Connected Magma Plumbing System Between Cerro Negro and the El

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 Hoyo Complex, Nicaragua revealed by gravity survey

Patricia MacQueena, b,*, Jeffrey Zurek a, Glyn Williams-Jonesa aDepartment of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada bMicro-g LaCoste, 1401 Horizon Ave., Lafayette, CO 80026, U.S.A.

ARTICLE INFO ABSTRACT

Article history: Cerro Negro, near León, Nicaragua is a young, relatively small basaltic cinder cone volcano that has been Received 5 April 2016 unusually active during its short lifespan. Multiple explosive eruptions have deposited signiﬁcant amounts Received in revised form 5 September 2016 of ash on León and the surrounding rural communities. While a number of studies investigate the geo- Accepted 8 September 2016 chemistry and stress regime of the volcano, subsurface structures have only been studied by diffuse soil Available online 13 September 2016 gas surveys. These studies have raised several questions as to the proper classiﬁcation of Cerro Negro and its relation to neighboring volcanic features. To address these questions, we collected 119 gravity measure- Keywords: ments around Cerro Negro volcano in an attempt to delineate deep structures at the volcano. The resulting Gravity complete Bouguer anomaly map revealed local positive gravity anomalies (wavelength 0.5 to 2 km, magni- Nicaragua tude +4 mGal) and regional positive (10 km wavelength, magnitudes +10 and +8 mGal) and negative (12 Cerro Negro − − El Hoyo and 6 km wavelength, magnitudes 18 and 13 mGal) Bouguer anomalies. Further analysis of these gravity Structure data through inversion has revealed both local and regional density anomalies that we interpret as intru- Magmatic plumbing sive complexes at Cerro Negro and in the Nicaraguan Volcanic Arc. The local density anomalies at Cerro Negro 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 an interconnected magma plumbing system beneath El Hoyo, Cerro La Mula, and Cerro Negro, and more than seven other proximal volcanic features, implying that Cerro Negro should be considered the newest cone of a Cerro Negro-El Hoyo volcanic complex. © 2016 Elsevier B.V. All rights reserved.

1. Introduction of the volcano. Some authors (e.g., Wood, 1980; Hill et al., 1998) maintain that Cerro Negro is a temporary feature, either a long- Cerro Negro is a small basaltic volcano (∼108 m3) located approx- lived monogenetic cinder cone or a parasitic cinder cone. In contrast, imately 20 km northeast of León, Nicaragua (Fig. 1). Although very McKnight and Williams (1997) hold that Cerro Negro should be young (first eruption in 1850) it is quite active, with a recurrence considered a juvenile stratovolcano, the beginning of a longer-lived interval of 6 to 7 years. Eruptions are typically Strombolian in nature feature. The distinction is important for more than classification, as (Volcanic Explosivity Index, VEI, 2 to 3), featuring sustained eruptive the hazards associated with cinder cone volcanoes are significantly columns and significant effusive activity. Explosive eruptions at different from those expected for even juvenile stratovolcanoes Cerro Negro frequently deposit ash on León and many nearby rural (McKnight and Williams, 1997; Hill et al., 1998). These classification communities. arguments derive their evidence mainly from external observations As Cerro Negro presents a clear hazard to surrounding commu- of the volcano, such as cone morphology and eruptive rate, not from nities, proper classification of the volcano is necessary to better structural data about the magmatic plumbing system. forecast future behavior. There is some debate over the proper clas- Geophysical surveys, and in particular gravity surveys, are an sification of Cerro Negro, with implications for the projected hazards effective means of studying the subsurface structure of volcanic systems. The large density contrast between basaltic intrusions and volcanic tephra or sediments make gravity measurements a logi- cal choice for investigating the subsurface structure of Cerro Negro * Corresponding author. E-mail addresses: [email protected] (P. MacQueen), [email protected] and neighboring volcanic features. Using an approach similar to (J. Zurek), [email protected] (G. Williams-Jones). Barde-Cabusson et al. (2014) and Connor et al. (2000), we collected

