Excerpted with Benjamin’s permission from:

van Wyk de Vries. B., 1993. Tectonics and magma evolution of Nicaraguan volcanic systems. Unpub. Ph.D.

Thesis, Open University, Milton Keynes, UK, 328pp.

Abstract

Variation of volcano morphology, eruptive style and magma composition are studied with respect to structural

environment along the Central American volcanic arc in Nicaragua. Two main types of volcano are encountered:

shield, with low-alumina basalts and andesites, and stratocones with high-alumina basalts and andesites. The

shields are located near, or within, small grabens and fault zones, while stratocones are constructed on crust

unaffected by regional faulting. Two type volcanoes are chosen to study the origin of the observed variations: (1)

Concepcion stratocone stands on unfaulted crust, which is locally deforming under the weight of the volcano.

Thrusts propagate away from the volcano, while the central region is rifted. Eruptions occur mainly from a central

vent and are predominantly pyroclastic. Magmas are influenced by moderate to high-pressure fractionation,

creating high-alumina basalts, which upon ascent to a low-pressure environment, fractionate further to andesite,

with rapid alumina loss. Mixing is restricted, and has little effect on magma compositions. Magmas probably

reside for extended periods within the lower crust, before ascent along a single pathway into an upper crustal

chamber. (2) Zapatera, a shield volcano, lies within the Ochomogo fault zone. It is predominantly constructed of

thin lava flows erupted from multiple vents. Low-alumina basalts with slightly variable near-primary

characteristics are erupted, indicating that separate magma batches rise to high levels. Most rocks are hybrids and

magma composition is dependant on mixing of rapidly fractionated magmas at low pressure.

In the absence of structural pathways, magmas pond in a high-pressure environment, forming high-alumina

basalts. The magmas subsequently rise along a single path to erupt from a central vent, creating a stratocone.

Where structural pathways are present rapid magma ascent favours low-pressure differentiation and high eruption

rates, creating shield- like constructs.

B van Wyk de Vries September 1993

Contents

Chapter 1: Introduction

1.1. The Project 1

1.2. Background to Volcanic Studies in Nicaragua 2

1.3. Thesis Layout. 4

Chapter 2. Quaternary Volcanism and Tectonics in Nicaragua

2.0. Introduction 5

2.1. Geology and Structure or Western Nicaragua 5

2.1.1. Regional Setting 5

2.1.2. Morphology 7

2.1.3. General Geology 8

2.1.4. Structure in Western Nicaragua 10

I. The Nicaraguan Depression 10

II. Neotectonic Features: Fault Zones and Grabens 14

IIa. La Pelona Fault Zone. 14

IIb. La Paz Centro Fault Zone 16

llc. Mateare Fault 17

lld. Managua Graben 19

IIe. Ochomogo Fault Zone 21

Ill. Origin of Fault Zones 22

2.1.5. Summary 24

2.2. Quaternary Volcanoes in Nicaragua 25

2.2.1. Shield Volcanoes: Individual Descriptions 27

I. Cosigüina Volcano 27

II. Telica Massif 27

III. Rota Massif 27

IV. El Hoyo 29

V. Zapatera 29

2.2.2. Stratocones: Individual Descriptions 29

I. San Cristobal Massif 29

II. Cerro Montoso and Momotombo 29

III. Mombacho Volcano 31

IV .Concepcion and Maderas 31

2.2.3. Ignimbrite Shield Volcanoes 31

I Malpaisillo Ignimbrite Shield 32

II. Las Sierras and Chiltepe Shields 32

2.2.4. Summary 35

2.3. Magma Composition in Nicaragua 36

2.4. Discussion 46

2.5. Two Type Volcanoes: Choice of Concepcion and Zapatera 47

[Not Scanned]

Chapter 3: Geology and Evolution of Concepcion: an Example or a Nicaraguan Stratocone Volcano

Chapter 4: Geology and Evolution of Zapatera: an Example or a Nicaraguan Shield Volcano

Chapter 5: The Origin of High-Alumina Basalts at Concepcion

Chapter 6: A Comparison of Concepcion and Zapatera Volcanoes: the Formation of Stratocones and Shield

Volcanoes.

Chapter 1 Introduction

1.1. The Project

The material erupted at a volcano is the product of magma generation in the source region, its subsequent

evolution within the mantle and crust and the mode of eruption. In subduction related arcs the roles of fractional

crystallization, mixing and contamination within the crust have been clearly demonstrated (O'Hara 1977,

Gil11981, De Paolo 1981). The effect of many crustal parameters, such as its thickness, composition, and

structure in controlling the degree to which these processes operate to produce different magmas and eruptive

activity has been increasingly appreciated (Carr 1984, Singer and Myers 1991). Most recently it has been

suggested that gravitational deformation of volcanic constructs may also have an important role in determining

the evolution of the magmatic system immediately underneath (Borgia et al. 1990, van Wyk de Vries et al. in

press).

This thesis investigates the relationship between structures (faults and localized stress concentrations) and the

magmatic and volcanic evolution of selected Nicaraguan volcanoes. Three particular aspects are addressed in

detail. Firstly, the evolution of multi-vent shield volcanoes and single vent stratocones: secondly, the empirical

observation that low-alumina basalts and andesites are associated with shield volcanoes, while high-alumina

basalts and andesites occur at stratocones, and thirdly the response of high-level magma evolution to structures

originating in the crust, or by volcano spreading.

By studying one distinct area of the Nicaraguan volcanic arc this study provides a general understanding of the

processes of magmatic and structural interaction. However, the results can be applied to the regional

understanding of volcanic activity in Nicaragua and may be useful in the prediction, and monitoring of volcanic

hazards.

Nicaragua provides an excellent location in which to study the interaction of crustal structures with magmatic

systems.

Firstly, there is a clearly marked difference between the two main types of volcanoes: shields and stratocones, and

a distinct variation in magma composition and petrography. The shield volcanoes erupt low-alumina basalts and

andesites, with small phenocryst sizes (<0.5cm), while the stratocones erupt high-alumina basalts and andesites

often containing megacrysts (<3cm).

Secondly shield volcanoes are, with only one exception, located in areas of crustal faulting, either grabens or

diffuse fault zones, while stratocones are not: the crust on which they stand is unfaulted. Thus there is a clear

association of volcano type with structures in the crust.

Thirdly, the composition of the crust along the volcanic arc in Nicaragua is considered to be uniform (Carr 1984)

and volcano spacing is very close (-25km); thus the major variable in the environment through which magmas

pass is likely to be structure.

