**3. Magma evolution**

Although monogenetic volcanism is widely known as part of basaltic magmatic systems (e.g. [7, 27, 41], it is also known as accompanying more complex mafic or even intermediate to acidic systems [42–48], thus indicating magmatic evolution during ascent. This evolution points to significant magma differentiation necessarily associated with low ascent rates or even magma crustal stagnation, and therefore evolution through processes such as fractional crystallisation and assimilation. This evolution is evidenced in the erupted magmas by trails such as: 1) the common presence of significant amount of intermediate plagioclase and mainly amphibole that requires relatively low magma temperatures to crystallise (<1000°C) (e.g. [10, 49, 50], 2) the common presence of crustal xenoliths and xenocrysts indicating time for incorporation and partial or total dilution (e.g. [8]), 3) the almost ubiquitous wide range of liquid compositions of glass within the same products indicating microscale magma interaction/evolution while minerals are forming; this yields heterogeneous portions of magma (e.g. [51]), 4) the strong variation of trace elements at constant SiO2 or MgO values within the same volcanic field (e.g. [10]), and 5) the diverse isotopic ratios indicating strong assimilation from the basement, also within the same volcanic field (e.g. [8, 26]). Magma mixing and self-mixing are possible additional processes linked to the magma evolution (e.g. [13, 43, 52]). Evidences of these are mineral disequilibrium textures (e.g. coronate, embayment, sieve, skeletal), reverse compositional zoning in minerals others than plagioclase (e.g. [53]), and also glass compositional differences in the same products [51]).

### **4. Magmatic plumbing systems**

A magma plumbing system under a monogenetic volcanic field can be understood as a network of interconnected dikes and sills that reach the surface in several points via different pathways [54]. Usually, these fields are understood as originated by magma reaching the surface directly from the asthenosphere in

**139**

**Figure 5.**

*Effusive Monogenetic Volcanism*

in that chamber (**Figure 5**).

**4.1 Examples**

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

terms of weeks or months through simple conduits without any pattern [7]. This is evidenced in the very common primitive magmas and scattered volcanoes that characterise many volcanic fields (e.g. [55]). There is also a "common wisdom" that acidic compositions produce large monogenetic volcanoes only and that most of these volcanoes are related to magma chambers feeding polygenetic volcanoes [1] due to stagnation in the crust makes the magma batches un-eruptible [7]. However, typical (in volume) monogenetic volcanoes, which are intermediate to acidic in composition, are commonly forming monogenetic fields, thus indicating: 1) "normality" rather than "rarity", and 2) stalling magma zones en route without cooling and crystallisation inhibiting the eruptivity. This stagnation has been evidenced as occurring within the lithosphere (e.g. [9]), particularly in the upper mantle-lower crust limit, or within the crust itself (e.g. [10, 12, 56], occasionally leaving small intrusive igneous bodies underneath the surface (e.g. [57]). This stagnation forming melt storage zones is a common geological explanation for many evolved monogenetic volcanic fields on different tectonic settings on Earth (e.g. [8, 11, 13, 14, 43, 52]). Thus, magmas coming to the planet surface directly from the asthenosphere tend to be mafic, while those coming from crustal melt storages tend to be either intermediate or felsic (**Figure 5**). Already near the surface, the eruptive style is driven by the internal magma characteristics but also by the external conditions linked to the lithology and the environment [27]. If the magmas do not reach the surface, they could form what would receive a name such as "monogenetic plutonic field." Monogenetic volcanoes can also be associated with polygenetic volcanoes and therefore with magma chambers; in this case, the composition of the products is fully related to the processes involved

A well-known place on Earth where effusive monogenetic volcanoes are located is the Altiplano-Puna Volcanic Complex [58] in the Central Volcanic Zone in South America [59]. In this place, several of these volcanoes have been identified, usually

*Schematic framework of magmatic plumbing systems for monogenetic volcanic fields. LAB:* 

