January 20, 1999

FIRST QUARTER 1999 MILESTONE REPORT FROM HAWAII UNDERSEA RESEARCH LABORATORY TO DIRECTOR, NATIONAL UNDERSEA RESEARCH PROGRAM.

Objective: Sustain Healthy Coasts.

PM: Number of Coastal and Great Lakes States with Improved Predictive Capabilities and Understanding of Environmental Processes.

Temporal evolution of hydrothermal fluids produced by Loihi Seamount, following an intense seismic event in July - August, 1996

Introduction

Loihi Seamount, an active submarine volcano (Fig. 1), is the most recent expression of the hotspot that has produced the Hawaiian Islands chain. Located 29 km southeast of the Island of Hawaii, the summit of Loihi is 980 m below the ocean surface. This location makes Loihi an ideal site for submarine neovolcanic studies, as it is only 24 hours away, by ship, from the Hawaii Undersea Research Laboratory (HURL) at the University of Hawaii in Honolulu.

HURL's research at Loihi has the long-term aim of developing a fundamental understanding of the volcano's behavior. This will

(a) provide a benchmark for comparison with other submarine volcanoes;

(b) enable an improved assessment of the role of hotspot volcanoes as contributors of CO2 and CH4 to the global carbon cycle;

(c) enable assessment of Loihi's hazard potential, for the generation of tsunamis by summit or flank collapse, and for submarine explosive eruptions that affect the sea surface;

(d) enable assessment of Loihi's function as a bio-reactor, utilizing microbial ecosystems in extreme environments at high temperatures and pressures, both those associated with high-temperature fluid vents, and those occurring within the very porous interior of the volcanic edifice.

This report is directed particularly to item (c), above.

Changes in summit topography and fluid chemistry, and their implications

This seamount, on the south flank of Mauna Loa, is appropriately named Loihi, meaning "long" in Hawaiian, because of its narrow southeast rift zone (Fig. 1). Loihi is a 30 km long submarine volcano being built primarily by extrusion along its southeast rift. The maximum width at its base is 15 km, narrowing to 5 km at the summit plateau, a relatively flat area of approximately 25 km2, at the 1200 m depth contour.

Prior to the 1996 seismic event the southern summit region was the site of two pit craters and a hydrothermally active pillow cone, Pele’s Vents (PV, Fig. 2). This cone was the dominant hydrothermal feature of Loihi, and had been visited numerous times by HURL submersibles. The hydrothermal fluids were typically low temperature (30 ¡C), high in carbon dioxide, iron and manganese and low in pH. The basalt surfaces of the Pele’s Vents mound were completely covered with iron-rich deposits of nontronite and bacterial mat. These deposits, and the chemistry of the hydrothermal fluids that produce them, have been the focuses of extensive research by HURL PIs.

Three pit craters now occupy the southern half of the summit area (Fig. 3). This is a direct consequence of the 1996 seismic swarm, a three-week event marked by more than 4000 earthquakes up to magnitude 4.9. A new pit crater, Pele’s Pit, was formed by a collapse of the Pele’s Vents area on the southern portion of the summit. Pele's Pit is a vertical-walled, oval feature 750 x 500m across and up to 300 m deep. The collapse of the Pele’s Vents mound displaced approximately 100 million cubic meters of basalt. No tsunami was detected at Honuapu Bay, the nearest tide gauge monitored by the Pacific Tsunami Warning Center, so the collapse was not instantaneous. The low magnitude of the seismicity implies that collapse occurred gradually over a period of hours or days, as lava drained out of a magma chamber beneath Pele's vents.

Many changes were observed in the hydrothermal system at Loihi following the collapse of the southern summit region (Fig. 4). The hot dikes, which had been the heat source for Pele’s Vents, were exposed along the north face of the new pit crater. The resulting hydrothermal venting was much hotter than previously observed, up to 200 ¡C. The chemistry of the solutions was also much higher in dissolved metals and gasses: Fe, Mn, CH4, CO2, H2S, and H2. The increased hydrothermal tracers reflect a vigorous outflow of high-temperature fluid, never before observed at Loihi. In addition to the intense fluid outflow, high-temperature reaction products were also sampled. Sulfides, such as pyrrhotite, pyrite and wurtzite indicate a fluid history with temperatures greater than 250 ¡C.

Revisits to Loihi since the collapse have yielded data suggesting a cyclical pattern in the hydrothermal system. Long-term monitoring devices (OsmoSamplers), which continuously sample small volumes of water, have shown that the hydrothermal system may be returning to more pre-collapse conditions. The temperatures of the venting are decreasing. Vents that were greater than 150 ¡C are now less than 150 ¡C. The OsmoSampler records also show that the vent fluid has evolved from being enriched in K and depleted in salinity to depleted in K and enriched in salinity, relative to ambient seawater.

Another interesting discovery is the presence of explosive eruptive products at depths previously thought to preclude phreatomagmatic eruptions. Fresh Loihi glasses, formed from lava bubbles, have volatile concentrations consistent with extrusion at 100-140 bars pressure (1000-1400 meters water depth). This exceeds the pressure of 60-80 bars generally thought to limit steam explosions. These results will help refine growth and stability models for Loihi and all Hawaiian Islands.

Although the 1996 event did not generate a tsunami or an explosive eruption at the sea surface, Pele's Pit is the smallest of the three Loihi pit-craters. A much longer monitoring record will be required to establish the full range of behavior of this volcano, and its hazard potential.

Investigators contributing to this report:

Rodey Batiza, University of Hawaii

David Clague, Monterey Bay Aquarium Research Institute

James Head, Brown University

Michael Garcia, University of Hawaii

Francis Sansone, University of Hawaii

C. Geoffrey Wheat, NURP – Alaska

John R Smith, University of Hawaii

Alexander Malahoff, University of Hawaii

DIAGRAMS:

Fig. 1: SeaBeam bathymetry over Loihi seamount acquired aboard R/V Ka'imikai o Kanaloa, 100 m contours. Insert shows Hawaii Island and location of Loihi.

Fig. 2: Loihi Seamount summit, NOAA SeaBeam data pre-1996 crisis 20 m contours. Illumination from the NE. Map by J. R. Smith.

Fig. 3: Loihi Seamount summit, R/V Ka'imikai-o-Kanaloa SeaBeam data from post-crisis cruise, Sept. 1996, 20 m contours. Illumination from the NE. Map by J. R. Smith.

Fig. 4: Temperature records of Loihi hydrothermal vents, from 1982 until the present.

Alexander Malahoff

Director, Hawaii Undersea Research Laboratory

January 20, 1999