Page 171 - Fundamentals of Physical Volcanology

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148 CHAPTER 10

a pyroclastic surge or density current, and thus con- but they are both transient in nature, Strombolian
verting what began as a relatively harmless effusive eruptions being associated with basaltic magmas
eruption in which lava advances slowly into a very and Vulcanian eruptions generally with more evolved
much more dangerous explosive one. magmas. Similarly Hawaiian and Plinian eruptions
are both types of sustained explosive eruption
Thus effusive eruptions can occur due to a range
although they also differ in character (compare
of different circumstances. Magma composition
Figs 1.1 & 1.2), in products, and in the composition
does play a role in controlling whether an eruption
of the magma involved.
is effusive or explosive but factors such as the stor-
We have already seen that whether an explo-
age history of the magma and the environmental
sive eruption is transient or sustained is strongly
conditions under which the eruption occurs are
influenced by the rise speed of the magma. This is
often equally or more important. This is particularly
because composition influences viscosity, which in
true in extraterrestrial eruptions: the atmospheric
turn controls the rise speed of gas bubbles within
pressure at the surface of Mars is about 200 times
the magma and hence their ability to overtake one
less than on Earth, and on the Moon and Io there
another and coalesce (Chapters 5 and 7). Three
is essentially no atmosphere, so that eruptions on
main issues are important here.
these bodies (the Moon in the very distant past,
Mars in more recent geological times and Io now) 1 Magma viscosity affects the ability of bubbles
must almost always have an explosive component, to move upward through the rising magma. The
and indeed Io has the largest volcanic eruption greater the magma viscosity the slower the rise of
plumes ever seen in the Solar System (Chapter 13). the bubbles relative to the magma, which makes
In contrast the pressure at the surface of Venus bubble coalescence less likely.
ranges from 4 to 9 MPa, similar to that under 2 The magma viscosity also affects the rise speed
500–1000 m of ocean on Earth (see Table 10.2), of the magma itself, greater viscosities tending to
making explosive eruptions very much less likely reduce the rise speed. This increases the time avail-
there than on Earth. able for bubbles to rise relative to the magma and
thus for bubble coalescence to occur.
3 The gas content of the magma also affects the
10.3.2 Chemical composition and lava flows
likelihood of bubble coalescence occurring. Greater
The primary products of effusive eruptions are lava gas contents lead to a greater number density of

flows. The behavior of lava flows is strongly bubbles in the magma and more bubble growth
influenced by the composition of the erupting lava during rise, both of which also increase the oppor-
because lava viscosity is a major control on flow tunities for bubble coalescence to occur. Thus, as
dynamics. These links have been discussed in gas content tends to be greater for more silica-rich
sections 9.4–9.6. magmas this tends to increase the opportunity for
coalescence.
The net effect of these factors is not simple to
10.4 Chemical composition and
deduce because they can counteract each other.
explosive eruptions
For example, a basaltic magma tends to have both
low viscosity and low gas content, so that factor 1
tends to increase the opportunity for bubble coales-
10.4.1 Transient and sustained
cence (and transient eruption) whereas factors 2
explosive activity
and 3 tend to decrease bubble coalescence. Figure
The majority of subaerial eruptions on Earth are 10.3 shows the results of computer modeling car-
explosive in character. A fundamental distinction ried out to assess the influence of the above factors
which can be drawn between such eruptions is on basaltic activity. Figure 10.3a shows the effect
whether they are sustained or transient in nature. described in point 1 with increasing viscosity mak-
Strombolian and Vulcanian eruptions differ consid- ing coalescence less likely for a given rise speed.
erably in violence and in the deposits they generate Figure 10.3b shows the effect described in point 3