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 indicate gravity 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 the location 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 its Cerro Negro has been regularly active in its brief existence. There immediate vicinity to characterize Cerro Negro and the nearby have been 23 eruptions at Cerro Negro since the first eruption in volcanic features in the context of the regional tectonic forces. 1850, the most recent occurring in 1992, 1995, and 1999 (Díez, We argue that Cerro Negro is best characterized based on its 2005; Connor et al., 2001; Hill et al., 1998). From its first eruption in relation to neighboring volcanic features. In this study, we present 1850 to its most recent eruption in 1999, Cerro Negro has erupted gravity data collected at Cerro Negro in February and March of 2012 0.058 km3 dense rock equivalent (DRE) of tephra, and 0.040 km3 DRE and 2013 and the associated density anomalies recovered through of lava (Connor et al., 2001). inversion of the Bouguer gravity anomaly. We then discuss how the Some information on the magmatic plumbing system that con- recovered subsurface structure ties in with the current understand- trols eruptions at Cerro Negro is provided from melt inclusion studies ing of the volcanic plumbing system at Cerro Negro, and propose suggesting minimum depths for the melt sources that fed eruptions that Cerro Negro is in fact the newest polygenetic cinder cone in a at Cerro Negro. Roggensack et al. (1997) calculated that magmas larger volcanic complex comprising Cerro Negro, Cerro La Mula, and from the 1992 and 1995 eruptions came from depths of 6 km and El Hoyo. 1–2 km, respectively. Additionally, Portnyagin et al. (2012) suggest a source region for Cerro Negro magmas of 14 km depth based on studies of melt inclusions in tephras from the 1867, 1971 and 1992 2. Geological setting eruptions. Venugopal et al. (2016) proposes a multi-level plumbing system for Cerro Negro consisting of a shallow source zone at 2 km As relatively small basaltic volcanoes, Cerro Negro and nearby and deeper reservoirs at 7–8 km and 14 km. These data suggest that El Hoyo are typical for Nicaraguan Arc volcanoes. Relative to the Cerro Negro magmas begin crystallizing at both mid-crustal and rest of Central America, Nicaragua has thinner crust (about 32 km shallow crustal levels. However, these melt inclusion data do not on average), lower elevations and volcanic edifice heights, denser, indicate the location of possible magma storage areas, or lateral vari- more basaltic magmas, and a higher dip angle in the subducting ations in magma pathways that may connect neighboring volcanic slab (Carr, 1984). Many cones in this region, despite their small size, features. have polygenetic histories and composite morphologies (McKnight Although the close proximity of Cerro Negro to El Hoyo may and Williams, 1997). We use the term “polygenetic” here to mean suggest shared origins (Fig. 2), the larger, less active El Hoyo vol- a volcano that erupts repeatedly, as defined by Walker (2000). Carr cano (∼5 × 1011 km3) has received much less scientific study. The (1984) argues, using a hydrostatic model proposed by Rose et al. most recent eruptions in 1952 and 1954 were phreatic explosions (1977), that Nicaragua’s thinner crust and higher magma densities from a NNW trending fissure on the northeastern side of the El prevent Nicaraguan volcanoes from attaining greater edifice heights. Hoyo cone (McBirney, 1955). The only known earlier eruption was In the Nicaraguan volcanic arc, a smaller edifice does not necessarily reported in 1528 in the accounts of Spanish settlers in the area; the imply a short-lived volcano. nature 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 indicate gravity 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 to Cerro 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) complete table of all gravity data and data reductions, see Sections 1 are unknown (McKnight, 1995). Aside from the 1528 eruption, the and 3 in Supplementary Material. historical record does not document any eruptions at any of the other Height control on gravity measurements was accomplished with volcanic features mapped in Fig. 2. differential GPS measurements using a dual-frequency Leica SR530 Studies by Venugopal et al. (2016) and McKnight (1995) found system. Rover data were processed relative to data from a contin- geochemical links between Cerro Negro and the El Hoyo complex. uous GPS station (CNG2) operated by Pennsylvania State University In the work of McKnight (1995), Harker diagrams of trends in major (Fig. 1). The height measurements obtained have a median height elements in the bulk rock geochemistry of samples from Cerro