Lastly, near-primary magma compositions are common (Walker et al. 1990, Carr et al. 1993) allowing variations

in magma composition to be attributed to either primary (source- derived) or secondary (magma chamber) origins.

1.2. Background to volcanic studies in Nicaragua.

First descriptions of the volcanoes in Nicaragua come from Oviedo y Valdez (1855), Von Seebach (1892) and

Sapper (1937). In 1538, Oviedo and his compatriots aided in a detailed description of the Masaya volcano in their

search for the gold in the active lava lake. Their observations on the melting of iron buckets and disappointment

on the cooling of extracted liquid probably represent the first thermal and composition observations of arc

magmas. The German explorers of this and the last century were the first to apply scientific thought to the

volcanoes. Von Seebach (1892) made elegant sketches and descriptions of the volcanoes and noted that the shape

of them varied from low hills to steep cones. Sapper (1937) observed that they are arranged in a linear fashion

with distinct breaks, and suggested that magmas rise along fissures, possibly associated with the Nicaraguan

'Graben' (Sapper 1937). The term used by Sapper, who wrote in German, was a specific geomorphological term

(Manton 1987), 'quersank', which unfortunately was translated into the more specifically structural term, graben,

leading to misinterpretation of the origin of the Nicaraguan Depression in later works.

The whole of the Nicaraguan chain was described by McBirney and Williams (1965), who noted that some of the

volcanoes were cut by north-trending faults, along which vents had formed. Stoiber and Carr (1973) divided the

Central American volcanic arc into segments, broken by zones of transverse faulting, containing volcanoes with a

tendency for explosive eruptions. One segment boundary was located at Managua, where the 1972 earthquake had

drawn attention to the graben in which the city lies (Brown et al. 1973). Ui (1972) made a petrological study of

the volcanoes around Masaya and observed that different magma compositions were erupted at different

volcanoes, noting that high-alumina basalts were erupted at the stratocones of Mombacho and Apoyo, while iron

and magnesia-rich basalts were erupted at Masaya and the Granada alignment The occurrence of near primary

magmas erupted from fault zones was demonstrated by Walker (1984) and these, in conjunction with basic rocks

from other Nicaraguan volcanoes have been used to constrain the nature of primary magmas parental to all others

along the volcanic front (Carr et al.1993), providing a useful constraint for studies of processes within the crust

Detailed petrological investigation of magma evolution at Nicaraguan volcanoes has to date only been made on

the Nejapa-Granada alignments (Walker 1984), Cerro Negro (Walker and Carr 1986) and the Masaya volcano

(Walker et al. 1993). Studies of eruptive processes have concentrated on the major Plinian and ignimbrite deposits

from Apoyo (Sussman 1985), Masaya (Williams 1983a), Apoyeque (Bice 1980, Hradecky 1988) and Cosigüina

(Hradecky 1988, Self et al. 1989). Eruptive processes at other volcanoes have been neglected, but have formed the

focus of recent projects by INETER (Instituto Nicaraguiense de Estudios Territoriales), of which this thesis is

one.

Chapter 2 Quaternary Volcanism and Tectonics in Nicaragua

2.0 Introduction

The Central American Volcanic Front passes through the Pacific side of Nicaragua with 18 distinct volcanic

centres. They are so closely spaced that lavas from one volcano often lie on top of those from the next Yet each

centre appears to maintain a distinct and separate magmatic system, resulting in varied magma composition,

eruption types and morphology, Two main types of volcano are encountered; shield-like stratovolcanoes, which

erupt predominantly lavas with low-alumina and stratocones, which erupt a major pyroclastic component and

have high-alumina compositions. The shield like volcanoes are heavily faulted and stand on, or near trans-

tensional fault zones. Stratocones, on the other hand, are not located near fault zones, though they are often

associated with thin-skinned volcano- tectonic compressive structures.

In order to provide a base for detailed study of the origin of magma and morphologic variation in the Nicaraguan

volcanoes this chapter describes the location of active faulting along the volcanic front, and the morphology and

composition of volcanoes. Two type areas are chosen for study.

2.1. Geology and Structure of Western Nicaragua

2.1.1. Regional Setting

Nicaragua is situated in the central part of the isthmus of Central America, with both Caribbean and Pacific coasts

(figure 2.1.1). The Pacific side is underthrust by the Cocos plate along the Middle American trench, giving rise to

the Central American Volcanic Front (CA VF). The Atlantic coast grades into shallow seas above the Nicaragua

Rise, a continuation of the continental nucleus of the Chortis block, an area of Palaeozoic continental crust,

occupied by Honduras, Guatemala and El Salvador. South of Nicaragua the Santa Elena suture marks the

boundary of the Chortis and Chorotega blocks (Borgeois et al. 1984). The Chorotega block is characterized by

ophiolitic basement as opposed to continental Chortis basement. The Cocos Ridge collides with Central America

just south of the Santa Elena suture, causing major uplift due to compression, and changes in subduction

conditions in Costa Rica. North of Nicaragua the major tectonic feature is the North American plate boundary,

along the Motagua -Polochic fault zone on the Mexico - Guatemala border. The influence of this boundary on the

structures in northern Nicaragua is only slight, seen mainly in small pull-apart grabens, which accommodate

rotational movements along faults and fracture zones (Manton 1987).

The Pacific side of Nicaragua shows some structural effects of the Middle American Subduction Zone, such as

the folding of Tertiary rocks, and coast-parallel faulting (Manton 1987, Cruden 1989), but the dominant structural

features are associated with movements within the Caribbean plate. The main structural trends are formed by

north west-trending faults and folds, which are not active at present (Cruden 1989), and north east-trending faults,

associated with north-trending tensional faults (Cruden 1989, Manton 1987).

[Fig not scanned]

Figure 2.1.1. Neotectonic map of the Caribbean and Pacific regions surrounding Central America. Compiled

after Burke et al (1984), Dengo (1985), Cruden (1989), Mann & Burke (1984) and Malfait & Dinkelmann (1972).

Base map from Cruden (1989). Note that the Nicaraguan Depression is omitted, since it is here interpreted as a

non-tectonic feature.

2.1.2. Morphology

The morphology of the Pacific side of Nicaragua is dominated by a low plain, partially occupied by Lakes

Managua and Nicaragua (figure 2.1.2). This area is the 'Valle de Nicarao' encountered by the first Spanish

explorers, and has become known as the Nicaraguan Depression, or Nicaraguan Graben. Northeast of the

Nicaraguan Depression, the Interior Highlands form the bulk of Nicaragua. The Depression is open to the Pacific

in the northwest, but from Puerto Sandino southwards the Las Sierras hills and the Brito and Rivas 'cuestas' form

a coastal range. The Central American volcanic front passes through the Depression from Cosigüina volcano in

the northwest to Maderas volcano in Lake Nicaragua.

