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During July and early August, 1996, the largest swarm of earthquakes ever observed at any Hawaiian volcano occurred at Loihi Seamount. In response to this event, an initial cruise was dispatched to Loihi in early August (Rapid Response Cruise: RRC), and two previously planned cruises sailed in September and October (LONO Cruises) 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.
EARTHQUAKES and NOISE
Between July 16 and August 9, 1996, over 4000 earthquakes from Loihi were detected by the U.S.G.S. Hawaiian Volcano Observatory (HVO). The initial phase of activity, consisting of 72 located earthquakes, continued for two days. After 30 hours of quiescence, activity resumed and continued at a higher rate, averaging over 88 events per day for the next 10 days before slowing.
Preliminary locations calculated using the HVO seismic network data place the majority of events between depths of 10 and 14 km shallowing seaward. P-wave arrivals at an OBS deployed on Loihi summit during the third week of the swarm arrive about two seconds early, suggesting that the velocity model used for the island of Hawaii is inappropriate beneath Loihi, and that initial locations are suspect. Using HVO's preferred velocity model under Hawaii and considerably lower shallow velocities under Loihi, reasonable hypocentral locations are obtained near 8 km depth [as depicted in Fig 1] Despite the obvious topographic modifications of the summit, few shallow earthquakes (between 0 and 5 km) were located.
Figure 1 Earthquake hypocenter cross section at Loihi located using data from the Hawaiian Volcano Observatory and one ocean bottom seismometer at Loihi during August, 1996. Circle size is proportional to relative magnitude. The oval at about 9 km depth indicates the possible location of a magma chamber consistent with petrological data.
Sonobuoys dropped from the K-O-K to listen for earthquakes and eruption sounds detected bangs, pops, and grinding noises with frequencies from tens to several hundred Hz at three distinct locations on the NE side of the summit, moving from south to north with time. An active area just north of East Pit was detected during the RRC, an area on the east flank at a depth of about 1600m was located early, and one just north of the summit was heard later in the first LONO cruise. Other priorities prevented detailed surveys of these sites although turbid water was observed drifting in from these areas during submersible dives.
Figure 2 Summit map obtained from K-O-K SeaBeam bathymetry taken during the LONO cruises. Stars identify vent sites and numbers identify the following features: 1) Tow-Yo track (see Fig 4); 2) Pisces Peak (OBS station); 3) West Pit; 4) East Pit; 5) Sand Channel; 6) Pele's Pit.
SeaBeam surveys documented the bathymetric changes at Loihi summit [see Fig 2] 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. Portions of the West Pit rim and areas to the north have faulted down several meters towards the summit center, bisecting Pisces Peak.
The Sand Channel, a preexisting
large fissure extending westward from the East Pit, now intercepts
Pele's Pit. East pit crater has shallowed by ~23 m, apparently
as a result of infilling by mud. There is evidence for recent
fracturing, hydrothermal activity, and emplacement of black
sands along the upper south rift. The surveys reveal no obvious
changes on the northern half of Loihi.
The collapse forming Pele's Pit exposed massive columnar jointed flows and hydrothermally lithified talus now forming large spires on the near-vertical north face of Pele's Pit. Much of the southern summit appears to be newly shaken; previously light brown pillow surfaces are now littered with broken pillow debris and glassy rocks. The glassy rocks fine away from the edges of the craters, suggesting ejection of the rocks from the craters.
|Figure 3 Breccia deposit. Mix of nontronite- covered pillow fragments, fresh talus, and glassy shards at the edge of Pisces Peak (cliff in the background) taken on the first Pisces V dive after the July, 1996 event.|
Intense hydrothermal plumes resulting from the seismic event were studied using hydrocasts (vertical water sampling at a single site) and tow-yos (sampling by an instrument package raised and lowered behind a moving ship). Temperature anomalies of 0.5°C were common during the RRC in the water column around the summit at depths of 1050-1250m, with anomalies of 0.1°C at distances >8 km [see Fig 4 below]. In contrast, mid-ocean ridge plumes typically have maximum anomalies of 0.02-0.1°C, although event plume anomalies of up to 0.3°C have been observed (e.g., Baker et al. 1987). One surprise was the observation of a very intense plume at 1600-1800 m depth at a background station 50 km NNE of Loihi. A marked decrease in pH (0.2 units) and a remarkable 3He enrichment (150%) were measured, suggesting an injection of magmatic gasses to the water column during a large short-lived, but rapidly cooled, volcanic episode well below the summit of Loihi during the early stages of the seismic event.
