in Escargot and a temperature-pH-H2S instrument in a vent called Diva). A digital still camera provides imaging at Tiny Village (a site with many small chimneys), and a seismometer, bottom-pressure tilt instrument, and current meter are located nearby.
We will be spending the next few days deploying and recovering equipment at many of the sites in Axial caldera, so stay tuned for video of areas that few people have ever seen, and dynamic and colorful geological formations and animals!