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Dive 311 into Pele's pit starts shortly after midnight. Jason locates the deep-sea elevator to retrieve instruments for work in the pit. The summit of Lo'ihi, like Mauna Loa and Kileua, has summit craters. Lo'ihi has three: Pele's Pit, the Pit of Death (perhaps more commonly known as the West Pit crater) and the East Pit crater. At 370 m below the rim, Pele's Pit is the deepest. Submarine landslides have caused the east and west sides of the summit to drop away to depth accentuating its elongate shape. A rift zone of greater volcanic activity runs the length of Lo'ihi. The south rift connects the active vents in and around Pele's Pit with the less active deep site of Ula Nui.

Lo'ihi is a hotspot volcano like all the volcanoes of the Hawaiian chain. Plate tectonics drives the motion of the oceanic plates across the ocean basins, but far below the ocean basins are small regions of the Earth's mantle that have higher heat flow and are partially molten. These regions of greater heat flow are anchored in the mantle and don't move with the crust - the crust moves over them. The hotspot can melt through the crust causing volcanic eruptions on the seafloor.

If the eruptions are large enough the volcano will grow to the surface of the ocean to form a volcanic island. As plate tectonics moves the oceanic plate, the volcanic island is carried away from the hotspot, consequently new plate is constantly sliding over the hotspot. The process is repeated forming a chain of islands. In the Pacific, plate tectonics is moving the seafloor toward the northwest on a bearing that is marked by the Hawaiian island chain. At 6,000 kilometers (3,700 miles) the Hawaiian Islands - Emperor Seamount chain is the longest chain of volcanoes on Earth.

Lo'ihi is about 35 km (20 miles) south of the big island. It forms a north-south trending ridge south east of Mauna Loa. From a bathymetric map it appears that Lo'ihi is erupting out the side of Mauna Loa, but this is an illusion. According to Garcia et al. (2005) Lo'ihi probably started erupting through the ocean crust about 400,000 years ago when Mauna Loa was much smaller. As the two volcanoes grew, their lavas flowed together, with the lava of the much larger Mauna Loa flowing around Lo'ihi. Eventually the movement of the Pacific plate will carry Mauna Loa away from the Hawaiian hotspot and its source of lava. Lo'ihi will then likely become the most vigorous volcano of the Hawaiian chain growing to rival Mauna Loa and Mauna Kea in size.

Topside, during the day scientists go in and out of the control van to make sure their samples are collected correctly and from the correct locations. Olivier Rouxel watches the titanium water bottles get filled. First temperatures must be taken to determine the best location for the sample. Then the snorkel of the bottle must be placed in a crevice where the hottest water is flowing out, and then we must wait until that hot water is seen leaving the telltale port or peephole in the bottle. Finally the Jason pilot can trigger the bottle. Brian Glazer is in the control van with a computer that collects data from the ISEA electrochemistry probe and displays it graphically. As the pilot pushes the probe into the microbial mat or the hydrothermal vent Brian watches until the readings become stable and then tells the pilot to insert the probe another 5 cm.

Both these operations demand extreme precision from the Jason pilots. The snorkels on the titanium water bottles are a curving tube about a foot long that must be placed within a half inch of the vent by a robot arm on a floating submersible at the end of a 4,000 foot cable. The ISEA probe must be located correctly and then inserted 5 cm at a time using the same robot arms. Bob Waters, one of the pilots, describes operating the manipulators like this: "Inject your arm full of novocain, then go try to write your name in a mirror."

The dive lasts 24 hours and Jason is recovered at midnight. The deep-sea elevator is recovered the morning of October 19th when daylight makes it easier to find.

Shawn Doan onboard the R/V Kilo Moana
18 October, 2007

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