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Volcanism (second part)

Scientists are also intrigued by the distribution of volcanoes around Venus. On Earth, volcanoes are grouped around rifts. Venus, on the other hand, is scattered with hundreds or thousands or more volcanoes, more or less randomly distributed over the planet.

Large volcanic structures are interpreted as being surface signs of mantle ash, suggesting that the Venusian mantle is thermally dynamic. These structures are also widely distributed across the surface, unlike terrestrial volcano structures which are focused along plate borders.  

However, their distribution is not random. There seems to be a large dependence between the altitude and the type of volcanic structure. This dependence could be explained by a combination of atmospheric pressure as a function of altitude and the depth of the magma reservoir.

Roches craquelées photographiées par Venera 14  Cracked rocks photographed by Venera 14 (credit: NASA)

Most of the volcanoes are not very tall but are effusive volcanoes like the Hawaiin volcanoes, or Olympus Mons on Mars. Probes that analysed the surface allowed us to determine the composition of the lavas: viscous basalts similar to rhyolite or alacites on Earth.

So, in the plains, where presssure is higher, the magma reservoir would be less deep, producing very liquid, very hot lava. However, on peaks and plateaus, the lava would be more viscous, coming from deeper reservoirs.

Tectonic structures (rifts, folds etc) characterised by their morphology, altitude and distribution are found all over the planet. Some regions are more affected, like the high regions of Ishtar Terra, in northern latitudes and in Aphrodite Terra and Beta Regio along the equatorial belt.

3D images obtained from Magellan radar data allowed reconstruction of the volcanic landscape of Venus.

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