Bull Volcanol (1998) 59 :345–355 Q Springer-Verlag 1998

ORIGINAL PAPER

H. Rymer 7 B. van Wyk de Vries 7 J. StixG. Williams-Jones

Pit crater structure and processes governing persistent activity

at Masaya Volcano, Nicaragua

Received: 16 May 1997 / Accepted: 29 October 1997

Editorial responsibility: D. Swanson

Hazel Rymer (Y) 7 Benjamin van Wyk de VriesDepartment of Earth Sciences, The Open University,Milton Keynes, MK7 6AA, UKFax: c44 1908 652558e-mail: h.rymer6open.ac.uk

John Stix 7 Glyn Williams-JonesDepartment of Geology, Université de Montreal, CP.6100, Montreal, Canada

Abstract Persistent activity at Masaya Volcano, Nica-ragua, is characterised by cycles of intense degassing,lava lake development and pit crater formation. It pro-vides a useful site to study the processes which governsuch activity, because of its easy accessibility and rela-tively short cycles (years to decades). An understandingof the present activity is important because Masaya isvisited by large numbers of tourists, is located close tomajor cities and has produced voluminous lavas, plin-ian eruptions and ignimbrites in the recent past. Weprovide structural and geophysical data that character-ise the “normal” present state of activity. These indi-cate that the ongoing degassing phase (1993 to present)was not caused by fresh magma intrusion. It was asso-ciated with shallow density changes within the activeSantiago pit crater. The activity appears to be asso-ciated predominantly with shallow changes in the pitcrater structure. More hazardous activity will occuronly if there are significant departures from the presentgravity, deformation and seismic signatures.

Key words Masaya 7 Santiago 7 Pit 7 Structure 7Microgravity 7 Magma 7 Gas

Introduction

Masaya Volcano is persistently active, with a vigorouslydegassing lava lake periodically visible. Geological evi-dence shows several cycles of pyroclastic cone-building

eruptions, lava flows and pit crater formation. In histor-ic times lava lakes have been common, and two lavaflows have been erupted (1670 and 1772). In the past150 years the volcano has formed only two pit craters,with episodic lava lake development and feeble strom-bolian eruptions, but it has maintained a dynamic mag-matic system manifested by voluminous gas emission.Five locally catastrophic degassing crises since 1852have degassed approximately 10 km3 of basaltic magma(Stoiber et al. 1986), contributing significantly to globalSO2 and CO2 output. At least four large (1–10 km3)caldera-forming eruptions have also taken place be-tween 2700 and 30 000 BP (Williams 1983; van Wyk deVries 1991).

As with other active volcanoes in populated areas,Masaya is a major tourist attraction. As such, evensmall increases in activity pose a serious risk and arepotentially damaging to the local economy. For thisreason it is desirable to understand the processes thatcontrol variations in activity, with an aim to predictingchanges in activity, including any return to the more de-structive phenomena. On a broader scale, it is neces-sary to understand the mechanisms of long-term persis-tent activity in order to develop models for the rates ofmagmatic processes at these sites.

We present structural and geophysical (gravity,ground deformation and SO2 flux) data from the mostrecent degassing crisis at Masaya (1993 to present). Theaim is to constrain the shallow structure of the magmaplumbing system, determine the geophysical signaturesof the present activity and predict future changes. Aninitial gravity decrease between February 1993 andApril 1994 of up to 90 mGal close to Santiago pit craterindicates that a shallow density reduction occurred justbelow the crater bottom. This gravity decrease corre-lates with increasing gas flux, indicating a change in theshallow plumbing system. A subsequent gradual in-crease in gravity between April 1994 and March 1997 ofup to 56 mGal has occurred at the same time as minorpit crater collapse and a decline of degassing. The grav-ity and deformation data indicate that changes oc-

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Fig. 1a–d Masaya Volcano. a Regional location of Masaya. bMap showing the Las Sierras Caldera enclosing the Masaya cal-dera, which in turn encloses the recent vents (black dots). Mapshows proximity of major towns (circles) and cities to Masaya cal-dera: approximately 1.5 million people live within 25 km of thevolcano. Heavy dashed line indicates location of COSPEC trav-erses. c Map of active crater area showing structural features,such as eruptive fissures (dashed lines), pit crater edges (filledtriangles) and explosion crater edges (open triangles). Lava flowsand lakes (shaded) and tephra cover (blank) are also shown. Lav-as and fissures are given dates of eruption (e.g. L1772 is the 1772flow and F1906 is the fissure that erupted gas in 1906). d Aerialphoto of the same area as the map in c

curred at shallow depths, and that the crisis was not ac-companied by major intrusion.

