**9. Strombolian eruption**

Phase 1: The magmatic column rises at a speed of 0.5–1 m/s. The rising speed of the bubbles is greater than that of the magma so very large bubbles form on the surface. At a depth of about 100 m, the fraction of gaseous bubbles reaches 75%,

#### **Figure 24.**

*(a): Fall-out bedded typical with an enlargement of its constituent (pumice) of a Plinian eruption from Somma-Vesuvius volcano (author's collection). (b): A model to visualize the spread of the pyroclastic deposits around a volcano. Modified from Cortini and Scandone [25].*

and the magma fragments erupt explosively. The decrease in viscosity produces an increase in speed from subsonic to supersonic (**Figure 26**).

Phase 2: The expulsion of materials occurs through a series of explosions with short time intervals (0.1 sec to 1 h). The exit speed of the mixture of gas and fragmented particles is about 200 m/s.

When the explosions occur in short intervals of time, an eruptive column is formed in which the height of the jet does not exceed 200 m and the convective column reaches heights of 5–10 km (**Figure 26**).

The Strombolian eruption has taken the name from the Stromboli Island part of the Eolian Islands, characterizing such volcanic activity worldwide. The style of the Stromboli eruption has been continuously active in the last 2000 years and it was very important for obsidian trade and as a lighthouse of the Mediterranean Sea. Some examples of Strombolian activity are listed and shown in figures. Eruption column of a Strombolian eruption of Vesuvius, the 1944 AD shown in **Figure 27**.

**Figure 25.** *Eruptive columns of Plinian eruptions (taken from the web).*

Scoria layers deposed during a Strombolian activity of Vesuvius in the medieval period are shown in **Figure 28** [37].