Figure 2.1.2 Topographic map of Western Nicaragua illustrating morphological features

2.1.3. General Geology

The gross morphological features in Western Nicaragua reflect the major changes in rock type (figure 2.1.3). The

Brito and Rivas 'cuestas' are formed by uplifted Tertiary marine rocks, and are highest to the southeast, where

deformation is most intense (table 2.1.1). To the north west they are overlain by the only slightly tilted El Fraile

formation, a marine near- shore sequence, which in turn passes laterally north into undeformed Tamarindo

formation rocks. The Tamarindo formation is a sequence of shallow marine, lacustrine and terrestrial sediments,

interspersed with ignimbrites, and is comparable in age to the El Coyol group to the north east of the Nicaraguan

Depression. The Coyol group borders the Depression from Honduras to Costa Rica. Some volcanic centres are

still distinguishable as constructional landforms (Ehrenborg 1987). El Coyol rocks also outcrop in Lake Nicaragua

on the Solentiname and Puerto Diaz islands and around El Limon and Telica in northwest Nicaragua (figure

2.1.3).

Table 2.1.1. Stratigraphic correlation chart of southwest Nicaragua adapted from "Mapa Geologico de Nicaragua

Occidenta11: 250,000", Managua (1972) and Weyl (1980).

Quaternary volcanic rocks are found in the Depression, notably the 18 major volcanic centres (figure 2.1.3 and

2.1.4), but also tephra deposits and volcaniclastic sediments. In the extreme north west Cosigüina volcano

occupies a peninsula in the Gulf of Fonseca. As with most Holocene volcanoes it is built on an earlier Quaternary

centre (Pre-Cosigüina), which is itself built on Tertiary volcanics and sediments. The Cordillera Marrabios

continues to the southeast with San Cristobal, Casita, La Pelona, Telica, Rota, El Hoyo, Monte Galan,

Momotombo and Momotombito. The latter four volcanoes are constructed on and interfinger with, the ignimbrite

deposits of the Malpaisillo caldera (Van Wyk de Vries 1990b). Further to the south-east, Chiltepe with the twin

calderas of Apoyeque and Jiloa, the Nejapa alignment, Masaya, Apoyo and Mombacho overlie and interfinger

with the Las Sierras basic ignimbrites, which erupted from the large Las Sierras Caldera surrounding Masaya

volcano (van Wyk de Vries 199Gb, 1991). South of Las Sierras the three volcanoes of Lake Nicaragua; Zapatera,

Concepcion and Maderas are the last Nicaraguan volcanic structures before Costa Rica.

Figure 2.1.3 Geological map of the major formations in western Nicaragua. Taken from 'Mapa GeologicoPreliminar 1: 1.000,000', Managua 1973, with minor corrections of the Quaternary volcanics and Las Sierrasformations from field data collected by the author. Names of volcanoes: 1. Cosigüina; 2.San Cristobal; 3 Casita;4. La Pelona; 5 Telica; 6 Rota; 7 El Hoyo; 8 Monte Galan; 9 Momotombo; 10 Momotombito; 11 Malpaisillocaldera; 12 Chiltepe; 13 Masaya/Las Sierras; 14 Apoyo; 15 Mombacho; 16 Zapatera; 17 Concepcion; 18Maderas.

Between the volcanoes, sequences of alluvium and lacustrine sediment are interlayered with

tephra deposits. The actual thickness of Quaternary rock in the Depression has been much debated. The

original sections of western Nicaragua drawn by McBirney and Williams (1965) show over 2km of

sediment. Electrical soundings in the Depression (Nissen et al. 1986) gave a possible depth to basement

of between 300 to 600m, while an interpretation of gravity data (Elming 1986) gave more than 3km

depth. These discrepancies arise because the term 'basement' was not clearly defined in each study. Bice

(1980), proposed a minimum estimate by calculating that at least 300 m of Quaternary volcanic deposits

should be present. M-16, an exploratory borehole at the Momotombo geothermal plant, penetrated over

2000 m of pyroclastic rocks and sediments. The well-log indicates that the base of white, dacitic, pumice

flows associated with the Malpaisillo caldera is only 150 m below ground level (confidential data,

supplied by Instituto Nicaragense de Energia (INE), author's interpretation reports). As elsewhere, the

pumice unconformably overlies Tertiary ignimbrites: this boundary probably reflects the depth of the

Quaternary near Momotombo. Since the Tertiary erosion surface dips gently below Quaternary

sequences into the Depression, and Tertiary rocks outcrop within the Depression it is more likely that

there is only a thin sequence of Quaternary rocks (figure 2.1.4).

2.1.4. Structure in Western Nicaragua

I. The Nicaraguan Depression

The structural interpretation of western Nicaragua is primarily dependant on how the Nicaraguan

Depression is explained. If, as has been suggested (McBirney and Williams 1965), the feature is a deep graben, or

half graben, then other structures must be interpreted within this context. If, however, as will be argued here, the

Depression is merely a low- lying area, marginal to the Pacific Ocean, associated with no large-scale structures,

then small graben and fault zones within it may be interpreted in other ways. The nature of the Depression is

important for volcanological studies since the composition of, and stress regime in, the upper crust may

significantly affect magma transport, evolution and eruption.

Figure 2.1.4. Map of western Nicaragua illustrating the degree of Tertiary outcrop within the NicaraguanDepression, depth to Tertiary basement below Qualernary deposits. and structures of Quaternary and Tertiary age.

Before proceeding it is worth marking the distinction between the Nicaraguan Depression and

the Nicaraguan Trough. The latter. as described by Weyl (1980) and more recently Schmidt (1989). was

a major offshore basin, related to Miocene -Pliocene subduction. In it, the Rivas, Brito, Masachapa and

El Fraile formations were laid down. The landward side of the trough is now buried in the Nicaraguan

Depression, except in northwest Nicaragua, where the sea-land transition is preserved in the Tamarindo

Formation (figure 2.1.3). The Nicaraguan Trough is therefore a Cretaceous- Tertiary feature. The

Nicaraguan Depression, in contrast is, sensu strictu, a morphological feature representing the low lying

area between the Central Highlands and the Pacific Ocean separated in part by low coastal hills.

