Eos, Vol. 80, No. 48, November 30,1999 face sediments. Planning for a second long-term observatory would begin at this point as well. Following the installation of the second observa tory the research efforts would focus on the recovery of a sediment core from the lake. Throughout this project careful attention will be paid to developing the appropriate technology to sample the lake with minimum impact. Also, it will be important to have ongoing data ex change workshops and close international col laboration to ensure success. A report on a workshop on Lake Vostok late last year, sponsored by NSFfis on the Web (http://www.ldeo.columbia.edu/vostok/). Hard copies can be requested from Margie Turrin (
[email protected]).The workshop and report were sponsored and supported by NSF
grant OPP-9820596 and Lamont-Doherty Earth Observatory contribution 5983. An international coordinating workshop sponsored by the Scien tific Committee on Antarctic Research was held in Cambridge, England, in September 1999. Authors Robin E. Bell and David M. Karl For more information, contact Robin E. Bell, Lamont-Doherty Earth Observatory of Columbia University PO Box 1000, Palisades, N.Y, 10964 USA; E-mail:
[email protected] or David M.Karl,School of Ocean and Earth Science and Technology University of Hawaii, Honolulu, HI 96822, USA; Tel: +1-808-956^8964; Fax: +1-808^956-5059
Origin, Effects of Masaya Volcano's Continued Unrest Probed in Nicaragua
Fig. 1. Digital elevation model of the Masaya area showing the Masaya caldera, its pit craters, and the Llano Pacaya ridge (courtesy of the Nicaraguan Institute of Territorial Studies). Superimposed over the model is a contour map showing the dispersion of the S0 plume measured in February-March 1999 downwind from the Masaya volcano. Near-ground SO concentrations are given in ppb. Inset map shows outline of Nicaragua and location of Masaya (black triangle). 2
2
References Abyzov,S.S., I. N. Mitskevich,and M. N. Poglazova, Microflora of the deep glacier horizons of Central Antarctica (trans, from the Russian),Microbiology, 67,451458,1998. Karl, D. M., In the microbiology of deep-sea hydrothermal vents, edited by D. M. Karl, pp. 35-124, CRC Press, Boca Raton, Fla., 1995. Petit, J. R.,et al., Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Nature, 399,429-436,1999. Siegert, M. J., J. A. Dowdeswell, M. R. Gorman, and N.EMcIntyre.An inventory of Antarctic subglacial \akes,Antarct ScL, 8,281-286,1996. Siegert, M. J., and J. K. Ridley, An analysis of the icesheet surface and subsurface topography above the Vostok Station subglacial lake, central East Antarctica.i Geophys. Res., 103,10,195-10,208, 1998.
PAGES 575,579,581 An international team of researchers is hop ing to pin down the origin and determine the effects of 6 years of continued unrest at Nicaragua's Masaya volcano. With the coun try's capital, Managua, close by, several million people are at risk if a large, explosive eruption were to occur. Masaya is distinctive not only for its explo sive basaltic eruptions of the past, but also for its postcaldera activity. Combined geophysical and geochemical surveys are improving un derstanding of volcanic activity there, and new techniques are providing exciting inroads into deciphering the atmospheric chemistry in its plume dispersal, addressing the nature of relatively rapid variations in its gas emission rates, and monitoring its gas hazards. Many issues remain to be resolved, however. The new work, involving researchers from Nicaragua, the United Kingdom, and Cana da, builds on pioneering efforts by Stoiber et al. [1986],who studied Masaya in the 1970s and early 1980s. It already has confirmed aspects of more recent studies, for example Metaxian et al.'s [1997] conclusion that the source of Masaya's permanent tremor is shallow and probably associated with mag ma movements or magma degassing activity or both. New gravity and petrological data confirm the presence of a low-density highly degassed magma beneath Santiago crater, the currently active vent. We believe that low-density gas-rich mag ma is periodically transported upward from depth, resulting in observed changes, such as significant decreases in gravity when gas emission increases.Yet the mechanism in volved is not clear. Our working hypothesis is that both convec tion and intrusion of magma are possible, and indeed they may be part of the same overall process. Cardosso and Woods [1999] inferred that a stagnant, gas-bubble-rich layer may de-
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velop on top of a cooling, crystallizing, vola tile-saturated basaltic magma reservoir, while the lower part of this reservoir is vigorously mixed by convection. Such a process may explain Masaya's recent behavior. In this case, resumption of surface activity may correspond to transport upward of a gas-rich, bubbly batch of melt separated from the volatile-saturated magma reservoir. Subsequently this shallow, low-density magma releases gases with uniform composition. Masaya is a low basaltic shield volcano consisting of a nested set of calderas and craters that lies on the Quaternary volcanic front of Cen tral America about 20-25 km southeast of Man agua (Figure 1). Masaya appears to be mostly producing lava flows. Nevertheless, it is one of the few known volcanoes that also has experi enced basaltic explosive activity The volcano's shallow, elongate caldera (6 x 11.5 km) is the product of an 8 km ignimbrite eruption about 2500 years ago [Williams, 1983]. This large-scale explosive activity is remarkable because basaltic magmas are generally consid ered to be nonexplosive.Yet explosive activity and significant degassing at Masaya point to the capability for arc basaltic magmas to be volatilerich [Sisson and Layne, 1993]. 3