General geology

Masaya Volcano is a large basaltic shield volcano, lo-cated 20 km south of Managua, Nicaragua (Fig. 1a). Itis composed of a nested set of calderas and craters, thelargest of which is Las Sierras shield and caldera (vanWyk de Vries 1993). Within this caldera lies MasayaVolcano sensu stricto, a shallow shield composed ofbasaltic lavas and tephras (Fig. 1b). This hosts Masayacaldera, formed 2.5 ka ago by an 8-km3 basaltic ignim-brite eruption (Williams 1983). Inside this caldera anew basaltic complex has grown from eruptions mainlyon a semi-circular set of vents that include the Masayaand Nindiri cones. The latter host the pit craters ofMasaya, Santiago, Nindiri and San Pedro. Observationsin the walls of the pit craters indicate that there havebeen several episodes of cone and pit crater formation(Fig. 2).

The floor of Masaya caldera is covered by poorlyvegetated lavas, indicating resurfacing within the past1000 or so years, but only two lava flows have eruptedsince the sixteenth century. The first, in 1670, was anoverflow from the Nindiri pit, which at that time hosteda 1-km-wide lava lake. The other, in 1772, issued from afissure on the flank of the Masaya cone. Since 1772 lavahas appeared at the surface only in the Santiago pitcrater (presently active) and possibly within Nindiricrater in 1852.

Cone construction

Historic activity has been confined to the Masaya andNindiri cones. They are steep sided on their westernand southern sides, where exposures in the pit cratersindicate a predominance of pyroclastic material, eithercoarse scoria and spatter layers, or fine, laminated ashbeds. The northern and eastern sides are less steep andsurfaced mostly by lavas. Exposures in the pit cratersrecord a higher proportion of lava flows on these sides.This asymmetrical construction is due to the prevailingeasterly wind. The deposits are consistent with thecones being built by lava fountain and strombolian ac-tivity accompanied by lava flows.

Pit Crater structure and history

Masaya pit crater

Masaya pit crater, located centrally within the Masayacone, is now vegetated but probably formed after thelate sixteenth century, as it was not described in itspresent state by Oviedo (1855) writing in 1525. It hasnearly vertical upper walls of scoria and lava above

steep scree slopes that lead to a flat base and a 10-m-high spatter cone (Fig. 1c). This cone is fresher than therest of the crater and was probably erupted concurrent-ly with the 1772 lava flow. East of Masaya pit crater is alava-filled crater, which is probably the “funnelshaped” crater that Oviedo (1855) mentions, and whichwas probably filled sometime between 1529 and 1670(McBirney 1956). The “funnel shaped” nature of thiscrater, and the local abundance of scoria, suggest that itoriginated from pyroclastic cone-building activity rath-er than pit collapse.

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Fig. 2a, b Structure of pitcraters. a Panoramic sketch ofthe walls of Santiago mainand inner craters, showingcrater walls and major struc-tural features. Lava-dominatedwalls are indicated by L, teph-ra layers by dotted lines, andcrater breccias by speckledpattern. Pit crater walls indi-cated by Pc and closed trian-gles. The sketch of the innerpit crater shows the locationof caverns and vents seenopening up from 1986 to 1989.b Maps of crater evolution atSantiago from 1986 to 1997.These show locations of vents,the enlarging of the innercrater and fissures and col-lapses in the main crater. Afault in 1965 lava lake; B col-lapse fault on edge of Nindiriwall, which produced a majorrockfall in November 1986; Ctwo caverns open and degass-ing in 1987–1988; D faultsopening at the surface from1988 to present; E lava-filledvent of February 1989; Fchamber with lava fountainwithin, during February 1989;G the “Canon” vent, briefly abroad glowing hole in 1989,subsequently blocked, whichstill has fumarolic activity; Hsmall strombolian vent activein February 1989; I collapsealong faults on south side ofSantiago; J intermittent ventfrom 1990 to 1993; K ventopened in June 1993; L theenlarged vent observed on 27May 1997

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a Panorama of Santiago main crater

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v. 12/90 vi. 6/93-3/97

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Nindiri pit crater

At the time of Oviedo’s observations (1525), Nindiricrater was approximately 210 m deep, with a vent at thebottom hosting a lava lake (McBirney 1956). Nindiri isnow filled by frozen lava lakes erupted between 1570and 1670 (Fig. 1c). A lava lake overflowed Nindiri tothe north in 1670, forming a thick 3-km-long aa flow(Fig. 1c). The lava surface in the crater subsequentlysagged downward on circular faults. These faults, nowcut by the San Pedro and Santiago crater walls, dip out-ward at approximately 807, and the lava flows are tiltedinward. The faults extend down to the former craterfloor of Nindiri, where they widen into broad fracture

zones on the north side and join with a few main faultson the south side (Fig. 3).

