**12. Surtseyan and Phreatoplinian eruptions**

They are explosive eruptions that produce fall deposits deriving from watermagma interaction processes, but pyroclastic density currents are much more abundant [8] (**Figure 32A** and **B**). As an example, a Surtseyan eruption is an explosive style of a volcanic eruption that takes place in shallow seas or lakes when rapidly rising and fragmenting hot magma interacts explosively with water and with water-steam-tephra. The eruption style is named after an eruption off the southern coast of Iceland in 1963 that caused the emergence of a new volcanic island, Surtsey (**Figure 32B**).

#### **Figure 34.**

*Origins of pyroclastic density currents. (A): short single-surge current derived by momentary collapse from a Plinian eruption column. (B): sustained current is derived from prolonged pyroclastic fountaining. The height of the jet (gas thrust) that feeds the current may vary and is transitional into (C). (C): A sustained current derived from a prolonged low pyroclastic fountaining explosive eruption. (D): current with a single (or multiple) surges derived from lateral blasts initiated by catastrophic decompression of a magmatic and/or hydrothermal system. (E): single-surge current derived from a collapsing lava dome or flow front. Hot rock avalanches generate turbulent density currents. (F): deposit-derived pyroclastic density current caused by gravitational collapse and avalanching of unstable loose ignimbrite. The current may be a single surge or more sustained where the collapse is retrogressive. Most large-volume ignimbrites derive from current types (B) and (C), which may involve periods of quasi-steady flow. Many may include significant components derived from currents of type (F) from Branney and Kokelaar [31]. For this paper, we chose the following three types: Soufriere, Pelee, and Merapi types (modified from Francis [10]).*
