the ∼ 1245 yr bp asososca maar eruption: the youngest event along the nejapa–miraflores...

Journal of Volcanology and Geothermal Research 184 (2009) 292–312

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The ∼1245 yr BP Asososca maar eruption: The youngest event along theNejapa–Miraflores volcanic fault, Western Managua, Nicaragua

Natalia Pardo a,⁎, José Luis Macias a, Guido Giordano b, Paola Cianfarra b,Denis Ramón Avellán a,c, Fabio Bellatreccia b

a Departamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, C.P. 04510, México, D.F., Mexicob Dipartimento di Scienze Geologiche, Universitá Roma Tre, Largo S. Leonardo Murialdo 1, 00146 Roma, Italyc Centro de Investigaciones Geocientíficas (CIGEO), Managua, Nicaragua

⁎ Corresponding author. Tel.: +52 55 5622 4119; fax:E-mail address: [email protected] (N. Pardo).

0377-0273/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.jvolgeores.2009.04.006

a b s t r a c t
a r t i c l e i n f o
Article history:Received 10 November 2008Accepted 5 April 2009Available online 21 April 2009

Keywords:Base surgephreatomagmatic eruptionpyroclastic density currentSEMvolcanic glasslineament swarm

The Asososca Tephra comprises the asymmetrically distributed, well-stratified phreatomagmatic products ofthe 1245 +125/−120 yr BP Asososca maar eruption West of Managua, Nicaragua. It is the youngest eruptiveunit related to the monogenetic volcanism concentrated along the N–S trending Nejapa–Miraflores activefault and intersecting E–W to NE–SW and NW–SE minor faults. The close relationship between monogeneticvolcanism and the structural-local tectonic framework is evidenced by the lineament domains and ventalignment patterns identified by remotely sensed images and DEMs of the research area. The Asososca maarlake bathymetry suggests that the crater was formed by coalescing vents migrating during the eruptive event.The Asososca Tephra dominantly consists of accidental lithics disrupted from the underlying stratigraphicunits observed inside and around the maar crater. Dry base-surge bedsets are dominant throughout theeruptive unit, showing facies variation from proximal, cross-stratified beds to mid-distal plane-parallel andwavy-parallel beds. Grain-size analyses indicate that base surges transported dominantly−1, 0, and 1ϕ sizedparticles at the bedload in form of traction carpets, while fine ash material in continuous suspension wasminimal. Juvenile fragments were only identified in the finer fractions, varying from vesiculated scoriae todense sideromelane fragments. SEM analyses suggest that the Asososca eruption resulted from a highlyefficient fuel–coolant interaction between a tholeiitic basaltic melt and an aquifer hosted in a shallow level(between 50 and 200 m) of olivine-bearing scoria ash and lapilli beds. The explosion triggered the country-rock comminution and the production of moss-like, fused-shaped, and blocky ash-shards with steppedsurfaces, quenching cracks, pitting, alteration skins, and adhered particles, all indicative of phreatomagmaticfragmentation. The very recent age of the Asososca maar eruption confirms that the densely inhabitedManagua area is volcanically active and that hazard studies should focus on the potential forphreatomagmatic eruptions.

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1. Introduction

More than 21 volcanic structures have been identified along theN–S trending Nejapa–Miraflores fault (NMF), West of Managua,Nicaragua (Walker 1984a; Bice, 1985). At least nine among thesevolcanic foci have erupted in the last 12,700 years (Pardo et al., 2008),mainly corresponding to clustered and aligned scoria cones, tuff rings,and maars (Bice, 1985). Although the Managua area is located in anactive, subduction-related arc setting, the importance of the mono-genetic volcanism in terms of hazards has been often neglectedcompared to the majestic stratovolcanoes and calderas, whichsurround the capital city of Nicaragua. However, the volcanic hazardof monogenetic volcanic fields has been recognized worldwide (Allen

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et al., 1996; Connor and Conway, 2000; Lorenz and Kurszlaukis, 2007;Lesti et al., 2008); therefore, it is essential to define the potential formonogenetic eruptions to occur in the Managua area, along with theeruptive style (magmatic vs. phreatomagmatic). So far, there are nospecific studies on hazard and risk assessment along the Nejapa–Miraflores fault (NMF), similarly to monogenetic fields elsewherewhich all have very little historical records (e.g. Kienle et al., 1980).However, the great preservation of many of the aligned vents alongthe NMF represents an excellent opportunity for exploring andadvancing our knowledge on monogenetic volcanoes, particularly onmaar-forming events. That is why detailed stratigraphic investiga-tions and extensive research on eruptive mechanisms, ejectedparticles transport and deposition are essential pre-requisites tohazard studies; and this is even truer in highly populated areas suchas Managua, where nearly 1 million inhabitants live at threat by thenext monogenetic eruption.

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293N. Pardo et al. / Journal of Volcanology and Geothermal Research 184 (2009) 292–312

This paper provides a detailed account of the Asososca Tephra(ASOT), which records the youngest eruptive event (∼1245 yr BP) alongthe NMF (Pardo et al., 2008). We address the eruptive mechanisms,products transport/deposition, and the Asososca maar formation byusing an integrated field/laboratory approach. Besides the knowncontrols over magma/water interaction (Heiken, 1972; Lorenz, 1973;White, 1991b; Zimanowski et al., 1997), such as water:magma ratio,interaction depth, mode of interaction, magma ascent rate, andtopography, we discuss the influence of the aquifer properties (e.g.Aranda-Gómez and Luhr,1996; Auer et al., 2007). We show howa smallmagma volume, rising along a right-lateral slip regime, could intersectmatrix porosity-controlled aquifers (i.e. Large storage capability but lowdischarge), resulting into highly explosive and hazardous interactions.

In this paper we refer as “vent” to the opening at the earth's surfacethroughwhich volcanicmaterials are erupted (Blates and Jackson,1984);avent canbe focused in aneruptive center, ormaycorrespond to afissure.One volcano can have nested vents migrating over short periods of time,but we still consider it monogenetic if only one eruptive unit is identifiedas produced by the volcano. Then, for monogenetic volcanoes weconsider one simplemagma conduit system (Cas andWright,1987) evenif one resulting crater is formed by two ormore nested and/ormigratingvents. If more than one eruptive unit is identified as originated in thesame volcano, we consider the latter as polygenetic. In this case, acomplex conduit system guarantees the occurrence of many eruptions(and/or eruptive phases) separated by long repose lapse-times (Cas andWright, 1987; Davidson and De Silva, 2000).

1.1. Maars, phreatomagmatic eruptions and base surges

In the volcanological literature, maars are known as the secondmost abundant volcanoes on Earth after scoria cones (Cas andWright,1987; Wohletz and Heiken, 1992; Vespermann and Schmincke, 2000).However, there are few historical records that could help to under-stand their formation, evolution, and hazard implications (Müller andVeyl, 1956; Kienle et al., 1980).

Commonly, maars are monogenetic volcanoes originated duringeruptions that excavate the preexisting surface (Ollier, 1967; Lorenz,1973; Fisher and Schmincke, 1984; Lorenz, 1986; White, 1991a;Gençalioğlu-Kuşcu et al., 2007; Lorenz and Kurszlaukis, 2007; Lorenz,2007). Rarely, polygeneticmaars also develop on larger volcanoes (e.g.Albano maar, Giordano et al., 2006; De Benedetti et al., 2008).

Nowadays there is a general consensuson thephreatomagmatic originofmaars (Ollier,1967;Wohletz andSheridan,1983; FisherandSchmincke,1984; Cas and Wright, 1987; Schmincke, 2004; Lorenz and Kurszlaukis,2007; Lorenz, 2007); which form when rising magma explosivelyinteracts with external water in a process known as Fuel–CoolantInteraction (Wohletz and Sheridan, 1983; Fisher and Schmincke, 1984;Wohletz,1986;Cas andWright,1987).Magma thermal energy is suddenlyconverted into mechanical energy. The resulting water deformation andexpansion lead to magma brittle failure and sudden external watervaporization; this processes result in the production of mechanical workwhich is evidenced byabrupt chilling andmagmaquenching,magma andcountry-rock fragmentation, crater excavation, tephradispersion, acousticand seismic perturbations (Wohletz and Heiken, 1992).

During maar formation, the initial magma–water mixture forms avertical jet at the explosion center promoting the opening of the ventthat with time becomes wider and deeper while a conical depressionis created (Lorenz and Kurszlaukis, 2007). Subsequently, the craterexperiences successive inner erosion and collapses, while theexcavated material is ejected and incorporated within base-surgeclouds as the eruption continues. In this way, maars have cratersb3 km in diameter and hundreds of meters deep, surrounded by arelatively thin phreatomagmatic deposit succession (≤50 m, rarely100 m thick; e.g. Vespermann and Schmincke, 2000), and mainlycomposed of base surge and fallout beds (Wohletz and Sheridan,1983; White, 1991b; Vespermann and Schmincke, 2000).

Pyroclastic surges are low-density currents mainly consisting ofsuspended particles in a gas phase (Valentine and Fisher, 2000). Theymove at high speeds (100 to 300m/s;Wilson andHoughton, 2000) onthe earth surface, mainly controlled by gravity (Sparks, 1976; Fisherand Heiken, 1982; Cas and Wright, 1987; Carey, 1991; Druitt, 1998;Belousov et al., 2002; Burgisser and Bergantz, 2002), and frequentlyimplying a turbulent regime (Valentine, 1987; Fisher, 1990; Carey,1991; Dellino and La Volpe, 2000; Valentine and Fisher, 2000). Theterm “base surge”was introduced by Glasstone (1950) to describe thehorizontal-moving, ring-shaped cloud, originated at the base of avertical column produced in thermonuclear experiments. Similarclouds were identified in phreatomagmatic eruptions by Moore(1967), who described them as fast gas–particle mixtures blastingtrees and destroying infrastructure, while leaving ephemeral deposits(Valentine,1998). Detailed studies onpyroclastic and base surgeswerecarried on by Schmincke et al. (1973), Sheridan and Wohletz (1981),Valentine (1987), Sohn and Chough (1989), Chough and Sohn (1990),Cole (1991), Dellino and LaVolpe (2000), and Sohn and Park (2005)among others, with the aim of understanding the physical processes.

According to temperature and the amount ofwater involved, surgesassociated to phreatomagmatic eruptions, either base surges orpyroclastic surges, can bewet or dry:wet surgesmove at temperaturesb100 °C allowing water vapor condensation and then, promoting thecoexistence of three phases (water droplets, solid particles, and gases).Dry surges flow at temperatures N100 °C, being biphasic systems ofsolid particles and gases (Derosa et al., 1992; Lajoie et al., 1992; Allenet al., 1996; Sohn, 1996), favoring the emplacement at higher speeds(Carey, 1991), and deposition of bedsets (Walker, 1984b; Dellino et al.,2004a,b; Sohn and Park, 2005). Paleomagnetic measurements oflithics inside phreatomagmatic surge deposits indicate emplacementtemperatures below 200 °C (Porreca et al., 2008).

Lateral and vertical facies variation of pyroclastic and base-surgedeposits was largely studied byWohletz and Sheridan (1979), Valentine(1987), Sohn and Chough (1989), Lajoie et al. (1992), Németh et al.(2001), and Giordano et al. (2002) while transport and depositionprocesses have been detailed by Crowe and Fisher (1973), Schminckeet al. (1973), Walker (1984b), Fisher and Schmincke (1984), Cas andWright (1987), Sohn and Chough (1989), Chough and Sohn (1990),Dellino and LaVolpe (2000), Dellino et al. (2004a,b), Vázquez and Ort(2006), Wohletz and Sheridan (1979), Wohletz (1986), Valentine(1987), Zimanowski et al. (1991, 1997), and Zimanowski (1998).

