Journal of Volcanology and Geothermal Research 327 (2016) 375–384

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Journal of Volcanology and Geothermal Research

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Connected magma plumbing system between Cerro Negro and El HoyoComplex, Nicaragua revealed by gravity survey

Patricia MacQueena, b,*, Jeffrey Zureka, Glyn Williams-Jonesa

aDepartment of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, CanadabMicro-g LaCoste, 1401 Horizon Ave., Lafayette, CO 80026, U.S.A.

A R T I C L E I N F O

Article history:Received 5 April 2016Received in revised form 5 September 2016Accepted 8 September 2016Available online 13 September 2016

Keywords:GravityNicaraguaCerro NegroEl HoyoStructureMagmatic plumbing

A B S T R A C T

Cerro Negro, near León, Nicaragua is a young, relatively small basaltic cinder cone volcano that has beenunusually active during its short lifespan. Multiple explosive eruptions have deposited significant amountsof ash on León and the surrounding rural communities. While a number of studies investigate the geo-chemistry and stress regime of the volcano, subsurface structures have only been studied by diffuse soilgas surveys. These studies have raised several questions as to the proper classification of Cerro Negro andits relation to neighboring volcanic features. To address these questions, we collected 119 gravity measure-ments around Cerro Negro volcano in an attempt to delineate deep structures at the volcano. The resultingcomplete Bouguer anomaly map revealed local positive gravity anomalies (wavelength 0.5 to 2 km, magni-tude +4 mGal) and regional positive (10 km wavelength, magnitudes +10 and +8 mGal) and negative (12and 6 km wavelength, magnitudes −18 and −13 mGal) Bouguer anomalies. Further analysis of these gravitydata through inversion has revealed both local and regional density anomalies that we interpret as intru-sive complexes at Cerro Negro and in the Nicaraguan Volcanic Arc. The local density anomalies at CerroNegro have a density of 2700 kg m−3 (basalt) and are located between −250 and −2000 m above sea level.The distribution of recovered density anomalies suggests that eruptions at Cerro Negro may be tapping aninterconnected magma plumbing system beneath El Hoyo, Cerro La Mula, and Cerro Negro, and more thanseven other proximal volcanic features, implying that Cerro Negro should be considered the newest cone ofa Cerro Negro-El Hoyo volcanic complex.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Cerro Negro is a small basaltic volcano (∼108 m3) located approx-imately 20 km northeast of León, Nicaragua (Fig. 1). Although veryyoung (first eruption in 1850) it is quite active, with a recurrenceinterval of 6 to 7 years. Eruptions are typically Strombolian in nature(Volcanic Explosivity Index, VEI, 2 to 3), featuring sustained eruptivecolumns and significant effusive activity. Explosive eruptions atCerro Negro frequently deposit ash on León and many nearby ruralcommunities.

As Cerro Negro presents a clear hazard to surrounding commu-nities, proper classification of the volcano is necessary to betterforecast future behavior. There is some debate over the proper clas-sification of Cerro Negro, with implications for the projected hazards

* Corresponding author.E-mail addresses: patricia@microglacoste.com (P. MacQueen), jmz3@sfu.ca

(J. Zurek), glynwj@sfu.ca (G. Williams-Jones).

of the volcano. Some authors (e.g., Wood, 1980; Hill et al., 1998)maintain that Cerro Negro is a temporary feature, either a long-lived monogenetic cinder cone or a parasitic cinder cone. In contrast,McKnight and Williams (1997) hold that Cerro Negro should beconsidered a juvenile stratovolcano, the beginning of a longer-livedfeature. The distinction is important for more than classification, asthe hazards associated with cinder cone volcanoes are significantlydifferent from those expected for even juvenile stratovolcanoes(McKnight and Williams, 1997; Hill et al., 1998). These classificationarguments derive their evidence mainly from external observationsof the volcano, such as cone morphology and eruptive rate, not fromstructural data about the magmatic plumbing system.

