INTRODUCTION
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.
STRUCTURE
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. |
HYDROTHERMAL PLUMES
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). |
HYDROTHERMAL VENTS
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
PETROLOGY:
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
RADIOCHEMISTRY
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.
MICROBIOLOGY
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.
MACROBIOLOGY
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.
CONCLUSION
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.
http://www.soest.hawaii.edu/GG/HCV/loihi.html
F. K. Duennebier N. C. Becker J. Caplan-Auerbach D. A. Clague J. Cowen M. Cremer M. Garcia F. Goff |
A. Malahoff G. M. McMurtry B. P. Midson C. L. Moyer M. Norman P. Okubo J. A. Resing |
J. M. Rhodes K. Rubin F. J. Sansone J. R. Smith K. Spencer X. Wen C. G. Wheat |
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Last page update on 22 Jul 1998
This page created and maintained by
Ken Rubin©,
krubin@soest.hawaii.edu.
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.