**2.2 Magma conduit and 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 ideal geoforms when related to conduit and topography.

### **Figure 4.**

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

### **2.3 Magma releasing**

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 Earth's surface.