*Lithosphere-Asthenosphere Boundary. Not to scale.*

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

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

Magma fragmentation is associated with bubble nucleation and growth. Thus, fragmentation occurs when the gas volume fraction reaches a critical value, i.e. when the magma changes from a liquid with bubbles to a medium of bubbles with liquid [40]. Bubbles, in turn, are a function of water diffusivity and melt viscosity during magma ascent and decompression; diffusivity is important for the feeding of the bubbles, while viscosity for allowing their growing [39]. Considering high efficiency of bubbles feeding and growing in a magma, it is possible to state that: a rapid decompression linked to a relative high ascent time, produces a high rate of bubbles nucleation, expansion and coalescence, and therefore a magma fragmentation to form a scoria/spatter cone. On the contrary, a slow decompression linked to a relative low ascent time, produces a low rate of bubbles nucleation; this yields to expansion, coalescence, channelling and the generation of a permeable network, which allows outgassing; the result is a magma reaching the surface without being fragmented, thus forming an effusive monogenetic volcano. In conclusion, effusive volcanoes in general are indicative of slow ascent times, at least, in the last part of their journey before reaching the

Although monogenetic volcanism is widely known as part of basaltic magmatic systems (e.g. [7, 27, 41], it is also known as accompanying more complex mafic or even intermediate to acidic systems [42–48], thus indicating magmatic evolution during ascent. This evolution points to significant magma differentiation necessarily associated with low ascent rates or even magma crustal stagnation, and therefore evolution through processes such as fractional crystallisation and assimilation. This evolution is evidenced in the erupted magmas by trails such as: 1) the common presence of significant amount of intermediate plagioclase and mainly amphibole that requires relatively low magma temperatures to crystallise (<1000°C) (e.g. [10, 49, 50], 2) the common presence of crustal xenoliths and xenocrysts indicating time for incorporation and partial or total dilution (e.g. [8]), 3) the almost ubiquitous wide range of liquid compositions of glass within the same products indicating microscale magma interaction/evolution while minerals are forming; this yields heterogeneous portions of magma (e.g. [51]), 4) the strong variation of trace elements at constant SiO2 or MgO values within the same volcanic field (e.g. [10]), and 5) the diverse isotopic ratios indicating strong assimilation from the basement, also within the same volcanic field (e.g. [8, 26]). Magma mixing and self-mixing are possible additional processes linked to the magma evolution (e.g. [13, 43, 52]). Evidences of these are mineral disequilibrium textures (e.g. coronate, embayment, sieve, skeletal), reverse compositional zoning in minerals others than plagioclase (e.g. [53]), and also glass

A magma plumbing system under a monogenetic volcanic field can be understood as a network of interconnected dikes and sills that reach the surface in several points via different pathways [54]. Usually, these fields are understood as originated by magma reaching the surface directly from the asthenosphere in

compositional differences in the same products [51]).

**4. Magmatic plumbing systems**

**2.3 Magma releasing**

Earth's surface.

**3. Magma evolution**

**138**

terms of weeks or months through simple conduits without any pattern [7]. This is evidenced in the very common primitive magmas and scattered volcanoes that characterise many volcanic fields (e.g. [55]). There is also a "common wisdom" that acidic compositions produce large monogenetic volcanoes only and that most of these volcanoes are related to magma chambers feeding polygenetic volcanoes [1] due to stagnation in the crust makes the magma batches un-eruptible [7]. However, typical (in volume) monogenetic volcanoes, which are intermediate to acidic in composition, are commonly forming monogenetic fields, thus indicating: 1) "normality" rather than "rarity", and 2) stalling magma zones en route without cooling and crystallisation inhibiting the eruptivity. This stagnation has been evidenced as occurring within the lithosphere (e.g. [9]), particularly in the upper mantle-lower crust limit, or within the crust itself (e.g. [10, 12, 56], occasionally leaving small intrusive igneous bodies underneath the surface (e.g. [57]). This stagnation forming melt storage zones is a common geological explanation for many evolved monogenetic volcanic fields on different tectonic settings on Earth (e.g. [8, 11, 13, 14, 43, 52]). Thus, magmas coming to the planet surface directly from the asthenosphere tend to be mafic, while those coming from crustal melt storages tend to be either intermediate or felsic (**Figure 5**). Already near the surface, the eruptive style is driven by the internal magma characteristics but also by the external conditions linked to the lithology and the environment [27]. If the magmas do not reach the surface, they could form what would receive a name such as "monogenetic plutonic field." Monogenetic volcanoes can also be associated with polygenetic volcanoes and therefore with magma chambers; in this case, the composition of the products is fully related to the processes involved in that chamber (**Figure 5**).
