Volcanic structures
Large volcano
Volcanic edifices more than 100 km across. These are shield volcanoes: the kind we usually picture when we think of a volcano. Venus has about 140–170 of them.
About 10 volcanoes on Venus are higher than 5 km. They measure some 500–700 km across, similar to Olympus Mons on Mars. Olympus Mons, however, is 24 km high. Compared with the volcanoes we know on Earth, these giants are very flat: their slopes are only 1–3° (Stofan et al. 2001). They are made of basalt.
Their lavas spread over areas hundreds of kilometres wide. The reason is the same one that let some volcanoes on Mars grow so large. There is no plate tectonics, so the crust does not move over the mantle upwelling beneath it. Magma therefore breaks through in the same place for a long time. On Venus the surface heat also plays a part. Lava cools more slowly there, so it can flow farther. Other possible causes are the composition of the lava, a higher eruption rate or more active dyke formation.
Like volcanoes on Earth and Mars, the large volcanoes of Venus have calderas in their summit regions. Maat Mons (see plate 1) is 9.6 km high, almost the highest volcano on the planet. It is unusual in many ways. It is probably very young, and it may even be active. Several signs point to this: the absence of a radar anomaly, the calderas near the summit, and pit craters that preserve very fresh, finely detailed forms.
Sapas Mons (see plate 2) is also interesting. It has twin summits and a rather cone-like shape. Yet it is low (2.4 km) and broad (600 km across).
Large volcanoes are not spread at random on Venus. They are more common in the Beta–Atla–Themis (BAT) regions.
Some tall volcanoes: Ongwuti Mons, Rhea Mons, Polik-Mana Mons, Yunya-Mana Mons, Yolkai-Estsan Mons, Ixtab Mons, Theia Mons, Tepev Mons, Nyx Mons.