1.2. The Asososca maar

The Asososca maar is located on the western outskirts of Managua at162 km east from the Middle America Trench where the Cocos Plate iscurrently subducting beneath the Caribbean Plate (Fig. 1). It is one of the∼21 aligned monogenetic volcanoes that delineate the N–S trendingNejapa–Miraflores fault (Bice,1985; La Femina et al., 2002). The crater ofAsososca hosts a 0.7 km2 lake with a volume of 44.4×106 m3 (CEDEX-ENACAL, 1999), which is a source of drinkable water for the city,supplying at least 10% of the population. The Asososca crater has apristinemorphology if comparedwith the other eroded, sediment-filled,or partially collapsed and faulted aligned volcanoes. The crater issurrounded by an asymmetrically distributed, well-stratified eruptiveunit definedasAsososca Tephra (Pardoet al., 2008), and it is theyoungestmaar on the western outskirts of Managua (1245 +125/−120 yr BP),where N–S, E–W to NE–SW, and minor NW fault systems interact witheach other (Viramonte, 1973; La Femina et al., 2002; Espinoza, 2007).

2. Geological setting

2.1. The Nejapa–Miraflores fault and related volcanic activity

The NMF (Fig.1) defines thewestern border of theManagua Graben(La Femina et al., 2002; Frischbutter, 2002), inside the trench-parallel

Fig. 1. Geological framework. A. Regional tectonic regime showing the active subduction of Cocos Plate beneath the Caribbean Plate, resulting in the Middle America Volcanic Arc. InNicaragua active volcanism is concentrated inside the Nicaragua Depression (ND). The dark gray box indicates the zoomed area shown in B, where the Nejapa–Miraflores fault (NMF)marks a right-step offset in the main arc. Along the fault recent mainly monogenetic volcanoes have been formed westward of Managua city that are shown in C.

294 N. Pardo et al. / Journal of Volcanology and Geothermal Research 184 (2009) 292–312

Nicaragua Depression (McBirney and Williams, 1964). The NMF isvisible on satellite images and is characterized by a right-lateralmovement with a significant normal component (La Femina et al.,2002; Espinoza, 2007). The NMF is one of a series of N–S trending faultsthat mark offsets along the main Central America Volcanic Arc (Stoiberand Carr, 1973; Walker, 1984a; Bukart and Self, 1985; Marshall, 2007).These faults are marked by the alignment of small monogeneticvolcanoes, including tuff rings, maars, and scoria cones (Walker,1984a; Bice, 1985; Havlicek et al., 1997). We refer to the aligned, oftenclusteredmonogenetic volcanoes located between theChiltepeVolcanicComplex to the North and Ticomo craters to the South, as the Nejapa–Miraflores Volcanic Field (NMVF). It includes mainly N–S aligned ventsand fewer E–W to NE–SW, and NW–SE aligned vents, corresponding tothe main active fault trend described by La Femina et al. (2002), andswarm lineaments that we identified in remotely sensed images andDigital Elevation Models (DEMs) of the research area (Fig. 2).

To further define lineaments of structures in the NMVF we appliedan automatic lineament detection with SID software (Salvini, 1985)over synthetically generated images. These images render the DEMdata (Shuttle Radar Topography Mission (SRTM) 3 arc-secondresolution) of the NMVF according to 9 different directions ofsynthetic lighting condition (every 20° of azimuth). This multipleanalysis (Fig. 2) allowed to overcome the bias induced by lightingconditions in lineaments identified from a single image (Wise et al.,1985). SID automatically identified 147 lineaments that were saved ina database and recorded with azimuth, length, latitude and longitudeof the end points. Results from each SID analysis were then merged in

this single dataset for cumulative statistic analysis. Statistical analysisof the detected lineaments by polymodal Gaussian fit distributionsshowed the existence of clusters of lineaments along preferentialorientations called domains (Wise et al., 1985).

The NMVF is characterized by one principal lineament domaintrending N13°W, and other three minor sub-ordered domains trendingN17°E, N69°E, and N48°W (Fig. 2B). The main domain is nearly parallelwith ∼NNW compression and ∼ENE dilation stress conditions. Thehighest lineament density (55–68 lineaments per km2) is located in thecentral part of the NMF, in the surroundings of Asososca and Nejapacraters (Fig. 2). In addition, DEM and satellite image (Landsat TM) of thearea allowed us to identify a dominant N–S vent alignment pattern,followed by a secondary E–W to NE pattern, and a subordinated NWpattern, with a maximum vent density of 4–5 vents per km2. Ventalignment has often been interpreted to reflect crustal tectonicstructures aswell as the associated stressfield (Connor,1990;Mazzarini,2007; Lesti et al., 2008). In this work spatial density analysis of volcanicstructures was done to highlight the structural control and relationshipwith the volcanism of the area. Identification of the aligned and oftenclustered eruptive vents (including tuff rings, maars, and scoria cones)was visually based on the assumption that volcanic features, in absenceof external influence, are characterized by circular morphology both incaseof constructive activityandof destructive activity (Lesti et al., 2008).Data confirm a straight linkage between volcanism and the structural–local tectonic framework West of Managua.

The geological record, including structural geology, stratigraphyand geochemistry as reported by McBirney (1955), Bice (1985),

Fig. 2. A. Automatically detected lineaments in the NMVF. Results from the analysis of each “pseudo-shadow” image were merged in a single dataset, for polymodal Gaussian fittingand cumulative representation on a rendering of the DEM. Image processing for lineament enhancing includes the following steps: preparation of 9 directional-derivative filteredimages (“pseudo-shadow” images); Laplacian filtering, threshold slicing, edge continuity enhancement and LIFE filtering (a specific filter to eliminate randomly scattered pixels) ofeach image. Eventually, the prepared images were processed by SID software for automatic lineament detection. B. Four lineament domains were found in the investigated area: adominant N13°W domain followed by N17°E to N68°E and N48°W minor domains. Total data: 147, max: 7, min: 0, mean: −4.940, standard deviation: 3.793, mode: −12.RMS=0.1694001. Best fit value=0.2799953. C. Lineament density map zoomed in D, where a significant density is located in the western outskirts of Managua. E. Vent density map,zoomed in F. Identification of the aligned and often clustered eruptive vents (including tuff rings, maars, and scoria cones) was visually based on the assumption that volcanicfeatures, in absence of external influence, are characterized by circular morphology both in cases of constructive activity and of destructive activity (Lesti et al., 2008). A subset of thepanchromatic band 8 (0.52–0.9 mm) of the Landsat scene (path: 017; row: 052; acquisition date: 5/11/1999) was processed by highpass spatial filter (3×3 kernel size) to emphasizethe morphological contrast. As many as 50 vents were detected by visual inspection (Goward et al., 2001) both in the enhanced satellite image and in the “pseudo-shadow” DEMimage of the investigated area. The density map derived from the spatial distribution of the identified vents (by the Interpreter Module in Erdas Imagine 9.0) shows a dominant N–Salignment that correlate with the location and direction of the highest lineament density area.


Walker (1984a), Havlicek et al. (1997), Girard and van Wyk de Vries(2005), Pérez and Freundt (2006), and Kutterolf et al. (2007),suggests that NMVF related eruptions have occurred since thePleistocene, and have produced basaltic, basaltic–andesitic, andrhyolitic pyroclastic deposits, together with basaltic and andesiticlava flows, that are all grouped in the Managua Formation. The highfrequency of Holocene eruptions westward of Managua is wellevidenced inside and around the Asososca crater by the occurrenceof 18 volcanic stratigraphic units related to the NMVF, 16 of themrecorded in the last 12,730 +255/−250 yr BP (Pardo et al., 2008).Four of them are magmatic in nature and are related to scoria conesand effusive activity of fissural vents; two are also magmatic butfelsic in composition and generated by Plinian activity related to thenorthern Chiltepe Volcanic Complex; five are phreatomagmatic innature and related to tuff rings and maar events originated in thecentral–south part of the fault.

2.2. Asososca maar morphology, stratigraphic position, andrelated products

The Asososca maar is characterized by an East–West elongatedirregular crater with a maximum diameter of 1100 m, steep innerwalls (40 to 57- locally 86°) that become gentler with depth as theresult of collapses. The lake bathymetry reported by UNAN-EAM(1978) shows the probable coalescence of vents similar to the Albanomaar in the Roman Volcanic Province (Funiciello et al., 2003; Anzideiet al., 2008). The eastern one is 0.23 km2 and has a slightly inclined,104 m deep, flat bottom, while the western one is 67 m deep with anearly funnel-shaped bottom (Fig. 3). They are separated by an archedseptum that might represent the eastern wall remnant of the western,younger vent. The resultant crater is excavated through the surround-ing topography and exposes awide Holocene record on its inner walls.The stratigraphic position and radiometric age of the Asososca Tephra

Fig. 3. A. Asososca area digital elevation model (DEM) showing the principal volcanoes of the central part of the Nejapa–Miraflores fault. B. Asososca bathymetry showing thepresence of two bottoms of different depth that could correspond to two craters. The western one is shallower and it presents an arched wall remnant. The current depressionexcavated through the surrounding topography might be the result of the interception of at least these two craters. Bathymetric data are taken from UNAN-EAM (1978).


(ASOT) relative to products of nearby volcanoes was established byPardo et al. (2008). It consists of finely stratified base-surge depositsthat are found atop the Managua Formation, overlaying the MasayaTuff (∼2000 yr BP, according to Perez and Freundt, 2006) (Fig. 4),which is the last phreatoplinian deposit of the southeastern MasayaCaldera. A maximum ASOT age estimate was determined by radio-metric dating of the underlying paleosol as 1245 +125/−120 yr BP(Pardo et al., 2008). The deposits are asymmetrically distributedaround Asososca, being thicker toward SW and W and thinner, topractically absent, toward E andN respectively; which is concordant tothe dominant local wind direction from East toward West. Thisasymmetric distribution is a common feature in some maars, such asUkinrek–Alaska (Kienle et al., 1980), Atexcaco–Mexico (Carrasco et al.,2007), and Pinacate–Mexico (Gutmann, 2002).

3. The ∼1245 yr BP Asososca eruption

We carried on a detailed study of the ASOT deposits by thedescription of stratigraphic sections and the systematic sampling ofindividual pyroclastic surge bedsets, as defined byDellino and La Volpe(2000) and Sohn and Park (2005). Laboratory researchwas performedto both analyze the granulometry and componentry of the depositsand to characterize themorphologyof ash particles, the composition ofjuvenile fragments, and to obtain complementary data on magmavolatile contents. The whole eruptive unit was subdivided into strat-igraphic sequences, each consisting of several bedsets and separatedby the contiguous ones by stratigraphic discontinuities, abruptchanges of lithological features as grain size, sedimentary structures,and composition. Representative bedsets of each sequence weresampled: one sample per each bedset layer, ensuring that contamina-tion of other layers and/or bedsets could be completely avoided. A totalof 26 samples were collected and sieved at 1ϕ intervals to carry outgrain-size analyses. 500 grains were separated from each of the−1, 0,and 1ϕ size-fractions, to carry out component analyses. Juvenile frag-ments were also identified and collected for each size fraction to

perform morphological investigations under the binocular micro-scope, while the 2ϕ fraction was observed under a scanning electronmicroscope (SEM-EDS Philips XL30, at the Interdepartmental Labora-tory of Electron Microscopy-L.I.M.E., Universitá degli Studi Roma-Tre,Italy). SEM studies were carried out to identify different ash mor-photypes, as defined in previous studies by Heiken (1972), Wohletzand Krinsley (1982), De Astis et al. (1997), Büttner et al. (1999), andDellino et al. (2001).

