Geology Reference
In-Depth Information
Figure 6.16. A segment of a venusian channel ~2 km wide in
Guinevere Planitia and the apparent spillover of radar-bright
material to form a massive
flow-lobe; also visible to the left of the
channel is a volcano (arrow) about 20 km across with a small summit
crater (NASA Magellan MRPS 41387).
and are hundreds of meters high. They are generally
circular in plan-form and have
flat, upper surfaces, leading
to the informal term
Figure 6.15. Abundant lava
flows are seen in the lowland plains and
in some upland regions of Venus. These
flows in the Lada Terra
region breached a large north
south ridge belt and can be traced
more than 600 km from their source; differences in radar brightness
indicate lava surface texture roughness; the area shown is 550 km by
630 km (NASA Magellan MRPS 37755).
-
volcanoes. Many of the
constructs have small craters on their surfaces, suggestive
of volcanic vents or explosive blasts. One
field of domes is
in the general vicinity of the Venera 8 landing site, where
compositional data suggest more silicic rocks, supporting
the suggestion that the domes were formed by more vis-
cous lavas.
“
pancake
”
6.5.4 Tectonic features
In the early period of exploration in the 1960s, there was
speculation that Venus might experience plate tectonics in
the style of Earth (i.e., spreading centers and zones of
subduction) because of the similarities in planetary size
and density. However, the global data sets from Ve ne r a
15 and 16 and Magellan clearly show that this is not the
case and that a different style of tectonism must be respon-
sible for the loss of heat from the planet
'
s interior. One clue
to this different style comes from surface features called
coronae (from the Latin for crowns). These are circular
features 200
600 km across consisting of ridges and
grooves arranged in concentric patterns
(Figs. 6.21
-
6.23)
.
The ridges and grooves are often found on the
flanks of
gentle domes or surrounding shallow depressions, some of
-
Figure 6.17. This small cinder cone (arrow) is surrounded by a radar-
dark zone, which has been interpreted as pyroclastic deposits
superposed on plains that have been extensively fractured by
tectonism; the area shown is 110 km by 90 km (NASA Magellan
F-MIDRP 50S345).
