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
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1.3. Thesis Layout.
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Chapter 2. Quaternary Volcanism and Tectonics in Nicaragua 2.0. Introduction
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2.1. Geology and Structure or Western Nicaragua
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2.1.1. Regional Setting
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2.1.2. Morphology
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2.1.3. General Geology
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2.1.4. Structure in Western Nicaragua
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I. The Nicaraguan Depression
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II. Neotectonic Features: Fault Zones and Grabens
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IIa. La Pelona Fault Zone.
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IIb. La Paz Centro Fault Zone
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llc. Mateare Fault
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lld. Managua Graben
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IIe. Ochomogo Fault Zone
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Ill. Origin of Fault Zones
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2.1.5. Summary
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2.2. Quaternary Volcanoes in Nicaragua
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2.2.1. Shield Volcanoes: Individual Descriptions
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I. Cosigüina Volcano
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II. Telica Massif
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III. Rota Massif
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IV. El Hoyo
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V. Zapatera
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2.2.2. Stratocones: Individual Descriptions
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I. San Cristobal Massif
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II. Cerro Montoso and Momotombo
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III. Mombacho Volcano
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IV .Concepcion and Maderas
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2.2.3. Ignimbrite Shield Volcanoes
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I Malpaisillo Ignimbrite Shield
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II. Las Sierras and Chiltepe Shields
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2.2.4. Summary
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2.3. Magma Composition in Nicaragua
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2.4. Discussion
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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 (
