**14. Diluted pyroclastic currents**

Base surge and pyroclastic surge originate from the base of eruptive columns of magmatic and phreatomagmatic eruptions [4, 8–10]. They move radially and are made up of one turbulent cloud of water vapor and ash at high temperatures (**Figures 36** and **37**). For all kinds of volcanic forms, we can look to **Figure 38**

**Figure 38.**

*Morphologies of volcanic structure. Modified from Nemeth and Martin [11].*

**Figure 39.** *Santorini caldera (author's collection).*

### *Introduction to the Volcanology DOI: http://dx.doi.org/10.5772/intechopen.102771*

depending on the size of the eruption and the volume emitted. For large eruptions, often after the volcanic activity, there is a collapse of the roof of the magma chamber that brings to a structure called Caldera. A classic example can be the Santorini Caldera, as shown in **Figure 39**. For a thoughtful explanation of the diverse type of caldera refer to Chris Newhall caldera book [39].