A final lava lake was erupted onto the sagged craterfloor (Fig. 3). Spatter on the edge of San Pedro andwithin Nindiri indicates that this may be the lava flowdescribed by Montesssus de Ballore (McBirney 1956).He reported that the flow was extruded in July 1852, afew years before the formation of Santiago and San Pe-dro pit craters in 1858–1859 (McBirney 1956).

San Pedro pit crater

San Pedro pit crater is a vertically sided cylinder, 400 mwide and approximately 200 m deep. The base is mostly

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Fig. 3 Photograph and sketch of the Nindiri wall of Santiagocrater, including the inner pit crater formed since 1986. Thesketch shows the main concentric faults and fractures in the 1670lava lake of Nindiri. Shaded areas are probably intrusions or sol-idified magma-filled caverns. Textured areas indicate intercraterbreccias

scree, descending to a small flat area under a slightoverhang directly below the eastern Nindiri side. Thisflat area may be the remains of a lava lake.

Santiago pit crater

Since its formation in 1858–1959, Santiago has been themain site of activity at Masaya, hosting the vent for allmajor degassing episodes except that of 1906 from fis-sures on the north flank of Masaya cone (McBirney1956). After its initial collapse, Santiago pit crater was avertical cylinder approximately 150 m deep and 600 mwide. The original scree-covered floor was resurfacedin 1948 and 1965 with lava lakes, which are broken byconcentric faults and are similar to those seen in Nin-diri (McBirney 1956). One of these faults is still visibleon the north side of the crater (Fig. 2b). From 1948 un-til 1986 a small circular vent was the locus of activity(Fig. 2b, i). Santiago now consists of the main crater150 m deep, and an inner crater 150 m deep, in whichthe active vent is located (Fig. 2b, iv).

The walls of the east and north side expose flatlayers of lava, either thin lava flows or lakes (Fig. 2a).These lavas lap onto a cinder cone to the southeast andabut old pit crater walls to the southwest and northeast(Pc in Fig. 2a). These old crater walls are composedmainly of fine ash and scoria layers. The bedding be-comes indistinct approximately 150 m down the south-western side, and below this a massive breccia formsthe wall of the inner crater (Fig. 2a). On the north sidethe layering is distinct to 30 m above the 1965 lavalayer, below which there is a massive breccia.

The western side of the crater wall is composed ofthe lava infill of Nindiri crater (Fig. 3). The upper crat-er walls of Nindiri dip steeply inward, gradually becom-ing less steep 100–200 m down, where there are largescree fans. The base of the walls, now exposed in theinner crater, consist of a massive homogeneous brec-cia.

The concentric downthrown faults in Nindiri arewell exposed in the walls. They dip approximately70–807 outward and extend into highly fractured areas,where no discrete fault planes can be seen. In additionto the fractures and faults, several areas of massive rocklocate solidified magma-filled cavities in the lower partof the crater, such as those described below during theformation of the inner crater of Santiago (Fig. 2a).

Santiago pit crater development 1986–1997

Progressive deepening of the Santiago crater began inNovember 1986 and continues through 1997. Observa-tions in this period illustrate how pit crater formationproceeds.

In 1986 there was an 80-m-wide vent at the bottomof Santiago crater (Fig. 2b, i). In 1979–1980 magma hadbeen occasionally visible near the surface, but in 1986only a red glow was visible. During early to mid 1986the vent was degassing vigorously, although less so thanin the crisis of 1980–1983. By late 1986 the vent hadenlarged toward the southwest, and a large portion ofthe southwest crater wall had collapsed (Fig. 2b, ii).The collapses blocked the vent and the degassing stop-ped. Two new vents had opened by 1987, and observa-tions by van Wyk de Vries (1993) during this periodindicated that the vents widened with depth. The ventsslowly enlarged by the collapse of their edges, progres-sively exposing a large hollow area beneath the craterfloor. This process is very similar to the formation ofthe Bocca Nuova on Etna in 1968–1969 (Murray1977).

By January 1988 these two vents were both degass-ing at varying rates (Fig. 2b, iii). Gas then obscured thenew inner crater until late 1988, by which time it hadwidened 50 m and deepened by approximately thesame amount to an uneven base. In February 1989 sev-eral new vents opened in the inner crater bottom; onewas lava filled, one produced small strombolian erup-tions and one glowed red. A fourth vent in the north-

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east corner of the crater opened into a subcrater cav-ern. It first glowed red, but as the vent enlarged lavafountaining became visible below an overhanging wall.This lasted for approximately 1 day, after which thevent glowed red. (Fig. 2b, iv). Each of these ventsformed by the opening of cracks and then the collapseof material into the vent.