Geophysical surveys, and in particular gravity surveys, are aneffective means of studying the subsurface structure of volcanic sys-tems. The large density contrast between basaltic intrusions andvolcanic tephra or sediments make gravity measurements a logi-cal choice for investigating the subsurface structure of Cerro Negroand neighboring volcanic features. Using an approach similar toBarde-Cabusson et al. (2014) and Connor et al. (2000), we collected

http://dx.doi.org/10.1016/j.jvolgeores.2016.09.0020377-0273/© 2016 Elsevier B.V. All rights reserved.

376 P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384

Fig. 1. Shaded relief map of Cerro Negro, the El Hoyo volcanic complex, and surrounding region. Inset map shows location of study area in Nicaragua (red box). Red dots indicategravity measurement locations. HOTEU is the location of the secondary gravity base station in León, BOUG1 is the location of the primary gravity base station, and CNG2 is thelocation of the continuous GPS station, operated by Pennsylvania State University, used as our GPS base station.

a broad network of gravity measurements at Cerro Negro and itsimmediate vicinity to characterize Cerro Negro and the nearbyvolcanic features in the context of the regional tectonic forces.

We argue that Cerro Negro is best characterized based on itsrelation to neighboring volcanic features. In this study, we presentgravity data collected at Cerro Negro in February and March of 2012and 2013 and the associated density anomalies recovered throughinversion of the Bouguer gravity anomaly. We then discuss how therecovered subsurface structure ties in with the current understand-ing of the volcanic plumbing system at Cerro Negro, and proposethat Cerro Negro is in fact the newest polygenetic cinder cone in alarger volcanic complex comprising Cerro Negro, Cerro La Mula, andEl Hoyo.

2. Geological setting

As relatively small basaltic volcanoes, Cerro Negro and nearbyEl Hoyo are typical for Nicaraguan Arc volcanoes. Relative to therest of Central America, Nicaragua has thinner crust (about 32 kmon average), lower elevations and volcanic edifice heights, denser,more basaltic magmas, and a higher dip angle in the subductingslab (Carr, 1984). Many cones in this region, despite their small size,have polygenetic histories and composite morphologies (McKnightand Williams, 1997). We use the term “polygenetic” here to meana volcano that erupts repeatedly, as defined by Walker (2000). Carr(1984) argues, using a hydrostatic model proposed by Rose et al.(1977), that Nicaragua’s thinner crust and higher magma densitiesprevent Nicaraguan volcanoes from attaining greater edifice heights.In the Nicaraguan volcanic arc, a smaller edifice does not necessarilyimply a short-lived volcano.

Cerro Negro has been regularly active in its brief existence. Therehave been 23 eruptions at Cerro Negro since the first eruption in1850, the most recent occurring in 1992, 1995, and 1999 (Díez,2005; Connor et al., 2001; Hill et al., 1998). From its first eruption in1850 to its most recent eruption in 1999, Cerro Negro has erupted0.058 km3 dense rock equivalent (DRE) of tephra, and 0.040 km3 DREof lava (Connor et al., 2001).

Some information on the magmatic plumbing system that con-trols eruptions at Cerro Negro is provided from melt inclusion studiessuggesting minimum depths for the melt sources that fed eruptionsat Cerro Negro. Roggensack et al. (1997) calculated that magmasfrom the 1992 and 1995 eruptions came from depths of 6 km and1–2 km, respectively. Additionally, Portnyagin et al. (2012) suggesta source region for Cerro Negro magmas of 14 km depth based onstudies of melt inclusions in tephras from the 1867, 1971 and 1992eruptions. Venugopal et al. (2016) proposes a multi-level plumbingsystem for Cerro Negro consisting of a shallow source zone at 2 kmand deeper reservoirs at 7–8 km and 14 km. These data suggest thatCerro Negro magmas begin crystallizing at both mid-crustal andshallow crustal levels. However, these melt inclusion data do notindicate the location of possible magma storage areas, or lateral vari-ations in magma pathways that may connect neighboring volcanicfeatures.

