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Timestamp: 2019-04-23 08:50:50+00:00

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Volcanic features are numerous and widely scattered on Venus. These features include widespread lava plains, extensive flows, lava channels, small shields, cones, domes, intermediate to large shields, and caldera-like structures not associated with shield volcanoes. Scientists suspect that some volcanoes are still active; however, clouds from volcanic erruptions are invisible to radar. In order to detect volcanic activity, scientists must compare images taken at different times and look for changes on the planet's surface.
The majority of volcanic materials on Venus are thought to be basaltic in composition, partly because of their appearence. This view is supported to some extent by data returned from the Soviet Vega and Venera space probes [Barsukov et al., 1982 and 1986].
Volcanic plains typically cover large lowland areas separated by mountains or ridge belts; they extend over 85% of the planetary surface. The plains range in elevation from about 1.5 kilometers (.9 miles) below to 2 kilometers (1.2 miles) above the mean planetary radius of 6,051 kilometers (3,761 miles) [Guest et al., 1992]. They consist of extensive sheets of flood lavas hundreds of kilometers in width and mostly 100 to 700 kilometers (62 to 430 miles) in length. Some plain regions are bright in radar images, but most are dark, indicating a smooth surface.
Lava flows may extend from a few to hundreds of kilometers in length. They can originate from volcanoes, cracks in the crust and depressions in the surface. Often, the sources of the flood lavas are unseen.
Lava flows can be characterized by smooth surfaces and the presence of channels, levees, pressure ridges, flow margins, and flow directions. These characteristics provide important information about lava flow formation and the underlying topography. Many flows contain small volcanic shields and cones.
The southern, topographically lower end of the trough was flooded with lavas that appear radar-dark in the image. At 51.5° S, 25.5° E, the flood spreads into a broad anastomosing reach. East of the highlands at this locality the channel branches into a distributary reach for about 130 kilometers (80 miles). Three radar-dark distributaries change to radar-bright with dark margins about midway along this reach. Bright flow deposits with lobate morphology are extensively distributed here. The deposits are ponded on the west side of a north-trending ridge belt for over 300 kilometers (186 miles). The main distributary channel extends through these deposits and terminates eastward at an extensive radar-bright plain east of a breach in the ridge belt. The radar-bright deposits from the outflow channel cover an area of about 100,000 km2; they show broad lobate margins typical of lava flows.
Volcanic constructs and edifices on Venus have been classified and subdivided on the basis of their size and morphology [Slyuta and Kreslavsky, 1990; Head et al., 1992]. Centers with a diameter less than 20 kilometers (12 miles) are considered small. They occur typically on the plains, but are also found on the flanks of large volcanoes and in association with coronae and arachnoids. They consist of small shields, cones, and some domes.
6mall shields have circular to elongated outlines. In general, they have very shallow slopes and are not associated with flow deposits. Many small shields have been identified by their smooth circular outlines and image tones that are darker than the surrounding plains. The outlines may also be diffuse and may be distinguished by the presence of a centrally located, circular summit pit about 1 kilometer (.6 miles) or less in diameter. Clusters of small shields approximately 10 kilometers (6 miles) in diameter are widely distributed, often in association with linear fracture belts on the plains.
Cones are circular features with steep slopes and a centrally located summit pit. They range in height from 200 to 1,700 meters (660 to 5,600 feet) with slopes from 12 to 23 degrees. Individual flows are not usually visible. Cones tend to occur in clusters on the plains. A temporal relation between cones or shields in a cluster and fractures on the plains is evident in cases where some of the cones or shields are cut by fractures and therefore are older. Other cones or shields in the cluster are superposed on the fractures and thus are younger.
Intermediate volcanoes are defined as centers between 20 to 100 kilometers (12 to 62 miles) in diameter. Typically they consist of relatively symmetrical shields characterized by radial lava flows and fracture patterns. Domes are prominent features in this size class.
The majority of Venusian domes range in diameter from less than 10 kilometers (6.2 miles) to about 100 kilometers (62 miles) with a mean of about 24 kilometers (15 miles). Their height range from 70 to 2000 meters (230 to 6,600 feet) with a mean of about 700 meters (2,300 feet) above the surrounding terrain. They are usually surrounded by a steep perimeter and have a relatively flat top. Images reveal that these features are remarkably circular in outline. The surfaces are slightly rough and have a slightly lower reflectivity and correspondingly higher emissivity than the surrounding terrain. They may have formed from viscous lava that erupted uniformly from a central vent [Pavri et al., 1992].
Small craters are a common feature of the surfaces of all domes; they may or may not be central. Breakouts occur on the flanks of some domes and radial fractures extend down the slopes into the surrounding plains. Many domes show evidence of gravitational collapse, slumping, tectonism, impact, and lava flooding.
Domes occur singly, in pairs, groups, or overlapping clusters. Many are associated with coronae, but the eruptive mechanism is not clearly understood. The domes are concentrated at elevations near or just below the mean planetary radius of 6052 kilometers.
Some volcanic domes have steep scalloped margins. The outline of the scallops and the presence of debris aprons in places around the margins suggests that the scallops were formed by slope failure. In addition, scalloped-margin domes are often surrounded by concentric fractures.
Large volcanoes have diameters mostly in the 100 to 600 kilometer (62 to 370 mile) range. Such edifices are characterized by a dominance of radial lava flows in association with positive topography. They occur mostly at higher elevations in broad rises and at tectonic junctions.
An impact crater with a diameter of 20 kilometers (12 miles) located in the northeast quadrant is partially buried by lava flows. A medium-to-light gray flow appears to be ponded to the west by the crater. This flow has been diverted south and east where it has buried a portion of the hummocky ejecta on the southeast side of the crater.
Calderas on Venus have been defined as circular to elongate depressions not associated with well-defined edifices. Characteristically they show the concentric patterns of surrounding fractures [Head et al., 1992]. They may lie in a broad region of elevated topography. They are distinct from impact craters in lacking a hummocky raised rim and an associated ejecta pattern.
This article was abstracted from Ford , chapter 9 by John P. Ford, Jeffrey J. Plaut, and Timothy J. Parker.
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