Page 95 - Fundamentals of Physical Volcanology

9780632054435_4_005.qxd 12/10/2007 12:20PM Page 72

72 CHAPTER 5

Fig. 5.7 A sequence of four frames
extracted from a movie showing the
rise of magma in a vent on Kilauea
volcano, Hawai’I, during an episode of
gas pistoning. In frames (a) to (c) the
accumulation of gas beneath the lava
crust causes the lava crust to rise
progressively higher in the vent. In
frame (d) the gas has escaped from
beneath the lava crust by tearing the
crust apart in a minor explosion and
the level of the lava in the vent has
fallen. (Photography by Tim Orr,
Hawaiian Volcano Observatory,
courtesy of the U.S. Geological
Survey.)

the overlying magma, whereas in 5000 seconds it with higher initial gas contents and with lower rise
will rise 50 m. Thus, during rise of magma over a speeds. Higher gas contents lead to larger bubbles
given distance, the slower rising magma allows the because the bubbles start to form at deeper levels
bubbles to travel further relative to their starting (supersaturation occurs at deeper levels) and thus
position in the magma. The further bubbles are able the bubbles grow more by decompression during
to rise through the magma, the greater the oppor- ascent. Smaller rise speeds lead to bigger bubbles
tunity for collision with other bubbles and there- because the bubbles have more time to grow by
fore for coalescence. In the extreme, the magma diffusion during ascent. For bubbles in basaltic
−1
itself may be stationary and the bubbles rise up magmas rising at speeds greater than 1 m s , the
through it to reach the surface of a lava pond in the maximum size that the bubbles reach is typically

vent. Ascent of the bubbles through such magma between 1 and 10 mm and essentially no bubble
−1
gives the initially largest bubbles the greatest pos- coalescence occurs. At rise speeds less than 1 m s ,
sible opportunity for overtaking smaller bubbles although the total gas content still influences bub-
and reaching the runaway stage in which single, ble size, coalescence is the dominant factor deter-
large bubbles form, filling the entire dike or con- mining the final bubble size – the slower the rise
duit. In basaltic magmas this effect can manifest speed the greater the final size of the bubble. When
itself in vigorous Strombolian explosions or in bubble growth is controlled by coalescence, bub-
more gentle “gas pistoning” (Fig. 5.7). These styles bles in basaltic magmas can grow to sizes greater
of eruption are discussed in detail in Chapter 7. than 1 m.
Figure 5.8 shows the influence of rise speed on Figure 5.8 shows one specific example of how
bubble growth for two different magma gas con- bubble growth is influenced by rise speed in a
tents (the amount of gas initially dissolved in the basaltic magma. In the case illustrated, the critical
magma at depth) in a basaltic magma that does not rise speed determining whether or not coalescence
−1
become supersaturated. The graph shows that at occurs is about 1 m s . The value of this critical rise
magma rise speeds of greater than 1 m s −1 the final speed varies as a function of magma gas content
size reached by a bubble by the time it is erupted and magma viscosity. At larger gas contents the crit-
depends linearly on the rise speed and also on the ical rise speed is greater. This is because, as long as
total gas content. Bubbles grow larger in magmas supersaturation does not become important, in