Eruptive activity stopped in April 1989, and in Maythe roof of the fourth cavern collapsed farther to reveala deeper vent. This became the focus of weak strombol-ian activity during May and June 1989 (Fig. 2b, v). Overthe latter half of 1989 eruption noises could be heard,but the source was too deep within the vent to be seen.During this period rockfalls became frequent in Santia-go, and fissures opened up above the southern wall. InNovember 1989 a large collapse occurred from this walland filled the crater bottom. From 1990 to 1993, therewas no distinct vent, and gas emission dwindled to lessthan 25!103 kg per day SO2 (Bulletin of the GlobalVolcanism Network 1992).

A vent became established again in June 1993, host-ing a new lava pool and building a small cone of ejecta(Fig. 2b, vi). This vent plunged approximately 807 un-der the northeast wall of the inner crater to approxi-mately 40 m depth, from where it appeared to extendvertically. The vent was partially blocked by a rockfallin April 1997, but gas still escaped through the bould-ers. Observations on 27 May 1997 by van Wyk de Vries(1993) revealed that a 20-m-wide vent had appeared.The vent had overhanging sides and was freely ejectinggas again. Park guards reported the resumption of a redglow on 3 June 1997.

Characteristics of the pit craters at Masaya Volcano

Each of the pit craters is a nearly vertical cylinderwhose walls may be slightly overhanging. They cutthrough all types of cone material without any apparentresponse to lithology. The bottoms of the craters arefunnel shaped unless they have been refilled by lavalakes. Sections though Nindiri indicate that collapse oc-curred mainly along outward-dipping faults. Such faultscarps are also present in Santiago. Vertical faults andfractures around Santiago indicate that the steep wallsof the pit craters are probably formed by collapseabove the outward-dipping fault set. Debris from col-lapsed slices of vertical faults forms large scree fans inthe craters.

Much of the collapse of the inner crater at Santiagosince 1986 has occurred by a process distinct from col-lapse along faults: that of unroofing small chambers, asobserved during the progressive collapse of Santiagoinner crater between 1988 and 1991 and again during1997. These collapses have happened episodically andrapidly; usually a small hole appears first, which thencollapses to reveal a cavern. The caverns have varied insize from 5 to 30 m diameter. During the 1989 activityfour cavernous vents were present. The vents them-

selves covered approximately 15% of the inner craterfloor area. Subsequent collapse revealed the cavernsbelow, which took up approximately 25% of the area ofthe inner crater, although not all of these were open atthe same time.

There have been many small collapses of this typesince 1986, and they have been more common in thewaning period of the degassing cycle. Some of theseevents have exposed lava-filled vents, such as in Fe-bruary 1989 and May 1989 (Fig. 2b). The appearance ofthese lakes may be caused by the level of the craterfloor reaching down to the magma level, not the usualopposite case where magma rises to the surface.

Geophysical data

In order to model the distribution and movement of gasand magma, microgravity and ground deformationmeasurements were made on five separate occasionsbetween February 1993 and March 1997. Gas flux data,providing information about the amount of SO2 degass-ing and by inference the total degassing budget, wereobtained using a correlation spectrometer (COSPEC)in March 1996 and March 1997.

Sulphur dioxide flux

Following a degassing crisis in 1979, activity wanedgradually into the late 1980s, until a weakly degassinglava lake appeared in 1989 (Bulletin of the Global Vol-canism Network 1991). In April 1992 the SO2 flux wasestimated (S. N. Williams, pers. commun.) at less than25!103 kg per day. In mid-1993 strombolian eruptionsand a lava lake indicated reactivation, which continuedwith a persistent incandescent vent and gas plumescomparable to those of 1980 (Bulletin of the GlobalVolcanism Network 1993a, b, 1994b). Measurements ofSO2 flux were made with a COSPEC on 16 March 1996over eight traverses under the gas column from Santia-go crater (Fig. 1b). On this occasion, the column wasbeing blown northeastward, and the errors are less than30%. Measured SO2 flux values ranged from a low of2.4!105 kg per day to a high of 1.1!106 kg per day).The mean flux was 6.0B2.9!105 kg per day (1 sigma).Our data fall between the averages of 3.8!105 kg perday prior to 1979 and 1.2!105 kg per day during 1980(Stoiber et al. 1986).

The continuous gas release was augmented in earlyOctober 1996 by short bursts of increased gas outputaccompanied by minor ash emission. Access to the San-tiago crater viewing area was restricted during this peri-od because of the obvious hazard to tourists. Rapid andshort-lived changes in the amount of gas released havebeen notable features at Masaya and are most likelydue to small alterations in the structure of the Santiagopit crater. Although we were unable to make new COS-PEC measurements during this phase, seismic data are