Figure 4 Tow-yo cross section. Cross section of tow-yo data taken during the RRC showing the thermal plume west of Loihi (see Figure 2 above for track).
Vent fields with temperatures of up to 77°C were discovered during the LONO cruises, one near the bottom and two on the north wall of Pele's Pit, two on the south rift, and one west of the summit [see Fig 2 above]. Venting was generally diffuse, exiting through nontronite coated talus, although 13 m wide fissures vented large volumes of water in the south rift vent area. Rocks bearing several hightemperature sulfide minerals were collected, suggesting that vent waters had been very hot (at least 250°C) during formation of these deposits. This apparent decrease in temperature with time will be verified by temperature recorders and samplers emplaced for yearlong sampling of vent fluids.
Vent fluid samples were collected for analysis from seven vents. Gas content of the fluids was >52 mmole/kg, or 17 times the background seawater value. Carbon dioxide continues to be the dominant vent fluid gas, but its ratio to dissolved silica and vent temperature (heat) has decreased dramatically during the last decade: dissolved CO2/heat ratios decreased by about 30% between 1987 and 1992 (Sedwick et al., 1994). The LONO CO2/heat ratios continued this trend, decreasing by over 90% relative to the values measured in 19871992. These decreases have been ascribed to progressive degassing from a magmatic intrusion. The much lower CO2 values in 1996 (~10 mmol/kg CT (total carbon), compared with ~300 mmol/kg CT in 1987 and ~200 mmol/kg CT in 1992) could reflect a continuation of this degassing trend; RRC plume CO2 measurements suggest that the magma supplying gases to the vents degassed substantially during the seismic event
Rocks and sediment were collected using the Pisces V submersible during the RRC and LONO cruises. The RRC sampled a "young" breccia on the western rim of West Pit [see Fig 3 above], yielding three of the freshest lavas collected from Loihi. The LONO cruises collected talus fragments, some with sulfides (pyrite, bornite, sphalerite) and amorphous silica coatings, in situ lavas, and sediment. The sediment consists of coarse black sand, Pele's hair, and paper-thin bubble-wall fragments produced by the reaction of fluid lava with seawater, mixed with planktonic foram tests.
The RRC lavas are low SiO2 tholeiites, typical of recent Loihi lavas (Garcia et al., 1995). They contain 1 to 2 vol.% clinopyroxene phenocrysts (rare in Hawaiian tholeiites) and two chemically distinct populations of olivine. Some of the clinopyroxenes have inclusions of strongly resorbed olivine and reversely zoned rims. This reverse zoning and two olivine populations indicate magma mixing, probably just before or during eruption. MgO contents of these rocks range from 8.2 to 10.3 wt% and the olivines are in equilibrium with these compositions, assuming 10% oxidized iron. The olivine inclusions in clinopyroxene apparently formed at moderate pressures (~2.8 kb) based on modeling of equilibrium crystallization of these rock compositions using the MELTS program (Ghiorso and Sack, 1995). Analyses of RRC glasses yield trace element ratios (e.g., La/Yb) which continue a decreasing temporal geochemical trend for young Loihi tholeiites (Garcia et al., 1995), supporting the hypothesis that they were recently erupted. These results indicate that the RRC rocks are young and were stored at 8.5 to 9 km (compared to 2-6 km depth for the nearby Kilauea magma chamber (Klein et al, 1987)) before being mixed with a more mafic magma, which may have triggered the eruption of these rocks.
Editor's Note: Additional details regarding the "rock story" can be found on the Rock Geochemistry and Rock Gallery web pages
The ages of two fresh-appearing RRC lavas are being determined using the 210Po-210Pb technique which can provide ages of lavas erupted within the past 2.5 years (Rubin, et al. 1994). Three to four analyses made several months apart are required to establish the age of the RRC rocks, so final ages will not be available until summer, 1997. At this stage, we can conclude that the rocks were both erupted within the past year, that their minimum ages predate the July, 1996 swarm by up to 4 weeks, and the two samples are probably at least 1 month different in age.
Lead isotopes, 210Po, and other volatile metals in RRC particle-enriched sea water (0.22 g/L) are being analyzed to detect anomalies expected from magmatic degassing of metals. Particulates show 10-40x enrichments of Pb, Po, Mo, Sb, As, Te and Tl relative to Loihi summit lavas,and a Pb isotopic composition indistinguishable from Loihi summit lavas, strongly implicating Loihi as the primary source of trace elements adhering to the particulates.
