The oldest terrain on Venus appears to be about 800 million years old. Extensive volcanism at that time took out the earlier surface including any large craters from early in Venus' history. Venus shows no evidence for plate tectonics. There are no long, straight volcano chains. Also, there are no clear subduction zones.0 Although rifts are common, none look like the mid-ocean ridges on Earth. Continent-like regions are also rare, and show none of the jigsaw fits common on Earth. Thus, where volcanism on Earth mostly marks plate boundaries and plate movements, volcanism on Venus is much more regional and much less systematic. Over 1600 major volcanoes or volcanic features are known on Venus, and this is more than on any other planet. The volcanoes come in a variety of forms. Most of them are either Large Shields or Smaller Shield volcanoes, but there are also many Complex Features, several Unusual Constructs, and a few Large Flow Features.
The surface of Venus is covered by about 20 percent lowland plains, 70 percent rolling uplands, and 10 percent highlands. The surface has been shaped by volcanism, impacts, and reshaping of the crust. No clear evidence of currently active volcanoes has been found, although large variations of sulfur dioxide in the atmosphere lead some scientists to assume that volcanoes may be active. Although no rainfall, oceans or strong winds exist to erode surfaces, some weathering and erosion does happen. Radar images of the surface show wind streaks and sand dunes. In addition, the corrosive atmosphere is likely to chemically alter rocks. Impact cratering is also affected by the dense atmosphere.
Venus has no water erosion and little wind erosion. Instead, volcanic eruptions are a major force reshaping the landscape. Differences in the types of erupted magma and the eruption rate lead to a wide variety of surface features.
|
Smooth plains formed by fluid lava cover most of Venus. A dense pattern of fractures covers the plains in this image. Narrow ridges and fractures such as these form when the crust is pulled and pushed by geologic forces.
Width of image area: 198 km (123 mi.) |
|
|
|
Volcanoes of all sizes are found on Venus, from thousands of small domes that dot the plains to large mountains. This volcano, Sapas Mons, is 4 kilometers (2.5 miles) tall and surrounded by lava flows. Two steep volcanic domes occur at the summit. Both show evidence of massive landslides along their flanks. The image color is computer generated.
Width of image area: 617 km (383 mi.) |
|
Large, steep-sided volcanic domes occur in clusters on Venus. They are often covered with complex fracture patterns.
Width of image area: 86 km (53 mi.) |
|
|
In some areas, volcanic activity persisted for a long time or produced such great volumes of magma that fields of lava flows were produced. This one, Mylitta Fluctus, is one of the largest, extending northward nearly 1,000 kilometers (600 miles) from source vents along the lower part of the image.
Width of image area: 581 km (361 mi.) |
|
All Magellan images courtesy NASA
Other Surface Features
Geologic forces at work beneath the crust create mountains, rifts, and patterns of fractures, while the sluggish winds sculpt the surface in subtler ways.
|
Alpha Regio is an example of highly deformed crust called tessera. Such areas may be the oldest surfaces on Venus. They were produced by strong geologic forces that folded the crust and created interwoven patterns of ridges and valleys.
Width of image area: 2,164 km (1,345 mi.) |
|
|
The Maxwell Montes, the bright area in this image, rise steeply from the smooth plateau to the west to heights of almost 11 kilometers (7 miles). The large, dark circular feature is the impact crater Cleopatra. High elevations typically appear very bright in radar images of Venus. This may be caused by a thin surface coating of metallic material that only forms above a certain altitude.
Width of image area: 1,265 km (786 mi.) |
|
Winding across Venus are narrow belts where the surface has been pulled apart by geologic forces. Here such rifting has torn the crater Somerville in two. Usually less than a few hundred kilometers wide, rift zones contain dozens of nearly parallel ridges and valleys. Volcanoes often occur along these areas of weakened crust.
Width of image area: 178 km (111 mi.) |
|
|
The tight pattern of bright and dark ripples in the center of this image is an area where loose material was sculpted by the gentle surface winds into dunes. The bright streaks of material curve away from small hills, revealing which way the winds were blowing. Stronger winds caused by meteorite impacts may also help create such features.
Width of image area: 172 km (107 mi.) |
|
Unlike Earth, Venus shows no evidence of plate tectonics (the movement of large pieces of crust), a process that helps release interior heat. One way Venus releases heat is by the formation of a large number of features called coronae, circular patterns of fractures thought to form when hot material beneath the crust pushes up, warping the surface. Coronae are often accompanied by vast lava flows.
Width of image area: 528 km (328 mi.) |
|
All Magellan images courtesy NASA
|
|
