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10 Eruption styles, scales,

and frequencies

10.1 Introduction minutes to tens of minutes (Figs 1.12 & 1.13). By
contrast hundreds of thousands of years may elapse
Volcanic eruptions show a tremendous variety of between volcanic eruptions of other volcanoes. For
styles, products, scales, and frequencies. Each erup- example, the last three eruptions from the magma
tion is, of course, unique and the sheer variety of chamber underlying Yellowstone caldera in the
activity can give the impression that each eruption western USA have occurred at time intervals of
is completely different from any other. Our inten- about 600,000 years. There is a fundamental link
tion in this book is, however, to emphasize the between the frequency and scale of eruptions,
similarities in the underlying physics of eruptions however, so that the frequent eruptions at Strom-
rather than the uniqueness of individual events: boli produce very small quantities of volcanic mat-
to show that eruptions which may superficially erial (typically a few cubic meters to a few tens of
appear very different are, in fact, very similar in cubic meters), whereas the infrequent eruptions
terms of their basic mechanisms. For this reason the of volcanic centers such as Yellowstone generally
preceding chapters have emphasized the physical produce vast quantities of volcanic material. The
processes which control the basic character of last major eruption at Yellowstone occurred about
3
eruptions (such as whether an eruption is effusive 600,000 years ago and produced ∼1000 km of
11
or explosive, sustained or transient) and of their pyroclastic material, that is about 100 billion (10 )

products (pyroclastic deposits or lava flows). These times as much volcanic material as a typical erup-
are the most fundamental characteristics of vol- tion at Stromboli! The second half of this chapter
canic eruptions and can be understood without looks at what is currently understood about the
reference to the specific details of an eruption. physical processes which control the frequency
However, the more detailed character of any given and scale of volcanic eruptions.
eruption is determined by a number of factors, the
most important of which is the composition of the
10.2 Chemical composition and styles of
magma involved. Chemical composition influences
volcanic activity
an eruption by affecting the physical properties of
the erupting magma such as the viscosity and gas
content. The first half of this chapter concerns how As we have already seen, broad distinctions can be
chemical composition affects eruption character drawn between volcanic eruptions on the basis of
and products. whether they are effusive or explosive, steady or
Volcanic eruptions vary widely not only in char- transient in character (Chapters 5–7). In practice,
acter but in the scale and frequency of activity. however, volcanologists have developed a system of
An observer watching activity at Stromboli, a vol- naming which refers more specifically to the char-
cano in the Aeolian Islands north of Sicily, often has acter of any individual eruption. So, for instance,
the opportunity to witness eruptions every few the sustained lava fountaining eruptions commonly