Negro, quality (standard deviation of the height component) of approxi- Cerro La Mula, and Las Pilas consistently show a geochemical trend mately 2 cm and an average accuracy of approximately 10 cm, and between Cerro Negro and Las Pilas. Cerro Negro defines the mafic standard deviation of approximately 30 cm, due to three stations end of the trend and Las Pilas the more evolved end. Venugopal et al. with >1 m error. (2016) expand on this work, further establishing a geochemical trend between Cerro Negro and El Hoyo with an analysis of the geochemistry of melt inclusions, host crystals, and matrix from Cerro Negro and El Hoyo tephra samples. Trends in major elements define an evo- 4. Results lutionary trend between Cerro Negro and El Hoyo, while analysis of volatiles defines a possible genetic link between the two volca- In total, 119 gravity measurements were made on Cerro Negro noes. Interestingly, based on analysis of incompatible elements in El and in the surrounding region, covering a total area of approximately Hoyo samples, Venugopal et al. (2016) suggest that the more evolved 660 km2. Due to access limitations, station spacing is variable, with composition of El Hoyo magmas may be due to magma mixing with spacings from 250 to 500 m in a tight grid on Cerro Negro’s cone and evolved residual magmas from previous eruptions. the immediate surroundings, whereas station spacing ranged from 1 to 5 km for more distal stations (Figs. 3 and 4). After being corrected for Earth tide, all gravity data were corrected for the free-air gradient (approximate), Bouguer slab, Bullard- 3. Gravity survey B, latitude, terrain, and bathymetry, using the values in Table 1. Tide, free-air gradient, latitude, and Bouguer slab corrections fol- Gravity data were collected at Cerro Negro and the surrounding low the methods described in Telford et al. (1990), the Bullard-B area in an irregular grid over two field seasons in February/March of correction follows the methods of LaFehr (1991), and the terrain 2012 and 2013 (Fig. 1). The gravity data collected in 2013 extended and bathymetry correction were performed using a digital eleva- and filled in the survey locations measured in 2012. At the start of tion model following the methods of Olivier and Simard (1981). each survey day, a reading was taken at either the primary base sta- The latitude correction uses the Geodetic Reference System (GRS- tion at Cerro Negro (BOUG1) or a secondary base station in León 1967) as a reference (Kovalevsky, 1971). Although it is often (HOTEU) (Fig. 1). One or both of these base stations was measured standard procedure to estimate a suitable value for terrain den- at the end of the day. All gravity measurements were normalized to sity by minimizing the correlation between the gravity anomaly BOUG1. For information on the gravimeters used in the survey and a and 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 station in 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 our GPSbasestation.

here given the existence of a suitable published value for terrain defined by only a few points with large (>3 km) spacing between density in Elming and Rasmussen (1997) and the wide variety measurements. of terrain densities expected in the survey area. The combined daily magnitude of drift and tares were constrained by the twice 5. 3-D gravity inversion daily gravity measurements at BOUG1, and included in the error budget. 5.1. Modeling method Possible errors in the data set arise from a combination of effects. These effects include (but are not limited to) the precision of the The GROWTH2.0 inversion package was chosen to invert grav- gravimeters, tares, wind noise, uncertainty in GPS measurements, ity data in this study for its superior handling of irregularly gridded and the limited resolution of the DEM used for terrain corrections. data, automatic subtraction of a linear regional trend, and easily Taking into account all these sources of uncertainty, a maximum accessed inversion statistics for comparing models (Camacho et al., error of 0.5 mGal was assumed for all stations. 