Previous interpretations of the Nicaraguan Depression placed long boundary faults along each side

(Weyl1980, McBirney and Williams 1965). The Depression was seen as being part of a line of grabens, extending

from El Salvador to Costa Rica. McBirney and Williams (1965) drew a cross section showing the Depression as a

half graben, while Cruden (1989) draws it as a full graben formed by coast parallel strike-slip faulting.

Interpretations of a graben origin for the Depression are critically dependant on two areas of faulting; the

Mateare fault zone and the northeast boundary of the depression at Cuesta Coyol (figure 2.1.4).

The Mateare fault zone has been described as a 70km long fault extending along the northeast side of the

Las Sierras hills (McBirney and Williams 1965; Weyl 1980; Parsons Corporation 1972). Although the Las Sierras

do continue for 70km, the scarp of the fault is limited to a length of about 30km, extending from just north west of

Mateare, to west of Managua. South of its termination, strata in the Las Sierras dip gently into the Depression,

disturbed only by the Masaya -Las Sierras Caldera and two small (<50m) fault scarps. Furthermore, the strata in

Las Sierras dip not westward, but radially from an axis located in the Masaya Caldera. The Mateare fault is

therefore a local feature, and cannot have created the Depression, although it does modify the morphology of one

small part.

At Cuesta Coyol there are faults downthrowing toward the Depression (McBirney and Williams 1965,

Ehrenborg 1987), but on close inspection these are found to be overlain by undeformed Quaternary Las Sierras

ignimbrites or sediments. Thus these faults which are of limited length, generally less than 10km, have not moved

in the Quaternary, and could not have contributed to the morphology of the Depression. The Tertiary El Coyol

rocks outcrop within the Depression below Cuesta Coyol and in other regions (figure 2.1.4), thus no great depth

of sediment has accumulated during the Quaternary, and no large scale faulting is required to explain the geology.

An alternative explanation for the origin of the Nicaraguan Depression, which can combine all observed

geological features, is that it is merely a lowland area occupying the coastal region between the Interior Highlands

and the Pacific. To the southeast the plain is separated from the Ocean by a continuation of the uplifted Nicoya

and Santa Elena ophiolite complexes. Further to the north it is separated by the Las Sierras shield volcano, but

towards El Salvador it dips gently into the Pacific. One possible explanation for the low- lying nature of the area,

which agrees with geological evidence is that the mass of Tertiary volcanics emplaced in the Interior Highlands

has caused isostatic readjustment of surrounding regions. Such an effect is observed in the dip of Matagalpa

strata, underlying the El Coyol in the Central Highlands (Weinberg 1993).

Figure 2.1.5. The location of active grabens in Nicaragua and southern Honduras. Data from 'Mapageologico preliminar', Managua (1972), from field studies by the author and Esperanza (1990). Locationof Honduran faults and grabens from Manton (1987). The fault zones in Nicaragua are; 1, La Pelona; 2,La Paz Centro; 3, Mateare fault; 4, Managua graben; 5, Ochomogo, 6; Achuapa; 7, El Sauce; 8,Teustepe; 9, Esteli; 10, Sebaco.

II. Neotectomc Features: Fault Zones and Grabens.

There are many small grabens and faults in the Nicaraguan Depression and Interior Highlands, but no

active or recent structures have been reported along the Pacific coast or in the coastal ranges. Recent seismicity is

concentrated in a 10-15 km area either side of the volcanic front with the exception of a zone of activity near El

Sauce. There are also historical accounts of earthquakes in the Interior highlands (Segura and Strauch pers. com.).

Areas with prominent faulting are Ochomogo, Managua. La Paz Centro, La Pelona, El Sauce and Sebaco (figure

2.1.5). Each area has north east-trending sinistral strike-slip faults, with associated north-trending normal faulting

and occasionally north west-trending normal faulting. None of the active faults are very long (<30km) and fault

scarps are generally low (<50m), and they appear to be recent phenomena. postdating the initiation of Quaternary

volcanism. Each of the fault zones along the volcanic front was mapped using aerial photography, topographic

maps and field observations, and they are described in turn below.

lla. La Pelona Fault Zone

North-north east of the village of Cristo Rey strong faulting affects the caldera of La Pelona, which by

morphological comparison with other Nicaraguan calderas is older than 30,000 years (figure 2.1.6). Most of the

faults follow a 030oN trend with few deviating to 010°N. One fault follows a more north west trend (300°N) and

provided the conduit for two eruptions from Casita, one a lava flow and one phreatomagmatic eruption which

formed the Guanacastal crater. Drainage on La Pelona deviates from the radial pattern expected around a

stratocone not only along obvious faults, but also in many minor gullies. Approaching Cristo Rey the landscape is

covered by thick tephra from the Telica volcano which has buried any possible structures, but a number of clear

030° orientated lineations and possible fault scarps cross Cerro Montoso on the Telica Massif. These cut strongly

dissected relief indicating that they are considerably later than the eruption of Montoso. One lava flow from Cerro

Aguero (Telica massif) is diverted by a probable fault scarp, which continues from a lineation on Cerro Montoso.

Thus the fault movement dates back as far as the activity on Cerro Aguero. Seismic events have been recorded in

the La Pelona area, both by the national seismic network and local stations while monitoring Telica volcano, but

no detailed seismic study has yet been made.

Figure 2.1.6. Structural map of the La Pelona fault zone, compiled from fieldwork: and aerialphotography and data from Hazlet (1987) and Martinez & Viramonte (1971).

Both Martinez and Viramonte (1971) and Hazlet (1987) draw faults with 030°N orientation in

gullies cutting the Casita volcano ridge, however no field evidence was found in this study for their

existence and the gullies have the expected orientation for drainage of the elongate 120° Casita ridge. To

the east of La Ollada crater, however, a fumarole field is conspicuously aligned along 030°N and small

scarps also follow that trend. In other areas of Casita and San Cristobal volcano, the few small faults are

predominantly north-orientated. To the south east of the La Pelona fault zone, on Telica volcano, most

faulting is north- orientated. Further north east-trending faulting occurs between Telica and Rota

volcanoes, where extensive solfatara fields are encountered at San Jacinto.

IIb. La Par. Centro Fault Zone.

To the west and north of La Paz Centro there are a number of conspicuous north west- orientated fault

scarps (figure 2.1.7). In road cuttings the faults cut both Malpaisillo ignimbrites and tephra layers from

Momotombo. In most cases they downthrown to the north east, creating a shallow half graben occupied by an arm

of Lake Nicaragua at the other side of which is the Momotombo-Monte Galan massif, which is not noticeably

affected by faulting.

