Trying to understand the geology of Venus is a very difficult task, but has been facilitated by a series of radar missions that are able to peer through the planet's atmosphere with radio waves. From the surface maps made in this unusual way, we are able to discern surface features that seem similar to our terrestrial volcanoes. Some of these volcanoes are higher than the highest mountains on Earth. Numerous smaller volcanoes seem to be similar to terrestrial shield volcanoes. There are also unique round features with steep sides and flat tops called "pancake domes" that probably form when volcanoes erupt with very thick, viscous lava, which oozes out slowly in all directions.
Composite radar map of Venus from Magellan. Click here for original source URL.
Are all these volcanoes active? Some pieces of evidence, including rapid changes in amounts of sulfur gases in Venus' atmosphere, indicate that the volcanoes probably have been active in recent geological time. In fact, they may have produced fresh lava and sulfurous gases as recently as the last few decades. This should not be surprising. The principles of comparative planetology say that similar size and mass planets should share broadly similar attributes. As the near twin of the Earth, Venus should be able to retain similar gases in its atmosphere, and its mass should drive tectonic activity and volcanism.
Several "pancake dome" volcanoes on the surface of Venus imaged by the Magellan spacecraft. Click here for original source URL.
Chemical measurements of rocks on Venus, carried out by a series of Soviet landers, suggest that most of the rocks are made of basalt. This makes them similar in composition to the rocks that make up the sea-floor crust of the Earth. Basalt is produced by volcanic activity — molten lava erupted onto the surface of Venus and solidified to form these rocks, just as on Earth. Pancake domes may have formed from lava with a slightly different composition, making it more viscous.
The most detailed radar mapping of Venus, by the American Magellan probe from 1990 to 1992, revealed only a modest number of impact craters. The number of impact craters gives us clues to the age of the surface. There are somewhat more craters on Venus than on Earth, but Venus has no ancient, heavily cratered areas, like the highlands of the Moon or Mercury. Instead, eighty percent of the surface of Venus is covered in basaltic lava flows that are all the same age. These findings indicate that Venus' surface is geologically young. Estimates range from 500 to 800 million years old, older than most surface regions of the Earth, but still relatively young compared to the other terrestrial planets.
Some event flooded most of the planet with lava about 500 to 800 million years ago, erasing older craters and effectively “resurfacing” the planet. Without plate tectonics, heat may build up in Venus's mantle because it can't escape at plate boundaries. Some researchers believe that this buildup of heat led to convective "super plumes" of hot magma. These rose in Venus' mantle, leading to simultaneous, massive eruptions in many spots on the surface. An alternate theory involves a large impact by an interplanetary body, like those that caused several mass extinction in Earth's history.
Since Venus and the Earth are about the same size, and they originally accreted from the same type of material, it’s reasonable to assume that Venus and Earth will have about the same amount of radioactive elements in their interiors. So we would expect Venus to have about the same level of geologic activity, driven by its internal heat. This may be true in the long term; however, it appears that Venus's volcanic activity is episodic — occurring in short bursts, separated by longer quiet periods — as opposed to the more steady eruptions through the Earths history.
Although Venus's surface features are geologically young, the planet as a whole, like the Earth and Moon, is 4.6 billion years old. Venus may have undergone many episodes of volcanic “re-paving” in the past. No one knows why such a dramatic resurfacing occurred at one particular time, and no one knows when it may happen again. Mysteries like this are part of what make science an adventure.