School of Ocean and Earth Science and Technology

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Loihi after the July-August event

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Loihi is an active submarine volcano that may become the next Hawaiian Island. In July 1996, Loihi became the site of a major seismic event that occurred over a two week period. This was recorded on the Hawaii Volcano Observatory's seismometer grid. The effects of oceanic volcanic eruptions and collapses may be manifested in increased carbon dioxide emissions in the atmosphere, generation of tsunamis by landslides, and destruction of biota living there. A quick response cruise funded by the National Science Foundation and led by Frederick Duennebier (P.I.) and Francis Sansone (co-P.I.) responded to this event and laid the foundation for the cruises that followed.

The quick response dives (Figure 1) were followed by extensive NOAA-funded work in September and October. Principal Investigators included Gary McMurtry and Francis Sansone, Alexander Malahoff, and James Cowen. These investigators found that the southern part of Loihi's summit, shaken by swarms of seafloor earthquakes and the withdrawal of magma, had collapsed. A crater one kilometer across and three hundred meters deep had formed, perhaps involving the downward movement of 100 million cubic meters of volcanic material. A four to five square mile area of the summit was altered, strewn with bus-size volcanic rocks, some precariously perched along the rim of the crater. "Pele's Vents," an area on the southern rim of the volcano, previously considered very stable, had disappeared into a giant pit, now named "Pele's Pit" (Figure 2). Seawater is flowing down into the newly formed pit on the northern end of the volcano where it percolates through the volcanic structure, mixes with minerals and bacterial matter, then flows out over a lip on Loihi's western edge. As a result, unpredictable bottom currents make submersible diving hazardous in this area.

The most hydrothermally active area of the volcano is presently located along the southern rim and rift. Dives on the hydrothermally less active northern rim showed relatively stable terrain as compared to that observed earlier, and high lava columns were still standing upright. A new SeaBeam bathymetric map of Loihi Figure 3) was generated aboard Ka'imikai-o-Kanaloa and verified during Pisces V dives, precisely documenting the morphological changes at the summit.

During the dives, visibility was reduced due to high concentrations of dissolved minerals and large floating mats of chemosynthetic bacteria in the water. The bacteria, which feed on dissolved nutrients, have already begun colonizing the new hydrothermal vents and may be indicators of the kinds of inorganic material ejected from the new vents (Figure 4). They were carefully sampled for further analysis in the laboratory.

Future work on Loihi will carefully monitor ongoing changes and assess the risks of explosive volcanism or devastating landslides in preparation for the installation of the Hawaii Undersea Geological Observatory (HUGO) on Loihi. HUGO will be connected to shore, 34 km away, by a fiber optic cable. It will give scientists real-time seismic, chemical and visual information about Loihi, which is now an international natural laboratory for the study of undersea volcanism and mid-plate island formation.

Figure Captions

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