Connected Magma Plumbing System Between Cerro Negro and the El

Journal of Volcanology and Geothermal Research 327 (2016) 375–384 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores 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 sys- tems. 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 http://dx.doi.org/10.1016/j.jvolgeores.2016.09.002 0377-0273/© 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 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.

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