2011, 2002; Del Potro et al., 2013). The GROWTH2.0 inversion begins Owing to the irregular distribution of gravity measurement loca- with a skeletal model, and then grows this model by selecting cells to tions, the resolution of the measurement grid depends on location add to the initial model according to a balance between model fit to within the grid. Neglecting noise, in the area of interest within 10 km data and model smoothness (minimization of anomalous mass). The of Cerro Negro, the smallest wavelength structure that could be balance between model fit and model smoothness is chosen by the imaged without aliasing ranges from 500 m to 2000 m (Fig. 5). Far- user with the “balance factor”; a lower balance factor favors model ther than 10 km from the cone, resolution is poorer, as the more fit and a higher balance factor favors model smoothness (Camacho widely spaced distal stations were measured primarily to constrain et al., 2011, 2002). The GROWTH2.0 inversion also simultaneously regional trends, not for resolving structures of interest close to Cerro inverts for and subtracts a linear regional trend and offset. The user Negro. can also select a value for the homogeneity of the model (sharpness As seen in Fig. 4, the Bouguer gravity anomaly after corrections of the boundaries of density anomalies), incorporate stratified back- shows three small (∼500 m across) negative anomalies, magnitudes ground density contrasts, and recalculate the terrain density (see −4to−6 mGal, centered on the cone of Cerro Negro and two of the Camacho et al. (2002) for further details). Model quality is primarily cones in the El Hoyo complex (“2” in Fig. 4). Positive anomalies in the evaluated using the standard deviation of the residuals, the flatness vicinity of Cerro Negro (“1” in Fig. 4) include a 2 km by 2 km anomaly of the autocorrelation function of the residuals, and the visual aspect to the east of Cerro Negro in the northern portion of the El Hoyo of the model, which includes the amount of noise or the presence of complex, and a 500 m by 500 m anomaly to the southeast of Cerro inflated or skeletal anomalies. Negro (Fig. 4), each of these have a magnitude of +4 mGal. Fig. 3 also shows regional negative anomalies to the southwest and north- 5.2. Inversion parameters east of Cerro Negro (∼12 and 6 km wavelength, magnitudes −18 and −13 mGal) and large positive anomalies to the northwest and south- To limit the range of possible density models obtained through east of Cerro Negro (∼10 km wavelength, magnitudes +10 and +8 inversion and ensure that our results tied in well with previous mGal, respectively). It should be noted that these large anomalies are research 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 Bouguer anomalies, 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 Arc significant stratified background density contrast should be expected in Elming and Rasmussen (1997). Table 2 shows the three sets of den- in the area of investigation. As discussed in Camacho et al. (2011), sity contrast ranges used for gravity inversion in GROWTH2.0. The the primary effect of a too-small background density contrast would positive density contrast bounds correspond to the density of lava be distortion of the density anomalies in which the anomalies would and volcanic rocks (2700 to 2800 kg m−3) in the density model in appear to bulge at their top. As part of our analysis of the gravity Elming and Rasmussen (1997). The negative density contrast bounds data, we tested a number of increasingly extreme background den- correspond to the density of colluvial deposits (2110 kg m−3)and sity contrasts, and observed no significant distortion of the anomalies sedimentary rocks (2180 kg m−3). The set 2 density contrast bounds proximal to Cerro Negro. are the average of the set 1 and 3 density contrast bounds. In the first rounds of inversion, 9 stations were removed as out- We chose these different sets of density contrast ranges in order liers due to high residual error values. Gravity data were weighted to explore the range in the volumes of recovered anomalies. As the uniformly for these inversions, as our lack of constraints on error due true density contrast bounds are not known, we chose the middle- to near-terrain corrections, which could range from 0.01 mGal to 0.5 range density contrast bounds for our final model, and used the mGal, makes any assumptions of differential error between stations higher and lower range density contrast bounds to generate upper difficult to substantiate. and lower bounds on anomaly volumes. We did not use a stratified background density contrast for any 5.3. Model characteristics of the models, as the models did not require a significant stratified background density contrast to reproduce the data. Additionally, The final best fit model, shown in depth slices in Fig. 6, reveals there was no evidence in the literature specific to the region that a density anomalies most likely related to volcanism at Cerro Negro