Figure 2.1.7. Structural map of the La Paz Centro fault zone compiled from aerial photography andfieldwork observations.

The longest fault, which has the highest scarp is the El Recreo Fault, which stretches for 17km from El

Recreo on the shore of Lake Managua to Laguna Asososca on the El Hoyo Massif. Laguna Asososca is elongated

along fault strike, with some fractures following the same along-strike trend on El Hoyo, including those which

opened during the 1953/4 eruptions (McBirney and Williams 1965). Seismic activity is common, and well

documented historically, including the destruction of Leon Viejo in 1609 by a combination of earthquake, and

eruption from Momotombo. The town was no more than 1.5km from the El Recreo fault.

The main fault trend in the south eastern area is the same as the Mateare fault (320°N), which is along

strike to the south east Bathymetry of Lake Managua (Archive INETER) shows a distinct scarp joining the two

fault zones. Where the faults near El Hoyo massif, the direction changes to a north east and north-trend. A strong

north east set of faults cuts north of Cerro Montoso reaching the Recreo fault near La America. Numerous craters

and cones are found in the area, most aligned along a northwards trend.

IIc. Mateare Fault

The Mateare fault forms part of a spectacular escarpment stretching just over 30km from the Sierras de

Managua to Nagarote (figure 2.1.8). Early interpretations (McBirney and Williams 1965) described the Mateare

fault as running some 50 km southeast from Mateare, forming the north east face of the Pacific ranges. The length

of the fault was reduced by Schwartz et al, (1975), when the southern end was terminated in the Las Nubes fault,

running north east in the Sierras de Managua. Seismic hazard assessments in the 70’s, after the Managua

earthquake (Brown et al. 1973), suggested that the fault was active. However no seismic events have been located

along it in the last 20 years (Segura pers com) and no evidence of recent fault movement was seen during

fieldwork.

The scarp is deeply incised by gullies, but as a whole retains a remarkably regular profile due to the

homogeneity of Las Sierras formation rocks exposed in the face. The profIle begins to break down near Mateare,

where the majority of rocks are less resistant scorias and ashes from the Chiltepe volcano.

The scarp summit reaches its highest point above sea level near El Crucero south of Managua, though the

scarp height is greatest (300m) near La Palanca (figure 2.1.8). To the north west, at Mateare its height is 200 m

and from Mateare to Nagarote it splits into various branches formed by separate faults with smaller scarps

(Arroyo Grande fault, La Trinidad fault). South of La Palanca at La Trinidad the fault is a composite scarp of 200

m. By the Carretera Leon Vieja the scarp is only 100 m high. Further south east there is no clear sign of faulting

and the scarp appears to be an erosional feature, which eventually curves round to become the north face of the

Las Nubes ridge. There are a number of northwest -trending lineaments on the north slopes of the Sierras de

Managua, and on the eastern part of the Las Nubes ridge.

Within the scarp there is a clear change from Las Sierras ignimbrites and waterlain sandstones to airfall

and loess, indicating that the fault initiated during Las Sierras volcanism. The scarp is highest nearest to the

Chiltepe volcano, which might indicate that the displacement is related to magrnaticall y induced extension; the

tirnespan of it's movement corresponds roughly to major dacitic eruptions on Chiltepe.

Figure 2.1.8. Map of the Mateare fault system made from fieldwork observations, from maps and aerialphotography.

IIc. Managua Graben

Whether the Managua graben is actually the most active tectonic feature in Nicaragua or whether seismic

activity has been more noticeable because of dense population is not clear. However the graben certainly has the

most well preserved and obvious neotectonic features in all the Pacific region. This is in part due to the lack of

erosion and deposition which tend to obscure features in other areas.

The graben is bounded in the west by the Nejapa -Miraflores alignment and to the east by the Cofradla

fault (figure 2.1.9). Both have associated volcanism; in the case of the Nejapa - Miraflores, no clear fault exists,

the line being distinguished by more than 20 craters and cones, and a drop of 150 m in the land level from west to

east In places north-orientated faults have large (<50m) down-throws to the east, but are covered with volcanic

products. The volcanics on the lineament are some of the most primitive basalts erupted in Nicaragua. However

andesites and dacites similar in composition to the Chiltepe rocks are also found and the lineament is probably an

extension of the Chiltepe volcanic system.

Figure 2.1.9. Structural map of the Managua Graben using data from aerial photography, field mappingand the Fault Map of Managua (INETER archives 1974).

Along the Cofradia fault, f1SSure eruptions, producing lava flows, spatter cones and two tuff rings, are of

basaltic composition, indistinguishable in composition from rocks at Masaya (Williams 1983b). At the village of

Cofradia the fault forms a sharply defined scarp about 20 m high. To the north and south the fault is less distinct,

but can be traced to the outline of the Las Sierras -Masaya Caldera and to Tipitapa. The shore of Lake Managua

follows the fault strike, and thermal springs extend as far north as Laguna El Playon, 25 km from Tipitapa.

From Laguna El Playon a distinct north east trending lineament is occupied by the Rio San Antonio and

then by the shore of Lake Managua to Punta Huete. The lineament can be picked out for a short distance on the

bottom of the lake on the bathymetric map of lake Managua (INE1ER archives). If the strike is continued, it

coincides with the northern limit of the Nejapa -Miraflores alignment This feature has been called the Punta Huete

lineament (Ward 1974) and forms the north side of the graben. No evidence of the fault is seen south west of the

graben, nor to the north west (Ehrenborg 1987), thus the interpretation of Hodgson and Lilliquist (1983), Ward

(1974) and Carr (1984) of the feature as a large-scale fault zone extending through Nicaragua to the Caribbean is

not supported.

The southern area of the graben is not constrained by one fault. In Las Sierras de Managua most of the

faults within the graben swing to a north south direction, and die out. Farther east the faults enter the Las Sierras

Caldera complex and are lost

Inside the graben the majority of faults are orientated at Q40oN and have moved in a sinistral manner

during the earthquakes of 1936, 1968 and 1972. The north-orientated faults had normal displacements. The most

active faults in the last 15 years have been the Cofradia and Veracruz faults. The outflow of Lake Managua to

Lake Nicaragua through Tipitapa has been blocked by this movement

lIe. Ochomogo Fault Zone

The Rio Ochomogo gives its name to a fault zone which extends in a north easterly direction between the

Mombacho and Zapatera volcanoes (figure 2.1.10). The river itself flows into Lake Nicaragua between an offset

ridge of Lamas del Brujo and Lamas San Ramon. The ridge is composed of Tertiary El Brito formation rocks

tilted to the north east and there appears to have been a dextral offset of about 500 m. This is in the opposite sense

to other Quaternary faults and conflicts with the sense of movement indicated by the geometry of the fault zone as

a whole. The displacement might be Tertiary in age making the Ochomogo fault a reactivated feature. The strike

of this fault is picked out by the Rio Ochomogo and is followed on Zapatera island by the voclanic structures of

Cerro El Menco, Loma Atrevasada. Sierra Santa Julia and minor fault features near Punta Zapatillo.

