Planetary Volcanism 837
parent asteroids with as much certainty, but we know from
their composition that the Aubrites and the Ureilite me-
teorites are rocks from the mantles of two different aster-
oids that had violently explosive eruptions, which ejected
what should have become their crustal rocks into space at
escape velocity. And the Acapulcoites and Lodranites are
rocks from the shallow crust or upper mantle of a body
that produced rather small amounts of gas during melting
in its mantle so that in these meteorites we see gas bub-
bles trapped in what was once magma traveling through
fractures toward the surface. The importance of these me-
teorites is that they give us copious samples of the very
deep interiors of their parent bodies as well as the sur-
faces; such samples will not be available for a very long time
for Venus and Mars and are rare even for the Earth. [See
Meteorites; Main-Belt Asteroids.]
2. Classification of Eruptive Processes
Volcanic eruption styles on Earth were traditionally classi-
fied partly in terms of the observed composition and dis-
persal of the eruption products. Over the last 20 years, it
has been realized that they might be more systematically
classified in terms of the physics of the processes involved.
This has the advantage that a similar system can be adopted
for all planetary bodies, automatically taking account of the
ways in which local environmental factors (especially sur-
face gravity and atmospheric pressure) lead to differences
in the morphology of the deposits of the same process oc-
curring on different planets.
Eruptive processes are classified as either explosive or
effusive. An effusive eruption is one in which lava spreads
steadily away from a vent to form one or more lava flows,
whereas an explosive eruption is one in which the magma
emerging through the vent is torn apart, as a result of the co-
alescence of expanding gas bubbles, into clots of liquid that
are widely dispersed. The clots cool while in flight above the
ground and may be partly or completely solid by the time
they land to form a layer of pyroclasts. There is some am-
biguity concerning this basic distinction between effusive
and explosive activity because many lava flows form from
the recoalescence, near the vent, of large clots of liquid that
have been partly disrupted by gas expansion but that have
not been thrown high enough or far enough to cool appre-
ciably. Thus, some eruptions have both an explosive and an
effusive component.
There is also ambiguity about the use of the word “ex-
plosive” in a volcanic context. Conventionally, an explosion
involves the sudden release of a quantity of material that has
been confined in some way at a high pressure. Most often
the expansion of trapped gas drives the explosion process.
In volcanology, the term “explosive” is used not only for this
kind of abrupt release of pressurized material but also for
any eruption in which magma is torn apart into pyroclasts
that are accelerated by gas expansion, even if the magma is
being erupted in a steady stream over a long time period.
Eruption styles falling into the first category include Strom-
bolian, Vulcanian, and phreato-magmatic activity, whereas
those falling into the second include Hawaiian and Plinian
activity. All of these styles are discussed in detail later.3. Effusive Eruptions and Lava Flows
Whatever the complications associated with prior gas loss,
an effusive eruption is regarded as taking place after lava
leaves the vicinity of a vent as a continuous flow. The mor-
phology of a lava flow, both while it is moving and after
it has come to rest as a solid rock body, is an important
source of information about the rheology (the deformation
properties) of the lava, which is determined largely by its
chemical composition, and about the rate at which the lava
is being delivered to the surface through the vent. Because
lava flows basically similar to those seen on Earth are so
well exposed on Mars, Venus, the Moon and Io, a great deal
of effort has been made to understand lava emplacement
mechanisms.
In general, lava contains some proportion of solid crystals
of various minerals and also gas bubbles. Above a certain
temperature called the liquidus temperature, all the crystals
will have melted, and the lava will be completely liquid.
Under these circumstances, lavas containing less than about
20% by volume of gas bubbles will have almost perfectly
Newtonian rheologies, which means that the rate at which
the lava deforms, thestrainrate, is directly proportional to
the stress applied to it under all conditions. This constant
ratio of the stress to the strain rate is called the Newtonian
viscosity of the lava. At temperatures below the liquidus
but above the solidus (the temperature at which all the
components of the lava are completely solid), the lava in
general contains both gas bubbles and crystals and has a
non-Newtonian rheology. The ratio of stress to strain rate
is now a function of the stress, and is called the apparent
viscosity. At high crystal or bubble contents, the lava may
develop a nonzero strength, called the yield strength, which
must be exceeded by the stress before any flowage of the lava
can occur. The simplest kind of non-Newtonian rheology is
that in which the increase in stress, after the yield strength
is exceeded, is proportional to the increase in strain rate:
The ratio of the two is then called the Bingham viscosity,
and the lava is described as a Bingham plastic.
The earliest theoretical models of lava flows treated them
as Newtonian fluids. Such a fluid released on an inclined
plane will spread both downslope and sideways indefinitely
(unless surface tension stops it, a negligible factor on the
scale of lava flows). Some lavas are channeled by preexisting
topography, and so it is understandable that they have not
spread sideways. However, others clearly stop spreading
sideways even when there are no topographic obstacles, and