Page 98 - Fundamentals of Physical Volcanology

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THE ROLE OF VOLATILES 75

trapped inside the giant bubble is generally at a
higher pressure than the atmosphere and so it
expands, throwing clots of lava from the torn skin
upward and outward in a Strombolian explosion. It
was shown earlier that bubble coalescence occurs
more easily in basaltic than more evolved magmas,
and Fig. 5.8 demonstrates that, even for basaltic
magmas, there is quite a sharp divide, in terms of
magma rise speed and volatile content, between
eruptions in which bubble coalescence is or is not
important.
As a result of the above issues, explosive erup-
tions are split into two main classes. Where magma
rise speed is high enough to prevent significant
coalescence, gas bubbles stay coupled to the magma
within which they form and this leads to “steady”
eruptions – eruptions in which a continuous stream
of gas and magma clasts is erupted (see Chapter 6).
In basaltic eruptions with low enough rise speeds,
bubble coalescence dominates and the bubbles
decouple from the magma in which they form.
Bubbles rising and coalescing lead to a situation in
which the distribution of gas through the magma
column is uneven. This can lead to a range of be-
haviors from relatively minor fluctuations in the
Fig. 5.11 Scoria clasts in the Montana Colorada cone,
intensity of the eruption (like a pulsing in the height
Lanzarote. The cone formed during an eruption in April
of lava fountains in Hawaiian eruptions) through
1736. (Photograph by Elisabeth Parfitt.)
to intermittent, discrete explosions occurring as
successive pockets of gas rise through the magma
same process, and a relatively uniform and steady to be erupted. These eruptions are referred to as

stream of gas and pyroclasts emerges through the “transient” and they are discussed in Chapter 7.
vent. This is the commonest way in which viscous
magmas are erupted. However, if a great deal of
coalescence of bubbles occurs, especially to the 5.7 Summary
point where giant bubbles fill nearly the whole
width of the dike, then the magma rising though the • Most volcanic eruptions are to some extent
dike is far from uniform. As the magma between explosive. In the context of volcanology this
the giant bubbles reaches the surface, it flows out of means that the magma is torn apart and ejected
the vent as lava, containing whatever gas bubbles from the vent as clots or blobs within a stream of
have avoided being swept up by the giant bubbles. gas. Explosive eruptions can be transient events,
As the giant bubbles reach the surface they up- occurring every few seconds to minutes or hours,
dome the surface of the lava above them forming or can be continuous, steady eruptions which
a skin. This skin may be plastic if lava is flowing out last hours or days.
of the vent fairly quickly and will stretch and tear • Most explosive eruptions owe their explosive
easily; alternately, if the lava is moving away only character to the presence of gases dissolved
very slowly, the skin may have cooled significantly within the rising magma. The solubility of vol-
since the arrival of the previous giant bubble and canic gases in magma decreases with decreasing
will fracture in a brittle manner as it bursts. The gas pressure, so as magma rises towards the surface a