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Coyotes

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CerroMontoso

SantiagoPit

MasayaPit

1 km

MediaLuna

Coyotepe

Managua

Masaya

N

Line of section in fig. 7

NindiriPit

Fig. 4 Location of gravity stations in the Masaya caldera

A2A2

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A3

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Masaya Gravity Changes

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Coyotes

Fig. 5 Gravity changes at stations within Masaya caldera mea-sured using Lacoste and Romberg meter G513. Station locationsare shown in Fig. 4. The data clearly fall into two groups duringthe period 1993–1994 (months 0–14): (a) no significant change ingravity through time at stations along the road from the northerncaldera margin toward the active crater up to and including sta-tion A6, and (b) significant decreases at stations in and aroundthe active crater area. Stations Coyotes and B were not measuredin February 1993, so their position on the figure is arbitrary, al-though the gradients are comparable to the others

available (W. Strauch, pers. commun.). There was anincrease only in shallow tremor, which is normally asso-ciated with degassing at Masaya. COSPEC measure-ments were again made on several occasions duringJanuary to April 1997 and a lower flux of3.3B1.8!105 kg per day was observed.

Assuming an average gas flux since reactivation in1993, then our observations of approximately4.0!105 kg per day SO2 suggest that at least7.2!108 kg of sulphur have been released from 1993through mid-1997. If magma releases ca. 240 ppm sul-phur during passive degassing (Stoiber et al. 1986, Ta-ble 5), then petrological calculations (Stix and Layne1996) suggest that 1.5!1012–kg magma have been de-gassed since reactivation. The total amount of gas re-leased can also be calculated using appropriate H2O/SO2 and CO2/SO2 molar ratios of 3.86 and 2.18, respec-tively (Williams et al. 1992; Symonds et al. 1994, Ta-ble 5). Applying these ratios to the amount of SO2 gives2.6!109 kg of gas (H2OcCO2cSO2) released from1993 through mid-1997.

Microgravity and ground deformation

Microgravity measurements were made in February1993, April 1994, October 1994, March 1996 and March1997 at 17 stations within the caldera (Fig. 4) and forreference in Masaya city using LaCoste and Rombergmeter G513. Leica GPS 200 dual-frequency differentialreceivers were used in April and October 1994, andMarch 1996 and 1997, to provide ground deformationdata. Strict survey procedures were followed to reduceerrors in the field (Rymer 1996; Murray et al. 1995).The microgravity measurements are subject to errors of

much less than B25 mGal (twice the standard devia-tion) and the GPS data have errors of B7 cm (vertical)for April 1994, reducing to B2 cm for October 1994and March 1996 and 1997. Between February 1993 andApril 1994, precision levelling (B1 cm) was carried outwithin the Masaya caldera, and there were no heightchanges anywhere in the caldera greater than 6 cm inthat period (J. B. Murray, pers. commun.; SmithsonianInstitution Bulletin of Global Volcanism Network1994a).

The gravity data obtained between February 1993and April 1994 (months 0 and 14) fall into two distinctgroups (Fig. 5). More than 2 km from the crater (sta-tions A1–A6), all data lie within 25 mGal of zero andindicate no significant gravity changes at the 95% confi-dence level. In contrast, there was a marked decrease ingravity at stations near the active crater area. StationE3 appears to behave anomalously between 1993 and1994. This may be due to the dilation of fissures on theedge of the crater (H on Fig. 3b) or possibly to an unre-liable initial reading in 1993, or, being the station clo-sest to the crater, it may indicate that gravity decreasesare much larger within the crater. The rate of gravitydecrease between 1993 and 1994 was approximately–70 mGal/year for near-crater stations. During Februa-

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Fig. 6 Contour maps showing schematically the approximategravity changes through time for the periods a 1993–1994, b1994–1996 and c 1994–1997. Numbers at stations indicate changesin the gravity difference (mGal) between each station relative tothe value at A1. Contour interval is 30 mGal

ry 1993 to March 1994, precision levelling within thecaldera revealed an uplift of just 2–3 cm at the summitrelative to a station 5 km east (J. B. Murray, pers. com-mun.; Smithsonian Institution Bulletin of Global Vol-canism Network 1994a). The GPS data indicate thatthere have been no vertical or horizontal movements inexcess of 2 or 1 cm, respectively, between 1994 and1997. The gravity data have not been height corrected,because the maximum size of the correction (ca. 9 mGalfor 3 cm at –308 mGal m–1 assuming the free air gra-dient, or ca. 6 mGal assuming the Bouguer-correctedfree air gradient) is insignificant compared with the ob-served gravity changes. Since 1994 there is some evi-dence for an increase in gravity in the crater area(Fig. 5), although the shape of the change is not veryclear (Fig. 6). It would also appear that the values ob-tained at stations A6 and A9 in 1996 are inconsistentwith the rest of the data; this may be due to local vege-tation effects.

In summary, the gravity data indicate a crater-cen-tred decrease in excess of 90 mGal (largest value ob-served is –192 mGal and maximum may be even largerwithin the active crater, closer to the vent) between Fe-bruary 1993 and April 1994, followed by a less well-de-fined increase over the following years until February1997 of approximately 30 mGal in approximately thesame location.