Selected juvenile particles were used for chemical analysis at theelectronmicroprobe of the Geophysical Institute at UNAM (Mexico) todetermine glass composition on polished samples. Fresh, microlite-and-vesicle poor or free glass shards were picked and prepared foranalyses with a NicPlan infrared microscope at Universitá degli StudiRoma-Tre in order to identify and quantify dissolved volatiles in theglass matrix. Data were processed as absorptivity or transmittivityspectrogramswith the aid of the ThermoNicolet OMNIC FTIR software,based on the method of Ihinger et al. (1994), Devine et al. (1995),Libowitzky and Rossman (1997), Wallace (1998), Ohlhorst et al.(2001), King and Holloway (2002), and Mandeville et al. (2002).

3.1. The Asososca Tephra in the field

The Asososca Tephra reaches a maximum thickness of 20 m onthe southern and southwestern inner crater walls, decreasing withdistance over flat areas and varying as a function of the abrupt changesof the underlying paleotopography. For example, at 700 m to thesouth–southwest it gets thicker and some coarse-grained (blocks up to30 cm) massive beds develop, as the result of a blocking effect exertedby the ∼12,730 yr BP Cerro Motastepe scoria cone (Pardo et al., 2008).Although the whole unit is difficult to observe because of intenseManagua urbanization, a maximum run-out distance of 3 km is esti-mated for the dry base surges based on fewexposures and topography.

We identified seven subunits based on minor unconformities(e.g. erosive contacts, sharp changes in the stratification and/or in thegrain size) and particularly on microscopic variations evidenced in the

Fig. 4. Geology of Asososca area following Pardo et al. (2008) including the geological map, cross-sections, and the general stratigraphic column. In the latter, black boxes indicatemafic compositions (basaltic to basaltic–andesites); white boxes correspond to felsic units (rhyodacitic to dacitic), and gray-yellowish boxes are buried paleosols where radiocarbondata is available.


laboratory analyses. Inside the crater, ASOT overlies a 10 cm-thickpaleosol developed above the b2000 yr BP Masaya Tuff; locally, amassive, friable, very well sorted, fine sand to coarse sand, 30 cm-thickbed, is found between the Masaya Tuff and ASOT. This bed consists ofrounded to subrounded scoria, red, brown, and dark grey porphyriticlithic fragments, and white to yellowish pumice. Subsequently, it wasinterpreted as a local deposit probably accumulated in a shore-lakeenvironment. Inside the crater and in proximal outcrops (Fig. 5), thelower part of the eruptive unit (Subunit ASOT-A) is partially induratedand consists of a basal massive, poorly sorted, 30 to 50 cm-thick bedwhich pinches out abruptly. It is characterized by the prevailing an-desitic accidental lithic blocks supported by a highly altered ash matrix(no juvenile sideromelane clasts were identified at outcrop scale). Themassive basal bed is overlainwith a sharp-plane contact by a 1.5m thicksequence of mm-to-5 cm-thick, gray, red, yellowish and green,

discontinuous, highly altered, massive to thinly laminated, fine ashbeds; their thickness decreases rapidly with distance, being thicker tothe S and SE of Asososca. In general, the massive and poorly sortedtexture, theplastic and cohesive appearance, the occurrence ofwet-soft-deformation structures (e.g. bomb sags), and scattered ash aggregatessuggest that this subunit was mainly deposited by wet base surges.

Overlying subunit ASOT-A, with an erosive contact, a thickersuccession of friable, thinly stratified deposits crops out, were formingthe bulk of the eruptive unit (Subunits ASOT-B-to-ASOT-G). Thesesubunits are asymmetrically distributed being thicker to the W andSW. The subunits succession consists of multiple bedsets character-ized by a basal 10 to 30 cm-thick, massive to inversely graded bed ofcoarse ash, lapilli and a few blocks (fines-depleted); each basal bed isfollowed by well-stratified, laterally discontinuous, 5 to 10 cm-thickbeds of ash and fine lapilli that commonly shows cross and parallel

Fig. 5. ASOT-A wet surge deposits. In proximal outcrops it begins with a basal, massive, altered bed followed by thin, discontinuous green, gray and yellow beds (A), where soft-sediment deformation (B–E, shown by arrows), fine lamination (FL), and accretional lapilli (AcL) are common. A corresponds to an outcrop inside the crater, B and E are outcropsclose to Motastepe scoria cone, while C and D are outcrops located S to SE Asososca. (For interpretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)


stratification and lamination, dunes (some of them surmountingblocks), and impact sags. Occasionally, an upper massive ash thin bed(b5 cm) is observed (Fig. 6). The bedsets dip outwards the crater atangles lower than 15°, and become horizontal at ∼100 m of distance.In proximal outcrops dune-structures bounded by parallel-stratifiedbeds are dominant with amplitudes ranging from 10 to 30 cm andwavelengths from 2 to 20 m (Fig. 7). At intermediate distances,between 0.8 and 1 km from the crater, plane-parallel and wavy-parallel beds with low-angle cross-stratification prevail over massivebeds and dunes. At these distances, dune amplitudes range from 15 to10 cm while wave lengths vary from 10 to 20 m (Fig. 7).

At the outcrop scale, accidental basaltic–andesitic to basaltic lavalithics, accidental scoriae and accidental ignimbrite fragments are set ina fine-grained matrix composed of smaller accidental scoria lithics aswell asolivine, augite, andplagioclase crystal fragments. Fractionsb−1ϕconsist of fresh juvenile basaltic glass shards and accidental lithics,including subrounded gray olivine-pyroxene bearing scoriae, olivine-bearing lavas, plagioclase–clinopyroxene bearing lavas, red scoriae and

Fig. 6. Typical ASOT bedsets, each one composed of a basal, coarse-grained, massive to inversstratified, matrix supported bed showing dunes, impact sags, parallel and cross-stratificationby a massive thin ash-bed.

lavas, white and yellow altered pumice, together with red, yellow andgrey altered ignimbrite fragments. All these accidental lithics resemblethe stratigraphy underlying the Asososca Tephra observed inside andaround the crater and described by Pardo et al. (2008).

3.2. Asososca Tephra laboratory analyses

The basal subunit ASOT-A, which is composed of wet base-surgedeposits was not sampled because of its high alteration and fragmentweakness (i.e. friable to the touch) that would not give preciselaboratory results. The following analyses are presented for subunits Bto G, composed of dry base-surge deposits that make up the bulk ofthe eruptive unit.

3.2.1. GranulometryGrain-size data of sampled bedsets are presented in Figs. 8 and 9,

indicating that base surges transported dominantly−1, 0, and 1ϕ sizedparticles. Fine ash suspension material (N4ϕ) was minimal, probably

ely graded or poorly parallel-stratified, matrix or clast-supported bed, followed by a mid, U-shaped channels, and lithic-concentrated lenses. Occasionally the mid-bed is capped

Fig. 7. ASOT dry base-surge facies. A and B show proximal sedimentary structures, where impact sags point toward Asososca and identified dunes are similar to Types a, b, and c, asclassified by Cole (1991). C and D are mid distance structures, the former showing an obstacle (ballistic block) surmounted by dry surges and the latter a typical dune Type IIfollowing the classification of Schmincke et al. (1973). Types d and e as classified by Cole (1991), together with Type V as classified by Schmincke et al. (1973) were also distinguishedin the field, all indicating provenance form Asososca.


due to its easy elutriation during transport and considering that theproportion of ejecta carried completely away from the vent is usuallylarge in maar-type volcanoes (White, 1991b). Occasionally coarse-sized(N−4ϕ) fragments were ejected as ballistics (subunits ASOT-C and D)andwere not considered in the granulometric analyses. It is noteworthythat the coarser fragments were found in ASOT-D-E where severalmassive beds contain up to 40% of accidental ballistic coarse blocks.Grain-size parameters were plot in the Walker (1971) and fit into thesurges fields (Crowe and Fisher, 1973; Walker, 1983), particularly in thebasal and mid-portions subfields of typical base-surge bedsets, follow-ing Crowe and Fisher (1973) and Allen et al. (1996).

Grain-size data, when combined with structural and texturalfeatures of the bedsets (Figs. 8, 9) were particularly helpful forinterpreting the transportation and deposition processes of the basesurges, which we attribute to turbulent, low-density currents, thatdeveloped a basal traction carpet (e.g. Sohn, 1997; Dellino et al.,2004a). ASOT fining-upward bedsets record the deposition of a basal,relatively high-concentrated bed that was passively dragged by theoverlaying turbulent and low-concentrated flow. The formation of thebasal beds was followed by the deposition of plane-parallel and cross-stratified beds (mid-portion), that resulted from traction and saltationin the interface between the traction carpet and the turbulent cloud.Deposition from the cloud mainly occurred at the bedload as laminae-by-laminae aggradation, and sandwave bedforms were developed.Occasionally, bedsets mid-beds were capped by thin massive and

discontinuous upper fine-ash beds as a result of suspended particlesdeposition during the waning stage of the flow, although a few ash-fallout, unimodal deposits might be interbedded.

3.2.2. Componentry and juvenile magma compositionThe component analysis shows that mainly accidental lithics (i.e.

excavated from the basement, conduit walls or picked-up duringtransport) are preserved in the deposits; juvenile fragments instead,are less than 15 vol.% considering the mean of total size-fractions, andless than 25 vol.% in separated finer size-fractions. Since all countedfragments are volcanic in origin, shape, composition, crystallinity,alteration, and comparison with underlying stratigraphic units werefundamental criteria for discrimination. Juvenile sideromelane frag-ments (Plate IA) identified in the 1 and 2ϕ size-fractions weredistinguished by their glassy bright lustre, absence or very low-gradeof alteration (palagonitization), dark-black to dark brown colour, andtypical morphologies of phreatomagmatic fragmentation detailedbelow. Most of them are low-vesicular fragments, with few plagio-clase, augite, and olivine microcrystals in a glassy matrix of basalticcomposition, where microlites are much lower than in the prevailingaccidental black scoriae. Some loose olivine and augite crystals mightbe also juvenile in origin but they are not easily distinguished fromfresh accidental crystals derived from older stratigraphic units.

Based on the stratigraphy presented by Pardo et al. (2008), aidedby hydrogeological wells-log data provided by the Centro de

Fig. 8. Granulometric distributions of ASOT samples and their variation with stratigraphic position (A–B). In general, bedsets are normally graded, with means and modes that varyfrom fine lapilli to coarse ash. Negative values are common in the basal bed of each bedset (N°-a), corresponding to coarser grain sizes, while values are close to zero or slightlypositive in the middle stratified beds (N°-b). Standard deviations are indicative of better sorting in the basal, matrix to clast supported, fines-depleted beds of each bedset comparedto the middle and rare upper, matrix supported and fine-enriched beds. Size distributions vary from symmetric to slightly asymmetric, and most of them are mesokurtic to veryleptokutic. In the Walker (1971) diagram (C) plots are typical of pyroclastic surges, following Crowe and Fisher (1973), Walker (1983), Fisher and Schmincke (1984), Dellino and LaVolpe (1995), Allen et al. (1996) and Gençalioğlu-Kuşcu et al. (2007).