Table 1 Corrections 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 base station 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 uniform density contrast bounds to generate three different density contrast density surfaces). For reference, the base of Cerro Negro is at models to investigate the possible range of anomaly volumes. Vol- approximately 450 m a.s.l. (above sea level). The primary feature of umes were calculated using a density cutoff of 2690 kg m−3 (lower the model in the vicinity of Cerro Negro is a positive density anomaly end density for basalt; Telford et al., 1990). Using density contrast consisting of three main lobes, located between 250 and 2000 m models calculated from the three sets of density contrast bounds, the a.s.l., marked with arrows in Figs. 6 and 7. One lobe is located beneath Cerro Negro segment of the positive anomaly has a volume of 1 to the northern portion of El Hoyo (1), the second just to the south- 2km3, the Cerro La Mula segment has a volume of 0.1 to 0.4 km3 and east of Cerro Negro (2), and the third to the northeast of Cerro La the El Hoyo anomaly has a volume of 3 to 5 km3. The total volume of Mula (3). As seen in Fig. 6, the shared density anomaly is first present the positive anomaly above the 2960 kg m−3 cutoff ranged from 4 to at −2000 m a.s.l. beneath the northwestern portion of El Hoyo. The 7km3 (Table 3). Cerro Negro and Cerro La Mula portions of the anomaly become distinct from the El Hoyo anomaly at −500 m a.s.l. (Fig. 6A,B). The 5.4. Error sources and limitations of inversion method northern lobes of the shared density anomaly persist to −250 m a.s.l. The connections between the lobes of the anomaly are robust and The density contrast models produced by the inversion have well defined features with wavelengths above the Nyquist limit in limitations and restrictions when interpreting the model. First, that area (Fig. 5). Additionally, the three lobes of the density anomaly resolution is variable – shallow structures (< 2 km) will be resolved correspond well to the three positive Bouguer gravity anomalies in more precisely than deeper structures (> 2 km). Second, the models the vicinity of Cerro Negro (Fig. 7). Thus we can be reasonably confi- reveal density contrasts rather than absolute densities, meaning that dent in the first-order geometry of the recovered density anomalies the background density structure may be homogenous or stratified. in this region. To definitively distinguish between these end-members, surveying Fig. 8 shows the observed Bouguer gravity anomaly (Fig. 8A) and with a method sensitive to horizontal structures would be neces- the predicted gravity data calculated from the best fit model (Fig. 8B). sary. Third, noise in the models can distort the geometry and size of Most of the main features of the observed data are adequately repro- anomalies observed, such that detailed interpretation of fine-scale duced in the predicted data, with the exception of some fine scale structures should be avoided. anomalies in the immediate vicinity of Cerro Negro. While we can have a reasonable level of confidence in the model 6. Discussion geometry, the volumes of the density anomalies are more difficult to constrain. As discussed in the previous section, we used three sets of Given that the range of densities obtained for the positive anomalies at Cerro Negro, Cerro La Mula, and El Hoyo correspond to the density of basalt, it is likely that these structures represent shallow intrusive complexes associated with each volcanic center. As Table 2 the density anomalies are clearly connected with the exception of Density contrast bounds for GROWTH2.0 gravity inversions. The zero density contrast − the small positive density anomaly to the southwest of Cerro La is set to the Bouguer correction value of 2450 kg m 3. Mula (Fig. 6), this suggests that the magma plumbing systems of El −3 −3 Set Density contrast bounds (kg m ) Absolute density bounds (kg m ) Hoyo, Cerro La Mula, and Cerro Negro are also connected. Over time, 1 (Smallest)−270 to 250 2180 to 2700 numerous dike intrusions between El Hoyo, Cerro Negro, and Cerro 2 (Middle) −305 to 300 2145 to 2750 La Mula, exploiting the easy pathways afforded by the NNW trend- − 3(Largest) 340 to 350 2110 to 2800 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 correction value 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 El Hoyo 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 observed would be weakened by active slip in the current tectonic regime, between the positive anomalies. creating an easy pathway for intruding magmas. It