Page 24 - Fundamentals of Physical Volcanology

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1 Volcanic systems

1.1 Introduction deposited ash as much as 1500 km downwind from
the volcano, to huge explosive eruptions which
A volcanic eruption is an amazing event to watch: have occurred in the geological past, such as the
dangerous and frightening but also fascinating and eruption 600,000 years ago at Yellowstone which
awe-inspiring. While most people will never experi- covered half of the United States with ash. They
ence an eruption first-hand, accounts of volcanic vary in scale from tiny eruptions producing a few
eruptions in the media, television documentaries cubic meters of lava to eruptions which can pro-
3
and Hollywood films all mean that even those living duce up to ∼2500 km of ash or lava (enough to
far from an active volcano have some idea of what cover the whole of Great Britain with a layer more
volcanic eruptions are like. than 10 m thick, enough to bury all but the tallest
Volcanic eruptions vary tremendously in style buildings!). They vary in duration from a few seconds
and in the deposits they produce, from lava foun- to years or decades. They vary tremendously in
taining eruptions in Hawai’I (Fig. 1.1), through frequency – an observer at Stromboli volcano (in
moderately explosive eruptions, such as the 1980 the Aeolian Islands, north of Sicily) usually has only
eruption of Mount St Helens (Fig. 1.2) which devast- to wait a matter of minutes to see an eruption,
ated the area immediately around the volcano and whereas an observer at Yellowstone National Park
could wait 100,000 years or more to see a volcanic
eruption!
Why does a volcanic eruption occur where it
does and when it does, and what controls what the
eruption is like? Physical volcanology is the branch
of geology which seeks to answer these questions
by applying basic physical principles to find out
how volcanoes work. The study of volcanoes in
this way over the last 30 years or so has shown
that, despite the apparent complexity of individual
volcanic eruptions, the basic physical processes
which govern them are often surprisingly simple,
and furthermore the processes can be very similar
in eruptions which superficially appear very differ-
ent from each other. In this book we seek to
Fig. 1.1 An approximately 300 m high lava fountain
describe these physical processes and to underline
eruption from the Pu’u ‘O’o vent on the East Rift Zone of
Kilauea volcano, Hawai’I. (Photograph by Pete Mouginis- their similarities in eruptions which are apparently
Mark, University of Hawai’I.) so different in character.