Although the close proximity of Cerro Negro to El Hoyo maysuggest shared origins (Fig. 2), the larger, less active El Hoyo vol-cano (∼5 × 1011 km3) has received much less scientific study. Themost recent eruptions in 1952 and 1954 were phreatic explosionsfrom a NNW trending fissure on the northeastern side of the ElHoyo cone (McBirney, 1955). The only known earlier eruption wasreported in 1528 in the accounts of Spanish settlers in the area; thenature of this eruption, its duration, and even if the eruption was

P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384 377

Fig. 2. Shaded relief map focusing on Cerro Negro and the El Hoyo volcanic complex. Black dashed lines indicate faults mapped by La Femina et al. (2002). Red dots indicategravity measurement locations, and BOUG1 is the location of the primary gravity base station. Green triangles mark the location of other volcanic features in close proximinty toCerro Negro and the El Hoyo complex (Saballos, 2016, unpublished data). “CLM” is Cerro La Mula, “CBV” is Cerro Cabeza de Vaca, “CN” is Cerro Negro, ”VLP” is Volcán Las Pilas,“VEH” is Volcán El Hoyo, “Cerro Las Flores”, “CA” is Cerro Asososca, “CLT” is Cerro Los Tacanistes, “LA” is Laguna de Asososca, and “CEP” is Cerro El Picacho.

located at El Hoyo (the report only references the Marabios Range)are unknown (McKnight, 1995). Aside from the 1528 eruption, thehistorical record does not document any eruptions at any of the othervolcanic features mapped in Fig. 2.

Studies by Venugopal et al. (2016) and McKnight (1995) foundgeochemical links between Cerro Negro and the El Hoyo complex.In the work of McKnight (1995), Harker diagrams of trends in majorelements in the bulk rock geochemistry of samples from Cerro Negro,Cerro La Mula, and Las Pilas consistently show a geochemical trendbetween Cerro Negro and Las Pilas. Cerro Negro defines the maficend of the trend and Las Pilas the more evolved end. Venugopal et al.(2016) expand on this work, further establishing a geochemical trendbetween Cerro Negro and El Hoyo with an analysis of the geochem-istry of melt inclusions, host crystals, and matrix from Cerro Negroand El Hoyo tephra samples. Trends in major elements define an evo-lutionary trend between Cerro Negro and El Hoyo, while analysisof volatiles defines a possible genetic link between the two volca-noes. Interestingly, based on analysis of incompatible elements in ElHoyo samples, Venugopal et al. (2016) suggest that the more evolvedcomposition of El Hoyo magmas may be due to magma mixing withevolved residual magmas from previous eruptions.

3. Gravity survey

Gravity data were collected at Cerro Negro and the surroundingarea in an irregular grid over two field seasons in February/March of2012 and 2013 (Fig. 1). The gravity data collected in 2013 extendedand filled in the survey locations measured in 2012. At the start ofeach survey day, a reading was taken at either the primary base sta-tion at Cerro Negro (BOUG1) or a secondary base station in León(HOTEU) (Fig. 1). One or both of these base stations was measuredat the end of the day. All gravity measurements were normalized toBOUG1. For information on the gravimeters used in the survey and a

complete table of all gravity data and data reductions, see Sections 1and 3 in Supplementary Material.

Height control on gravity measurements was accomplished withdifferential GPS measurements using a dual-frequency Leica SR530system. Rover data were processed relative to data from a contin-uous GPS station (CNG2) operated by Pennsylvania State University(Fig. 1). The height measurements obtained have a median heightquality (standard deviation of the height component) of approxi-mately 2 cm and an average accuracy of approximately 10 cm, andstandard deviation of approximately 30 cm, due to three stationswith >1 m error.

4. Results

In total, 119 gravity measurements were made on Cerro Negroand in the surrounding region, covering a total area of approximately660 km2. Due to access limitations, station spacing is variable, withspacings from 250 to 500 m in a tight grid on Cerro Negro’s cone andthe immediate surroundings, whereas station spacing ranged from 1to 5 km for more distal stations (Figs. 3 and 4).

