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ERUPTION STYLES, SCALES, AND FREQUENCIES 145

observed on Hawaiian volcanoes (Fig. 1.1), which still possible, for instance, to have a basaltic Plinian
generate long, fluid lava flows, came to be called eruption if the erupted magma is unusually volatile-
Hawaiian eruptions and this term is used to denote rich or if it incorporates water from an extensive
any eruption of this character wherever it occurs in groundwater system as it nears the surface.
the world. The term Plinian is used to denote any During an eruption of any given type of magma
eruption which, like a Hawaiian eruption, is explo- the eruption style may change, so composition is not
sive and sustained in character but which generates the only control on eruption style. During a basaltic
a tall eruption plume and pyroclastic fall deposits eruption, for instance, there may be a change in
(Fig. 1.2). Other terms commonly used to describe character from Hawaiian to Strombolian or in a rhy-
different volcanic eruptions include Strombolian, olitic eruption from Plinian to ignimbrite-forming
Vulcanian, subPlinian, ultra-Plinian, and hydromag- to effusive. Thus for any magma composition a cer-
matic. These terms have already been described in tain range of behaviors is likely. It is important to
Chapter 1 and used throughout this book. recognize also that, despite the different eruption
When volcanologists examined and named dif- styles observed for different magma compositions,
ferent types of eruption they found that there are there are still similarities between the types of
general links between the composition of the magma behavior seen for each composition. The change
erupted and the types of eruptive activity seen. So, from Hawaiian to Strombolian activity in basaltic
for instance, Hawaiian and Strombolian eruptions eruptions, for example, mirrors the change from
usually involve basaltic magmas; Vulcanian eruptions Plinian to Vulcanian activity commonly observed in
usually involve compositions ranging from basaltic intermediate eruptions because both reflect a change
andesite to dacite; and Plinian eruptions occur with from sustained to transient explosive activity. Both
high-silica andesites to rhyolites (Fig. 10.1). By con- changes may occur for similar reasons (a reduction
trast, phreatomagmatic activity and effusive activity in the magma volume flux) even though the style of
can occur in association with magma of any com- the eruptions themselves differ.
position. The links between magma composition Nevertheless, the association of certain erup-
and eruption style are only general patterns; it is tion styles with certain magma types shows that

16
Phonolite
14
12 Tephri- Trachyte
phonolite
10
Na 2 O + K 2 O 8 Tephrite tephrite Basaltic Trachy- Trachydacite Rhyolite
Foidite
Phono-
andesite
trachy-
basanite
6
Trachy- andesite
basalt
4 Basaltic Andesite Dacite
Basalt
andesite
2 Picro-
basalt
0
40 50 60 70 80
SiO 2
Fig. 10.1 Diagram showing the classification of magma types in terms of their total alkali (Na O + K O) content and silica
2 2
content in weight percent. (Based on fig. 1 in Rogers, N. and Hawksworth, C. (2000) Composition of magmas. Encyclopedia
of Volcanoes. Academic Press, pp. 115–131, copyright Elsevier (2002).)