Page 25 - Fundamentals of Physical Volcanology

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2 CHAPTER 1

Fig. 1.2 Eruption cloud from the
sustained phase of the May 18, 1980
eruption of Mount St Helens volcano,
Washington State, USA. The lower
part of the cloud is almost vertical but
near the top the cloud and the small
particles falling from it are being
carried away downwind. (Photograph
taken by Robert Krimmel, courtesy
of U.S. Geological Survey/Cascades
Volcano Observatory.)

1.2 Styles of volcanic eruptions gularly shaped fragments of solid rock. Explosive
activity happens when the magmatic liquid rising
Direct observations of volcanic eruptions, recorded from great depth is torn apart by the expansion of
on film, in photographs, and in written eye-witness gas bubbles formed by the release of volatile com-

accounts, combined with geological observations pounds dissolved in the liquid. The term effusive
of older volcanic deposits, show that the styles, eruption is often used, incorrectly, to denote any
scales, and products of volcanic eruptions vary tre- eruption in which lava flows are produced. Hawaiian
mendously from volcano to volcano. In this sec- eruptions, in particular, are often referred to as effu-
tion we will look at these different styles of volcanic sive because they produce lava flows. However, in
activity, but we will start by drawing a basic distinc- fact very many of these eruptions are explosive, and
tion between effusive and explosive eruptions. the lava flows they produce are formed indirectly,
Volcanic eruptions can be divided into two main by coalescence of lava clots accumulating near the
classes: effusive and explosive. An effusive erup- vent, rather than by the direct oozing of the magma
tion is one in which molten rock called magma, ris- from the vent.
ing from the deep interior of the Earth, flows out of We now present a broad overview of the range of
a vent as a coherent liquid called lava. Both while styles of volcanic activity that occur on Earth. All but
they are still molten and after they cool, the bodies the first style – effusive – are explosive in character.
of rock formed on the surface in this way are called
lava flows. An explosive eruption is one in which
1.2.1 Effusive eruptions
the magmatic material is torn apart as it is erupted
into pieces called pyroclasts. These can range from An effusive eruption is an eruption in which lava
hot clots of still-molten liquid to much cooler, irre- flows away from a vent as a coherent liquid. For