7. Venus: The veiled planet
Tectonics on Venus
The hot rising material has buckled, fractured and stretched the crust on Venus, like a crumpled piece of paper or a face seamed and thickened by age. It has split the crust open and spread it apart, forming rift valleys with steep sides and sunken floors. Some of them are found in Alta Regio alongside its volcanoes. The linear rift zones in the equatorial highlands can extend for millions of meters, but are cracked apart by just a few thousand meters. In contrast, rifts that split open the Earth’s continents can widen up to make way for its biggest oceans.
When a bubble of molten rock, or magma, rises to just below the surface, it presses against the crust, causing the ground to bulge and crack. Circular and radial fractures are created around the edges of the rising dome, forming a network of radar-bright features that resemble a spider. Some of them have therefore been nicknamed arachnoids, from the Greek and modern Latin words for ""spider"". The term coronae, the Latin word for ""crown"" is used for the larger, elevated, circular structures that are also pushed up from below by rising molten rock trying to get out. Both arachnoids and coronae are unique to Venus and have not been found on any other planet.
Coronae have concentric ridges and fractures that are hundreds of thousands of meters across, and large volcanic outpourings have occurred within them. When enough lava spills out into a corona, the upwelling subsides and it is no longer supported from below. The bulge will deflate and buckle the surrounding terrain, producing an annulus of ridges and troughs that often surrounds coronae, like the moat around a castle. Or else, the magma cools and retreats as it ages and the molten rock drains back down the vent from whence it came. Then the dome will collapse like a giant fallen soufflé, creating ring-like fractures and a crumpled, cracked surface.
The increasing pressure of the upwelling magma can stretch the planet's skin until it bursts, like the broken cheese bubbles in a pizza or a split in an overcooked hotdog. Small volcanic domes, known as pancakes, are sometimes formed when pasty, sluggish lava breaks through and flows along the surface like toothpaste. In other places the crust breaks and spreads open and lava flows into the gap like olive oil.
As the surface moves up in some locations and down in others, the associated stresses pull the surface apart or push it together. Over time, these stresses can be created in different directions, producing a regularly spaced, gridded pattern of fractured terrain that is only found on Venus at this scale. Some of the cracked patterns of the tessarae have regular six-sided shapes that can be attributed to global heating and cooling of the surface. Repeated episodes of surface deformation in some highlands have additionally created a chaotic network of ridges, troughs and depressions with linear and curved structures.
On Venus, the dominant movement is often vertical, or up and down. Upwelling material pushes against the ground, creating arachnoids, coronae and tessarae, and punctures the surface to form volcanoes. Volcanic rises are held up by the hot rising material, and long mountain ranges may have been built during sinking, downward compression. Moreover, vast regions of the planet consist of flat, lowland plains with no substantial motion, either vertically or horizontally.
Although astronomers know virtually nothing about the first 4 billion years on Venus, they have been able to piece together a sequence of events during the last 500 million years. At the beginning of the record, they see complex and extensive surface deformation giving rise to an intensely fractured crust over nearly the entire globe. Widespread lava flooding created the flat lowland plains soon after this episode of tessera formation. After this brief but intense period of global volcanic floods, the style and rate of volcanism changed. Localized volcanoes grew on top of the vast plains and coronae were formed within them, but primarily in the equatorial regions where extensive rifts are also found.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University