After being corrected for Earth tide, all gravity data were cor-rected for the free-air gradient (approximate), Bouguer slab, Bullard-B, latitude, terrain, and bathymetry, using the values in Table 1.Tide, free-air gradient, latitude, and Bouguer slab corrections fol-low the methods described in Telford et al. (1990), the Bullard-Bcorrection follows the methods of LaFehr (1991), and the terrainand bathymetry correction were performed using a digital eleva-tion model following the methods of Olivier and Simard (1981).The latitude correction uses the Geodetic Reference System (GRS-1967) as a reference (Kovalevsky, 1971). Although it is oftenstandard procedure to estimate a suitable value for terrain den-sity by minimizing the correlation between the gravity anomalyand height (Nettleton, 1939), this methodology was not employed

378 P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384

Fig. 3. Interpolated complete Bouguer gravity anomaly map of Cerro Negro and surrounding region. The gravity scale is relative to gravity measurements at the base station,BOUG1. Black dots mark measurement locations. Interpolated gravity data are overlain on 100 m topography contours. HOTEU is the location of the secondary gravity base stationin León, BOUG1 is the location of the primary gravity base station, and CNG2 is the location of the continuous GPS station, operated by Pennsylvania State University, used as ourGPS base station.

here given the existence of a suitable published value for terraindensity in Elming and Rasmussen (1997) and the wide varietyof terrain densities expected in the survey area. The combineddaily magnitude of drift and tares were constrained by the twicedaily gravity measurements at BOUG1, and included in the errorbudget.

Possible errors in the data set arise from a combination of effects.These effects include (but are not limited to) the precision of thegravimeters, tares, wind noise, uncertainty in GPS measurements,and the limited resolution of the DEM used for terrain corrections.Taking into account all these sources of uncertainty, a maximumerror of 0.5 mGal was assumed for all stations.

Owing to the irregular distribution of gravity measurement loca-tions, the resolution of the measurement grid depends on locationwithin the grid. Neglecting noise, in the area of interest within 10 kmof Cerro Negro, the smallest wavelength structure that could beimaged without aliasing ranges from 500 m to 2000 m (Fig. 5). Far-ther than 10 km from the cone, resolution is poorer, as the morewidely spaced distal stations were measured primarily to constrainregional trends, not for resolving structures of interest close to CerroNegro.

As seen in Fig. 4, the Bouguer gravity anomaly after correctionsshows three small (∼500 m across) negative anomalies, magnitudes−4 to −6 mGal, centered on the cone of Cerro Negro and two of thecones in the El Hoyo complex (“2” in Fig. 4). Positive anomalies in thevicinity of Cerro Negro (“1” in Fig. 4) include a 2 km by 2 km anomalyto the east of Cerro Negro in the northern portion of the El Hoyocomplex, and a 500 m by 500 m anomaly to the southeast of CerroNegro (Fig. 4), each of these have a magnitude of +4 mGal. Fig. 3also shows regional negative anomalies to the southwest and north-east of Cerro Negro (∼12 and 6 km wavelength, magnitudes −18 and−13 mGal) and large positive anomalies to the northwest and south-east of Cerro Negro (∼10 km wavelength, magnitudes +10 and +8mGal, respectively). It should be noted that these large anomalies are

defined by only a few points with large (>3 km) spacing betweenmeasurements.

5. 3-D gravity inversion

5.1. Modeling method

The GROWTH2.0 inversion package was chosen to invert grav-ity data in this study for its superior handling of irregularly griddeddata, automatic subtraction of a linear regional trend, and easilyaccessed inversion statistics for comparing models (Camacho et al.,2011, 2002; Del Potro et al., 2013). The GROWTH2.0 inversion beginswith a skeletal model, and then grows this model by selecting cells toadd to the initial model according to a balance between model fit todata and model smoothness (minimization of anomalous mass). Thebalance between model fit and model smoothness is chosen by theuser with the “balance factor”; a lower balance factor favors modelfit and a higher balance factor favors model smoothness (Camachoet al., 2011, 2002). The GROWTH2.0 inversion also simultaneouslyinverts for and subtracts a linear regional trend and offset. The usercan also select a value for the homogeneity of the model (sharpnessof the boundaries of density anomalies), incorporate stratified back-ground density contrasts, and recalculate the terrain density (seeCamacho et al. (2002) for further details). Model quality is primarilyevaluated using the standard deviation of the residuals, the flatnessof the autocorrelation function of the residuals, and the visual aspectof the model, which includes the amount of noise or the presence ofinflated or skeletal anomalies.

