**8. Magmatic explosive eruptions that produce fall-out bedded deposits caused by Plinian eruption**

To explain such deposits, I have schematically subdivided the magmatic explosive eruptions into two phases and sections—how the magma act from inside to outside volcanoes.

Phase 1: The magmatic column rises with a speed of <0.5 m/s.

The volatiles that dissolves form gas bubbles whose upward movement is slow because it is hindered by the viscosity of the magma. The expansion stops when the ratio between gas and magma is about 3:1 causing fragmentation. At the level of fragmentation, the drastic decrease in viscosity causes the sudden increase in speed that passes from subsonic to supersonic (**Figure 23**).

Phase 2: The mixture of gases and particles expelled from the crater forms a Plinian eruptive column in which three zones can be recognized, which are as follows:

a. jet expulsion area with initial acceleration, of height 1–2 km


A typical fall-out bedded deposit is shown in **Figure 24A**. This massive fall-out bedded is a characteristic Plinian deposit of the Somma-Vesuvius volcano. The main

**Figure 23.**

*Eruptive Plinian column from the source up to the surface. Modified from Schmincke [14].*

components are pumice, as seen in **Figure 25A** [32]. To compute such deposits is not very easy, the measurements of thickness and pumice size called, respectively, isopach and isopleths can be a difficult task [8]. **Figure 25B** shows a model of isopach calculation seen in three-dimensional space [24]. Each isopach is characteristic on just one thickness that can be followed and extrapolated in the field around a volcano that forms tephra fall-out bedded eruption, many model computations have been published, I just quote some: [33–36]. Generally, Plinian eruptions are characterized by strong volcanic plumes coupled with vertical columns (**Figure 25**), although as shown from **Figure 25** even a strong volcanic plume can have a characteristic bent-over feature like in a weak plume [36].
