846 Encyclopedia of the Solar System
from normal, purely magmatic eruptions. When the equiva-
lents of Strombolian or Hawaiian explosive events take place
from eruption sites located in shallow water, they lead to
much greater fragmentation of the magma than usual be-
cause of the stresses induced as pyroclasts are chilled by
contact with the water. This activity is usually called Surt-
seyan, named after the eruption that formed the island of
Surtsey off the south coast of Iceland. A much more vig-
orous and long-lived eruption under similar circumstances
leads to a pyroclastic fall deposit similar to that of a Plinian
event, but again involving greater fragmentation of magma:
The result is called phreato-Plinian activity. Since the word
“phreatic” does not specifically refer to water as the non-
magmatic volatile involved in these kinds of explosive erup-
tion, it seems safe to apply these terms, as appropriate, to
the various kinds of interactions between magma and liq-
uid sulfur or sulfur dioxide forming the plumes currently
seen on Io. These eruptions appear to involve about 30%
by weight volatiles mixed with the magma; these propor-
tions are close to the optimum for converting the heat of the
magma to kinetic energy of the explosion products. Phreatic
and phreato-magmatic eruptions should also have occurred
on Mars in the distant past if, as many suspect, the atmo-
spheric pressure was high enough to allow liquid water to
exist on the surface.
4.7 Dispersal of Pyroclasts into a Vacuum
The conditions in the region above the vent in an explo-
sive eruption on a planet with an appreciable atmosphere
(e.g., Venus, Earth, or Mars) are very different from those
when the atmospheric pressure is very small (much less
than about 1 Pa), as on the Moon or Io. If the mass of at-
mospheric gas displaced from the region occupied by the
eruption products after the magmatic gas has decompressed
to the local pressure is much less than the mass of the mag-
matic gas, there is no possibility of a convecting eruption
cloud forming in eruptions that would have been classed as
Hawaiian or Plinian on Earth. In the region immediately
above the vent, the gas expansion must be quite complex
and will involve a series of shock waves. Relatively large
pyroclasts will pass through these shocks with only minor
deviations in their trajectories, but intermediate-sized par-
ticles may follow very complex paths, and few studies have
yet been made of these conditions. The magmatic gas even-
tually expands radially into space, accelerating as it expands
and reaching a limiting velocity that depends on its initial
temperature. As the density of the gas decreases, its ability
to exert a drag force on pyroclasts also decreases. On bodies
the size of the Moon, even the smallest particles eventually
decoupled from the gas and fell back to the planetary sur-
face, though in gas-rich eruptions on asteroids they were
commonly ejected into space.
These are the conditions that led to the formation of the
dark mantle deposits on the Moon, with ultimate gas speeds
on the order of 500 m/s, leading to ranges up to 150 km for
small pyroclasts 30–100 micrometers in size. They are also
the conditions that exist now in the eruption plumes on Io,
though with an added complication. The driving volatiles in
the Io plumes appear to be mainly sulfur and sulfur dioxide,
evaporated from the solid or liquid state by intimate mixing
with rising basaltic magma in what are effectively phreato-
magmatic eruptions. The plume heights imply gas speeds
just above the vent of∼1000 m/s, and these speeds are con-
sistent with the plume materials being roughly equal mix-
tures of basaltic pyroclasts and evaporated surface volatiles.
However, as the gas phase expands to very low pressures,
both sulfur and sulfur dioxide will begin to condense again,
forming small solid particles that rain back onto the surface
along with the silicate particles to be potentially recycled
again in future eruptions.
A final point concerns pyroclastic eruptions on the small-
est atmosphereless bodies, the asteroids. Basaltic partial
melts formed within these bodies were erupted at the
surface at speeds that depended on the released volatile
content. This is estimated to have been as much as 0.2–
0.3 weight percent, leading to speeds up to 150 m/s. These
speeds are greater than the escape velocities from aster-
oids with diameters less than about 200 km, and so instead
of falling back to the surface, pyroclasts would have been
expelled into space, eventually to spiral into the Sun. This
process explains the otherwise puzzling fact that we have
meteorites representing samples of the residual material
left in the mantle of at least two asteroids after partial melt-
ing events, but have no meteorites from these asteroids with
the expected partial melt composition.5. Inferences about Planetary Interiors
The presence of the collapse depressions called calderas
at or near the summits of many volcanoes on Earth, Mars,
Venus, and Io suggests that it is common on all of these bod-
ies for large volumes of magma to accumulate in reservoirs
at relatively shallow depths. Theories of magma accumula-
tion suggest that the magma in these reservoirs must have
an internal pressure greater than the stress produced in the
surrounding rocks by the weight of the overlying crust. This
excess pressure may be due to the formation of bubbles by
gas exsolution, or to the fact that heat loss from the magma
to its cooler surroundings causes the growth of crystals that
are less dense than the magmatic liquid and so occupy a
larger volume. Most commonly, a pressure increase leads
to fracturing of the wall of the reservoir and to the propa-
gation of a magma-filled crack, called a dike, as an intrusion
into the surrounding rocks. If the dike reaches the surface,
an eruption occurs, and removal of magma from the reser-
voir allows the wall rocks to relax inward elastically as the
pressure decreases. If magma does not reach the surface,
the dike propagates underground until either the magma