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ERUPTION STYLES, SCALES, AND FREQUENCIES 153

strongly influenced by the storage history of the prior to removal and removal only occurs when
magma and the environmental conditions at the sufficient gas pressure builds to cause brittle failure
eruptive vent. Composition does, however, play of this plug. This gives rise to the more violent
a critical role in determining the nature of effu- explosions characteristic of Vulcanian events.
sive eruptions because magma viscosity (which is • For sustained explosive activity a key difference
strongly linked to composition – Fig. 10.2) strongly exists between the size distribution of the material
influences the dynamics of lava flows (see sections ejected in Hawaiian and Plinian eruptions. Hawaiian
9.4–9.6). eruptions produce large, fluid lava clots whereas
• Whether an explosive eruption is transient or Plinian eruptions produce smaller clasts. This dif-
sustained in character is largely determined by ference is crucial in determining the very differ-
the rise speed of the magma and the ability of gas ent styles of these two types of eruption. The links
bubbles to rise through, and segregate from, the between clast-size distribution and fragmentation
magma. Transient explosions occur when gas can are not well understood but current ideas suggest
segregate and rise through the magma in the form that the difference in degree of fragmentation
of a few large bubbles whereas sustained explosive between Hawaiian and Plinian eruptions is mainly
activity occurs when little segregation of gas from due to the differences in viscosity between the
the magma occurs and a large number of small bub- magmas involved. In Hawaiian eruptions the low
bles is present. Composition plays a significant role viscosity of the magma causes relatively low strain
in determining whether gas rise and segregation rates during ascent and progressive fragmentation
can occur. In low and intermediate magmas vis- occurs. For more viscous magmas the strain rates
cosity is sufficiently small to allow gas bubbles to are higher and fragmentation is a brittle and
rise through the magma and transient explosions more complete process resulting in smaller clasts.
can occur. The high viscosity of high-silica magmas Magma gas content may also influence the degree
such as rhyolites prevents gas bubble segregation of fragmentation by affecting the acceleration
and thus suppresses transient explosions caused by and strain rates experienced by magma during
magmatic gases (they can, of course, still be associ- ascent.
ated with transient explosions caused by interac-
tion with meteoric water).
10.6 Magnitudes and frequencies
• Two main types of transient explosion are
of volcanic eruptions
known – Strombolian and Vulcanian – and they

are strongly associated with certain magma types.
Strombolian eruptions occur with basaltic magmas At any given moment in time typically about 20 vol-
whereas Vulcanian explosions are associated with canoes will be erupting on Earth. Usually we are
intermediate magmas. The differences in style of aware of this fact only if we live close to an active
these two types of transient explosion are thought volcano or the eruption is dramatic enough to make
to be caused by differences in the amount of cool- the news. Through news coverage and television
ing experienced by the top of the magma column documentaries most people have some idea of re-
between explosions. In Strombolian explosions the cent damaging eruptions such as the 1980 erup-
low viscosity of the magma allows rapid rise of gas tion of Mount St Helens in the western USA (Fig.
bubbles through the magma and hence the surface 1.2), the 1991 eruption of Mount Pinatubo in the
of the magma column has little time to cool before Philippines, the eruptions on Montserrat which
more gas bubbles rise and disrupt it. The bursting of started in 1995, or the 2002 eruption of Nyiragongo
these bubbles gives rise to the characteristically in the Democratic Republic of Congo. People usu-
weak explosions of Strombolian events. For more ally also have some idea of the destructive effects
viscous magmas, however, the rise speed of gas of “large” eruptions which have occurred further
bubbles is slower and accumulation of gas beneath in the past. For instance, the 1883 eruption of
the “plug” at the top of the magma column is Krakatau (the eruption was so loud that it woke
slower. This means that the plug cools more people sleeping in southern Australia more than