5.2. Inversion parameters

To limit the range of possible density models obtained throughinversion and ensure that our results tied in well with previousresearch on crustal structures in this area, we used a set of density

P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384 379

Fig. 4. Interpolated complete Bouguer gravity anomaly map, focused on Cerro Negro and vicinity. Features marked with a boxed “1” or “2” are positive and negative Bougueranomalies, respectively, discussed in the text. The gravity scale is relative to gravity measurements at the base station, BOUG1. Black dots mark measurement locations. Interpo-lated gravity data are overlain on 100 m topography contours. BOUG1 is the location of the primary gravity base station, and CNG2 is the location of the continuous GPS station,operated by Pennsylvania State University, used as our GPS base station.

contrast bounds drawn from a density model of the Nicaraguan Arcin Elming and Rasmussen (1997). Table 2 shows the three sets of den-sity contrast ranges used for gravity inversion in GROWTH2.0. Thepositive density contrast bounds correspond to the density of lavaand volcanic rocks (2700 to 2800 kg m−3) in the density model inElming and Rasmussen (1997). The negative density contrast boundscorrespond to the density of colluvial deposits (2110 kg m−3) andsedimentary rocks (2180 kg m−3). The set 2 density contrast boundsare the average of the set 1 and 3 density contrast bounds.

We chose these different sets of density contrast ranges in orderto explore the range in the volumes of recovered anomalies. As thetrue density contrast bounds are not known, we chose the middle-range density contrast bounds for our final model, and used thehigher and lower range density contrast bounds to generate upperand lower bounds on anomaly volumes.

We did not use a stratified background density contrast for anyof the models, as the models did not require a significant strati-fied background density contrast to reproduce the data. Additionally,there was no evidence in the literature specific to the region that a

significant stratified background density contrast should be expectedin the area of investigation. As discussed in Camacho et al. (2011),the primary effect of a too-small background density contrast wouldbe distortion of the density anomalies in which the anomalies wouldappear to bulge at their top. As part of our analysis of the gravitydata, we tested a number of increasingly extreme background den-sity contrasts, and observed no significant distortion of the anomaliesproximal to Cerro Negro.

In the first rounds of inversion, 9 stations were removed as out-liers due to high residual error values. Gravity data were weighteduniformly for these inversions, as our lack of constraints on error dueto near-terrain corrections, which could range from 0.01 mGal to 0.5mGal, makes any assumptions of differential error between stationsdifficult to substantiate.

5.3. Model characteristics

The final best fit model, shown in depth slices in Fig. 6, revealsdensity anomalies most likely related to volcanism at Cerro Negro

Table 1Corrections applied to gravity data.

Correction Value/Resolution Source

Free air 0.3086 mGal m−1

Bouguer Slab, Bullard-B 2450 kg m−3 Carr (1984),Elming and Rasmussen (1997)

Terrain 2450 kg m−3/30 m Carr (1984),Elming and Rasmussen (1997)ASTER Global DEM (NASA Land Processes Distributed Active Archive Center, 2001)

Bathymetry 1030 kg m−3/90 m General Bathymetric Chart of the Oceans (British Oceanographic Data Centre,2009)

380 P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384

Fig. 5. Calculation of Nyquist resolution for the gravity survey grid used in this study overlain on 100 m topographic contours. BOUG1 is the location of the primary gravity basestation and CNG2 is the location of the continuous GPS station, operated by Pennsylvania State University, used as our GPS base station.

and El Hoyo (See Fig. S1 for anomalies projected as 3D uniformdensity surfaces). For reference, the base of Cerro Negro is atapproximately 450 m a.s.l. (above sea level). The primary feature ofthe model in the vicinity of Cerro Negro is a positive density anomalyconsisting of three main lobes, located between 250 and 2000 ma.s.l., marked with arrows in Figs. 6 and 7. One lobe is located beneaththe northern portion of El Hoyo (1), the second just to the south-east of Cerro Negro (2), and the third to the northeast of Cerro LaMula (3). As seen in Fig. 6, the shared density anomaly is first presentat −2000 m a.s.l. beneath the northwestern portion of El Hoyo. TheCerro Negro and Cerro La Mula portions of the anomaly becomedistinct from the El Hoyo anomaly at −500 m a.s.l. (Fig. 6A,B). Thenorthern lobes of the shared density anomaly persist to −250 m a.s.l.The connections between the lobes of the anomaly are robust andwell defined features with wavelengths above the Nyquist limit inthat area (Fig. 5). Additionally, the three lobes of the density anomalycorrespond well to the three positive Bouguer gravity anomalies inthe vicinity of Cerro Negro (Fig. 7). Thus we can be reasonably confi-dent in the first-order geometry of the recovered density anomaliesin this region.

