Page 223 - Fundamentals of Physical Volcanology

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200 CHAPTER 13

Fig. 13.9 The ∼180 km diameter
martian shield volcano Hecates
Tholus. The area to the west of the
summit caldera complex has fewer
small impact craters per unit area than
other parts of the volcano and may
represent a recent pyroclastic fall
deposit. (Mosaic of infrared Mars
Odyssey THEMIS images courtesy of
NASA/JPL/Arizona State University.)

deposit from a Plinian eruption has been identified to a hybrid may have existed between the fountain-
with much confidence is on the western flank of ing over the vent that on Earth feeds pyroclastic
Hecates Tholus (Fig. 13.9). The inference is that the density currents, and the nearly ballistic dispersal of
eruption cloud was about 70 km high and the erup- pyroclasts that dominated eruptions on the Moon
7
−1
tion rate was about 10 kg s . However, there is a and is common now on Io. No attempts have yet
large area to the west of Tharsis where the surface been made to develop theoretical models of such

is known to be a very poor reflector of radar sig- eruptions.
nals, and some scientists think that this whole area The difficulty of detecting both widespread pyro-
may be blanketed by tens of meters of very fine clastic fall deposits and the martian equivalents of
pyroclastic dust. Indeed, it is just possible that the near-vent cinder, spatter, or scoria deposits has led
dust which is present everywhere on Mars, being to the suggestion that, even if pyroclastic materials
sculpted into dunes by the wind, is not the result of are not distributed planet-wide, at least the shield
the slow chemical weathering of solid rocks but is volcanoes may contain great quantities of pyroclas-
just an endlessly reworked planet-wide accumula- tics interspersed with the lava flows that are much
tion of primary volcanic pyroclastic material. easier to identify. This would be hard to verify with-
There is one caveat to this speculation. It may out seismic data or drill cores, but it would not be
not be safe to extend models of eruption clouds on surprising given that the low atmospheric pressure
Earth to the current martian environment. The low would be expected to enhance explosive activity.
surface pressure means that beyond a height of The average acceleration due to gravity on Mars
somewhere between 10 and 20 km some of the is only 38% of that on Earth. As mentioned earlier,
simplifying assumptions used in these models may this means that at a given distance below the sur-
not be valid. This suggests that very high eruption face the pressure due to the weight of the over-
clouds may never have formed on Mars in very high lying rocks is nearly three times less. However, the
discharge rate eruptions. Instead, something closer strengths and other elastic properties of martian