Page 226 - Fundamentals of Physical Volcanology

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VOLCANISM ON OTHER PLANETS 203

flow lengths averaging 350 km, the longest having
double this length. Scattered around the plains are
more than 20,000 small dome-like volcanoes up to
15 km in diameter; these are grouped into more
than 500 clusters with the clusters averaging 150 km
in diameter. Additionally about 270 features are
classified as intermediate-sized volcanoes averaging
25 km in diameter. Finally, many of the mountains
are large shield volcanoes, of which there are more
than 150, typically 400 km in diameter and 2 to 4 km
high (Fig. 13.12). Some of the flows down the
flanks of these volcanoes can be several hundred
Fig. 13.11 Mosaic of RADAR scans of a 210 km by 140 km
kilometers long. More than 80 of the large shield
area of volcanic plains on Venus from NASA’s Magellan
spacecraft. The dark patches are relatively young lava flows volcanoes have summit calderas, averaging about
superimposed on older, lighter flows. Darker areas are 60 km in diameter. These calderas are much wider
rougher and lighter areas area smoother in these images. than those on Earth – in fact bigger than most of
(NASA Magellan RADAR image.) those on Mars. However, the acceleration due
to gravity is only a little less on Venus than on
to show us that gently undulating volcanic plains Earth, so this underlines the suggestion made ear-
occupy about 80% of the surface area while moun- lier that magma reservoir width is not controlled
tainous regions occupy the rest. Most of the plains by gravity.
consist of various kinds of lava flows, more than The mountainous areas on Venus that are not
50 of which are classified as flood lavas with volcanoes generally form linear belts that seem to

Fig. 13.12 The shield volcano Sapas
Mons on Venus stands 1.5 km above
the surroundings plains and lava flows
spread out down its flanks in all
directions for more than 150 km.
(NASA Magellan RADAR image.)