Fig. 9. Synthesis of ASOT characteristics with stratigraphic position. A: Grain-size distributions show bimodal to polymodal populations and a general lack of coarse fragments. Basalportions of each bedset are coarser-grained compared to mid-portions. B. Componentry: note the low content of juvenile fragments in comparison with lithics derived from countryrock. Black, accidental olivine-bearing scoriae are dominant while deep fragments input, probably derived from Las Sierras Formation, appears in the second uppermost mid-part ofthe stratigraphic column, in ASOT-D subunit. C. Juvenile glass-ash morphologies. Low-vesiculated shards, moss-like, fused-shaped, and drop-like fragments show oscillatory trends,reach maximum contents of 80%, and prevail over the blocky-equant fragments. ASOT-B records the ejection of accidental fragments derived from the shallowest levels of theprevious stratigraphy, with prevalent moss-like, fluidal/fused-shaped juvenile ashes. ASOT-C is characterized by the abrupt decrease of juveniles, while dominance of shallowaccidental lithics suggests continuous crater widening. ASOT-D is characterized by the abrupt decrease in felsic pumice fragments, by the input of deep (N200 m) andesitichypocrystalline lithics, and by the greater amount of blocky and low-vesiculated shards. ASOT-E shows a new increase of juveniles, while hypocrystalline fragments and lithic-richignimbrites detached from Las Sierras Formation reach their maximum in ASOT-F (Maximum deepening andmagma rise facilitated). ASOT-G shows a general mixture of componentand ash-morphotypes populations (crater widening). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


Investigaciones Geocientíficas (CIGEO, Managua), and the University ofNicaragua (UNAN), it was possible to associate different accidentallithic groups to stratigraphic units underlying the Asososca Tephra:

a) Lithics derived from shallow units (Plate IB, C, and D): They includedark gray, pale gray and brown Olivine–Augite bearing scoriae withvarying vesicularity, abundantmicrolites, and lowdegree of alteration,aswell as fresh, hypohyaline basalts and andesites. Loose crystalswereconsidered as accidental, since most olivines are altered to iddingsite,and most of the augites are rounded with embayments and corrosiontextures. In addition, these crystals are similar to the phenocrystsfound in the accidental scoriae and lavas. These lithic fragmentsprobably derive fromshallowstratigraphic levels of low-alteration andmainly originated during explosive events related to scoria cones, tuffrings and maars in the central NMVF (Motastepe Tephra, RefineríaTephra, Satélite Tephra, Nejapa Tephra, and El Hormigón Tephra as

definedbyPardoet al., 2008). The lava fragmentsderived fromshallowlevels are related to effusive events along the NMF (e.g. BataholaLavas). Fromwells-log data these unitsmight be lying in the first 50mbelow the actual surface level.

b) Lithics derived frommid-depth units (Plate IE, F): They includewhite,yellowish, and gray altered pumice fragments, lithic tuffs with whitepumice, and altered red lavas and scoriae, probably deriving fromintermediate levels (at least below the felsic Plinianunits known in thearea: Apoyeque tephras and Apoyo tephras, Fig. 4). From wells-logdata, these units might be lying between 50 and 200 m deep.

c) Lithics derived from deep units (Plate IG, H, I): They include highlyaltered, lithic-rich ignimbrite fragments and highly altered redhypohyaline fragments typical of Las Sierras Formation; black tubularpumices typical of the Fontana Tephra as described by Wehrmannet al. (2006); rounded hypocrystalline andesites that were found asaccidentals in all the units that underlay the Asososca Tephra in

Plate I.Main components identified in ASOT samples. A. Juvenile, poorly vesiculated, glass shards. B –D. Shallow lithic fragments (above the Upper Apoyeque Tephra): B – olivine andaugite bearing scoriae, C – loose crystals, D – hypohialine fresh basalts and andesites. E–F – typical mid-depth lithic fragments (below Upper Apoyeque Tephra): E – dacitic torhyodacitic altered pumices, F – red altered lavas and scoriae. G–I – deep lithic fragments (Las Sierras Formation): G – lithic-rich ignimbrites, H – palagonitized tubular mafic pumice,I – rounded hypocrystalline andesites.


western Managua. Inwells data Las Sierras Formation ignimbrites arefound below a 274 m depth.

The average percentage of each identified component in thedeposits (Fig. 9) and their variation up through stratigraphic positionwere obtained in order to understand the evolution of the Asososcamaar eruption. Results confirm the dominance of country-rock lithicsderived from stratigraphic levels lying above ∼200 m depth, whereasthe proportion of juvenile sideromelane derived directly fromphreatomagmatic fragmentation is clearly subordinated (around15 vol.%). It is noteworthy that in the lowermost part of thestratigraphic column (i.e. ASOT-B-and-C in Fig. 9), shallow lithics(≤50 m below ground-surface) are mixed in different proportions,whereas in the upper mid-portion of the column there is a markedinput of lithics derived from deeper levels (N250–300 m), particularlyof rounded, hypocrystalline andesites (i.e. ASOT-D-to-G in Fig. 9).

3.2.3. Morphology of fine ash (SEM petrography)Eight juvenile ashmorphotypes, named from A to H, were identified

based on shape, surface, border, vesicularity, fracture pattern, presence

or absence of “v”-shaped pits, grooves, up-turned plates, cracks,adhering particles, and corrosion, dissolution, and precipitation textures(Table 1). Moss-like, fused-shaped, and blocky morphology withstepped surfaces, quenching cracks, pitting, alteration skins, andadhering particles are all indicative of phreatomagmatic fragmentation(Plates II, III). A ninth raremorphotypewas only recognized among dustparticles: it is irregular, splinter-like, with planar fractures, highlyvesiculated with tube-like vesicles (Plate III-J3).

The detailed characteristics and distinguishing features of eachtype are shown in Table 1. Vesicle percentage, forms, and size varyamong each morphological type. Blocky-shaped (A-type), fused-shapes (E-and-F-types), and Pelée-tears (H-type) are non vesiculatedor present less than 3% of spherical to irregular vesicles, the latter dueto coalescence. Types B, C, and D present a greater percentage ofvesicles (up to 40, 15, and 20 vol.% respectively) and the highestvesiculated particles are the G-type (up to 70 vol.% of bubbles).

In general, the different ash morphologies do not show any sig-nificant trend with the stratigraphic position, but present a sinusoidalpattern (i.e. peaks and lows) throughout the Asososca Tephra (Table 1and Fig. 9). Blocky, equant shards without vesicles (Type-A) vary

Table 1Glass shards morphotypes. A. Main distinguishing features under a binocular microscope and a scanning electron microscope (SEM). B. Variation of single morphotypes with stratigraphic position.

303N.Pardo

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Plate II.Main glass shards morphotypes and detailed features, typical of phreatomagmatic fragmentation. A1. Blocky-shaped ash with microstructures detailed in A2, which indicatemagma fragile rupture and particle traction transport. B-1 Equant vesiculated particles with spherical bubbles, evidence of pitting and grooves structures, and adhered ash zoomedinB2. C-1. Poligonal vesiculated ash with spherical vesicles separated by thickwalls and showing ash infill. Microstructures indicating fragile behavior and the effect of corrosive fluids(pits) are also common as zoomed inC2. D-1. Moss-like ash with irregular and stepped surfaces (Zoomed in D2). E-1. Equant, fused-shaped ashes without vesicles withmicrostructures (Detailed in E 2) that indicate a plastic behavior after fragmentation. F-1. Elongated fused-shaped ashes with adhered ash (Detailed in F2).


between 0 and 10 vol.% from ASOT-B to ASOT-D showing a relativeincrease from subunit ASOT-E to ASOT-G, where they reach up to33 vol.%. Similarly equant but vesiculated fragments (Type-B) varybetween 15 and 30 vol.%, reaching maximum values in the lowerASOT-C (up to 72 vol.%). Low-vesiculated trapezoidal particles (Type-C) show similar proportions but reach their maximum at ASOT-E (upto 67 vol.%). Types B and C are the most abundant throughout theAsososca Tephra. Moss-like particles (Type-D) and fused/fluidal-shaped particles (Types-E and F) are the second most abundantmorphologies throughout the unit. Moss-like varying between 7 and50 vol.%, while Types E and F reach maximum values of 80 vol.% inASOT-C. Irregular, highly vesiculated particles (Type-G) are present in25 vol.% at the base of the unit and then are less common, alwaysbeing below 10 vol.%. Rare Pelee's tears particles occur in the deposit(b3 vol.%).

In general, D-E-and-F-types are more significant in the lowersubunit ASOT-B, while the contribution of A-B-and-C-types is moresignificant in subunits ASOT-D-and-E. In the upper subunits ASOT-F-and-G there is a general mixture of different morphotypes (Fig. 9),that might be the result of recycling of pyroclast in the late stage of theeruption (Houghton and Smith, 1993; Houghton et al., 1999). By theother side, different morphotypes are also indicative of the rheologicalbehavior under phreatomagmatic fragmentation and give informationabout the chilling speed and style. A, B, and C morphotypes suggestrapid chilling, quenching, and fragile fragmentation, while D, E, F, andH types are indicative of a ductile deformation under slower chillingrates. G type is more typical of a viscous behavior.

An important feature of Asososca ashes is that, although theymight be externally dense, when cut and polished in thin sections,most of them show some degree of vesicularity (Fig.10). Inner vesiclesare mainly spherical or slightly ellipsoidal, less than 1 mm in diameter

and well separated. Some vesicles might be irregular showingevidence of being formed by coalescence of smaller vesicles.

Microprobe analysis of different glassy ash morphotypes give atholeiitic basaltic composition with high MgO and CaO contents, andlow SiO2, MnO, Na2O, and TiO2 contents (Fig. 10; Table 2).

3.3. Infrared spectroscopy (Micro-FTIR)

The Fourier Transform infrared technique performed on selectedjuvenile, fresh, ash fragments allowed the identification and estima-tion of the dissolved volatile contents in the glass, as representative ofthe volatile content of the erupting magma. Although more detailedstudies are needed, our data are consistent with a low H2O content.The analyses show a typical peak of molecular H2O at a wavelength of3550 cm–1 (Fig. 11). H2O content (C (H2O)) in wt.% was estimatedusing the Beer–Lambert Law:

C H2Oð Þ = A × 1:8t × D × e

whereA corresponds to the absorptionpeak amplitudemeasureddirectlyfrom the infrared spectra in cm−1; 1.8 is the H2O molecular weightalready converted in the corresponding units; t is the thickness of thesamplemeasured in cm,D is the basaltic glass densitygiven in g/cm3, andε is the molar absorptivity coefficient given in L/(mol×cm). ConsideringIhinger et al. (1994), Libowitzky and Rossman (1997), Wallace (1998),Ohlhorst et al. (2001), King and Holloway (2002), and Mandeville et al.(2002)molar absorptivitycoefficients (ε3550),H2O contentwasestimatedas lower than 0.14% (Table 3). These results are in agreementwithWalker(1984a) who characterized the magmas related to the aligned volcanoesof Nejapa–Miraflores as low in pre-eruptive H2O (b1 wt.%).


4. Discussion: maar formation and eruptive mechanism

The thin, cohesionless, and well developed stratification, the finegrain size (ash range) and the very low proportion of juvenilecomponent, measured at a maximum at 15 vol.% (4% in average), arethe main characteristics of the Asososca dominantly dry base-surgedeposits. The high content of dark gray accidental scoriae containingplagioclase, augite, and olivine throughout the Asososca Tephrasuggests that magma/water interaction took place at one or severalshallow to mid-depth stratigraphic levels made of this lithology (first200 m below current ground-surface). Several scoriae levels under-neath andesitic and basaltic lava flows were identified in the field andin exploratory well-logs between 70 and 190 m deep (UNAN-CIGEO,unpublished data). In fact, the principal aquifers of Managua have beenidentified in basaltic to andesitic scoriae deposits, as well as in basalticlava flows (CIEU, 2001), which correlates with the prevalence of thesekind of fragments in ASOT.

The typical stratified and “dry” characters of the Asososca Tephrasuggest phreatomagmatic pulses related to discrete steam explosionsand a limited flux of groundwater into the explosion chamber, becauselarger amounts of water would have favored thicker and moistureddeposits (Wohletz and Heiken, 1992; Lorenz and Kurszlaukis, 2007):based on Aranda-Gómez and Luhr (1996), the generation of maar “drythin ash layers” requires the pre-existence of an aquifer of large storagecapability and high specific retention which might provide a limitedhydraulic conductivity and a reliable source of water for a long time.Therefore, the Asososca Tephra suggests a matrix porosity-controlledaquifer rather than a fracture-controlled aquifer, which would providehighwater:magma ratios andwouldmore likely generatewet, probablythick, poorly sorted, and generally massive phreatomagmatic deposits.