In view of the negligible volume change of the edif-ice (determined from the lack of deformation), the de-crease in gravity between February 1993 and April 1994near the crater must reflect a subsurface mass and den-sity decrease. In order to model this decrease, an inter-active 2.5D forward-modelling program GRAVMAG(Pedley 1991) was employed. This program calculatesthe gravitational effect at the topographic surface alonga profile of polygons of any size and density beneaththe surface. The models are 2.5D rather than truly 3D,because each polygon must be symmetrical about theprofile, although its thickness or “half strike” is userdefined. Obvious causes of subsurface mass and densitydecreases at an active volcano are (a) water-tablemovement, (b) magma drainage and (c) magma vesicu-lation within a feeder pipe. The observed gravitychanges are so localised, and the water table so deep inthe area (1300 m), that (a) is not considered likely inthis case. Magma drainage is also unlikely, since the pe-riod was characterised by increased degassing and somestrombolian activity.

The plausible dimensions for a feeder pipe (up toseveral tens of metres across) are too small for the max-imum possible density change (–2600 kg m–3; basaltmelt replaced by air) to produce the observed gravitysignature. The gravity changes during this period arecentred about the crater (Fig. 6a) and extend over adistance in excess of 1800 m (the wavelength of theanomaly). Density changes within the feeder pipewould produce an anomaly with a much smaller wavel-ength. On the other hand, density changes within adeep magma chamber (even in only the upper part)

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Fig. 7 a The observed andcalculated gravity changes forthe period February1993–April 1994 along theprofile shown in Figs. 4 and 5for the model illustrated inpart b. b Subsurface body ofreduced density (–600 kg m–3

for A and –100 kg m–3 for B).The gravity effect of this bodyis shown in a. A thinner bodyof density change –300 kg m–3

produces a similar calculatedgravity profile. c Interpretativecross section through Santia-go, taking into account the ob-served structure, the observeddevelopment of the inner crat-er by cavern collapse and thestructural features in Nindiricrater. This diagram alsoshows proposed area of accu-mulation of the gas-rich frothymagma layer beneath the San-tiago pit crater (shown bywhite “bubbles”)

observedcalculated

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vity

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North South

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Interconnectedmagma-filled

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pre-1986crater floor

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1948 suface

1948 suface

Active vent

Bubble-richmagma

Magma reservoirat -0.5-?5 km

depth

Craterbreccia(stippled)

?

?

Topography

Rough location ofpit boundary faults

?

would produce an anomaly with a much larger wavel-ength than that observed. The wavelength of the ano-maly, therefore, constrains the depth and approximatedimensions of the causative body. The magnitude ofthe anomaly is determined largely by the densitychange within the causative body, but since there areno observations inside the crater, the gravity change inthis region must be inferred.

For a N–S profile crossing the active crater (Fig. 4),the model that best fits the observed anomaly suggestsa region of reduced density approximately 440 m in di-ameter at a depth of only a few tens of metres belowthe active vent (Fig. 7). This diameter is similar to thatof Santiago and its inferred boundary faults. The corre-

lation with observed structure lends strong support tothe model. Models consistent with the data include abody approximately 100 m thick with a density contrastof –300 kg m–3, or a stratified body approximately200 m thick, with a thin (25 m) upper layer of high con-trast (e.g. –600 kg m–3) above a smaller contrast, closeto –100 kg m–3.

It is not possible to resolve the differences betweenthese end-member models with this data set, but it isclear that a shallow body of relatively low density de-veloped during the period of this study. These modelssuggest a mass decrease of approximately 3.7!109 kgto 4.0!109 kg. The low-density bodies probably repre-sent an accumulation of highly vesiculated magma with-

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in several caverns whose horizontal distribution is com-parable with the diameter of Santiago crater. A 10%increase in the degree of magma vesiculation wouldproduce the inferred density decrease. The gravity in-creases observed between 1994 and 1997 (Figs. 5, 6b, c)occur over the same region as the preceding decreasesand probably reflect a slight decrease in thickness ofthe vesiculated layer.