is also worth noting how well the orientation of the density 6.1. Role of regional tectonics anomalies to the northwest and southeast of Cerro Negro agrees with the zone of static stress change calculated by Díez (2005) for The orientation of the inferred zones of dike intrusions likely the 1999 eruption of Cerro Negro. Díez (2005) calculated that the reflect the influence of regional tectonics. La Femina et al. (2002) pro- three earthquakes preceding the 1999 eruption could have caused pose a model in which crustal blocks in the Caribbean plate rotate stress reduction along a plane oriented north-south along the Cerro along northeast trending strike-slip faults that favors east-west ori- Negro-Cerro La Mula eruption sufficient to trigger the 1999 erup- ented extension in the center of the blocks. This east-west extension tion. It is thus possible that residual magmas from the 1999 eruption gives rise to north-northwest trending extensional features (Díez, contribute to the density anomalies imaged to the northwest and 2005). The northwest orientation of the dikes between Cerro La Mula southeast of Cerro Negro. and El Hoyo and the location of the Cerro Negro anomaly are most likely the result of magmas taking advantage of east-west exten- 6.2. Multi-level magma plumbing system sion. Fault capture, in which intruding magmas exploit faults as a mechanically efficient path to the surface likely explains the north- The density anomalies imaged in this study likely represent east orientation of the dikes between Cerro Negro and El Hoyo the shallowest portion of a multi-level magma plumbing system (Gaffney et al., 2007). Northeast trending faults in the area (Fig. 2) that 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. P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384 383

Table 3 may host hundreds of cones (Connor and Conway, 2000), and Cerro Density anomaly volume ranges. Anomaly volumes calculated using a density cutoff Negro (and likely many of the cones in the El Hoyo complex) is of 2960 kg m−3. demonstrably polygenetic, unlike the smaller monogenetic cinder Density anomaly Volume range (km3) cones found in larger fields of distributed volcanism (i.e., Paricutin Cerro Negro 1–2 and Jorullo, Mexico; Connor and Conway, 2000). However, there is no Cerro La Mula 0.1–0.4 single dominant peak in the Cerro Negro-El Hoyo complex, and the El Hoyo 3–5 distribution of cones is over a relatively large area (approximately Total 4–7 12 km × 13 km). The results of our gravity survey display what may be a central magma chamber connecting to a similarly sized parasitic magma chamber feeding Cerro Negro volcano and other volcanic fea- features in its immediate vicinity. Melt inclusion studies of erup- tures in the complex. Accordingly, Cerro Negro should be classified tive products at Cerro Negro consistently show evidence for magma not as a separate entity, but as part of a larger system fed by a well storage at multiple depths (Portnyagin et al., 2012; Roggensack, established magma plumbing system. 2001; Roggensack et al., 1997; Venugopal et al., 2016). The depth of the Cerro Negro anomaly is in agreement with storage depths for 7. Conclusion 1995 eruption magmas obtained from melt inclusion data (Roggen- sack et al., 1997; Venugopal et al., 2016), so this shallow anomaly The inversion of gravity data collected at Cerro Negro has revealed may represent a zone of shallow magma storage, where un-erupted several interconnected intrusive basaltic bodies in the vicinity of the magma cools to form an intrusive complex of dikes and sills. volcano. These bodies most likely represent a combination of shal- Although the El Hoyo anomaly has a greater vertical extent than the low intrusions where magma was temporarily stalled on its way to Cerro Negro anomaly, its depth is also in general agreement with the surface and a surrounding network of dikes. The geometry of the existing melt inclusion data (Portnyagin et al., 2012; Roggensack et bodies, with lobes beneath Cerro Negro, the El Hoyo complex, and al., 1997; Venugopal et al., 2016), opening the possibility that the Cerro La Mula, suggests that Cerro Negro is not a separate entity, El Hoyo complex may also be a storage site for magmas eventually but instead shares a plumbing system with a volcanic complex com- erupted at either Cerro Negro or El Hoyo. prising the cones and volcanic features in the immediate proximity. Volcanic activity at Cerro Negro may represent an intermediate style 6.3. The Cerro Negro-El Hoyo complex of edifice building between the end