Plate III.G1. Vesiculated particles showing elongated or ovoid bubbles and intense dissolutioeffect during transport (Shown in H2). I1. Blocky-like dust particle. I2. Moss-like dust particle.bubble. J2-3. Progressive dissolution effect caused by corrosive fluids during magma–water

This hypothesis is consistent with our observations and interpretations,since the scoria ash and lapilli beds present in theAsososca substrate areunconsolidated porous media, which host aquifers.

The high efficiency of the fuel–coolant interaction is reflected bythe high fragmentation, because more than 90 vol.% of particles arefiner than 2 mm throughout the whole unit. In addition, the lowjuvenile content suggests a small volume of magma involved in theeruption per time unit. The absence of deep xenoliths (e.g. marinesediments or metamorphic rock fragments), in turn, reflects a slowmagma ascent which implies a low magma supply rate. Moreover, thedominant C-D-E-and-F ash morphotypes suggest that fluid instabil-ities and viscous stresses, instead of passive magma quenching, werethe principal phreatomagmatic fragmentation mechanisms, favoringrelatively dry eruptions. However, the complexity of the explosiveinteraction is attested by the variety of juvenile ash morphologies(Fig. 12):

a) Blocky shards (A-type) represent the brittle end-member ofmagma fragmentation and probably were generated at themagma–water interaction front. Then, they represent magmathat directly interacted with the external water, quenchinginstantaneously (e.g. Heiken, 1972). Quench-fragmentationoccurred before any vesiculation could occur. Similar equant ortrapezoidal but poorly vesiculated shards (B-and-C-types) alsorepresent brittle fracture due to rapid chilling and quenching butthemagma portion involved was already subject to some degree ofgas exsolution and bubble nucleation before fragmentationoccurred (e.g. Houghton et al., 1999).

b) Moss-like particles (D-type) and fluidal/fused shards (E-and-F-types) are typical of low-viscosity magmas that form Taylor and

n pits (Zoomed in G2). H-1. Pelee-hair ash with microstructures that suggest the tractionI3. Acicular dust with tube-like vesicles. J1. Dissolution pit in the inner wall of a sphericalinteraction.

Fig. 10. Microprobe, single-point, bulk glass composition of fresh ash particles. SEM, backscattered images of typical blocky and fused-shaped ashes are shown. Note that althoughthose morphotypes are externally low vesiculated or completely dense, internally they show different proportions of spherical and irregular vesicles.


Kelvin–Helmholtz instabilities when they interact with externalwater, due to the density contrast between both fluids during theexpansion (Heiken, 1972; Dellino and Kyriakopoulos, 2003).Fluidal and drop-like morphologies were probably formed whenthe magma remained in a semi-ductile state after fragmentation.Then, the outer particle hot surface could plastically deform andcould flow before complete solidification. Their morphology isthen determined by the domain of surface tension over viscosity athighly efficient interactions, as experimentally demonstrated byHeiken (1972). Where viscous effects overcome surface tension,particularly at zones of rapid vapor formation, moss-like particleswere more likely.

c) Highly vesicular shards (G-type) and Pelee's particles (H-type)probably represent the viscous and ductile end-members ofmagma fragmentation, respectively, and probably derive fromthose magma portions that did not have enough contact withexternal water. The highly vesiculated particles with ovoid andelongated bubbles probably derived from shear zones near to theconduit walls. Together with the highly vesiculated dust (Plate III-H3), they indicate that some magma fragmentation due to bubbleburst was also allowed; then, during a typical and highly efficientphreatomagmatic eruption local regions of magma decompressionmight be also favored (e.g. Dellino et al., 2001). Furthermore, the

Table 2Glassy ash-shards ion-microprobe analyses carried out at LUP-Instituto de Geofísica-UNAM

Sample 11-3b-4-3 11-6b-2-2 11-6b-2-3 11-6b-2-4 11b-2a-7-2

Unit ASOT

Lat. N 12° 8′ 9″

Long. W 86° 19′ 17″

SiO2 48.093 48.108 47.562 48.99 48.469Al2O3 12.206 11.524 13.193 9.665 13.116Fe2O3(T) 12.595 11.845 14.512 13.666 16.59MnO 0.313 0.284 0.339 0.327 0.364MgO 8.278 8.983 6.894 9.587 5.044CaO 16.262 17.057 14.465 15.618 10.907Na2O 1.386 1.148 1.409 1.144 2.304K2O 0.312 0.241 0.419 0.325 0.632TiO2 0.946 0.854 0.97 0.887 1.171P2O5 0.163 0.118 0.126 0.123 0.237Total 100.554 100.162 99.889 100.332 98.834

fact that all analyzed particles are internally variably vesicularsuggests that some degree of magma vesiculation occurred beforethe interaction with external water and further fragmentation. Inthe vesiculated shards we observed up to four bubble-sizepopulations (Table 1), among spherical to irregular due tocoalescence. There are probably first generation bubbles thatwere heterogeneously nucleating and growing in a low-viscositymagma. Second generation bubbles nucleated in a low-to-midvesiculated melt and were inferred from aligned small sphericalbubbles along stepped surfaces.

In summary, ash-particles analysis with the scanning electronmicroscope and infrared microscope evidenced that most of theparticles are poorly to moderately vesiculated (0–40%), commonlywith a few isolated spherical vesicles, some of them formed by thecoalescence of smaller ones. This indicates that the rising basalticmagma was a low-viscosity melt that allowed bubble nucleation andfree, spherical expansion. These features are in agreement with thebasaltic tholeiitic composition obtained with the electron microprobefor the juvenile shards as representative of the pre-eruptive liquidphase. Bubble growth and interconnection was enhanced duringmagma ascent, increasing magma permeability and facilitating gasescape. Besides the high fluidity of the melt, the low H2O content also

(México).

11b-2a-7-4 11b-2a-7-5 11b-7b-2-5 11b-7b-4-1 11b-7b-4- 4

47.632 47.78 47.275 47.892 48.38812.576 12.584 13.929 14.057 11.75717.498 15.852 14.819 14.098 14.2790.365 0.299 0.358 0.26 0.2514.951 5.406 5.394 6.267 8.21711.063 11.556 12.003 13.003 14.7872.339 2.308 2.12 2.408 1.5680.62 0.724 0.686 0.782 0.2811.225 1.167 0.993 0.974 0.9230.241 0.168 0.183 0.232 0.117

98.51 97.844 97.76 99.973 100.568

Fig. 11. Preliminary results of micro-FTIR spectroscopy of juvenile glass shards. Note the typical H2O peak at a 3550 cm−1 wavelength, indicating the presence of dissolved water inthe glass.


suggests that magma fragmentation was not effectively driven bygas exsolution processes. These observations are similar to thosepresented by Houghton et al. (1999), and indicate that without theinteraction with external water, vesiculation alone would not havefragmented magma.

Based on field, granulometry, and componentry data (Fig. 9), theeruption of Asososca can be characterized as follows (Fig. 12):

a) The eruption onset is marked by a massive, highly altered, coarse-grained deposit that suggests ballistic ejection of country-rockblocks as vent opened, followed bywet base surges (Subunit ASOT-A). The limited distribution and the stratigraphic position of thosewet deposits suggest that they might be produced by a first ventopening to the East in the pre-existent hanging wall of the grabenarea west of Managua (Pardo et al., 2008). The initial high water:magma ratio was allowed by the interactionwith pre-existent lakewater, suggested by the unconsolidated, very well sorted sandybed that underlay the Asososca volcanic deposits inside the crater.However subsequent eruptive activity shifted to the prevailingconditions were external water could not easily flow into the vent:

b) A westward migration of the vent probably occurred as indicatedby the asymmetric distribution of the dry base-surge deposits(Subunit ASOT-B) and by the superposition of the western de-pression over the eastern one shown in the bathymetry (Fig. 3).The lake water was likely already exhausted and more efficientfragmentation was enhanced by magma interaction with water-saturated scoriae levels. When the external portion of magma(magma front and conduit walls) firstly interacted with ground-water, it quenched and fragmented, forming the blocky shardswhile triggering a shock-wave over the rising magma. Thisdecompressionwavewas able to propagate inward and downward,favoring further vesiculation and the emission of particles thatinteracted in progressively lower amounts with the external water,as represented by type B to H shards (Fig. 12). In that way, anoptimum quantity of thermal energy was transformed in mechan-

ical energy that fractured the country rock and widened the craterwhile dry base surges were generated. Base-surge clouds weredensity stratified as indicated by sedimentary structures suchas dunes, plane-parallel stratification, good grain-size sorting,chute and pool structures, and by the evidences of blocking (e.g.Valentine, 1987) when they encountered topographic obstaclessuch as the Motastepe scoria cone (Pardo et al., 2008). Coarserparticles (−1bϕb−3) were mainly deposited by traction carpets,although pulses of ballistic input were also attained. Finer materialwas deposited from traction sensu stricto and saltation by theoverriding turbulent and diluted flow.

The progressive widening of the crater enhanced inner, unstablewalls collapses, partially and temporally obstructing the conduit asindicated by ballistic-block-rich levels (Subunit ASOT-C).

c) In general, the component analysis does not show significant shiftsin the amount or nature of accidental lithics along the stratigraphicsequence, which indicates that the fragmentation level did notsignificantly migrate downward or upward during the eruption(e.g. Németh et al., 2001; Auer et al., 2007). A little variation oflithic type proportions is registered in subunit ASOT-D, whichsignals the input of deeper material that progressively increased inASOT-E-to-F, shown by the higher content of hypocrystallineporphyries, black tubular pumice, and highly altered ignimbritefragments likely derived from the Pleistocene Las Sierras Forma-tion; ASOT-D also records a significant ballistic input with coarsergrain-size fragments and a relative increase of blocky and low-vesiculated shards (A-B-and-C-types) compared to moss-like andfused-shaped shards (D-E-F-types). Then, ASOT-D-E subunitssuggest that there were eruptive pulses or phases of lowermagma and country-rock fragmentation, probably due to a greatercontribution of groundwater flux into the vent (i.e. larger amountof water involved), and a probable increment of the explosiveinteraction depth.

Table 3Dissolved volatile calculations using the Beer–Lambert Law with absorption data obtained in the infrared microscope. We considered ε3550 values from different authors.

