Planetary Volcanism 839
more exotic volcanic fluid (or some other process) must be
assumed.
There are numerous uncertainties in using the forego-
ing relationships to estimate lava eruption conditions. Thus,
there have been many studies of the way heat is transported
out of lava flows, taking account of the porosity of the lava
generated by gas bubbles, the effects of deep cracks extend-
ing inward from the lava surface, and the external environ-
mental conditions—the ability of the planetary atmosphere
to remove heat lost by the flow by conduction, convection,
and radiation. However, none of these has yet dealt in suffi-
cient detail with turbulent flows, or with the fact that cool-
ing must make the rheological properties of a lava flow a
function of distance inward from its outer surface, so that
any bulk properties estimated in the ways described earlier
can only be approximations to the detailed behavior of the
interior of the lava flow. There is clearly some feedback be-
tween the way a flow advances and its internal pattern of
shear stresses. For example, lava flows on Earth have two
basic surface textures. Basaltic flows that have erupted at
low effusion rates or while still hot near their vents have
smooth, folded surfaces with a texture called pahoehoe (a
Hawaiian word), the result of plastic stretching of the outer
skin as the lava advances; at higher effusion rates, or at lower
temperatures farther from the vent, the surface fractures in
a more brittle fashion to produce a very rough texture called
‘a’a. A similar but coarser, rough, blocky texture is seen on
the surfaces of more andesitic flows. Because there is a
possibility of relating effusion rate and composition to the
surface roughness of a flow in this way, there is a growing
interest in obtaining relatively high resolution radar images
of planetary surfaces (and Earth’s surface) in which, as in
theMagellanimages of Venus, the returned signal intensity
is a function of the small-scale roughness.
4. Explosive Eruptions
4.1 Basic Considerations
Magmas ascending from the mantle on Earth commonly
contain volatiles, mainly water and carbon dioxide together
with sulfur compounds and halogens. All of these have solu-
bilities in the melt that are both pressure- and temperature-
dependent. The temperature of a melt does not change
greatly if it ascends rapidly enough toward the surface, but
the pressure to which it is subjected changes enormously.
As a result, the magma generally becomes saturated in one
or more of the volatile compounds before it reaches the
surface. Only a small degree of supersaturation is needed
before the magma begins to exsolve the appropriate volatile
mixture into nucleating gas bubbles. As a magma ascends to
shallower levels, existing bubbles grow by decompression,
and new ones nucleate. It is found empirically that after
the volume fraction of the magma occupied by the bubbles
exceeds some value in the range 65–80%, the foam-like
fluid can no longer deform fast enough in response to the
shear stresses applied to it and as a result disintegrates into
a mixture of released gas and entrained clots and droplets
that form the pyroclasts. The eruption is then, by definition,
explosive. The pyroclasts have a range of sizes dictated by
the viscosity of the magmatic liquid, in turn a function of
its composition and temperature, and the rate at which the
decompression is taking place, essentially proportional to
the rise speed of the magma.
It is not a trivial matter for the volume fraction of gas in
a magma to become large enough to cause disruption into
pyroclasts. The lowest pressure to which a magma is ever
exposed is the planetary surface atmospheric pressure. On
Venus, this ranges from about 10 MPa in lowland plains to
about 4 MPa at the tops of the highest volcanoes; on Earth,
it is about 0.1 MPa at sea level (and 30% less on high vol-
canoes) but much higher, up to 60 MPa, on the deep ocean
floor; on Mars it ranges from about 500 Pa at the mean
planetary radius to about 50 Pa at the tops of the highest
volcanoes; and it is essentially zero on the Moon and Io. If
the magma volatile content is small enough, then even at
atmospheric pressure no gas will be exsolved—or at least
too little will be exsolved to cause magma fragmentation.
Using the solubilities of common volatiles in magmas, cal-
culations show that explosive eruptions can occur on Earth
as long as the water content exceeds 0.07 weight percent
in basalt. On Mars, the critical level is 0.01 weight percent.
On Venus, however, a basalt would have to contain about
2 weight percent water before explosive activity could oc-
cur, even at highland sites; this is greater than is common
in basalts on Earth and leads to the suggestion that explo-
sive activity may never happen on Venus, at least at lowland
sites, or may happen only when some process leads to the lo-
cal concentration of volatiles within a magma. Examples of
this are discussed later. Finally, the negligible atmospheric
pressures on the Moon and Io mean that miniscule amounts
of magmatic volatiles can in principle cause some kind of
explosive activity there.
The preceding discussion assumes that released mag-
matic volatiles are the only source of explosive activity. How-
ever, many Vulcanian and all phreato-magmatic explosive
eruptions involve interaction of erupting magma with solid
or liquid volatiles already present at the surface (always wa-
ter or ice on Earth and probably on Mars; mainly sulfur
compounds on Io). The total weight fraction of gas in the
eruption products in such cases will depend on the detailed
nature of the interaction as well as the composition and
inherent volatile content of the magma; this is a critical fac-
tor in understanding explosive activity on Io.4.2 Strombolian Activity
Strombolian eruptions, named for the style of activity
common on the Italian volcanic island Stromboli, are an