160 Encyclopedia of the Solar System
FIGURE 8 Radar image of a lava flow field at 60◦N, 183◦Ein
the plains of Venus. The flow field is approximately 540×
900 km. The name of the flow field is Mamapacha Fluctus, and it
is made up of lava flows of moderate radar brightness or
moderate roughness.
River basalts or the Deccan Traps on Earth. In some plains
regions, the surface is clearly built up of multiple, super-
posed lava flow deposits, while other regions are more fea-
tureless. Lava flows have varying brightness in theMagellan
SAR images. Most lava flows are of intermediate brightness.
Comparisons to radar images of lava flows in Hawaii indi-
cate that the venusian flows have similar roughness, though
some flows on Venus are unusually smooth.
The plains are also covered with abundant small (<5km
across) shield and cone-shaped volcanoes (e.g. Fig. 9).
Thousands of these volcanoes have been mapped, and they
may contribute as much as 15% of the plains volcanic de-
posits. Other flows in the plains may have originated at
fissures, which were then obscured by later eruptions. Tim-
ing of the plains flows is a subject of debate, with some
FIGURE 9 Radar image of small shield volcanoes and polygonal
terrain,∼30 by 30 km, centered at 28.8◦N, 142.2◦E. Polygons
range in size from the limit of resolution to several kilometers in
diameter. The volcanoes at the north overlay the polygons.
Polygons are superimposed on the volcanoes in center right of
the image, where calderas indicate the top of the volcanoes. On
the western side of the image, various volcanic flows bury
polygons. Thus, the formation of the polygons appears
synchronous with the volcanism in this region.advocating that the plains formed relatively synchronously
across Venus in a single resurfacing event. Others argue
that the data support a slower, nonsynchronous formation
for the plains. Unfortunately, the impact crater population
can be interpreted to support either hypothesis, and it will
take future mission data to constrain plains formation on
Venus.
Large volcanoes on Venus (those with diameters
>100 km) are found at topographic rises, along rift zones,
and concentrated in the region bounded by Beta Regio, Atla
Regio, and Themis Regiones. Over 100 large volcanoes have
been identified. Large volcanoes have average heights of
about 1.5 km and aprons of lava flows that extend hundreds
of kilometers from their summits. Maat Mons, the largest
volcano on Venus is about 8.5 km high and 400 km across
(Fig. 10). In comparison, Mauna Loa, the largest volcano
on Earth, is about 9 km high and 100 km across. Detailed
studies of individual large volcanoes have revealed their
complex histories. Many volcanoes show evidence of mul-
tiple eruptions from their summits as well as sites on their
flanks. Some large volcanoes have calderas at their summits
similar to volcanoes on Earth and Mars, formed by col-
lapse of the underlying magma chamber. Others have radi-
ally fractured summits, with the radial fractures interpreted