Sample Maximum infrared absorption peak

11-1b-5a Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)

D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.72 2.68 2.8 2.72 2.68 2.8 2.72 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.087 0.090 0.091 0.076 0.079 0.080 0.083 0.086 0.086 0.081 0.084 0.085 0.075 0.077 0.078 0.095 0.097 0.099ppm 867.681 899.818 906.533 764.753 793.077 798.996 827.009 857.639 864.039 814.286 838.646 850.746 745.473 767.775 778.852 945.153 973.429 987.473⁎Obtained with a 0.03 mm thickness, and with A (a.u.) of 0.24711-1b-5b Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.718667 2.68 2.8 2.718667 2.68 2.8 2.718667 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.122 0.126 0.127 0.107 0.111 0.112 0.116 0.120 0.121 0.114 0.118 0.120 0.105 0.108 0.109 0.133 0.137 0.139ppm 1218.97 1264.117 1273.55 1074.37 1114.161 1122.476 1161.83 1204.861 1213.853 1143.956 1178.179 1195.178 1047.284 1078.615 1094.177 1327.806 1367.53 1387.26⁎Obtained with a 0.03 mm thickness, and with A (a.u.) of 0.34711-3a-2d Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.718 2.68 2.8 2.718 2.68 2.8 2.718 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.072 0.075 0.075 0.064 0.066 0.066 0.069 0.071 0.072 0.068 0.070 0.071 0.062 0.064 0.065 0.079 0.081 0.082ppm 720.843 747.541 753.120 635.333 658.864 663.781 687.054 712.500 717.817 676.484 696.893 706.774 619.316 638.000 647.046 785.204 808.893 820.362⁎Obtained with a 0.025 mm thickness, and with A (a.u.) of 0.17111-3a-2d1 Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.718 2.68 2.8 2.718 2.68 2.8 2.718 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.075 0.077 0.078 0.066 0.068 0.069 0.071 0.074 0.074 0.070 0.072 0.073 0.064 0.066 0.067 0.081 0.084 0.085ppm 746.136 773.770 779.545 657.626 681.982 687.072 711.161 737.500 743.004 700.220 721.345 731.573 641.046 660.386 669.750 812.755 837.275 849.147⁎Obtained with a 0.025 mm thickness, and with A (a.u.) of 0.17711-3a-2a Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.718667 2.68 2.8 2.718667 2.68 2.8 2.718667 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.065 0.068 0.068 0.057 0.060 0.060 0.062 0.064 0.065 0.061 0.063 0.064 0.056 0.058 0.059 0.071 0.073 0.074ppm 651.732 675.870 680.914 574.420 595.695 600.141 621.182 644.189 648.996 611.625 629.923 639.011 559.939 576.690 585.010 709.922 731.161 741.710⁎Obtained with a 0.038 mm thickness, and with A (a.u.) of 0.23511-3a-2b Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.718 2.68 2.8 2.718 2.68 2.8 2.718 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.071 0.073 0.074 0.062 0.065 0.065 0.067 0.070 0.070 0.066 0.068 0.069 0.061 0.063 0.063 0.077 0.079 0.080ppm 707.198 733.391 738.864 623.307 646.393 651.217 674.048 699.013 704.230 663.678 683.701 693.395 607.593 625.924 634.799 770.341 793.582 804.834⁎Obtained with a 0.038 mm thickness, and with A (a.u.) of 0.25511-3a-2c Pandya et al. (1992) Mandeville et al. (2002) Yamashita et al. (1997) Ohlhorst et al. (2001)D (g/cm3) 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.7 2.68 2.8 2.718 2.68 2.8 2.718 2.68 2.8 2.718 2.68ε (l⁎mol−1⁎cm−1) 61 61 61 69.21 69.21 69.21 64 64 64 65 65 65 71 71 71 56 56 56C (wt.%) 0.045 0.047 0.048 0.040 0.042 0.042 0.043 0.045 0.045 0.043 0.044 0.045 0.039 0.040 0.041 0.050 0.051 0.052ppm 454.826 471.671 475.191 400.872 415.719 418.822 433.506 449.561 452.916 426.836 439.714 445.948 390.766 402.555 408.263 495.435 510.382 517.619⁎ Obtained with a 0.038 mm thickness, and with A (a.u.) of 0.164

308N.Pardo

etal./

Journalof

Volcanologyand

Geotherm

alResearch

184(2009)

292–312

Fig. 12. The figure is not to scale. Asososca maar formation and eruptive mechanism. A. A single batch of basaltic and degassed magma rose towards the surface penetrating theNicaragua Depression volcanic infill, favored by the NMF. At depths shallower than 190m, according to well-log data (UNAN-CIGEO, unpublished data) therewas an aquifer hosted inone or some of the scoria levels, confined between impermeable lavas. At the time immediately before the Asososca eruption, most of the Managua Formationwas already depositedwith the Masaya Tuff atop. B. Zoom at the contact zone, where different glassy ash morphologies resulted according to the degree of magma/water interaction, the vesicularity ofmagma at that locus, and depending on the different behavior of magma upon stresses generated during the contact with external water. A-type: Blocky-equant ashes derived frompassive magma quenching in the contact front. B–C-types: Equant or pyramidal/trapezoidal ashes derived from already vesiculated magma portions that solidified prior toquenching and fragmentation. D-type: Moss-like particles were produced in zones of fluid instabilities development where magma behaved viscously during a highly efficientinteraction. E–F-types: Fused and drop-like shapes derived from regions where surface tension effects were dominant during dynamic fluid instabilities. Particles were semi-plasticafter fragmentation and prior to complete solidification. G-type: Highly vesiculated particles reflect zones of considerable gas exsolution andminimal interactionwith water. H-type:Pelee-hair ashes probably reflect magma portions that did not interacted with water. C–D. The sudden water vaporization and expansion caused the drastic explosion, vent openingand base surges generation. C. Initially an eastern vent opened with the generation of wet base surges; the pre-existence of a surface lake probably contributed to the initial highwater/magma ratio. D. As the lake got empty, the low-storage aquifer favored the generation of dominantly dry base surges and the migration of the vent westward. E. Subsequentlyan irregular, bowl-shaped maar crater was formed as dry base surges continued, accompanying wall-rock collapses, vent coalescence, and minor deepening occurred.


Subunit ASOT-G evidences a decrease in deep fragments and ageneral mixture of shallow volcanic lithics, suggesting juvenilematerial recycling during the late stages of the eruption (Houghtonand Smith, 1993; Houghton et al., 1999).

Well-log data and previous works, suggest that Las SierrasFormation (ignimbrites and lava flows) appears at a depth of 274 mdeep and is ∼680 m thick (Kutterolf et al., 2007), constraining themaximum depth reached by the Asososca excavation. In general


terms, the whole stratigraphic sequence is dominated by volcanicdebris which forms the upper ∼200 m without any interveningsedimentary rocks (older than Pliocene), which lie below ∼700 m(Bice, 1985). This suggests that crater widening was dominant overdeepening, which is consistent with the Asososca maar irregular andbowl-shapedmorphology. Following Németh et al. (2001) and Auer etal. (2007), these features are typical of maar emplacement in a soft-substrate.

5. Synthesis: main results and hazard implications

The alignment of volcanoes along the NMF is related to the localstructural framework determined by N–S, E–W to NE–SW and minorNW lineaments, which correlate with the main geological andregional tectonic structures. For example, the N–S Nejapa–Mirafloresfault is parallel to the strike of the East-Pacific-Rise that borders theCocos Plate; the E–W lineaments are parallel to the Galápagos Rift,which also borders the Cocos Plate; and NW lineaments are parallel tothe Middle America Trench, the Mateare fault and the NicaraguaDepression. These results are in agreement with the findings of Wiseet al. (1985) who proposed a relationship between lineament swarmsand the stress field, in which main lineament domains are normal tothe minimum horizontal stress. In this case the N13°W lineamentdomain could be the result of stretching brittle surface rocks above a“more-ductile” basement undergoing regional compression (model Iin Wise et al., 1985). In this case, the N13°W lineament domain mayrepresent the NNW–SSE transtensive fault zone that is favoring theascent of magma and volcano generation on the surface.

At the western outskirts of Managua, the area of largest lineamentdensity coincides with the area of largest vent density, and with theexistence of the youngest volcanism. There, the explosiveness anderuption style might have a topographic control (Lorenz, 1984; White,1991b) but particularly depend on the probability of rising magmainteraction with the available aquifers (e.g. Aranda-Gómez and Luhr,1996; Németh et al., 2001; Auer et al., 2007). The substrate ofManagua is largely heterogeneous, including a wide spectrum ofindurated/consolidated interbedded primary and secondary volcani-clastic deposits with lava flows and ignimbrites, together with acomplex structural domainwhichmight influence the geometry of thevolcanic conduits. In case of interaction, further phenomena (includ-ing base surges, fallouts, lahars) and related hazards may varyaccording to the magma supply volumes and rates, and to the wateravailability. In the studied area, the latter is not strongly controlled byclimate variations, but depends on the aquifer nature (i.e. porosity-controlled or fracture-controlled).

The model presented here for the ∼1245 yr BP Asososca maar issimplified, since the maar morphology is the result of various factors,among which the conduit (s) or feeder dike (s) heterogeneity,structural controls determined by N–S, NE and NW fault systems, andthe interaction with previous volcanic structures should be consid-ered. However, Asososca is a typical maar surrounded by dominantlydry, thin ash layers resulted from magma/water interactionsoccurring in a matrix porosity-controlled aquifer hosted in scoriaash and lapilli beds, present in the Asososca maar substrate. The veryrecent age of the Asososca Tephra indicates that monogeneticeruptions might occur again at western Managua; there, the localstructural and tectonic regime provides extensional zones, facilitatingmagma ascent, with a potential for phreatomagmatic explosiveinteraction of even volatiles-poor, low-viscosity basaltic magmaswith aquifers and/or superficial water (e.g. Managua Lake). Hazardsestimation for future eruptions along the NMF should take intoaccount the probable change of the eruption dynamic and style:magmatic to phreatomagmatic, “wet” phreatomagmatic to “dry”phreatomagmatic, Hawaiian to Strombolian, etc. (e.g. Gutmann,2002; Abrams and Siebe, 1994; Houghton et al., 1999.). Although wedo not knowwhen, where and how future eruptions will occur, events

similar to the Asososca maar-forming eruption along the NMF woulddefinitively affect the Managua urban area. Earthquakes, eruptionclouds, tephra fallouts, ballistic bombs and blocks, base surges, as wellas surface excavation and emission of toxic volcanic gases wouldthreaten nearly 1.8 million people, essential infrastructures (e.g. mainroads, hospitals, aqueducts), industrial lands, and vegetation, whereaspollution of drinkable water sources would also be significant.

Western Managua population is currently unprepared to managevolcanic emergencies because no eruptions have occurred since∼1250 years ago. Emergency plans and strategies regarding scientificknowledge socialization and practical labor within communitiesshould be adopted, while increasing cooperation efforts amongnational and international entities on monitoring systems are beingdeveloped.

Acknowledgements

This study was co-financed by the Centro de InvestigacionesGeocientíficas (CIGEO, Nicaragua), the Mexican Consejo Nacional deCiencia y Tecnología (CONACYT grant 47226 to J. L. Macías), theGeological Society of America research grant – Harold T. StearnsFellowship Award (to N. Pardo-UNAM), and by an internationalmobility scholarship given to N. Pardo-UNAM by the DirecciónGeneral de Estudios de Posgrado (DGEP-UNAM-Mexico). Laboratoryanalyses were performed at Universitá degli Studi Roma-Tre, Italy. Wethank T. Scolamacchia (UNAM, Mexico) and F. Espinoza (CIGEO) fortheir valuable support during the fieldwork; C. Muñóz, F. Mendiola,and C. Linares (UNAM, Mexico), as well as Sergio Lo Mastro (LIME,Roma-tre, Italy), for the valuable technical support with lab analyses.We particularly thank F. Salvini, A. De Benedetti, E. Caprilli (Universitádegli Studi Roma-Tre, Italy), as well as J. Roeberge (UNAM, Mexico DF)and H.F. Murcia (INGEOMINAS, Colombia) for helpful comments anddiscussion. Dr. Karoly Németh and Dr. Pierofrancesco Dellino arestrongly thanked for their valuable suggestions and reviews.

References

Abrams, M., Siebe, C., 1994. Cerro Xalapaxco: an unusual tuff cone with multipleexplosion craters, in central Mexico. J. Volcanol. Geotherm. Res. 63, 183–199.

Allen, S.R., Bryner, V., Smith, I., Balance, P., 1996. Facies analysis of pyroclastic depositswithin basaltic tuff-rings of the Auckland volcanic field, New Zealand. N. Z. J. Geol.Geophys. 39, 309–327.

Aranda-Gómez, J.J., Luhr, J.F., 1996. Origin of the Joya Honda maar, San Luis Potosi,Mexico. J. Volcanol. Geotherm. Res. 74 (1–2), 1–18.

Auer, A., Martin, U., Németh, K., 2007. The Fekete-hegy (Balaton Highland Hungary)“soft-substrate” and “hard-substrate”maar volcanoes in aligned volcanic complex –

implications for vent geometry, subsurface stratigraphy and the palaeoenviron-mental setting. J. Volcanol. Geotherm. Res. 159, 225–245.

Anzidei, M., Carapezza, M.L., Esposito, A., Giordano, G., Lelli, M., Tarchini, L., 2008. TheAlbano Maar Lake high resolution bathymetry and dissolved CO2 budget (ColliAlbani volcano, Italy): Constrains to hazard evaluation. J. Volcanol. Geotherm. Res.171 (3–4), 258–268.

