Geology Reference
In-Depth Information
that some complex ridged terrain is among the oldest
recognizable geologic units on Venus.
Mare-type ridges are common on nearly half of the
venusian plains
(Fig. 6.24)
, mostly at lower elevations.
The ridges are several hundred meters high, as wide as a
kilometer or two across, and can be hundreds of kilo-
meters long. More than 65,000 mare ridges have been
identi
ed, and they are tectonic compression features.
Detailed mapping reveals the global lithospheric stress
field in which lowland plains were in compression at the
time of ridge formation, while some uplands were in
extension, exempli
ed by chasma formation.
6.5.5 Gradation features
Even in the absence of water and
fluvial streams
-
the
dominant agent of gradation on Earth
Figure 6.21. Thouris Corona is about 190 km across and is identi
ed
by the concentric set of fractures surrounding a gentle dome. The
structure is set within regional fractured plains (NASA Magellan
mosaic ASU-IPF).
the harsh surface
environment of Venus can be expected to lead toweathering
-
Figure 6.22. Classi
cation of venusian
coronae (from Stofan et al.,
1992
).