Fig. 8 shows the observed Bouguer gravity anomaly (Fig. 8A) andthe predicted gravity data calculated from the best fit model (Fig. 8B).Most of the main features of the observed data are adequately repro-duced in the predicted data, with the exception of some fine scaleanomalies in the immediate vicinity of Cerro Negro.

While we can have a reasonable level of confidence in the modelgeometry, the volumes of the density anomalies are more difficult toconstrain. As discussed in the previous section, we used three sets of

Table 2Density contrast bounds for GROWTH2.0 gravity inversions. The zero density contrastis set to the Bouguer correction value of 2450 kg m−3.

Set Density contrast bounds (kg m−3) Absolute density bounds (kg m−3)

1 (Smallest)−270 to 250 2180 to 27002 (Middle) −305 to 300 2145 to 27503 (Largest) −340 to 350 2110 to 2800

density contrast bounds to generate three different density contrastmodels to investigate the possible range of anomaly volumes. Vol-umes were calculated using a density cutoff of 2690 kg m−3 (lowerend density for basalt; Telford et al., 1990). Using density contrastmodels calculated from the three sets of density contrast bounds, theCerro Negro segment of the positive anomaly has a volume of 1 to2 km3, the Cerro La Mula segment has a volume of 0.1 to 0.4 km3 andthe El Hoyo anomaly has a volume of 3 to 5 km3. The total volume ofthe positive anomaly above the 2960 kg m−3 cutoff ranged from 4 to7 km3 (Table 3).

5.4. Error sources and limitations of inversion method

The density contrast models produced by the inversion havelimitations and restrictions when interpreting the model. First,resolution is variable – shallow structures (< 2 km) will be resolvedmore precisely than deeper structures (> 2 km). Second, the modelsreveal density contrasts rather than absolute densities, meaning thatthe background density structure may be homogenous or stratified.To definitively distinguish between these end-members, surveyingwith a method sensitive to horizontal structures would be neces-sary. Third, noise in the models can distort the geometry and size ofanomalies observed, such that detailed interpretation of fine-scalestructures should be avoided.

6. Discussion

Given that the range of densities obtained for the positive anoma-lies at Cerro Negro, Cerro La Mula, and El Hoyo correspond to thedensity of basalt, it is likely that these structures represent shal-low intrusive complexes associated with each volcanic center. Asthe density anomalies are clearly connected with the exception ofthe small positive density anomaly to the southwest of Cerro LaMula (Fig. 6), this suggests that the magma plumbing systems of ElHoyo, Cerro La Mula, and Cerro Negro are also connected. Over time,numerous dike intrusions between El Hoyo, Cerro Negro, and CerroLa Mula, exploiting the easy pathways afforded by the NNW trend-ing extensional zone and NE trending faults (La Femina et al., 2002)

P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384 381

Fig. 6. Depth slices through Set 2 (Table 2) GROWTH2.0 model. Gravity measurement locations are marked with black dots. Density values are relative to the Bouguer correctionvalue of 2450 kg m−3, overlain on 200 m topographic contours. In A, CN marks the location of Cerro Negro, CLM for Cerro La Mula, and EH for El Hoyo. In B, “1” indicates the ElHoyo lobe of the positive density anomaly, “2” the Cerro Negro lobe, and “3” the Cerro La Mula lobe.

