National Geographic : 1993 Feb
Overall, plate tectonics on Earth is a dramatically horizon tal process, creating whole ocean floors and moving conti nents. On Venus the movement is thought to be largely vertical. Volcanoes and highland regions arise over mantle upwellings, while sinking regions may form mountain belts due to thicken ing and compression of the crust. The eastern slope of Freyja Montes (left) in Ishtar Terra has undergone uplift and compression but is now fractur ing as portions of the crust col lapse under their own weight. The lack of surface water and strong winds means that Venus's features are largely preserved between periods of volcanic or tectonic activity. That's good news for scien tists who study mountains here on Earth. Erosion from rain, wind, and glaciers, as well as breakdown from the freeze thaw cycle, have long since removed the outer layers of our oldest mountain chains, leaving only their cores. Thanks to Magellan's images we can get some idea of how Earth's great mountain ranges would appear in the absence of erosion.