Interpretation

Geophysical data

Integrating the microgravity, ground deformation andgas flux data, we note that between February 1993 andApril 1994 there was a shallow mass decrease of ap-proximately 4.0!109 kg, followed over the next 3 yearsby a slight mass increase, that a minimum of2.6!109 kg of gas has been released between 1993 andmid-1997, and that 1.5!1012 kg of magma has de-gassed. Shallow intrusion of new magma is clearly notindicated by the microgravity data, since gravity de-creased during the early period of reactivation, and theslight increase in gravity was associated with steadylow-level activity. However, the replacement of rela-tively dense, degassed magma by gas-rich material be-tween February 1993 and April 1994 is consistent withthe data. We suggest that convection within a deepchamber caused magma overturn during early 1993 andthat, as a result, gas-rich magma was transported to thetop of the chamber while denser magma was displacedto a greater depth (11 km) the magma level beneaththe crater floor may also have fallen but not so deep tohave precluded occasional ejecta at the surface. Thegas-rich magma began to vesiculate due to decompres-sion, and a layer of relatively vesiculated, fizzing mag-ma formed at the top of the magma column within cav-erns beneath the Santiago vent, causing a shallow den-sity reduction and a corresponding decrease in gravity(Fig. 7c). The gradual gravity increase since 1994(Figs. 5, 6) suggests that the thickness of the vesiculatedlayer has reached a maximum and may be decreasing(in thickness or bubble density).

If the gas flux were reasonably constant during1993–1997, then the mass of gas lost in 1993–1994 wasapproximately 7.0!108 kg, approximately six times lessthan the mass decrease according to the gravity calcula-tions. If magma containing 2–3 wt.% dissolved waterrose to replace magma with little or no water, the mag-ma density would decrease by approximately200 kg m–3 (Kazahaya et al. 1994). To satisfy the300 kg m–3 decrease required by the gravity data, thefurther 100 kg m–3 reduction could be caused by a 5%increase in the degree in vesiculation. In this scenario, acombination of shallow gas-rich magma intrusion andvesiculation processes caused the decline in gravity.The second possibility, which we favour, is that the gasflux was substantially higher in 1993–1994 compared

with that in 1996–1997. If this were the case, then anSO2 flux into the vesiculated layer of approximately2.6!106 kg per day would correspond to a total gasflux in this period comparable with the mass loss calcu-lated from the gravity data (ca. 4.0!109 kg). In thiscase, an increase of approximately 10% in the degree ofvesiculation occurred early and is the main cause of thegravity decrease. The COSPEC data suggest that atleast 2.0!109 kg of gas has been erupted, correspond-ing to 8.85!1011 kg of degassed magma. The gas withinthe vesiculated layer accounts for a further 2!109 kg,which means that at least twice the calculated mass ofmagma has actually been degassed (e.g. 1.77!1012 kg).The amount of gas stored within the vesiculated layerindicates that gas release is a slow process and that thesystem could continue to vent gas at the same rate (ap-proximately 3.0!105 kg) for 4 more years – or longerat a reduced flux, even if additional deep magma doesnot degas. The calculated mass of degassed magma(1.8!1012 kg) relates to material within a deep magmareservoir. The volume of degassed magma, assuming adensity of 2700 kg m–3, is 0.6 km3, half that calculatedby Stoiber et al. (1986) for the 1979-1985 episode.There is no net change in mass within the deep reser-voir level and therefore no gravity change because vo-latile-rich material leaving the chamber is replaced bydownward convection of denser degassed magma fromthe shallow system above. This process is similar to oneproposed by Andres et al. (1991) to account for excessSO2 observed at Lonquimay and Lascar volcanoes(Chile) using COSPEC observations and petrologicalcalculations.

We interpret the observed increase in activity atMasaya in terms of a single convective pulse in 1993.We suggest that the cyclic degassing crises at Masayasince the mid-19th century have been caused by period-ic convective overturns of magma. Convection may beinitiated by intrusion of fresh material from below, butit may also be triggered by density contrasts producedeither by magma crystallisation or shallow degassing(Jaupart and Vergniolle 1989; Kazahaya et al. 1994).Thus, dramatic changes in surface activity are not nec-essarily preceded by subsurface intrusion. This classicexample of open system degassing is maintained atMasaya by the combination of a long-lived magma res-ervoir and an open vent.

Discussion and conclusion

We propose a structural model for the Masaya pit crat-ers illustrated along with the gravity interpretation inFig. 7c. This model is based on the study of pit craterwalls (i.e. Fig. 2), observation of the progressive open-ing of the inner crater of Santiago since 1987 (Fig. 3)and the gravity data (Figs. 5, 6). Cylindrical faults, dip-ping slightly outward, guide collapse into a magma res-ervoir. The vertical sides of the craters are produced bycollapse on fractures formed above the concentric

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faults (Fig. 7c). The inner material is broken into acoarse breccia, and continued collapse occurs either bythe formation of new inner faults or by the coalescenceand unroofing of caverns. These caverns may beformed by magma intrusion and withdrawal or by local-ised subsidence of the breccia, possibly caused bystopping of material into the magma reservoir. Whatev-er the mode of formation, the caverns appear to forman interconnected network through which magma andgas can reach the surface.

