Venus has volcanoes. Lots of them. More than any other planet (or moon) in the solar system. But we never hear about them. Why?
Several reasons, actually. For one, Venus is less hospitable than places like Mars and the Moon. And, being human, we like to study places that people might actually go to someday. People will never go to Venus. We can send robots, like the Magellan. But robots are expensive, and we want our money to go to places that we, you got it, find interesting and might go to someday. But we do know some things about Venus from robots that have gone there already.
The Magellan Spacecraft radar mapped the surface of Venus extensively, revealing details that are invisible form the Earth. Venus has surface features unlike any found elsewhere, including dome volcanoes referred to as euphemistically as “pancakes”, “coronae”, “arachnoids” and “ticks”. Surface tests performed by the Venera 13 spacecraft determined that much of the ground is covered by basalt flows. Nine varieties of Venerean dome volcanoes have been identified. Of these nine, four are characterized by circular bases; the five remaining “modified” types show varied morphological differences from the first four. Presumably, basalt lavas behave differently than they do on Earth under the temperatures and pressures of Venus’ atmosphere. The temperature planet wide (including the night side) is 875° F, making its surface hotter than that of Mercury, whose daytime side only reaches 800°F.
Air pressure on Venus is something else, also! The pressure at sea level is around 1000 millibars (1 bar) on Earth. Mars’ atmospheric pressure is about 1mb (1000 times less than ours), while the atmospheric pressure of Venus is 95,000 mb! That pressure, at the surface, is 1360 lbs per square inch, compared to the paltry 14 pounds per square inch we experience here on earth.
Gas content of the lavas affects how explosive the eruptions are. A small dome with relatively little gas (or one whose gases are held in solution under the pressure) may form a low lying, flat dome. As the layers of lava in one of these domes build upon one another over time, the sub-surface magma has to travel farther vertically to reach the top. Changes in pressure cause the dissolved gasses to come out of solution and bubble; much like the CO2 in a bottle of soda once the 2000-mb (2 atmosphere) pressure has been released. These bubbles, in turn, start to expand the higher they go, causing that lava to have less density than the cooled lava upon which it rests. These “bubbly” lavas form high central peaks on many Venerean domes. Explosive pyroclasts are found at the tops of otherwise non-explosive volcanoes because of this decompression of the contained gases The gaseous lavas froth as they gain altitude, inducing greater pressures within, and when the pressure of the volatile contents exceed the elastic capacity of either the magma chamber or the vents leading to the surface, and explosive eruption occurs.
If the gasses start to expand within the lava and never reach the surface, dimpling occurs on the top of the volcano as the lava degases. If the supporting magma chamber collapses from below, the volcano then becomes a corona. These pancake volcanoes can be described as “Ground-hugging pyroclastic flows…”, and their abundance is a result of “… the most likely product of explosive volcanoes on Venus” (Bulmer and Guest 1996).
Age estimates of the surface infer Venus is an estimated 300- 500 million years old, assuming a planet-wide cataclysmic event may have resurfaced the planet at that time. However, atmospheric tests by the Venera 14 Lander measured anomalous gas readings, concluding volcanic eruptions are ongoing, and new surface is being created. Perhaps Venus, like Io, replaces all or part of its surface over time.
Coronal domes, however, tend to form in clusters; often a more recent dome will form over an older one, making a double or triple corona. Sachs Patera is such a volcano; the remnants of former coronae are visible along the edge of it. Multiple eruptions and the flexing and swelling of the magma chamber below have formed concentric rings and folds around the base of the volcano. These domes are more mafic than the domes of the “tick” type volcanoes, whose sub-Venerian lavas force lateral spreading of the volcano’s flanks. As new flows overtake older flows that have begun to harden, internal pressure forces the sides of the volcano to steepen. Mass wasting-landslides- occur more readily, leaving steep-sided escarpments.
Not to say that volcanoes on Earth aren’t fearsome things. But we have only a few volcanoes here: a few hundred, at most, and only 25 or 30 of them are considered “active”.
The Venerean surface is covered with them. Thousands. We have no idea how many are currently active because the gases escaping the volcanoes are similar temperature and chemical composition to Venus’ nasty atmosphere.
Which is sad, in a way. Volcanoes can be a threat to life on Earth. We have a handy, nearby Earth-like (in many ways) planet with plenty of volcanoes we could study to learn more about them. But the planet that there are on is inhospitable to humans landing there. Even the robots we send there don’t last more than a few hours before being melted, crushed, and dissolved by the Venerean weather.
Too bad Martian volcanoes are all extinct.
Oh, yeah. Mars has them, too!
Picture is interplanetary pancakes: Venus style!