Cover image: Scientists aboard the Okeanos Explorer plan the remote operated vehicle (ROV)’s course to explore the emerging lava core inside Vailulu’u Seamount. This image shows a topographic map of the summit of Vailulu’u created using multibeam sonar imaging, with the ROV’s path charted along the red line. Image courtesy of NOAA Office of Exploration and Research.
The NOAA Ship Okeanos Explorer has kicked off its 2017 field season so far with amazing dives in the waters off American Samoa, a US territory in the southern Pacific Ocean. In February 2017, the expedition team explored the Vailulu’u Seamount, an underwater volcano located east of the Samoan Island of Ta’u. This offered scientists a rare and exciting opportunity to observe the geological and ecological characteristics of an active underwater volcano.
Seamounts, like Vailulu’u, are undersea mountains formed by volcanic activity. As lava from the Earth’s interior erupts through the seafloor, it meets cold ocean water and hardens. As more lava erupts, hardens, and piles up, a seamount is formed. If the eruptions continue for long enough, a seamount can actually rise tall enough to break the water surface, which is how islands are formed. Once seen as little more than hazards to undersea navigation, oceanographers have discovered that seamounts are hotspots of biological diversity. Underwater exploration has shown many seamounts to support a vast array of marine species and emphasized their importance as vital marine habitats.
An opportunity to explore Vailulu’u Seamount is particularly intriguing because of its status as the most active submarine volcano in the waters of American Samoa. Vailulu’u is thought to have erupted sometime between 2001 and 2005. This eruption formed a new 239 meter (nearly 1000 feet) tall lava cone inside the volcano’s crater. The lava cone, dubbed Nafanua after the fierce Samoan goddess of war, was discovered during an expedition in 2005 by scientists aboard a University of Hawaii research vessel using NOAA’s Pisces V manned submersible vehicle.
The discovery of the lava core was an important finding, as the chance to study an actively-forming underwater lava core is uncommon. Woods Hole Oceanographic Institution’s (WHOI) Senior Scientist, Stan Hart, said, “To actually have a documented case of an underwater volcano that has been constructed within a known period of time is very rare—this is one of those cases.” The area of Vailulu’u Seamount had originally been mapped using multibeam sonar imaging in 1991. From this baseline data, scientists estimated that the growth rate of the new lava core could have reached an average rate of 8 inches per day.
One particularly striking discovery of the 2005 University of Hawaii expedition occurred as the team explored the volcano’s hydrothermal vent system. Hydrothermal vents form at tectonically active areas under the ocean, where the seafloor is spreading or where tectonic plates are coming together. The seawater escaping from hydrothermal vents can reach temperatures of over 700° F. This superheated water is saturated in chemicals that fuel unique biological communities, many of which are found only in these very specific ecosystems. As the expedition team exploring Nafanua approached the lava cone’s hydrothermal vent area, they began to see large numbers of eels lurking among the surrounding rock pillars. When the submersible landed, scientists were surprised to see huge numbers of foot-long eels emerging from the rock caves and crevices surrounding the area – a dramatic illustration of the sort of unique communities that can found around vent systems. The memorable experience led scientists to dub this area “Eel City.”
During the February 2017 dive on Vailulu’u Seamount, the Okeanos Explorer collected further data on this active volcano in the Samoan region. The information collected during this dive will provide scientists with a critical view of the geochemistry of the early stages of a young volcano. In addition, comparing this dive to the ones in previous years will allow scientists to study the changes in biological communities that occur as an active volcano alters its environment.
Investigating the Vailulu’u Seamount has been just one highlight of the 2017 Okeanos Explorer field season thus far. With many more ROV dives planned, there are doubtless many more intriguing discoveries to be made. Stay tuned to see what else the Okeanos Explorer finds as it continues to explore the unknown waters of the Pacific Ocean.