Lōihi Volcano

Lōihi Seamount is an active volcano built on the seafloor south of Kilauea about 30 km from shore. The seamount rises to 969 m below sea level and is marked by a caldera-like depression 2.8 km wide and 3.7 km long. Three collapse pits or craters occupy the southern part of the caldera; the most recent pit formed during an intense earthquake swarm in July-August 1996. Named Pele's Pit, the new crater is about 600 m in diameter and its bottom is 300 m below the previous surface! Like the volcanoes on the Island of Hawaii, Lōihi has grown from eruptions along its 31-km-long rift zone that extends northwest and southeast of the caldera. During July and early August, 1996, the largest swarm of earthquakes ever observed at any Hawaiian volcano occurred at Lōihi Seamount. In response to this event, an initial cruise was dispatched to Lōihi in early August, and two previously planned cruises sailed in September and October on the R/V Kaimikai-O-Kanaloa (K-O-K). Calm weather and a newly refurbished ship combined to provide excellent conditions for documenting the volcanic, plume, vent, and biological activity associated with this swarm. These cruises conducted a total of 15 PISCES V submersible dives, 41 water sampling operations, and 455 km of SeaBeam surveys, and deployed 40 sonobuoys and one ocean bottom seismometer (OBS). The most obvious result of the activity was the formation of a large summit pit crater similar to those observed at Kilauea. Greatly expanded hydrothermal activity was also observed resulting in the formation of intense hydrothermal plumes in the ocean surrounding the summit. SeaBeam surveys documented the bathymetric changes at Lōihi summit corresponding to the seismic swarm. Pele's Vents, previously the prime locus of hydrothermal activity at a depth of 980m, has collapsed forming a pit crater (Pele's Pit) approximately 600 m in diameter with its bottom 300 m below the previous surface. In 1996, manned submersible exploration with the Pisces V revealed high-temperature hydrothermal fluids issuing from vents in Pele's Pit but it was too hazardous to get close enough for accurate temperature recording. Return dives in 1997 documented water temperatures up to 200°C and in 1998 the hydrothermal fluids had cooled to about 160°C. In the years since then, dives have been conducted on an ongoing basis to monitor activity of the volcano. Nearly 170 submersible dives to the volcano have been conducted by the Hawaii Undersea Research Laboratory since 1987.