Page 29 - Fundamentals of Physical Volcanology

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6 CHAPTER 1

1.2.5 Ignimbrite-forming eruptions
The Hawaiian and Plinian eruption styles were
defined based on observations of modern and his-
toric eruptions. By contrast the eruption style referred
to by the (rather cumbersome) term ignimbrite-
forming was defined based on eruption deposits:
geologists observed and mapped ignimbritedeposits
long before they witnessed or understood what
type of volcanic eruption formed them. Decades
went by after the term was defined before the-
oretical modeling of how eruptions work, together
with observations of certain modern eruptions, gave
volcanologists an understanding of the types of
Fig. 1.9 A Plinian fall deposit from the ∼3.3 ka Waimihia eruption in which ignimbrites form. Ignimbrites
rhyolitic eruption of Taupo volcano, New Zealand.
(originally known as ash-flow tuffs in the USA) are
(Photograph courtesy of Stephen Self.)
the deposits produced by very large-scale pyro-
clastic density currents. A pyroclastic density
current is a hot cloud of volcanic ash, magmatic gas
clasts emerges from the vent at speeds of ∼100– and air which flows along the ground at a very high
−1
600ms and forms a convecting eruption plume in speed. Possibly the fastest example yet documented
the atmosphere as the surrounding air is sucked into in detail was produced during the AD 189 eruption
the jet (see Chapter 6). Clasts are carried upwards of Taupo, in New Zealand. Deposits from this
and progressively fall out from the eruption plume. flow are found on top of a ∼1600 m high moun-
Large clasts are carried upwards only a short dis- tain, which appears to imply a speed of at least
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tance and fall to the ground close to the vent, while ∼180ms . More typical, much thicker, ignimbrites
smaller clasts are carried to greater heights and fall include the Bandelier Tuff in the USA and the
out further from the vent, forming part of an un- Campanian Tuff in Italy.
consolidated airfall deposit (Fig. 1.9, see Chapter 8). Pyroclastic density currents form in various ways,
The eruptions are sustained, and can last for hours one of which involves an eruption column, like that

or days. Eruptions exhibiting this style of activity are formed in a Plinian eruption, becoming unstable
subdivided into sub-Plinian, Plinian, and ultra- and collapsing (Fig. 1.10). No human being has ever
Plinian based on their mass flux and plume heights experienced the formation of a really large pyro-
(factors which are linked). Mass fluxes across the clastic density current. However, a tragic example
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three subtypes range from ∼10 to 10 kg s , rates of the devastating effect of even a small event of this
which are generally greatly in excess of those for kind occurred during the 1902 eruption of Mount
recent basaltic eruptions. Mass fluxes during recent Pelée, a volcano on the Caribbean island of Martini-
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eruptions at Kilauea are ≤ 10 kg s , whereas the que. On May 8, 1902, Mount Pelee erupted produc-
highest mass flux during the (exceptional) Laki ing a large black cloud that rolled down the flanks
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eruption was ∼10 kg s . Generally, Plinian erup- of the volcano and spread out, generating a kind of
tions involve magmas which are relatively rich in pyroclastic density current called a pyroclastic
silica and in dissolved gases, are very viscous, and surge that engulfed the main town on the island, St
erupt at lower temperatures than basalts. They are Pierre, 6 km away from the volcano. The surge
the products of various kinds of interaction of cloud moved rapidly through the town, setting
more basaltic melts with other rocks in the crust. anything combustible on fire. In the space of 2 to
The commonest magma types involved in Plinian 3 minutes about 28,000 people were killed. Only a
eruptions are called dacites and rhyolites, but handful of people survived, possibly as few as two
rare basaltic Plinian eruptions are known. or three; one survivor was a prisoner locked in the