Belousov, A., Voight, B., Belousova, M., Petukhin, A., 2002. Pyroclastic surges and flowsfrom the 8–10 May 1997 explosive eruption of Bezymianny volcano, Kamchatka,Russia. Bull. Volcanol. 64, 455–471.

Bice, D.C., 1985. Quaternary volcanic stratigraphy of Managua, Nicaragua: correlationand source assignment for multiple overlapping plinian deposits. Bull. Geol. Soc.Am. 96, 553–566.

Blates, R.L., Jackson, J.A., 1984. Dictionary of Geological Terms, Third Edition. AmericanGeological Institute. 571 pp.

Burgisser, A., Bergantz, G.W., 2002. Reconciling pyroclastic flow and surge: themultiphase physics of pyroclastic density currents. Earth. Planet. Sci. Lett. 202(2), 405–418.

Bukart, B., Self, S., 1985. Extension and rotation of crustal blocks in northern CentralAmerica and effect on the volcanic arc. Geology 13, 22–26.

Büttner, R., Dellino, P., Zimanowski, B., 1999. Identifying magma–water interaction fromsurface features of ash particles. Nature 401, 688–690.

Carey, S., 1991. Transport and deposition of tephra by pyroclastic flows and surges.sedimentation in volcanic settings. SEPM Spec. Publ. 45, 39–57.

Carrasco, G., Ort, M.H., Romero, C., 2007. Evolution and hydrological conditions of amaarvolcano (Atexcaco crater, EasternMexico). J. Volcanol. Geotherm. Res. 159, 179–197.

Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions, Modern and Ancient. Allen & UnwinLtd., London. 546 pp.

CEDEX-ENACAL, 1999. Convenio para la realización de trabajos y actividades decooperación técnica, en materia de medio ambiente e infraestructuras, con los


países de Centroamérica afectados por el Huracán Mitch. Proyecto Nicaragua 2.Evaluación del comportamiento estacional de la Laguna de Asososca. Informetécnico final. Tomo único. Madrid. 122 pp.

Chough, S.K., Sohn, Y.K., 1990. Depositional mechanics and sequences of base surges,Songaksan tuff ring, Cheju Island, Korea. Sedimemtology 37 (6), 1115–1135.

CIEU, 2001. Evaluación de Recursos de agua de Nicaragua. Cuerpo de Ingenieros de losEstados Unidos de América-Distrito de Mobile y Centro de Ingeniería Topográfica.83 pp.

Cole, P.D., 1991. Migration direction of sand-wave structures in pyroclastic-surgedeposits: implications for depositional processes. Geology 19, 1108–1111.

Connor, C.B., 1990. Cinder cone clustering. J. Geophys. Res. 95, 19,395–19,405.Connor, C.B., Conway, F.M., 2000. Basaltic volcanic fields. In: Sigurdsson, H. (Ed.),

Encyclopedia of Volcanoes. Academic Press, pp. 331–344.Crowe, B.M., Fisher, R.V., 1973. Sedimentary structures in base-surge deposits with

special reference to cross-bedding, Ubehebe Craters, Death Valley, California. Bull.Geol. Soc. Am. 84, 663–682.

Davidson, J., De Silva, S., 2000. Composite volcanoes. In: Sigurdsson, H. (Ed.),Encyclopedia of Volcanoes. Academic Press, pp. 663–682.

De Astis, G., Dellino, P., De Rosa, R., La Volpe, L., 1997. Eruptive and emplacementmechanisms of widespread fine-grained pyroclastic deposits on Vulcano Island(Italy). Bull. Volcanol. 59, 87–102.

De Benedetti, A.A., Funiciello, R., Giordano, G., Caprilli, E., Diano, G., Paterne, M., 2008.Volcanology, history and myths of the Lake Albano maar (Colli Albani volcano,Italy). J. Volcanol. Geotherm. Res. 176, 387–406.

Dellino, P., La Volpe, L., 1995. Fragmentation versus transportation mechanisms in thepyroclastic sequence of Monte Pilato–Rocche Rosse (Lipari, Italy). J. Volcanol.Geotherm. Res. 64, 211–232.

Dellino, P., La Volpe, L., 2000. Structures and grain size distribution in surge deposits as atool for modelling the dynamics of dilute pyroclastic density currents at La Fossa diVulcano. J. Volcanol. Geotherm. Res. 96 (1–2), 57–78.

Dellino, P., Kyriakopoulos, K., 2003. Phreatomagmatic ash from the ongoing eruption ofEtna reaching the Greek island of Cefalonia. J. Volcanol. Geotherm. Res. 126,341–345.

Dellino, P., Isaia, R., La Volpe, L., Orsi, G., 2001. Statistical analysis of textural data fromcomplex pyroclastic sequences: implications for fragmentation processes of theAgnano–Monte Spina Tephra (4.1 ka), Phlegrean Fields, southern Italy. Bull.Volcanol. 63, 443–461.

Dellino, P., Isaia, R., La Volpe, L., Orsi, G., 2004a. Interaction between particlestransported by fallout and surge in the deposits of the Agnano–Monte Spinaeruption (Campi Flegrei, Southern Italy). J. Volcanol. Geotherm. Res. 133,193–210.

Dellino, P., Isaia, R., Veneruso, M., 2004b. Turbulent boundary layer shear flows as anapproximation of base surges at Campi Flegrei (Southern Italy). J. Volcanol.Geotherm. Res. 133, 211–228.

Derosa, R., Frazzetta, G., Lavolpe, L., 1992. An approach for investigating the depositionalmechanism of fine-grained surge deposits – the example of the dry surge depositsat La-fossa-di-Vulcano. J. Volcanol. Geotherm. Res. 51, 305–321.

Devine, J.D., Gardner, J.E., Brack, H.P., Layne, G.D., Rutherford, M., 1995. Comparison ofmicroanalytical methods for estimating H2O contents of silicic volcanic glasses. Am.Mineral. 80, 319–328.

Druitt, T.H., 1998. Pyroclastic density currents. In: Gilbert, J.S., Sparks, R.S.J. (Eds.), ThePhysics of Explosive Volcanic Eruptions. Geol. Soc. London Spec. Publ., vol. 145,pp. 145–182.

Espinoza, F.J., 2007. Neotectónica de la falla Nejapa, porción Oeste del Graben deManagua, Nicaragua. Tesis de Maestría. Posgrado en Ciencias de la Tierra,Universidad Nacional Autónoma de México, México D.F. 100 pp.

Fisher, R.V., 1990. Transport and deposition of a pyroclastic surge across an area of highrelief: the 18 May 1980 eruption of Mount St. Helens, Washington. Geol. Soc. Am.102, 1038–1054.

Fisher, R.V., Heiken, G., 1982. Mt. Pelée, Martinique: May 8 and 20, 1902 pyroclasticflows and surges. J. Volcanol. Geotherm. Res. 13, 339–371.

Fisher, R.V., Schmincke, H.-U., 1984. Pyroclastic Rocks. Springer-Verlag, Berlin. 472 pp.Frischbutter, A., 2002. Structure of the Managua graben, Nicaragua, from remote

sensing images. Geofis. Int. 41 (2), 87–102.Funiciello, R., Giordano, G., De Rita, D., 2003. The Albano maar lake (Colli Albani

Volcano, Italy): recent volcanic activity and evidence of pre-Roman Agecatastrophic lahar events. J. Volcanol. Geotherm. Res. 123, 43–61.

Gençalioğlu-Kuşcu, G., Atilla, C., Cas, R., Kuşcu, I., 2007. Base surge deposits, eruptionhistory and depositional processes of a wet phreatomagmatic volcano in CentralAnatolia (Cora Maar). J. Volcanol. Geotherm. Res. 159, 198–209.

Giordano, G., De Rita, D., Cas, R., Rodani, S., 2002. Valley pond and ignimbrite veneerdeposits in the small-volume phreatomagmatic ‘Peperino Albano’ basic ignimbrite,Lago Albano maar, Colli Albani volcano, Italy: influence of topography. J. Volcanol.Geotherm, Res. 118, 131–144.

Giordano, G., De Benedetti, A.A., Diana, A., Diano, G., Gaudioso, F., Marasco, F., Miceli, M.,Mollo, S., Cas, R.A.F., Funiciello, R., 2006. The Colli Albani mafic caldera (Roma,Italy): stratigraphy, structure and petrology. J. Volcanol. Geotherm. Res. 155, 49–80.

Girard, G., van Wyk de Vries, B., 2005. The Managua Graben and Las Sierras–Masayavolcanic complex (Nicaragua); pull-apart localization by an intrusive complex:results from analogue modeling. J. Volcanol. Geotherm. Res. 144, 37–57.

Glasstone, S., 1950. The Effects of Atomic Weapons. Los Alamos, N.M., Los Alamos Sci.Lab. U.S. Atomic Energy Comm. 456 pp. http://glasstone.blogspot.com/2006/03/samuel-glasstone-and-philip-j-dolan.html.

Goward, S.N., Masek, J.G., Darrel, L., Williams, D.L., Irons, J.R., Thompson, R.J., 2001. TheLandsat 7 mission: terrestrial research and applications for the 21st century.Remote Sens. Environ. 78, 3–12.

Gutmann, J.T., 2002. Strombolian and effusive activity as precursors to phreatomag-matism: eruptive sequence at maars of the Pinacate volcanic field, Sonora, Mexico,J. Volcanol. Geotherm. Res. 113, 345–356.

Havlicek, P., Hradecký, M., Navarro, M., Novák, Z., Staník, E., Sebesta, J., 1997. Estudiopara el reconocimiento de la amenaza geológica en el área de Managua, Nicaragua.Cesky Geologicky Ustav Praha, Instituto Nicaraguense de Estudios Territoriales,Managua (INETER). 81 pp.

Heiken, G., 1972. Morphology and petrography of volcanic ashes. Bull. Geol. Soc. Am. 83,1961–1988.

Houghton, B.F., Smith, R.T., 1993. Recycling of magmatic clasts during explosiveeruptions: estimating the true juvenile content of phreatomagmatic volcanicdeposits. Bull. Volcanol. 55, 414–420.

Houghton, B.F., Wilson, C.J.N., Smith, I.E.M., 1999. Shallow-seated controls on styles ofexplosive basaltic volcanism: a case study from New Zealand. J. Volcanol.Geotherm. Res. 91, 97–120.

Ihinger, P.D., Hervig, R.L., McMillan, P.F., 1994. Analytical methods for volatiles in glasses.In: Carroll, M.R., y Holloway, J.R. (Eds.), Volatiles inMagmas. Reviews inMineralogy,vol. 30, pp. 67–111.

Kienle, J., Kyle, P.R., Self, S., Motyka, R.J., Lorenz, V., 1980. Ukinrek Maars, Alaska, I. April1977 eruption sequence, petrology, and tectonic setting. J. Volcanol. Geotherm. Res.7, 11–37.

King, P.L., Holloway, J.R., 2002. CO2 solubility and speciation in intermediate (andesitic)melts: the roleofH2Oandcomposition.Geochim. Cosmochim.Acta 66 (9),1627–1640.

Kutterolf, S., Freundt, A., Pérez,W.,Wehrmann,H., Schmincke,H.U., 2007. Late Pleistoceneto Holocene temporal succession and magnitudes of highly explosive volcaniceruptions in west–central Nicaragua. J. Volcanol. Geotherm. Res. 163 (1–4), 55–82.

La Femina, P.C., Dixon, T.H., Strauch, W., 2002. Bookshelf faulting in Nicaragua. Geology30 (8), 751–754.

Lajoie, J., Lanzafameb, G., Rossic, P.L., Tranneb, C.A., 1992. Lateral facies variations inhydromagmatic pyroclastic deposits at Linosa, Italy. J. Volcanol. Geotherm. Res. 54(1–2), 135–143.

