Despite similar size and composition, Venus and Earth also differ greatly in their geology. Since Venus is covered by clouds, geological features like mountains and volcanoes were hidden from our view up to the time of the Magellan mission. The Magellan probe was equipped with radar, operating at a frequency at which the clouds remain transparent. The Magellan spacecraft put itself in a polar orbit around the planet, that is, an orbit flying over Venus' poles and along its meridians. Little by little, the Magellan probe constructed a detailed map of Venus' valleys, rifts, and mountains. Much of our knowledge about Venus' formation and past comes from radar mapping.

Venus does not display the widespread erosion phenomena typical of the Earth, where the main agent of erosion is water. Venus lost its water long ago. Most of the planet (65%) is a made of a series of volcanic plains, but Magellan also found high plains called 'crustal plateaus', volcanic formations close to continents in size. Crustal plateaus are like gigantic mesas, round in shape, flat at the top, and bounded by very steep rims. Magellan also found quite a few volcanoes divided into nine divisions or 'volcanic rises.'

These volcanic rises are dome-like structures 1000-2400 km in diameter. Their features indicate that many are still active or were active recently. Some of the solidified lava flows are as long as rivers.

Unlike on Earth, large but relatively shallow dome-like structures called 'coronae' are also observed on Venus; hundreds of miles wide, these coronae appear to be places in which magma from the interior of the planet never made it to the surface. They display characteristic concentric cracks.

Venus' volcanoes are like what we call 'shield' volcanoes on Earth. As the name indicates, volcanoes of this kind (those found on Hawaii, for example) look like shields when seen from space. On Earth, and probably on Venus, lava from these volcanoes comes from very deep in the mantle of the planet. The corridors of molten material fueling the volcanoes remain in the same position for a long time and are called 'hot spots.' On Earth the position of hot spots relative to the crust changes due to the slow motion of the tectonic plates. This gives rise to chains of volcanoes. Venus has no plate tectonics and therefore such a process is absent.

To the amazement of many scientists, Magellan also observed sharp mountains, rims, ridges and other geological features so steep on Venus that they would be unstable if they were made from the same rock as similar formations on Earth (even if erosion were absent). A series of laboratory tests on volcanic rocks made it clear that the rocks of the same type are much stronger on Venus because they do not contain even the minute quantities of water that Earth's rock always contain. The stiffly rising terrains do not crumble because of the enormous strength of the rocks that compose them.

Perhaps the most important finding of the Magellan mission is that all places on the surface of Venus seem to have approximately the same age--about 600-800 million years. This became clear when Magellan made a map of the craters on Venus' surface (which resulted from the impact of little asteroids). Craters like these erode away rapidly, and can be destroyed by plate tectonics.

On Venus, just as on the Moon, impact craters were preserved intact for many millions of years. One would expect that the meteoric impact would happen randomly around the planet and that younger regions should have fewer craters. Also, given the immense density of Venus' atmosphere, asteroids smaller than about a mile would burn up in the atmosphere before reaching the ground, but the bigger ones would make it to the surface. The big surprise is that all of Venus' planet surface seems to have more or less the same number of meteoric craters, suggesting that all features such as its plains and continents have the same age. Not only that: it turns out that, unlike on Mercury or the Moon, on Venus the total number of craters is consistent with an age of 'only' 600-800 million years for the crust, suggesting that a catastrophic event completely reshaped the planet's surface in a short interval of time.

Many geologists assume that, since Venus and the Earth probably have a very similar composition, they also have similar amounts of radioactive elements in their interior. Radioactivity keeps Earth hot inside, and such heat finds a way out of the planet through volcanoes and plate tectonics. Some speculate that, since Venus does not have plate tectonics, and sporadic volcanism is not sufficient in getting the heat out, Venus is like a gigantic pressure cooker, getting periodically hotter and hotter inside until the whole crust, or large portions of it, melt in oceans of lava. Scientists disagree on this point, since other theories point to a periodic rise in volcanic activity, on a planetary scale, rather than a catastrophic melting of its surface. The fact remains that the record of meteoric craters that we see today seem to be all from a time after these events took place.

Enormous quantities of lava, covering all of the planet in a relatively short time--probably less then 100 million years--rejuvenated the surface of Venus. The higher plateaus and especially the volcanic rises, which seem to be related to 'hot spot' volcanism, might have formed slightly after the low volcanic plains, at a time when the planet's crust was thicker. This hypothesis is still controversial.

Other theories even link the geology of Venus' crust directly to its atmosphere, claiming that the two strongly influence each other. Other scientists speculate that the lack of water oceans on Venus may be connected to the planet's lack of plate tectonics.