Page 31 - Fundamentals of Physical Volcanology

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8 CHAPTER 1

eruptions consist of transient explosions typically
lasting 1–2 seconds which occur in sequences. At
Stromboli itself, for example, explosions generally
happen at time intervals of a few minutes to a few
hours. Each explosion generates a small ash plume
which is usually less than 200 m high (Fig. 1.12)
and throws out large incandescent blocks which,
by night, can be seen following ballistic trajector-
ies (Fig. 1.13). The volume of material produced in
each explosion is small: it can be as little as a few
cubic meters. Though this type of activity is charac-
teristic of Stromboli itself, the term Strombolian is
used more broadly to denote a range of styles of
transient explosive activity. In active lava lakes, for
example, gas rising to the lake surface causes up-
doming and bursting of large bubbles (which can
exceed 1 m in diameter) resulting in very weak
explosions (Fig. 1.14). These eruptions can also be
described as Strombolian. In Strombolian activity at
Heimaey in Iceland in 1973 explosions occurred at
time intervals of 0.5 to 3 seconds. The close spacing
in time of each explosion meant that the eruption
was effectively sustained, not transient, and was able
to generate an eruption plume up to 10 km high.
So the term Strombolian can be applied to
eruptions as weak as the bubble bursting events in
Hawai’I through to ones which generate high erup-
Fig. 1.13 Pyroclasts following ballistic trajectories from
tion plumes and deposit ash over a relatively large a Strombolian explosion at Stromboli volcano, Italy.
area. A further problem arises in understanding (Photograph by Pete Mouginis-Mark, University of Hawai’I.)

Fig. 1.14 Clots of fragmented magma
ejected as gas bubbles burst on the
surface of a lava lake on Kilauea
volcano, Hawai’I. (Photograph by Pete
Mouginis-Mark, University of Hawai’I.)