Page 227 - Fundamentals of Physical Volcanology

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204 CHAPTER 13

Fig. 13.13 Bahet Corona, in the
Fortuna region of Venus, is about 230
by 150 km in size. Numerous tectonic
and volcanic features are visible,
suggesting that coronae and related
structures form above mantle plumes.
(NASA Magellan RADAR image.)

be produced by compression, although in many ated above mantle hot spots. In this regard there
places linear sets of fractures are seen implying that are more than 450 features on Venus that have
tensional forces have also been at work. Embedded no exact equivalent on Earth (Fig. 13.13). These

in these upland areas are terrains described as are the coronae (latin for crowns), arachnoids
tesserae, from the latin for tiles. These are local- (because of their alleged resemblance to spiders
ized regions of very intense deformation and frac- in their webs!) and novae (the analogy is to an
turing, and may represent the oldest rocks visible exploding star). These features range from less than
on the surface of the planet. 100 to as much as 2000 km in size and may repre-
Despite the abundance of volcanic features, sent different stages in the evolution of a single kind
nowhere on Venus are there clear analogs of the of structure located over a mantle plume that pene-
Earth’s spreading ridges and subduction zones. The trates to relatively shallow levels. Alternately they
best interpretation is that the surface of Venus may represent different ways in which the crust
has been fractured into a series of plates, but that responds to the deformation caused by such a
the plates simply jostle against one another rather plume. All of these features are circular to oval in
than being systematically enlarged from spreading shape, and have a central plateau or dome sur-
ridges or destroyed by subduction. This process rounded by a depressed moat that has annular
therefore does very little to help the interior of (and quite often also radial) fractures associated
Venus to lose heat. Instead, heat must be lost by with it. Lava flows are commonly found in the
conduction through the lithosphere and by erup- moat or associated with the fractures, and where
tions of magma at volcanic centers, presumably loc- the fractures radiate out for many hundreds of