**2.1 Evidences of internal dynamics**

Coherent lava bodies of effusive monogenetic volcanoes have usually a glassy groundmass, which is the evidence of the rapid cooling when magma reaches the surface (**Figure 3A**). Commonly, the magma hosts phenocrysts (i.e. crystals greater than 0.5 mm) and microphenocrysts (i.e. crystals between 0.5 and 0.05 mm), although they do not dominate in the products. Occasionally, when the magma reaches the surface, decompression triggering solubility decreasing, oversaturation and degassing, induces crystal nucleation and therefore growing of multiple small crystals [39]; if these crystals can be distinguished in type, they are called microliths (usually between 50 and 5 μm) and the groundmass can be defined as microcrystalline if they dominated (**Figure 3B**), on the contrary the crystals can be called nanoliths (<5 μm) and the groundmass denominated as cryptocrystalline (**Figure 3C**). This crystal nucleation, along with temperature, composition (mostly SiO2 but also MgO content) and dissolved volatiles (mostly H2O but also CO2), are the factors controlling the magma viscosity and somehow the volcano that is built (i.e. a lava dome, couleé, small-shield or lava flow). The higher the crystals and silica content, the higher the viscosity [39]; so, these magmas tend to form lava domes or couleés. On the contrary, small-shields and lava flows are related to low amount of crystals and low silica. Magma temperature tends to indicate relative low values in lava domes and high values in lava flows, while volatiles have a special behaviour [39]: their content is higher in viscous, high-silica magmas, but at the same time they keep viscosity lower; therefore, under a similar composition, a rapid degassing yields a lava dome formation, while a slow degassing leads to a lava flow geoform. Overall, slow ascent times are related to lava domes, while fast ascent times to lava

### **Figure 3.**

*Groundmass in effusive monogenetic products. (A, B) Glassy groundmass. (C, D) cryptocrystalline groundmass. (E, F) microcrystalline groundmass. Parallel nichols to the left, crossed nichols to the right.*

**137**

**Figure 4.**

*Effusive Monogenetic Volcanism*

*DOI: http://dx.doi.org/10.5772/intechopen.94387*

**2.2 Magma conduit and topography**

flows. The relationship between the mentioned elements, however, are somehow circular or themselves dependent, and consequently without a linear relation. Thus, although the groundmass and the major crystals are evidence for the dynamics of magma propagation during ascent, from our experience, no direct relationships can be drawn between any of the elements vs. the volcanoes, even in a thin section study of the eruptive products under the microscope. This is worth mentioning because it explains why the definition of these volcanoes is purely dependent on the geoform and do not consider, for instance, petrographic characteristics. In spite of this, we consent some approaches that can be made from a rock. For example, an increase in decompression rates results in (1) bubbles and crystals with smaller sizes, (2) a lower crystallinity and thus higher glass fraction, and (3) a higher abundance of unstable hydrous phases [17, 40]. This may help as a starting point for subsequent

studies when a rock from effusive monogenetic volcanoes is analysed.

ideal geoforms when related to conduit and topography.

*Volcanic geoforms vs. ascent conduit type and emplacement topography.*

Monogenetic effusive volcanoes are related to physical elements such as the conduit form and dimension, and the interaction with the surface, but also to the topography where the magmas are released. Thus, the volcanoes can be formed through a cylindrical vs. a fissural conduit and in a flat vs. a hilly topography. This complex emplacement can deviate the resulting geoforms from what we normally would expect. For instance, a lava flow volcano that could be linked to a low viscosity magma, could be really the result of a high viscosity magma released and emplaced through a long fissure in a flat topography; also a dome-like geoform that could be linked to high viscosity magma, could be really the result of a lava-type, low viscosity magma, released in a valley or basin that limited its movement. A more complex circumstance could also occur when the magma solidifies forming barriers for subsequent melt to come out, although clearly this situation would not play any role in large volume of magma outpourings. Thus, the upper dozens of meters of the conduit geometry in turn related to the shape of the crater and the magma rheology will be very important in the resulting landform type. Because of the obvious complexity and due to most of the times the construction of the volcanoes is not witnessed, the proposed classification scheme is based on geoforms, thus avoiding terminology complication associated with the source. **Figure 4** details the

### *Effusive Monogenetic Volcanism DOI: http://dx.doi.org/10.5772/intechopen.94387*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

elements is the topic of the following sections.

**2.1 Evidences of internal dynamics**

flows based on their geoform. The construction of every volcano is linked to the internal dynamics of the magma, but also to the form and dimension of the ascending conduit, the interaction of the conduit with the surface, and the topography where the magma is released. Every factor should be in-depth investigated. An overview of these

Coherent lava bodies of effusive monogenetic volcanoes have usually a glassy groundmass, which is the evidence of the rapid cooling when magma reaches the surface (**Figure 3A**). Commonly, the magma hosts phenocrysts (i.e. crystals greater than 0.5 mm) and microphenocrysts (i.e. crystals between 0.5 and 0.05 mm), although they do not dominate in the products. Occasionally, when the magma reaches the surface, decompression triggering solubility decreasing, oversaturation and degassing, induces crystal nucleation and therefore growing of multiple small crystals [39]; if these crystals can be distinguished in type, they are called microliths (usually between 50 and 5 μm) and the groundmass can be defined as microcrystalline if they dominated (**Figure 3B**), on the contrary the crystals can be called nanoliths (<5 μm) and the groundmass denominated as cryptocrystalline (**Figure 3C**). This crystal nucleation, along with temperature, composition (mostly SiO2 but also MgO content) and dissolved volatiles (mostly H2O but also CO2), are the factors controlling the magma viscosity and somehow the volcano that is built (i.e. a lava dome, couleé, small-shield or lava flow). The higher the crystals and silica content, the higher the viscosity [39]; so, these magmas tend to form lava domes or couleés. On the contrary, small-shields and lava flows are related to low amount of crystals and low silica. Magma temperature tends to indicate relative low values in lava domes and high values in lava flows, while volatiles have a special behaviour [39]: their content is higher in viscous, high-silica magmas, but at the same time they keep viscosity lower; therefore, under a similar composition, a rapid degassing yields a lava dome formation, while a slow degassing leads to a lava flow geoform. Overall, slow ascent times are related to lava domes, while fast ascent times to lava

*Groundmass in effusive monogenetic products. (A, B) Glassy groundmass. (C, D) cryptocrystalline groundmass. (E, F) microcrystalline groundmass. Parallel nichols to the left, crossed nichols to the right.*

**136**

**Figure 3.**

flows. The relationship between the mentioned elements, however, are somehow circular or themselves dependent, and consequently without a linear relation. Thus, although the groundmass and the major crystals are evidence for the dynamics of magma propagation during ascent, from our experience, no direct relationships can be drawn between any of the elements vs. the volcanoes, even in a thin section study of the eruptive products under the microscope. This is worth mentioning because it explains why the definition of these volcanoes is purely dependent on the geoform and do not consider, for instance, petrographic characteristics. In spite of this, we consent some approaches that can be made from a rock. For example, an increase in decompression rates results in (1) bubbles and crystals with smaller sizes, (2) a lower crystallinity and thus higher glass fraction, and (3) a higher abundance of unstable hydrous phases [17, 40]. This may help as a starting point for subsequent studies when a rock from effusive monogenetic volcanoes is analysed.
