Page 32 - Fundamentals of Physical Volcanology

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VOLCANIC SYSTEMS 9

Fig. 1.15 Walker (1973) devised this
classification scheme for different
types of explosive eruption based
on the degree of fragmentation, F,
a function of the range of sizes of
pyroclasts produced, and the dispersal, 100
D, defined as the area within the Ultra-
boundary where thickness of the Plinian
deposit decreased to 1% of its
maximum thickness. The circled
asterisk indicates where the deposits Fragmentation, F (%) 50 Vulcanian Plinian
of the 1959 Kilauea Iki eruption plot
using this classification system. After
fig. 6.2 in Cas, R.A.F. and Wright, J.V. Hawaiian
(1988) Volcanic Successions. A Sub-
Strombolian Plinian
Geological Approach to Processes,
0
Products and Successions. Chapman
0 0.05 5 500 5000
and Hall, 528 pp. With kind
2
permission of Springer Science Dispersal, D (km )
and Business Media.)
what is meant when an eruption is described as more evolved compositions (e.g., dacite). Typically
Strombolian, because the most widely used classifi- Vulcanian explosions last a number of seconds or
cation system defines an eruption as Strombolian if minutes and often occur in sequences with repose
the dispersal of ash during the eruption lies within times between explosions varying from tens of
certain specific limits (Fig. 1.15). This leads to prob- minutes to hours. It is common for the violence
lems because it is common for the deposits of modern of the explosion to be related to the repose time,
Hawaiian eruptions to lie within the Strombolian so that longer repose times result in more violent
field (Fig. 1.15). Thus an eruption could be defined explosions. Vulcanian explosions can be extremely
as Hawaiian based on observation of the eruption violent events: typical eruption velocities range
style but Strombolian based on the deposits gener- between 200 and 400 m s −1 and the eruptions can
ated. Extreme caution is needed, therefore, in clas- eject blocks a few meters in size out to distances of

sifying an eruption as Strombolian on the basis of as much as 5 km from the vent. One explosion at
ash dispersal alone. In this book we always use Ngauruhoe in New Zealand in 1975 ejected a dense
terms such as Strombolian to refer to the style block 27 m long and 15 m wide which is estimated
of an eruption, not the extent of the ash dispersal to weigh around 3000 tons! In addition to ejecting
involved. Characteristic features of Strombolian large blocks and volcanic bombs ballistically, the
eruptions are that they consist of transient explo- eruptions commonly generate eruption plumes
sions, closely spaced in time, involving low viscos- which carry finer material to heights of several kilo-
ity, generally basaltic magma, and occurring within meters. In eruption sequences in which explosions
volcanic systems which are open to the surface. occur with relatively short repose times, higher
plumes can be generated. These can be up to
∼20 km in height. The eruption plumes generate a
1.2.7 Vulcanian eruptions
fall deposit and, as the eruption columns are com-
Vulcanian eruptions are like Strombolian eruptions monly unstable, they can collapse to generate pyro-
in consisting of discrete or transient explosions, clastic density currents. For example, during the
but are associated with more evolved magmas 1975 Ngauruhoe eruption the eruption plume rose
than those causing Strombolian eruptions, typi- to heights of 4–5 km and deposited ash as much
cally ranging from intermediate compositions as 20 km from the vent, and the partial collapse of
(e.g., basaltic andesite to andesite) through to the plume generated pyroclastic currents which