834 Encyclopedia of the Solar System
FIGURE 5 A variety of radar-bright lava flows radiate from the
summit area down the flanks of a shield volcano on Venus.
(NASAMagellanimage.)
by the numbers of superimposed impact craters, were em-
placed within the last∼700 Ma. [SeeVenus:Surface and
Interior].
Many areas within the plains and within other geological
units contain groupings (dozens to hundreds) of small vol-
canic edifices, from less than one to several kilometers in
diameter, with profiles that lead to their being classified as
shields or domes. These groupings are called shield fields,
and at least 500 have been identified. Some of the individ-
ual volcanoes have small summit depressions, apparently
due to magma withdrawal and collapse, and others are seen
to feed lava flows. Quite distinct from these presumably
basaltic shields and domes is a class of larger, steep-sided
domes (Fig. 6) with diameters of a few tens of kilometers
and heights up to∼1 km. The surface morphologies of these
domes suggest that most were emplaced in a single episode,
and current theoretical modeling shows that their height-
to-width ratio is similar to that expected for highly viscous
silicic (perhaps rhyolitic) lavas on Earth.
Many much larger volcanic constructs occur on Venus.
About 300 of these are classed as intermediate volcanoes
and have a variety of morphologies, not all including exten-
sive lava flows. A further 150, with diameters between 100
and about 600 km, are classed as large volcanoes. These are
generally broad shield volcanoes that have extensive sys-
tems of lava flows and heights above the surrounding plains
of up to about 3 km.
Summit calderas are quite common on the volcanoes,
ranging in size from a few kilometers to a few tens of kilo-
meters. There are two particularly large volcano-related
depressions, called Sacajawea and Colette, located on the
FIGURE 6 A cluster of∼25 km diameter “pancake” domes on
Venus. These domes are evidence of the eruption of lava, which
is more viscous than that forming the majority of flows on Venus.
(NASAMagellanimage.)upland plateau Lakshmi Planum. With diameters on the or-
der of 200 km and depths of∼2 km, these features appear
to represent the downward sagging of the crust over some
unusually deep-seated site of magma withdrawal.
Finally, there are a series of large, roughly circular fea-
tures on Venus, which, though intimately linked with the
large-scale tectonicstressesacting on the crust (they range
from a few hundred to a few thousand kilometers in di-
ameter), also have very strong volcanic associations. These
are the coronae, novae, and arachnoids. Though defined in
terms of the morphology of circumferential, moatlike de-
pressions, radial fracture systems, and so on, these features
commonly contain small volcanic edifices (fields of small
shields or domes), small calderas, or lava flows, the latter
often apparently fed from elongate vents coincident with
the distal parts of radial fractures. In such cases, it seems
extremely likely that the main feature is underlain by some
kind of magma reservoir that feeds the more distant erup-
tion sites via lateral dike systems.1.5 Mercury
Much of the surface of Mercury is a heavily cratered ancient
terrain like that of the Moon. There are some relatively flat
plains-forming units dispersed among the craters, and it is
tempting to speculate that these contain lava flows. Half
of the surface of the planet was imaged by the flyby probe
Mariner 10, but the resolution of the images is too poor
to allow the lobate edges of any flow units to be identified
unambiguously. Earth-based spectroscopic measurements
suggest that many of the surface rocks are similar to basalts
in composition. In places, patches of materials with these
kinds of compositions have shapes consistent with explosive
volcanic processes like those that we know occurred early in