Figure 2.1.10. Map of the Ochomogo fault zone. Compiled from fieldwork. aerial photographs and datafrom Hradecky (1988) and Cruden (1989).

Between the two volcanoes much of the landscape is due to the southern collapse of Mombacho. In the

hummocky terrain, fault location is very difficult Outside the major debris flow, faulted rocks and irregular scarps

are found around Mecatepillo, Charco Muerto and on Zapatera island. Faulted tephra layers are seen in the river-

cutting at Cuartro Esquinas and a north east-trending fault runs from there into the Granada cinder cones on the

north side of Mombacho. Mombacho is cut by a number of north and north east-trending faults, two sector

collapses being guided along these directions. The strongest faulting is found on Zapatera island. A clear 040N

scarp cuts the north east part of the island and a 150 m high scarp cuts across the centre just east of a small

caldera. There are numerous 040N faults on the island and many eruptions have built up structures elongated

along this trend (Lorna Atrevasada, El Pilon, San Fernando, El Llano).

A Mercali scale 6.2 earthquake in 1986 was located between Zapatera and the mainland. The focal

mechanism gave a probable sinistral north-easterly movement (Segura pers. corn).

III. Origin of Fault Zones

The origin of faulting along the Pacific of Nicaragua received a number of studies after the 1972

Managua earthquake (Matumoto and Latham 1973, Brown et al. 1973, Schwartz et al. 1975). The consensus was

that the earthquake had occurred on a northeast-trending fault and that at Managua there was a concentration of a

number of fault lineaments. Stoiber and Carr (1973) carried this further, interpreting the Managua area in the

context of segmentation of the subducting slab along Central America, with resultant breaks in the volcanic front.

They state that the observed faulting and an assumed offset in the volcanic front at Managua were related to a

break in the subducting slab. Burbach et al. (1984), using related seismic data, showed that breaks occurred in

Costa Rica and in Guatemala, but that between these the slab only varied gradually in dip. Furthermore, Burkhart

and Self (1986) produced an interpretation of segmentation in Guatemala, which relied solely on the influence of

the Montagua fault zone. discounting any influence from the subducting slab.

Figure 2.1.11. Alignment of the volcanoes in Nicaragua. Left: Best fit regression lines for the vo1canoesbetween Cosigüina and El Hoyo and Momotombo and Maderas. Right map showing strike of Coastlineand Tertiary strata. Shaded area is a zone of increased seismicity (F. Segura. pers. com. 1990).

The evidence of an offset in the volcanic front at Managua was based on the presence of faulting and a

supposed change in direction of the volcanic axis. Stoiber and Carr, (figure 1. 1983) place their break to the north

east of Managua. A detailed analysis of the alignment of volcanoes (figure 2.1.11) shows that there is a change in

alignment at Momotombo, but not at Managua. The Pacific coastline, 50km from Momotombo, also changes

strike. Although some faulting occurs near Momotombo, none extends towards the Pacific, and the slight changes

in strike may be influenced by minor changes in the plate boundary , and not by active tectonic structures.

An alternative interpretation of the Managua graben and other fault zones follows the work of Manton

(1987), who interpreted small grabens on the west and south side of Honduras as being formed by the rotation of

faults or fracture systems under the influence of the northern Caribbean plate boundary .The morphotectonic

zones in Honduras extend into Nicaragua and thus rotation-induced accommodation of stress may occur (figure

2.1.12). The concentration of grabens along the volcanic front may be due to weakening of the crust by magma

intrusion.

Figure 2.1.12. Origin of small grabens and fault zones in Nicaragua explained by block rotation in the Caribbeanplate. At the boundary of each rotating block space is created which is accommodated by the formation <X"grabens. The exact location of fracture and fault zones in blocks is conceptual and extension may be taken upanywhere within a block. Extension may be accommodated preferentially along the volcanic front, where thecrust is weakened by magma intrusion.

2.1.5. Summary

In Nicaragua, the Central American volcanic front lies on a low plain, the Nicaraguan Depression,

partially occupied by two big lakes. Quaternary volcanics and sediments form a thin cover to the Tertiary

landscape, which still outcrops in places within the Depression. Interpretation of the Depression as a graben is

inconsistent with the observed geology; there are no large boundary faults and no large Quaternary displacements.

The most suitable model for the Depression is that it was initiated by isostatic subsidence of the crust caused by

the mass of Pliocene El Coyol volcanics. Localised areas of extension are present in small grabens and fault

zones. Most structures can be related to movements in the Caribbean plate or to magmatic influences. There is

little evidence to suggest that structures are at present affected by the subducting Cocos plate. Most seismic

activity is restricted to faults on the volcanic front, although restricted loci of activity are associated with grabens

away from the front. Fault zones are either north east-orientated, such as the La Pelona and Ochomogo zones, or

north-trending, such as the Managua graben.

2.2. Quaternary Volcanoes in Nicaragua

There are 21 Quaternary volcanic centres in Nicaragua, forming a portion of the Central American

volcanic front (Figure 2.2.1). They are closely spaced, averaging 25 km between central craters, and generally

small (-30km3). Volcano summits are lower than in other areas of the Central American volcanic front, and the

volumes are smaller. Three large ignimbrite shields form the basement, and interfinger with the more recent

volcanoes in centre of the chain.

On viewing the Nicaraguan chain from a distance the most striking feature is the difference in shape, or

outline, of each volcano. In general there are two forms; conical and shield-like (figure 2.2.2), though three big

calderas and three ignimbrite shields are also present. The shield volcanoes have many cones and craters, often

arranged in distinct orientations and extending 5-l5km from the summit Summits are less than 1050m above sea

level and slopes 15°-20°. The surfaces are predominantly composed of lava flows, and major accumulations of

pyroclastic material are restricted to vent areas. Lavas flow up to l5km from the summit and are usually thin

(<5m), narrow flows with aa, and occasionally pahoehoe surfaces. Faulting is common and shield volcanoes are

located on or near fault zones (figure 2.2.2).