382 P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384

Fig. 7. Recovered density model at −500 m a.s.l. (A) and complete Bouguer anomaly map (B). In (A), “1” marks the El Hoyo density anomaly, “2” marks the Cerro Negro anomaly,and “3” marks the Cerro La Mula anomaly. Black dots mark gravity measurement locations. In (B), CN marks the location of Cerro Negro, CLM for Cerro La Mula, and EH for El Hoyo.

could have cooled and crystallized to form the connections observedbetween the positive anomalies.

6.1. Role of regional tectonics

The orientation of the inferred zones of dike intrusions likelyreflect the influence of regional tectonics. La Femina et al. (2002) pro-pose a model in which crustal blocks in the Caribbean plate rotatealong northeast trending strike-slip faults that favors east-west ori-ented extension in the center of the blocks. This east-west extensiongives rise to north-northwest trending extensional features (Díez,2005). The northwest orientation of the dikes between Cerro La Mulaand El Hoyo and the location of the Cerro Negro anomaly are mostlikely the result of magmas taking advantage of east-west exten-sion. Fault capture, in which intruding magmas exploit faults as amechanically efficient path to the surface likely explains the north-east orientation of the dikes between Cerro Negro and El Hoyo(Gaffney et al., 2007). Northeast trending faults in the area (Fig. 2)

would be weakened by active slip in the current tectonic regime,creating an easy pathway for intruding magmas.

It is also worth noting how well the orientation of the densityanomalies to the northwest and southeast of Cerro Negro agreeswith the zone of static stress change calculated by Díez (2005) forthe 1999 eruption of Cerro Negro. Díez (2005) calculated that thethree earthquakes preceding the 1999 eruption could have causedstress reduction along a plane oriented north-south along the CerroNegro-Cerro La Mula eruption sufficient to trigger the 1999 erup-tion. It is thus possible that residual magmas from the 1999 eruptioncontribute to the density anomalies imaged to the northwest andsoutheast of Cerro Negro.

6.2. Multi-level magma plumbing system

The density anomalies imaged in this study likely representthe shallowest portion of a multi-level magma plumbing systemthat feeds Cerro Negro, El Hoyo, Cerro La Mula, and other volcanic

Fig. 8. Observed (A) and predicted (B) gravity data for the density contrast model presented in Figs. 6 and 7. Gravity measurement locations are marked with black dots.

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Table 3Density anomaly volume ranges. Anomaly volumes calculated using a density cutoffof 2960 kg m−3.

Density anomaly Volume range (km3)

Cerro Negro 1–2Cerro La Mula 0.1–0.4El Hoyo 3–5Total 4–7

features in its immediate vicinity. Melt inclusion studies of erup-tive products at Cerro Negro consistently show evidence for magmastorage at multiple depths (Portnyagin et al., 2012; Roggensack,2001; Roggensack et al., 1997; Venugopal et al., 2016). The depth ofthe Cerro Negro anomaly is in agreement with storage depths for1995 eruption magmas obtained from melt inclusion data (Roggen-sack et al., 1997; Venugopal et al., 2016), so this shallow anomalymay represent a zone of shallow magma storage, where un-eruptedmagma cools to form an intrusive complex of dikes and sills.Although the El Hoyo anomaly has a greater vertical extent than theCerro Negro anomaly, its depth is also in general agreement withexisting melt inclusion data (Portnyagin et al., 2012; Roggensack etal., 1997; Venugopal et al., 2016), opening the possibility that theEl Hoyo complex may also be a storage site for magmas eventuallyerupted at either Cerro Negro or El Hoyo.

6.3. The Cerro Negro-El Hoyo complex

The interconnected magma plumbing systems imaged in thisstudy are consistent with geochemical studies of the eruptive prod-ucts of Cerro Negro and El Hoyo that show evidence for chemicallinks between the magmas of the two volcanoes. The geometry of theshared density anomaly, tapering down to a single distinguishablesource at approximately −2000 m a.s.l., may represent one possiblepathway for magmas ascending to both El Hoyo and Cerro Negro,where magmas may stall before taking diverging paths to eruptionat the surface, as discussed in Venugopal et al. (2016) and McKnight(1995). It should be noted that the resolution of this gravity data setdoes not rule out the possibility of a separate, smaller conduit feed-ing only Cerro Negro, providing melt from magma storage areas atroughly 7 and 14 km depth (e.g., Portnyagin et al., 2012; Venugopalet al., 2016). In general, however, we now have both geophysical andgeochemical evidence for connections at shallow depths betweenCerro Negro and El Hoyo.