Shipboard 222Rn analyses during the LONO cruises revealed elevated water column Rn activities (60-75 dpm/100L) at 1000-1300 m depth and up to 30,000 dpm/100L in the vent fluids. This radon distribution is similar to that in 1993, indicating that any potential eruptive pulse of Rn in August had already gone by early October.
Microbial mats surrounding hydrothermal vent orifices were abundant and were observed to reform within a day following sampling. Samples of mats from different sites were brought into enrichment culture using a novel method designed to focus the bacteria along opposing iron-sulfur vs. oxygen gradients. These enrichments are being prepared into pure cultures so that physiology studies can be undertaken. Additionally, microbial mat samples were collected to address questions such as community succession. The majority of the taxa observed previously in Loihi microbial mats have been shown to be most closely related to mesophilic iron- and sulfur- oxidizing bacteria (Moyer et al, 1995).
The geomorphology and venting character of Pele's Pit has created a unique "chemostat-like" reservoir. As the entire circumference of Pele's Pit is comprised of relatively steep walls rising ~200 m from the pit bottom, fluid exchange is restricted to vertical mixing, presumably influenced by hydrothermally-induced buoyant updrafts and ambient cold water down drafts. These conditions may contribute to the high microbial biomass observed within the pit.
Loihi Seamount hydrothermal vent systems lack the luxuriant macrobenthic communities characteristic of vents from mid-ocean ridges. The only two vent-specific macrofaunal species described from Loihi have been a novel bresiliid shrimp, Opaepele loihi (Williams and Dobbs, 1995), and a unique lineage of pogonophoran worm (R. Vrijenhoek, pers. comm.). The post-event dives, however, found no evidence for either, and the long term impact of the event on these species is unknown.
The Loihi rapid response cruises and earthquake records have provided a wealth of data that has, for the first time, documented the submarine eruption and pit crater formation of a hot-spot volcano. While the synthesis of these data will take several years to complete, the value of obtaining a diverse database quickly is clearly demonstrated. Dives planned for the upcoming two years, and the concurrent establishment of an underwater observatory at Loihi, should more fully document these and future events at this active submarine volcano.
Baker, E.T., G.J. Massoth, and R.A. Feely, Cataclysmic hydrothermal venting on the Juan de Fuca Ridge, Nature, 329, 149-151, 1987.
Garcia, M. O., D. J. P. Foss, H. B. West, , and J. J. Mahoney, Geochemical and isotopic evolution of Loihi volcano, Hawaii, J. Petrol. 36, 1647, 1995.
Ghiroso, M. S, and R.O. Sack, Chemical mass transfer in magmatic systems IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures, Contrib. Mineral. Petrol., 119, 197, 1995.
Klein, F.W., R.Y. Koyanagi, J.S. Nakata, and W.R. Tanigawa, The Seismicity of Kilauea's Magma System, U.S. G.S. Professional Paper 1350, 1019-1186, 1987.
Moyer, C.L., F.C. Dobbs, and D.M.Karl, Phylogenetic diversity of the bacterial community from a bicrobial mat at an active hydrothermal vent system, Loihi Seamount, Hawaii. Appl. Environ. Microbiol. 61: 1555-1562, 1995.
Rubin, K.H., Macdougall, J.D. and Perfit, M.R. 210Po-210 Pb dating of recent volcanic eruptions on the sea floor, Nature, 368, 841-844, 1994.
Sedwick, P. N., G. M. McMurtry, D. Hilton, and F. Goff, Carbon dioxide and helium in hydrothermal fluids from Loihi Seamount, Hawaii, USA: temporal variability and implications for the release of mantle volatiles,Geochim. Cosmochim. Acta 58, 1219-1227, 1994.
Williams, A.B., and F.C. Dobbs, A new genus and species of caridean shrimp (Crustacea: Decapoda: Bresiliidae) from hydrothermal vents on Loihi Seamount, Hawaii, Proc. Biol. Soc. Wash., 108: 228-237, 1995.
F. K. Duennebier
N. C. Becker
D. A. Clague
G. M. McMurtry
B. P. Midson
C. L. Moyer
J. A. Resing
J. M. Rhodes
F. J. Sansone
J. R. Smith
C. G. Wheat
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Last page update on 22 Jul 1998
This page created and maintained by Ken Rubin©, firstname.lastname@example.org.
The content of this page was written by the 1996 Loihi Science Team.
Image credits are as follows:
*Event Summary earthquake depth image by Jackie Caplan-Auerbach
*Event Summary Loihi relief map by Craig Smith
*Event Summary Breccia Photo by Fred Duennebier
*Event Summary ToYo image by Brian Midson
Other credits for this web site.
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Last page update on 22 Jul 1998