Figure 2.2. I. Map of Western Nicaragua Showing the 1ocation and activity state of Quaternaryvolcanoes. Cross section in figure 2.2.2 is shown with dotted line.

The stratocones have one major central vent and few subsidiary vents, usually less than 3km from the

base of the volcano. Vents are sometimes aligned in north-trending alignments, but are more often scattered.

Summits are between 1()(X)m and 1700m above sea level. The volcanoes can be divided into an upper cone, on

which pyroclastic material outcrops most, and lower fans of lava, distal pyroclastic material, and laharic debris.

Slopes on the upper cone are between 25° and 30°. Lava flows may be thin and extensive, up to 12km from the

summit. but most are thicker than 5m and may have wide, lobate flow fronts. The predominant surface texture is

blocky , occasionally aa, but pahoehoe is not observed. Stratocones are not sited near fault zones, and faulting

seen in the edifice is usually in response to gravitational instability rather than tectonic forces.

Figure 2.22. Profile along the Central American volcanic front in Nicaragua. showing the different morphologyand heights of volcanoes. Large ignimbrite shields are numbered; 1 Malpaisillo; 2 Chiltepe; 3; Las Sierras. Lineof section shown in figure 2.2.1.

2.2.1. Shield Volcanoes: Individual Descriptions

I. Cosigüina Volcano

Cosigüina occupies a peninsula in the Gulf of Fonseca. separated by 50 km from the nearest

other Nicaraguan volcano. It is a large shield-like structure, partially eroded, and part submerged by the sea.

Lavas reach at least 15 km from the crater to the south east and south west The main central crater (2.5km

diameter) was formed during the 1835 eruption (Self et al. 1989, Hradecky 1990) and the same site has ~n the

source of most eruptions in the recent past. The 1835 eruption is the only recorded historic activity, but was one

of the largest volcanic events of the 19th Century (Self et al. 1989). Only three satellite vents have been detected

(El Barranco maar and the two Cerros Cachos). The volcano is built upon other earlier structures; the Filete Cresta

Montosa is a probable caldera rim; Cerro la Salva is an eroded tuff cone, Lorna San Juan and Lorna Filete

Quemado have been interpreted as early Quaternary volcanoes on a Tamarindo Formation basement (Hradecky

1990). Filete la Salva is probably the scarp of a major gravity slide rather than a caldera margin as proposed by

Hradecky (1990). It is the only major structural feature on the volcano.

II. Telica Massif

The Telica massif is made up of nine or more centres that coalesce into a shield -like east- trending ridge.

The south side of the main mountain is steep and built of scoria layers, but elsewhere dips are generally shallow,

formed by thin lava flows. The eroded Cerro Montoso, Tisate and El Carol are parts of an earlier Quaternary

volcano on which the recent craters grew. The summit vents are aligned on an east-trending line while Santa Clara

and Agtiero are on north-orientated lineaments. North-orientated faults are common on the volcano.

III. Rota Massif

Rota is a substantially eroded, irregular mountain with a central crater, surrounded by numerous

subsidiary vents. It is strongly faulted on the east side, and lava flows to the west are also cut by faults. The last

eruption formed thick andesite lavas which flowed from a north easterly aligned fissure to the north west of the

summit. To the north there is an extensive field of lava which was erupted from numerous small cones and maars.

The 1 km diameter central crater appears to have been the source of pumice deposits encountered around the base

of the volcano.

Figure 2.2.3. Topographic maps with simplified distribution of lava and tephra at shield volcanoes in Nicaraguamade from aerial photography and field mapping. Previous maps for Cosigüina (Hradecky 1990), Telica and ElHoyo (Bice 1980) were consulted. A Cosigüina; B El Hoyo; C Telica and Rota; D Zapatera. Key in B applies toC.

IV .El Hoyo

The El Hoyo massif has the most extensive lava field in Nicaragua. It stretches about 30km from north to

south and 15 km from east to west. Two vents have been active in recent times, that of Cerro Negro and the

summit fissure system of El Hoyo. The general symmetry is similar to Telica. There is a central east-trending

ridge formed by numerous craters and many subsidiary cones on north-trending lineaments. Outlying activity has

produced cinder cones and maars, while the central vent activity has been almost exclusively effusive, the only

pyroclastics being spatter layers and thin scoria deposits. Lavas flowing from the centre are long and lava tubes

and pahoehoe surfaces are a common feature.

V. Zapatera

Zapatera is an easterly elongated shield volcano with many vents. The Ochomogo fault zone has produced

heavy faulting in the edifice and many of the vents are elongated along north easterly and north-trending lines.

More than 30 vents are spread out over a 150 krn2 area.

2.2.2. Stratocones: Individual Descriptions

I. San Cristobal Massif

Hazlet (1987) has described the general geology of the San Cristobal massif, which includes

El Chonco, Moyotepe, San Cristobal, Casita and La Pelona. The volcanoes are stratocones of various sizes. San

Cristobal is the highest at 17lOm, and is active at present. Each structure has one major crater and a few flank

vents. Strong faulting is restricted to the La Pelona Fault zone, which postdates the formation of the La Pelona

Caldera.

II Cerro Montoso and Momotombo

The cones of Cerro Montoso and Momotombo are small simple stratocones with only one vent.

Momotombo has a small somma ring on its west side 200 m from the summit. Between the two cones is the

Monte Galan caldera, a circular structure, 6km-diameter, which was the source of at least one ignimbrite of the

Malpaisillo shield

III. Mombacho Volcano

Mombacho is a steep stratovolcano built upon a wide base of subsidiary cones. The edifice has suffered at

least three sector collapses, one to the south and another to the north east, and one to the south east Most recent

activity has been concentrated to the west of the old summit. There is a set of volcanic lineaments to the north and

east of the volcano and a collection of vents to the southeast, which merge into the Zapatera volcanic system.

IV. Concepcion and Maderas.

Concepcion and Maderas, on the island of Ometepe, are two large stratovolcanoes. Each has a major

central vent surrounded by numerous subsidiary vents arranged radially around the base of the cone. A few north-

aligned lineaments are found on each volcano. Large faults on Maderas are probably gravity collapse features,

while folding and thrust faulting around Concepcion is probably caused by volcano loading, leading to

decollement in the basement of weak lake sediments. There are no major non-volcanic faults near these

volcanoes.

