Page 103 - Fundamentals of Physical Volcanology

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80 CHAPTER 6

–1
Rise speed (m s )
Fig. 6.2 The variation with depth
2 5 10 20 50 100 200 beneath the surface of the rise speed
0 0
C of magma ascending in a dike. The
magma rises at a constant speed from
B its source until gas bubbles first start to
form at point A. Gas bubble expansion
releases energy and the magma
300 300 accelerates until bubbles become
Depth (m) close-packed and fragmentation
occurs at point B. Above this point
acceleration increases until the
600 600 wall friction becomes negligible and
gas–pyroclast mixture erupts at the
surface at point C. (Adapted from
fig. 3 in Wilson, L. & Head, J.W. (1981)
A
Ascent and eruption of basaltic magma
900 900 on the Earth and Moon. J. Geophys.
2 5 10 20 50 100 200 Res., 86, 2971–3001.)

friction becomes available for a different use. There rounding atmosphere into which they emerge, and
is no change in the amount of potential energy it determines the way large pyroclasts decouple
required to raise the magma through a given dis- from the stream of gas and small particles to fall to
tance toward the surface, and so the extra energy the surface. It is the measurement of the distribu-
goes into the kinetic energy term and causes a large tion of these large clasts around the vent that allows
increase in the acceleration of the gas–pyroclast us to analyze the conditions during prehistoric
mixture. eruptions.
The acceleration of the mixture during its ascent
towards the surface can be simulated using com-
6.4.1 Magmatic gas content and exit velocity
puter programs. Figure 6.2 shows one such
simulation. The ascending magma rises at a fixed
The exit velocity of the gas–pyroclast mixture in
speed of ∼2.5ms −1 until gas bubbles first start to any particular eruption is sensitive to the gas con-

form within it (point A). Once gas bubbles form tent of the magma, with larger gas contents leading
and expand, energy is released and the magma
to higher exit velocities. There are three main rea-
mixture accelerates (between points A and B). At
sons for this.
point B fragmentation occurs. Above this point wall
friction becomes negligible and acceleration of the • The larger the gas content, the greater the total
mixture becomes very much more pronounced. In energy available for release during gas expansion
this example, the gas–pyroclast mixture acceler- and therefore the greater the energy available to
ates from less than 20 m s −1 at the fragmentation accelerate the rising gas–pyroclast mixture.
level (at ∼100 m depth) to a velocity of ∼170ms −1 • The larger the gas content of the magma, the
upon eruption at the surface (Fig. 6.2). greater the depth at which bubbles will first nucle-
ate. The deeper the nucleation depth, the greater
the total pressure decrease experienced by the gas
6.4 Controls on exit velocity as it ascends and, therefore, the greater the energy
release through gas expansion.
The speed with which gas and entrained small • The larger the gas content, the deeper the frag-
pyroclasts leave the vent in an explosive eruption is mentation level. Deeper fragmentation means that
important for two reasons: it determines the rate at the change from high to low wall friction also
which the eruption products mix with the sur- occurs deeper, the overall friction losses during