Planetary Volcanism 841
emergence of successive bubbles or slugs from a vent may
range from seconds to at least minutes, making this a dis-
tinctly intermittent type of explosive activity. If the largest
rising gas bubble does not completely fill the vent, continu-
ous overflow of a lava lake in the vent may take place to form
one or more lava flows at the same time that intermittent
explosive activity is occurring, resulting in a simultaneously
effusive and explosive eruption.
A second method of producing gas slugs has been sug-
gested for some Strombolian eruptions on Earth, in which
gas bubbles form during convection in an otherwise stag-
nant body of magma beneath the surface and drift upward
to accumulate into a layer of foam at the top of the magma
body. When the vertical extent of the foam layer exceeds
a critical value, it begins to collapse. Liquid magma drains
from between the bubbles, and these coalesce into a large
gas pocket that can now rise through any available fracture
to the surface. The argument is that if a fracture was already
present, the high effective viscosity of the foam would have
inhibited its rise into the fracture, whereas the viscosity of
the pure gas is low enough to allow this to occur. If a frac-
ture was not already present, the changing stresses due to
the foam collapse may be able to create one.
As long as any volatiles are exsolved from a low-viscosity
magma rising sufficiently slowly to the surface, some kind
of Strombolian explosive activity, however feeble, should
occur at the vent on any planet, even at the high pressures
on Venus or on Earth’s ocean floors. Strombolian eruptions
commonly involve excess pressures in the bursting bubbles
of only a few tenths of a megapascal, so that the amount of
gas expansion that drives the dispersal of pyroclasts is small.
Pyroclast ranges in air on Earth can be several tens to at most
a few hundred meters, and ranges would be much smaller
in submarine Strombolian events on the ocean floor or on
Venus because of the higher ambient pressure. Subaerial
Strombolian eruptions on Mars would eject pyroclasts to
distances about three times greater than on Earth because
of the lower gravity; as a result, the deposits formed would
have a tenfold lower relief than on Earth, and so far few
examples have been unambiguously identified in spacecraft
images.
4.3 Vulcanian Activity
At the other extreme of a slowly rising viscous magma, it is
relatively difficult for gas bubbles to escape from the melt.
Particularly if the magma stalls as a shallow intrusion, slow
diffusion of gas through the liquid and rise of bubbles in the
liquid concentrate gas in the upper part of the intrusion,
and the gas pressure in this region rises. The pressure rise
is greatly enhanced if any volatiles existing near the surface
(groundwater on Earth; ground ice on Mars; sulfur or sulfur
dioxide on Io) are evaporated. Eventually the rocks overly-
ing the zone of high pressure break under the stress, and the
rapid expansion of the trapped gas drives a sudden, discrete
FIGURE 10 A dense cloud of large and small pyroclasts and gas
ejected to a height of a few hundred meters in a transient
Vulcanian explosion by the volcano Ngauruhoe in New Zealand.
(Image courtesy of the University of Colorado in Boulder,
Colorado, and the National Oceanic and Atmospheric
Administration, National Geophysical Data Center.)explosion in which fragments of the overlying rock and of
the disrupted magma are scattered around the explosion
source: This is called Vulcanian activity (Fig. 10), named
for the Italian volcanic island Vulcano. Again, as long as any
volatiles are released from magma or are present in the near-
surface layers of the planet, activity of this kind can occur.
Several Vulcanian events on Earth involving fairly viscous
magmas have been analyzed in enough detail to provide es-
timates of typical pressures and gas concentrations. Bombs
approaching a meter in size ejected to ranges up to 5 km
imply pressures as high as a few megapascals in regions that
are tens of meters in size and that have gas mass fractions
in the explosion products up to 10%.
On Mars, with the same initial conditions, the lower at-
mospheric pressure would cause much more gas expansion
to accelerate the ejected fragments, and the lower atmo-
spheric density would exert much less drag on them; also
the lower gravity would allow them to travel farther for