Figure 2.2.4. Topographic maps with simplified distribution of lava and tephra at stratocone volcanoesin Nicaragua made from aerial photographs and field mapping. A San Cristobal massif (after Hazlet1987); B Mombacbo (after Hradecky 1986); C Concepcion; D Maderas; E Cerro Montoso andMomotombo.

2.2.3. Ignimbrite Shield Volcanoes.

There are three coalescing shields built mainly of ignimbrites in the centre of the volcanic chain in

Nicaragua. The northern Malpaisillo shield has very little topographic expression, whereas the southern-most Las

Sierras shield reaches .9OOm above sea level. Each is formed of a number of sheets of pumice flows, ash flows

and surge deposits; none of the material is welded. The major difference between shields is the composition; the

Las Sierras shield has erupted mainly basic to intermediate compositions while the northern shields are acidic

(van Wyk de Vries 1990a). Little study has been made on them as yet, and the origin of such voluminous

volcanism in an area where the majority of volcanoes are so small is not yet understood.

I. Malpaisillo Ignimbrite Shield.

A large silicic ignimbrite field outcrops north and south of the El Hoyo and around the Monte Galan -

Momotombo area. The most recent ignimbrites (50,(XX) yrs) come from the Monte Galan caldera and older ones

probably from the Malpaisillo and San Fernando structures. Other centres may exist in Lake Nicaragua or

deposits may have come from Chiltepe to the south east. Basic lavas found as lithics in the ignimbrites attest to

the presence of stratovolcanoes during ignimbrite activity, and drill cores in the Momotombo geothermal field

show intercalations of scorias and ignimbrite. A few highly eroded structures, such as Cerro Las Palomas and

Lorna La Chistate are overlain at their base by the ignimbrites and are probably late- Tertiary or early Quaternary

volcanoes (figure 2.2.5).

Figure 2.2.5. Map of the Malpaisillo ignimbrite shield and adjacent areas. The known extent of outcrop andsubcrop below Marrabios range volcanoes is shown. Data from field mapping and Bice (1980).

II Las Sierras and Chiltepe Shields

A large ignimbrite shield volcano dominates the volcanic front south of Momotombito (figure 2.2.6). The

main caldera of the shield is at Masaya, where a 25 km wide structure entirely encloses the Masaya shield volcano

and the Masaya caldera. Ignimbrite flows reach up to 50 km from the centre. On the north east side of the

Nicaraguan Depression they have filled in valleys, for example at Las Maderas. On the Pacific coast the flows

outcrop at Montelimar, overlying the El Fraile formation. To the south, ignimbrites are found on the north bank

of the Rio Ochomogo, and to the north they interfinger with the Malpaisillo ignimbrites and deposits from the

Chiltepe centre. The shield slopes gently away from the caldera rim, which is 900 m high to the west and 250 m

to the east Modification by faulting has given the shield a half dome appearance to the north, but the asymmetric

height of the caldera rim is due to preferential tephra accumulation to the west, and to the presence of elevated

basement to the west The upper layer of ignimbrite near Diriamba (figure 2.2.5) has been dated from a carbon

sample at 29,500yr (Sussman 1985). A small ignimbrite was emplaced during the formation of Masaya caldera, at

about 4,000 years BP. As these two eruptions are so recent it is probable that the ignimbrite-forming system is

still potentially active.

The best exposures of the sequence are along the Mateare fault scarp and down the deep ravine sides

which extend into, and away from the caldera from El Crucero. In the Mateare fault many layers of scoria and

ash are included in the sequence, especially near Chiltepe and in the Masaya region, showing that ignimbrite

activity was interspersed with less explosive volcanism -perhaps representing a more effusive and less energetic

phase as at present.

Acidic ignimbrite with lag breccia is found in the Chiltepe peninsula in the walls of Jiloa Caldera and at

least two major silicic eruptions have occurred at Apoyo caldera. The modem Chiltepe volcano has produced at

least three Plinian dacite eruptions (Bice 1980).

Chiltepe, or Apoyeque, volcano is dominated by the calderas of Apoyeque and Jiloa. It is a small

collection of domes and lava flows, with a long north-trending lineament extending from Jiloa to Monte Tambor

15 km to the south. This has been called the Nejapa lineament by Walker (1984). It follows the west border of the

Managua graben. Apoyeque lies just outside the graben.

Tiscapa is a small monogenetic volcano in the centre of Managua and may either be an errant cone from

the Nejapa -Miraflores lineament. or a separate centre.

Figure 2.2.6. Topographic map of the Las Sierras shield and Masaya volcano, including the Chiltepe shield, andApoyo caldera. Outcrop distribution from field m3pping and aerial photography.

Masaya volcano lies within the Las Sierras Caldera breaching it only to the north, where lava flows and

fissure eruptions have extended along the west border of the Managua Graben. The present active volcano is

enclosed within the 2,000-4,000 year old Masaya caldera. All Masaya lavas are basaltic, and activity is usually

restricted to lava lakes and fissure eruptions. Occasional Plinian eruptions also occur and the Masaya caldera was

formed during an 8krn3 (DRE) ignimbrite and surge producing eruption (Williams 1983a).

Apoyo caldera cuts through a large sequence of Las Sierras ignimbrite, mudflow and scoria deposits. In

places lava flows of basaltic-dacitic composition are found both above and below ignimbrites. Little of the

original volcanic edifice is preserved but by the distribution of lava bodies, Sussman (1985) concluded that a

shield-like volcano was once present

2.2.4 Summary.

The Quaternary volcanoes in Nicaragua can be grouped into three morphological types; shield volcanoes,

stratocones, and ignimbrite shields. The shield volcanoes have many vents, usually aligned along lineaments and

are constructed by predominantly effusive activity. They are all sited near to, or on, fault zones and are

themselves heavily faulted. Stratocones have a main central vent and are composed of a large proportion of

pyroclastic material. They are situated away from fault zones, and structures are predominantly related to

gravitational instability of the edifice rather than regional tectonics. Since stratocones are located in regions with

no crustal faulting, whereas shields are ??, structure may control the type of volcano at a particular location. The

presence of faulting may provide pathways and produce stress concentrations in the crust, which place controls on

the storage, movement and eruption of magma. In addition, once high-level intrusive activity is in operation at

shield volcanoes, stress release in the Caribbean plate may favour these locations. The ignimbrite eruptions, some

of which are recent events, indicate that in the central area of the volcanic front in Nicaragua large volumes of

magma may collect and erupt from time to time. It is not yet known whether variations in magma influx from the

mantle cause greater volumes of material to be erupted, or whether large volumes are continually stored in the

crust.