Page 228 - Fundamentals of Physical Volcanology

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VOLCANISM ON OTHER PLANETS 205

dreds of kilometers) winding channels called
canali. These are typically only a few kilometers
wide but the longest extends for more than 6000
km! Of the many suggestions about their origin, the
most likely seems to be the long-duration eruption
of a large volume of very low-viscosity lava.
How can the predominance of volcanic features
be reconciled with the absence of plate tectonics
on Venus? The thick atmosphere shields the surface
of the planet from all but the largest impacting
meteorites. But enough have penetrated to the
surface and formed craters for us to estimate that
Fig. 13.14 So-called “pancake domes” in the Eistla region the average ages of the plains are in the range 600
of Venus, probably formed by the eruption of viscous lava. to 800 Ma. In fact, some interpretations of the
These large examples are up to 65 km in diameter and
crater distributions imply that “all” of the volcanic
∼600 m high. (NASA Magellan RADAR image.)
surfaces have about this age. Thus it has been sug-
gested that, with no plate tectonics, the relatively
kilometers it is suspected that they represent inactive Venus lithosphere slowly heats up with
the traces of swarms of giant dikes. time until a massive episode of flood volcanism
The dense atmosphere that makes visible obser- almost completely resurfaces the planet in a geo-
vations difficult effectively traps heat from the Sun logically short time interval. This exhausts the
so that most of the surface is hotter than 400°C, hot magma supply and re-sets the system to start
enough to melt lead. These high temperatures, cou- another several hundred million year cycle of heat-
pled with the high atmospheric pressure (∼9 MPa ing. It should be stressed, however, that this is not
in the lowlands and ∼4 MPa on mountain tops) and the only interpretation of the impact crater distri-
corrosive nature of the atmosphere (traces of HCl bution, and some volcanic areas may be much
and HF are mixed with the carbon dioxide) are very younger than this model implies.
damaging to any probes landing on the surface
of Venus. Nevertheless the Soviet Union landed a
total of nine spacecraft on the surface plains, six of 13.7 Mercury

which survived long enough to make various kinds
of compositional measurements that confirmed the Our knowledge of the surface of Mercury is very
presence of a range of different types of basaltic restricted because so far only one spacecraft has vis-
rocks. This is very much what would be expected ited the planet. Mariner 10 made a series of three
if most of the volcanism consists of the eruption of fly-bys in 1974 and imaged a total of just over half
mantle material melted in rising plumes. the surface at resolutions between about 100
There is evidence that other types of magma have and 4000 m per pixel. This was good enough to
erupted on Venus. There are about 80 near-circular, show that the surface is very old and dominated
“pancake”-shaped domes (Fig. 13.14), with dia- by impact craters and basins, just like the Moon and
meters in the range 20–30 km and thicknesses up to the highlands of Mars. However, there seems to be
∼1 km, that appear to consist of lava with the visco- a predominance of relatively flat ground between
sity of dacite or even rhyolite. Given that the high many of the craters, giving rise to the classifica-
atmospheric pressure reduces volatile exsolution tion of much of the surface as “intercrater plains”
from Venus magmas, it is possible that these lavas (Fig. 13.15). The inference is that this material con-
would have erupted explosively on Earth to form sists of lava flows, but this is by no means proven.
ignimbrite sheets. At the other extreme, on the The highest resolution images show lobate
plains of Venus there are a number of long (hun- boundaries in some areas that may possibly be the