Planetary Volcanism 845
FIGURE 15 The Plinian phase of
the explosive eruption of Pinatubo
volcano in 1991. A dense cloud of
large and small pyroclasts and
volcanic gases is ejected at high
speed from the vent and entrains
and heats the surrounding air.
Convections then drives the
resulting cloud to a height of tens of
kilometers, where it drifts
downwind, progressively releasing
the entrained pyroclasts. (Photo
credit: R. S. Culbreth, U. S. Air
Force. Photo courtesy of the
National Oceanic and Atmospheric
Administration, National
Geophysical Data Center.)Although the high magma gas contents needed suggest
that large-scale, steady (Plinian) explosive eruptions are rare
on Venus, it is possible to calculate the heights to which their
eruption clouds would rise. The high density and temper-
ature of the atmosphere lead to rise heights about a factor
of 2 lower than on Earth for the same eruption rate, and
very large (at least a few tens of meters) clasts may be trans-
ported into near-vent deposits. At distances greater than a
few kilometers from the vent, pyroclastic fall deposits will
not be very different from those on Earth. A few examples
of elongate markings on the Venus surface have been pro-
posed as fall deposits, but no detailed analysis of them has
yet been carried out.
The conditions that cause a steady explosive eruption to
generate pyroclastic flows instead of feeding a stable, con-
vecting eruption cloud are fairly well understood. If the
eruption rate exceeds a critical value (which increases with
increasing gas content of the mixture emerging through
the vent and decreases with increasing vent diameter), sta-
ble convection is not possible whatever the nature of the
atmosphere. Because pyroclastic flow formation is linked
automatically to high eruption rate and, in general, to high
eruption speed, which will encourage a great travel dis-
tance, it would not be surprising if large-scale pyroclastic
flow deposits distributed radially around a vent were the
products of high discharge rate eruptions of gas-rich mag-
mas. Many of the flanking deposits of some martian volca-
noes, especially Tyrrhena and Hadriaca Paterae, may have
been produced in this way.
Theoretical work has shown that pyroclastic flows on
Mars may be able to transport quite large blocks of rock (up
to several meters in size, similar to those found on Earth)
out of the vent and into nearby deposits. These pyroclast
sizes are much greater than those expected in fall deposits
on Mars, thus making it potentially possible to distinguish
flow and fall deposits in future, high-resolution spacecraft
images of martian vents. No equivalent work has yet been
carried out for Venus, again mainly because of the expecta-
tion that voluminous explosive eruptions may be rare under
the high atmospheric pressure conditions.
Short-lived or intermittent explosive eruptions (e.g., Vul-
canian explosions, phreato-magmatic explosions, or events
in which a gas-rich, high-viscosity lava flow or dome disinte-
grates into released gas and pyroclasts as a result of excessive
gas pressure) can also produce small-scale pyroclastic flows.
Because these are shorter lived and have characteristically
different grain size distributions, they are called surges. The
least well understood aspect of these phenomena is the way
in which the magmatic material interacts with the atmo-
sphere. As a result, it is currently almost impossible to pre-
dict in detail what the results of this kind of activity on Mars
or Venus would look like. Such deposits, by the nature of
the way they are generated, would not be very voluminous,
however, and so would be spread very thinly, and might not
be recognized if they were able to travel far from the vent.4.6 Phreato-Magmatic Activity
Some types of eruption on Earth are controlled by the vig-
orous interaction of magma with surface or shallow sub-
surface water. If an intrusion into water-rich ground causes
steam explosions, these are called phreatic events (from
the Greek word for a well). If some magma also reaches
the surface, the term used is phreato-magmatic, as distinct