A Natural Laboratory Masaya caldera hosts four young pit craters, San Fernando, Nindiri, San Pedro, and Santiago. Santiago, the active vent, has experienced five periods of ephemeral lava lake development, mild strombolian eruptions, intense gas emis sion, volcanic tremor, and inner crater wall col lapse since the mid-19th century [McBirney,
\%6;Stoiberetal., 1986;Metaxian etal, 1997; Rymer etal, 1998]. Almost no solid materials were erupted during these periods, a paradox that is poorly understood.The gas plume associated with the strong degassing episodes has adversely affected the region downwind from the volcano.The last reactivation of Masaya took place in Santiago crater in mid-1993 and continues today [Rymer et al, 1998]. Broad aspects of persistent volcanism, com bined with easy access, make Masaya an excel lent natural laboratory to study important issues such as the potential for basaltic magmas to pro duce large explosive eruptions, the origin of the long-lived noneruptive degassing, and the effects of acid gases on the surrounding environment. The current study hopes to infer the extent and structure of the shallow and deeper magmatic plumbing system; interpret the origin, nature,and timescales of magma movements; characterize the types and amounts of volcanic gases being emitted into the atmosphere; and evaluate the dispersion and deposition of the gas plume downwind from Masaya.The approach includes innovative applications of well-established and new geophysical and geochemical techniques, such as microgravity groundbased remote sens ing of gases, ground- and satellite-derived ther mal data, direct gas sampling, diffuse carbon dio xide monitoring, and melt inclusion petrology Onset, Intrusion Not Related Subsurface mass and density redistributions within the volcano can be quantified and locat ed using a combination of high-resolution ground deformation and microgravity monitor
ing techniques. Global Positioning System (GPS) measurements allow the vertical and horizontal coordinates of a point to be obtained at a preci sion of 1-3 cm. By repeating the determinations at different sites on the volcano, vertical and horizontal ground deformation can be detected through time. Having corrected for any height changes, temporal fluctuations in gravity detected on the ground surface are used to estimate subsurface movements of water or magma. Microgravity measurements also may be affected by the Earth tide or by changes in ground elevation during inflation or deflation of the volcano. Rymer et al. [1998] reported that vertical or horizontal movements as measured by GPS at Masaya did not exceed 2 cm between 1994 and 1997. However, microgravity data reveal substan tial variations (Figure 2). Data acquired at stations near Santiago crater show a maximum decrease of 90 uGal between February 1993 and April 1994, followed by a gradual increase of up to 60 uGal between April 1994 and March 1997. Based on the lack of ground deformation and the observed gravity decrease, it is likely that the onset of the ongoing gas crisis at Masaya was not related to intrusion of primitive, dense magma near the surface. Rymer et al. [1998] concluded that the 1993-1994 gravity decrease reveals the presence of a magma body of reduced density a few tens of meters beneath the Santiago crater. The maximum volume of this magma is 0.03 km . According to the model, the mass decrease may be accounted for by storage of 4 x 10 kg of gas ( H 0 + C 0 + S0 ),and in all alike combina tions throughout] in a vesiculated magma layer. The gradual increase in gravity observed be tween 1994 and 1997 may then reflect a de crease in thickness of this layer during continu ous gas emission. The most recent microgravity surveys between March 1997 and February 1999 suggest a consis tent decrease of about 60-70 uGal, which is of the same order as that observed between 1993 and 1994.There were no significant changes in elevation associated with these surveys. We pre sume that these changes involve similar process es to those inferred in 1993-1994. Short-term gravity variations were measured at Santiago crater every 15 min for 7 hours in 1997, during which gravity increases of 2045 uGal were recorded. In 1998, a more detailed survey was conducted at the same location, but for longer periods of about 13 hours. Atmospheric pressure and seismicity were monitored in con junction with the microgravity measurements. We observed gravity changes with amplitudes of 40,20, and 35 uGal on February 25, March 6, and March 13,1998.These fluctuations do not relate to atmospheric pressure and seismicity However, a tentative correlation appears to exist between the maximum gravity changes and the maximum daily Earth tide amplitude, particular ly on February 15 and March 13,1998 (Figure 3). The Earth tides may affect the level of magma in the shallow volcanic conduit, or possibly the density of the magma. 3
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