Venus - Introduction to Planetary Geomorphology - page 117

Figure 6.14. This volcanic sinuous rille is near Alpha Regio and
appears to have originated froman irregular caldera-like depression;
similar features are seen on the lunar maria; the area shown is
130 km by 160 km (NASA Magellan, F 25S345).

Figure 6.13. This 90 km in diameter impact crater, Addams, shows a
massive

diameter. Although the resolution is not suf cient for
detailed analysis, the shield volcanoes are similar to small
basaltic shields in the Snake River Plain of Idaho and else-
where on Earth. Other venusian plains are characterized by
fields of small cones, suggestive of cinder cones, some of
which include distinctive radar-dark halos that have been
interpreted as being pyroclastic deposits (Fig. 6.17) .
Venusian volcanoes are among the largest and most
complex seen in the Solar System. Hundreds of volcanic
constructs have been identi ed, many of which are more
than 100 km across. One of the largest, Maat Mons
(Fig. 6.18) , reaches 9 km above planetary datum. The
main construct is more than 500 km wide at its base and
displays flows that can be traced as far as 600 km from their
source to form gentle flank slopes. Some venusian shields
either lack, or have only small, summit craters, while others
display enormous complex calderas (Fig. 6.19) .
Evidence of more viscous lava flows than typical for
basalt is seen in the form of dome-shaped volcanoes
(Fig. 6.20) . These structures are as large as 25 km across

lobe of ejecta melt extending 550 km to the
bottom of the image; such morphologic features are not seen on any
planetary surface other than Venus (NASA Magellan F-MIDR
55S097).

“ out ow ”

including compositions that are high in magnesium and
iron (called ), such as komatiite, which is

thought to erupt at high temperatures and is of low vis-
cosity. Even more exotic lavas, such as liquid sulfur and
carbonatite, have also been considered. Carbonatite is
molten carbonate of magmatic origin and has been
observed erupting at the Oldoinyo Lengai volcano in
Tanzania, where its physical properties were measured,
including its very low viscosity. In any event, it is likely
that lava tubes, or crusted surfaces on flows through lava
channels, were important to retain heat and enable con-
tinued flow over long distances.
Fields of small shield volcanoes are found inmany plains
regions. Some fields cover areas a few hundred kilometers
across, while the individual shields are 5 - 15 km in

“ ultrama c ”