Changes in the pit crater structure will alter thenear-surface magma plumbing system, resulting inchanges in activity. Cavern collapse can expose magma,leading to mild strombolian activity and lava-filledvents; cavern blocking may cause pressure buildup andshort-term explosive activity. Small-scale magma-levelfluctuations may fill caverns or create more caverns byintrusion and subsequent withdrawal, leading to furthercollapse. Using the analogy of the activity at Nindiricrater in the seventeenth century, new magmatic inputwill cause lava lake formation and may slowly fill upSantiago but is unlikely to cause activity outside the pituntil it overflows. This scenario is valid as long as thecavern system provides free access to the surface.

For at least the last several thousand years Masayahas built pyroclastic cones with associated lava flowfields and has formed pit craters. For the past 150 years,activity has been confined to pit crater formation ac-companied by cycles of intense degassing. Evidencefrom the present pit craters suggests that they developepisodically and are associated with degassing activityand the presence of shallow magma in a cavernous re-gion below the pit crater floor. Geophysical data col-lected during the present reactivation at Masaya sug-gest that no new magma has entered the upper volcanicsystem and that the increase in activity is probably as-sociated with changes within the shallow pit craterstructure. We suggest that the beginning of this degass-ing episode may have been triggered by convectiveoverturn of magma left within the shallow plumbingsystem since the previous (1980s) episode. This de-gassed magma briefly appeared at the surface in Fe-bruary and May 1989 as collapse unroofed caverns inthe inner pit crater. With reactivation in 1993, a vesicu-lated layer of magma developed immediately below thepit crater, leading to local gravity decreases. As the de-gassing episode waned, this layer has become thinneror less vesiculated, leading to the observed gravity in-creases. We suggest that, toward the end of the presentcycle, there will be further collapse of the pit crater asthe pressure within this layer decreases, and magmamay again be uncovered. Indeed this was apparentlyoccurring in May to June of 1997.

The best-fit model for the gravity anomaly suggeststhat a region approximately 200 m thick, extending tothe structural margins of the pit crater, underwent adensity decrease as the degassing began. This indicatesthat all of the pit crater substructure is involved in themagmatic system, rather than a restricted pipe (Fig 7c).

Many small caverns were seen to form and collapseduring the formation of the inner crater; thus, the pres-ent pit crater structure probably also contains severalcaverns, filled with magma or empty.

Pit crater collapse occurs by two processes. Oneprocess is the large-scale collapse of material from out-wardly dipping planes (Figs. 3, 7c). This is responsiblefor the creation of the general structure of the pit crat-ers and is probably related to major episodes of magmawithdrawal in the main reservoir. It may tend to blockvents. The other process is the incremental collapse ofcaverns, formed within the boundaries of the pit crater(Figs. 2, 7c). This appears to occur toward the end of adegassing phase, from either a loss of gas pressure orthe gradual downward movement of breccia within thepit crater relative to magma levels. This incrementalcollapse tends to lower the crater floor relative to themagma level, thus exposing magma to the surface.

The onset of major explosive or effusive activity atMasaya will probably be preceded by a marked altera-tion in the present equilibrium state. We expect a majorchange in activity to be heralded by large increases ingravity over a wider area than the observed variationsreported here, perhaps accompanied by edifice infla-tion and a change in gas flux (possibly even a decreasefollowing vent blockage). We also expect any majorchange to be preceded by considerable increases inseismicity, well above the normal shallow tremor. Suchactivity would not be restricted to pit craters and couldproduce new structures.

Integrated studies, such as the preliminary micro-gravity, ground deformation and SO2 flux study pre-sented here, have a vital role to play in hazard assess-ment, as they provide independent data on the triggersand likely outcome of volcanic events. Our geophysicaldata could not be used to predict rapid minor changesin activity such as occurred in October 1996, but theydo show that the magmatic system at Masaya is in astable state. On the other hand, close observation ofstructural changes, such as vent blocking and caverndevelopment, may give clues to short-term minorevents. Our model for a single convective overturn in1993 followed by steady degassing is consistent withthis event being only a minor, short-lived departurefrom the equilibrium state related to ongoing pit craterevolution. The larger eruptions, such as plinian and ig-nimbrite-forming events, will probably be preceded bymuch larger structural changes and marked geophysicalanomalies, although the sequence of events leading upto these is still unknown and is an important area ofstudy.

Acknowledgements The authors gratefully acknowledge TheRoyal Society, the National Sciences and Engineering ResearchCouncil of Canada and The Leverhulme Trust for financial sup-port, and INETER, especially W. Strauch, H. Teleno and M. Nav-arro, for logistic and scientific support. We are grateful also toORSTOM, especially S. Bonvalot, for use of monumented gravitystations, to R. Hoff of Environment Canada for loan of a COS-PEC, and to W. D. Whitaker for assistance in the field. R. Blake-

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ly, Ch. Newhall, D. Rothery and S. Williams are also thanked forproviding very helpful reviews of a previous version of this manu-script.

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