Lesti, C., Giordano, G., Salvini, F., Cas, R., 2008. Volcano-tectonic setting of the intraplate,Pliocene–Holocene, Newer Volcanic Province (southeast Australia): Role of crustalfracture zones. J. Geophys. Res. 113 (7), 1–11.

Libowitzky, E., Rossman, G.R., 1997. An IR absorption calibration for water in minerals.Amer. Mineral. 82 (11–12), 1111–1115.

Lorenz, V., 1973. On the formation of maars. Bull. Volcanol. 37 (2), 1–22.Lorenz, V., 1984. Zur Geologie des Meerfelder Maares. Cour. Forsch. Inst. Senkenberg 65,

5–12.Lorenz, V., 1986. On the growth of maars and diatremes and its relevance to the

formation of tuff-rings. Bull Volcanol 48, 265–274.Lorenz, V., 2007. Syn- and posteruptive hazards of maar–diatreme volcanoes.

J. Volcanol. Geotherm. Res 159, 285–312.Lorenz, V., Kurszlaukis, S., 2007. Root zone processes in the phreatomagmatic pipe

emplacement model and consequences for the evolution of maar–diatremevolcanoes. J. Volcanol. Geotherm. Res. 159, 4–32.

Marshall, J., 2007. Geomorphology and physiographic provinces. Chapter 3 In:Bundschuh, J., Alvarado, G.E. (Eds.), Central America: Geology, Resources andHazards, vol. 1, pp. 75–122.

Mandeville, C.W., Webster, J.D., Rutherford, M.J., Taylor, B.E., Timbal, A., Faure, K., 2002.Determination of molar absorptivities for infrared absorption bands of H2O inandesitic glasses. Am. Min. 87, 813–821.

Mazzarini, F., 2007. Vent distribution and crustal thickness in stretched continentalcrust: the case of the Afar Depression (Ethiopia). Geosphere 3, 152–162.

McBirney, A.R., 1955. The origin of the Nejapa Pits near Managua, Nicaragua. Bull.Volcanol. 17, 145–154.

McBirney, A.R., Williams, H., 1964. The origin of the Nicaraguan Depression. Abstr. Bull.Volcanol. 27, 63.

Moore, J.G., 1967. Base surge in recent volcanic eruptions. Bull Volcanol 30, 337–363.Müller, G., Veyl, G., 1956. The Birth of Nilahue, A New Maar-type Volcano at Rininahue,

Chile. 20th Int. Geol. Congr., Mexico City,1956. Rep. Sec, vol. 1, pp. 375–396.Németh, K., Martin, U., Harangi, Sz., 2001. Miocene phreatomagmatic volcanism at

Tihany (pannonian Basin, Hungary). J. Volcanol. Geotherm. Res. 111, 111–135.Ohlhorst, S., Behrens, H., Holtz, F., 2001. Compositional dependence of molar

absorptivities of near-infrared OH− and H2O bands in rhyolitic to basaltic glasses.Chem. Geol. 174, 5–20.

Ollier, C.D., 1967. Maars: their characteristics, varieties and definition. Bull Volcanol 31,45–73.

Pandya, N., Sharma, S.K., Muenow, D.W., Sherriff, B.L., 1992. The effect of bulkcomposition on the speciation of water in submarine volcanic glasses. Geochemicaet Cosmochimica Acta 56, 1875–1883.

Pardo, N., Avellán, D.R., Macías, J.L., Scolamacchia, T., Rodriguez, D., 2008. The∼1245 yr BP Asososca maar: new advances on recent volcanic stratigraphy ofManagua (Nicaragua) and hazard implications. J. Volcanol. Geotherm. Res. 176 (4),493–512.

Pérez, W., Freundt, A., 2006. The youngest highly explosive basaltic eruptions fromMasaya Caldera (Nicaragua): stratigraphy and hazard assessment. In: Rose, W.I.,Bluth, G.J.S., Carr, M.J., Ewert, J., Patino, L.C., Vallance, J.W. (Eds.), Volcanic Hazardsin Central America. Spec. Publ., vol. 412. Geol. Soc. Am., pp. 189–207.

Porreca, M., Mattei, M., MacNiocaill, C., Giordano, G., McClelland, E., Funiciello, R., 2008.Paleomagnetic evidence for low-temperature emplacement of the phreatomagmaticPeperino Albano ignimbrite (Colli Albani volcano, Central Italy). Bull. Volcanol. 70,877–893.

Salvini, F., 1985. Slop–intercept–density plots: a new method for line detection in theimages. IEEE International Geoscience and Remote Sensing Symposium(IGARSS'85) Digest. IEEE Catalog 85CH2162-6, USA, pp. 715–720.

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Schmincke, H.-U., 2004. Volcanism. Springer Verlag, Berlin. 324 pp.Schmincke, H.-U., Fisher, R.V., Waters, A.C., 1973. Antidune and chute and pool

structures in the base surge deposits of the Laacher See area, Germany.Sedimentology 20, 553–574.

Sheridan, M.F., Wohletz, K.H., 1981. Hydrovolcanic explosions I. The systematics ofwater-pyroclast equilibration. Science 212, 1387–1389.

Sohn, Y.K., 1996. Hydrovolcanic processes forming basaltic tuff rings and cones on ChejuIsland, Korea. Geol. Soc. Am. Bull. 108 (10), 1199–1211.

Sohn, Y.K., 1997. On traction-carpet sedimentation. J. Sed. Res. 67 (3), 502–509.Sohn, Y.K., Chough, S.K., 1989. Depositional processes of the Suwolbong tuff ring, Cheju

Island (Korea). Sedimentology 36, 837–855.Sohn, Y.K., Park, K.H., 2005. Composite tuff ring/cone complexes in Jeju Island, Korea:

possible consequences of substrate collapse and vent migration. J. Volcanol.Geotherm. Res. 141, 157–175.

Sparks, R.S.J., 1976. Grain size variations in ignimbrites and implications for thetransport of pyroclastic flows. Sedimentology 23, 147–188.

Stoiber, R.E., Carr, M.J., 1973. Quaternary volcanic and tectonic segmentation of CentralAmerica. Bull. Volcanol. 37, 304–325.

UNAN-EAM, 1978. Proyecto Investigación de la calidad de agua de la “Laguna deAsososca”. Informe final, período de 4 de diciembre de 1976 al 15 de enero de 1978.Managua, Nicaragua. 163 p.

Valentine, G.A., 1987. Stratified flow in pyroclastic surges. Bull Volcanol 49, 616–630.Valentine, G.A., 1998. Damage to structures by pyroclastic flows and surges, inferred

from nuclear weapons effects. J. Volcanol. Geotherm. Res. 87, 117–140.Valentine, G.A., Fisher, R.V., 2000. Pyroclastic surges and blasts. In: Sigurdsson, H. (Ed.),

Encyclopedia of Volcanoes. Academic Press, pp. 571–580.Vázquez, J.A., Ort, M.H., 2006. Facies variation of eruption units produced by the passage

of single pyroclastic surge currents, Hopi Buttes volcanic field, USA. J. Volcanol.Geotherm. Res. 154, 222–236.

Vespermann, D., and Schmincke, H.U., 2000. Scoria cones and tuff-rings. In: Sigurdsson,H., Houghton, B.F., McNutt, S.R., Rymer, H., and Stix, J., (Eds.), Encyclopedia ofVolcanoes. San Diego, San Francisco, New York, Boston, London, Sydney, Toronto,Academic Press: 683–694.

Viramonte, J., 1973. Las últimas erupciones volcánicas en Nicaragua (Periodo 1968–1970). Publ. Geol. ICAITI 4, 69–80.

Walker, G.P.L., 1971. Grain-size characteristics of pyroclastic deposits. J. Geol. 79,619–714.

Walker, G.P.L., 1983. Ignimbrite types and ignimbrite problems. J. Volcanol. Geotherm.Res. 17, 65–88.

Walker, J.A., 1984a. Volcanic rocks from the Nejapa and Granada cinder conealignments, Nicaragua, Central America. J. Petrol. 25, 299–342.

Walker, G.P.L., 1984b. Characteristics of dune-bedded pyroclastic surge bedsets.J. Volcanol. Geotherm. Res. 20, 281–296.

Wallace, P.J., 1998. Pre-eruptive H2O and CO2 contents of mafic magmas from thesubmarine to emergent shield stages of Gran Canaria. 1998 In: Weaver, P.P.E.,Schmincke, H.-U., Firth, J.V., Duffield, W. (Eds.), Proceedings of the Ocean DrillingProgram, Scientific Results, vol. 157, pp. 411–420.

Wehrmann, H., Bonadonna, C., Freundt, A., Houghton, B.F., Kutterolf, S., 2006. FontanaTephra: a basaltic plinian eruption in Nicaragua. In: Rose, W.I., Bluth, G.J.S., Carr,M.J., Ewert, J., Patino, L.C., Vallance, J.W. (Eds.), Volcanic Hazards in CentralAmerica. Spec. Publ., vol. 412. Geol. Soc. Am., pp. 209–223.

White, J.D.L., 1991a. Maar–diatreme phreatomagmatism at Hopi Buttes, Navajo Nation(Arizona), USA. Bull. Volcanol. 53, 239–258.

White, J.D.L., 1991b. The depositional record of small, monogenetic volcanoes withinterrestrial basins. In: Fisher, E.V., Smith, G.A. (Eds.), Sedimentation in VolcanicSettings. SEPM Special Publication No, vol. 45, pp. 155–171.

Wilson, C.J.N., Houghton, B.F., 2000. Pyroclast transport and deposition. In: Sigurdsson,H. (Ed.), Encyclopedia of Volcanoes. Academic Press, pp. 545–554.

Wise, D.U., Funiciello, R., Parotto, M., Salvini, F., 1985. Topographic lineament swarms:clues to their origin from domain analysis of Italy. Geol. Soc. Amer. Bull. 96,952–967.

Wohletz, K.H., 1986. Explosive magma–water interactions: thermodynamics, explosionmechanics and field studies. Bull. Volcanol. 48, 245–264.

Wohletz, K.H., Sheridan, M.F., 1979. A model of pyroclastic surge. In: Chapin, C.E.,y Elston, W.E. (Eds.), Ash-flow Tuffs. Sp. Pap., vol. 180. Geol. Soc. Amer., pp. 177–194.

Wohletz, K., Krinsley, D., 1982. Scanning electronmicroscopy of basaltic hydromagmaticash. Los Alamos National Laboratory Report, LA-UR 82-1433. 43pp.

Wohletz, K.H., Sheridan, M.F., 1983. Hydrovolcanic explosions II. Evolution of basaltictuff rings and tuff cones. Amer. Journ. Sci. 283, 385–413.

Wohletz, K.H., Heiken, G., 1992. Volcanology and Geothermal Research. University ofCalifornia Press, Berkeley. Los Angeles- Oxford. 415 pp.

Yamashita, S., Kitamura, T., Kusakabe, M., 1997. Infrared spectroscopy of hydrous glassesof arc magma compositions. Geochem. J. 31, 169–174.

Zimanowski, B., 1998. Phreatomagmatic explosions. In: Freundt, A., Rosi, M. (Eds.), FromMagma to Tephra. Modeling Physical Processes of Explosive Volcanic Eruptions.Elsevier Science Publications, Amsterdam, pp. 25–54.

Zimanowski, B., Frohlich, G., Lorenz, V., 1991. Quantitative experiments on phreatomag-matic explosions. J. Volcanol. Geotherm. Res. 48, 341–358.

Zimanowski, B., Büttner, R., Lorenz, V., 1997. Premixing of magma and water in MFCIexperiments. Bull. Volcanol. 58, 491–495.

the ∼ 1245 yr bp asososca maar eruption: the youngest event along the nejapa–miraflores...

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