An alternate explanation for the shared density anomaly could bea series of co-located, but not connected, intrusions of cooled magmain the form of stacked dikes and sills, active at different times. Thedensity model in this study represents a single snapshot in time,such that the density model alone cannot constrain the temporalrelationship between the different lobes of the anomaly. However,combining this snapshot structural model with the evolutionary andmixing trends between Cerro Negro, El Hoyo, and other nearby vol-canic features revealed by previous geochemical studies (McKnight,1995; Venugopal et al., 2016) provides compelling evidence for asystem connected in time as well as space.

Given that the magma plumbing systems of Cerro Negro and ElHoyo are likely connected at depth, the two volcanoes should bemore properly viewed as elements of the same complex, with CerroNegro representing the newest active vent of the complex. Viewed inthis light, this complex can be considered as an intermediate mem-ber between the end-members of distributed volcanic fields (e.g.,Michoacán, Mexico and Springerville, Arizona; Connor et al., 1992;Connor and Conway, 2000) and single-edifice stratovolcanoes (e.g.,Concepción and San Cristóbal, Nicaragua; Carr, 1984; Siebert andSimkin, 2002). The Cerro Negro-El Hoyo complex has far fewer conesthan classic cases of distributed volcanism in which a volcanic field

may host hundreds of cones (Connor and Conway, 2000), and CerroNegro (and likely many of the cones in the El Hoyo complex) isdemonstrably polygenetic, unlike the smaller monogenetic cindercones found in larger fields of distributed volcanism (i.e., Paricutinand Jorullo, Mexico; Connor and Conway, 2000). However, there is nosingle dominant peak in the Cerro Negro-El Hoyo complex, and thedistribution of cones is over a relatively large area (approximately12 km × 13 km). The results of our gravity survey display what maybe a central magma chamber connecting to a similarly sized parasiticmagma chamber feeding Cerro Negro volcano and other volcanic fea-tures in the complex. Accordingly, Cerro Negro should be classifiednot as a separate entity, but as part of a larger system fed by a wellestablished magma plumbing system.

7. Conclusion

The inversion of gravity data collected at Cerro Negro has revealedseveral interconnected intrusive basaltic bodies in the vicinity of thevolcano. These bodies most likely represent a combination of shal-low intrusions where magma was temporarily stalled on its way tothe surface and a surrounding network of dikes. The geometry of thebodies, with lobes beneath Cerro Negro, the El Hoyo complex, andCerro La Mula, suggests that Cerro Negro is not a separate entity,but instead shares a plumbing system with a volcanic complex com-prising the cones and volcanic features in the immediate proximity.Volcanic activity at Cerro Negro may represent an intermediate styleof edifice building between the end members of stratovolcano anddistributed volcanism. The presence of a well established and con-nected magma plumbing system beneath the Cerro Negro-El Hoyocomplex suggests that Cerro Negro is likely to be a long lived fea-ture. Future study of Cerro Negro should incorporate observationsof the complete Cerro Negro-El Hoyo complex for an improvedunderstanding of volcanic behavior and potential hazards from thecomplex.

Acknowledgments

We thank the two anonymous reviewers whose comments andsuggestions significantly improved this paper. This study was sup-ported by an NSERC Discovery Grant to G. Williams-Jones. Manythanks to Gwen Flowers and Jackie Caplan-Auerbach for helpfulcomments and discussion. Thank you to Hazel Rymer for valuablediscussion and the use of the G-513 gravity meter. Our thanks toPete LaFemina and Halldor Geirsson for access to their continuousGPS site at Cerro Negro. We are also grateful to INETER (InstitutoNicaragüense de Estudios Territoriales) and the staff of the Coop-erativa Las Pilas-El Hoyo for their support, in particular ArmandoSaballos for the names and locations of the volcanic features nearCerro Negro. Thank you to Tim Niebauer of Micro-g LaCoste for theuse of facilities for final data processing and manuscript editing.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2016.09.002

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