844 Encyclopedia of the Solar System
as the products of extreme dispersal of the smallest, 30–100
micrometer-sized particles.
Nevertheless, it appears that hot lava ponds up to∼5km
in diameter could have formed around basaltic vents on the
Moon if the eruption rates were high enough—as high as
those postulated to explain the long lava flows and sinuous
rilles. The motion of the lava in such ponds would have been
thoroughly turbulent, thus encouraging thermal erosion of
the base of the pond, and this presumably explains why the
circular to oval depressions seen surrounding the sources of
many sinuous rilles have just these sizes. Similar calculations
for the Mars environment show that, as long as eruption
rates are high enough, the atmospheric pressure and gravity
are low enough on Mars to allow similar hot lava source
ponds to have formed there, again in agreement with the
observed sizes of depressions of this type that are seen.
Some noticeable differences occur when Hawaiian erup-
tions take place from very elongate fissure vents. Instead of
a roughly circular pyroclastic cone containing a lava pond
feeding one main lava flow, a pair of roughly parallel ridges
forms, one on either side of the fissure. These are generally
called spatter ramparts. Along the parts of the fissure where
the eruption rate is highest, pyroclasts may coalesce as they
land to form lava flows so that there are gaps in the ramparts
from which the flows spread out. One striking example of
this has been found so far on Mars (Fig. 14).
FIGURE 14 Mosaic of two images showing fissure vent near
Jovis Tholus volcano on Mars. The vent has produced multiple
lava flow lobes, probably of basaltic composition. The area shown
is 24 km wide. (NASAMars Odysseyimage.)
4.5 Plinian Activity
In the case of a basaltic magma that is very rich in volatiles,
or (much more commonly on Earth) in the case of a
volatile-rich andesitic or rhyolitic magma, fragmentation in
a steadily erupting magma is very efficient, and most of
the pyroclasts formed are small enough to be thoroughly
entrained by the gas stream. Furthermore, the speed of
the mixture emerging from the vent, which is proportional
to the square root of the amount of gas exsolved from the
magma, will be much higher (perhaps up to 500 m/s) than
in the case of a basaltic Hawaiian eruption (where speeds
are commonly less than 100 m/s). The fire fountain in the
vent now entrains so much atmospheric gas that it develops
into a very strongly convecting eruption cloud in which the
heat content of the pyroclasts is converted into the buoy-
ancy of the entrained gas. The resulting cloud rises to a
height that is proportional to the fourth root of the magma
eruption rate (and hence the heat supply rate) and that may
reach several tens of kilometers on Earth. Only the very
coarsest pyroclasts fall out near the vent, and almost all of
the erupted material is dispersed over a wide area from the
higher parts of the eruption cloud (Fig. 15). This activity
is termed Plinian, after Pliny’s description of thea.d. 79
eruption of Vesuvius. Not all eruptions of this type produce
stable convection clouds. If the vent is too wide or the erup-
tion speed of the magma is too low, insufficient atmospheric
gas may be entrained to provide the necessary buoyancy for
convection, and a collapsed fountain forms over the vent,
feeding large pyroclastic flows or smaller, more episodic
pyroclastic surges.
Mars is the obvious place other than Earth to look for
explosive eruption products: The low atmospheric pres-
sure encourages explosive eruptions to occur, and the atmo-
spheric density is high enough to allow convecting eruption
clouds to form, at least up to∼20 km. However, we think
that stable eruption clouds much higher than this cannot
form on Mars because the atmosphere becomes too thin
to provide the amount of entrained gas that is assumed in
current theoretical models. In fact, only one potential fall
deposit has yet been identified on Mars with any confidence.
This is a region on the flank of the shield volcano Hecates
Tholus, where, in contrast to the rest of the volcano, small
impact craters appear to be hidden by a blanket of fine ma-
terial in a region about 50 km wide and at least 70 km long.
The sizes of the hidden craters suggest that the deposit is
∼100 m thick, giving it a volume of∼65 km^3 ; if we allow for
the likely low bulk density of the deposit, this is equivalent
to a dense rock volume of 23 km^3. The volumes of the four
summit depressions on Hecates Tholus range from∼10 to
∼30 km^3 , suggesting that they may be calderas produced
by collapse of the summit to compensate for the volume
removed from a fairly shallow magma storage reservoir in
each of a series of eruptions, the most recent of which pro-
duced the deposit described above.