members of stratovolcano and distributed volcanism. The presence of a well established and con- The interconnected magma plumbing systems imaged in this nected magma plumbing system beneath the Cerro Negro-El Hoyo study are consistent with geochemical studies of the eruptive prod- complex suggests that Cerro Negro is likely to be a long lived fea- ucts of Cerro Negro and El Hoyo that show evidence for chemical ture. Future study of Cerro Negro should incorporate observations links between the magmas of the two volcanoes. The geometry of the of the complete Cerro Negro-El Hoyo complex for an improved shared density anomaly, tapering down to a single distinguishable understanding of volcanic behavior and potential hazards from the − source at approximately 2000 m a.s.l., may represent one possible complex. pathway for magmas ascending to both El Hoyo and Cerro Negro, where magmas may stall before taking diverging paths to eruption at the surface, as discussed in Venugopal et al. (2016) and McKnight Acknowledgments (1995). It should be noted that the resolution of this gravity data set We thank the two anonymous reviewers whose comments and does not rule out the possibility of a separate, smaller conduit feed- suggestions significantly improved this paper. This study was sup- ing only Cerro Negro, providing melt from magma storage areas at ported by an NSERC Discovery Grant to G. Williams-Jones. Many roughly 7 and 14 km depth (e.g., Portnyagin et al., 2012; Venugopal thanks to Gwen Flowers and Jackie Caplan-Auerbach for helpful et al., 2016). In general, however, we now have both geophysical and comments and discussion. Thank you to Hazel Rymer for valuable geochemical evidence for connections at shallow depths between discussion and the use of the G-513 gravity meter. Our thanks to Cerro Negro and El Hoyo. Pete LaFemina and Halldor Geirsson for access to their continuous An alternate explanation for the shared density anomaly could be GPS site at Cerro Negro. We are also grateful to INETER (Instituto a series of co-located, but not connected, intrusions of cooled magma Nicaragüense de Estudios Territoriales) and the staff of the Coop- in the form of stacked dikes and sills, active at different times. The erativa Las Pilas-El Hoyo for their support, in particular Armando density model in this study represents a single snapshot in time, Saballos for the names and locations of the volcanic features near such that the density model alone cannot constrain the temporal Cerro Negro. Thank you to Tim Niebauer of Micro-g LaCoste for the relationship between the different lobes of the anomaly. However, use of facilities for final data processing and manuscript editing. combining this snapshot structural model with the evolutionary and mixing trends between Cerro Negro, El Hoyo, and other nearby vol- Appendix A. Supplementary data canic features revealed by previous geochemical studies (McKnight, 1995; Venugopal et al., 2016) provides compelling evidence for a Supplementary data to this article can be found online at http:// system connected in time as well as space. dx.doi.org/10.1016/j.jvolgeores.2016.09.002 Given that the magma plumbing systems of Cerro Negro and El Hoyo are likely connected at depth, the two volcanoes should be more properly viewed as elements of the same complex, with Cerro Negro representing the newest active vent of the complex. Viewed in References this light, this complex can be considered as an intermediate mem- ber between the end-members of distributed volcanic fields (e.g., Barde-Cabusson, S., Gottsmann, J., Martí, J., Bolós, X., Camacho, A., Geyer, A., Planagumà, L., Ronchin, E., Sánchez, A., 2014. Structural control of monogenetic Michoacán, Mexico and Springerville, Arizona; Connor et al., 1992; volcanism in the garrotxa volcanic field (northeastern Spain) from gravity and Connor and Conway, 2000) and single-edifice stratovolcanoes (e.g., self-potential measurements. Bull. Volcanol. 76 (1), 1–13. Concepción and San Cristóbal, Nicaragua; Carr, 1984; Siebert and British Oceanographic Data Centre, 2009. Gebco Gridded Global Bathymetry Data. Camacho, A.G., Fernández, J., Gottsmann, J., 2011. The 3-D gravity inversion package Simkin, 2002). The Cerro Negro-El Hoyo complex has far fewer cones GROWTH2.0 and its application to tenerife island, Spain. Comput. Geosci. 37 (4), than classic cases of distributed volcanism in which a volcanic field 621–633. 384 P. MacQueen et al. / Journal of Volcanology and Geothermal Research 327 (2016) 375–384

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