Magellan SAR images show that volcanic features are abundant and widely distributed on Venus. Notable features include widespread, mostly lowland lava plains, extensive flows, lava channels, small shields, cones, domes, intermediate to large shields, and caldera-like structures not associated with shield volcanoes. Other large features such as coronae and arachnoids (considered to be volcanotectonic in origin [Stofan et al., 1992]) are discussed in Chapter 8. Outflow features associated with impact craters are covered in Chapter 7.
Observations from Magellan images supported by Magellan altimetry and radiometry data have provided quantitative information about the morphology (size, shape, and relief), radar backscatter, reflectivity, and emissivity of these volcanic features, as well as their frequency distribution, geologic associations, and general correlations with elevation. Such observations have been used to infer the physical properties of the planetary surface-for example, centimeter scale roughness, decameter-scale slope, lithologic composition, and dielectric properties.
The majority of volcanic materials on Venus (e.g., plains and shields) are thought to be basaltic in composition, partly because of their geomorphic expression. This view is supported to some extent by compositional information returned from Soviet Vega and Venera space probes [Barsukov et al., 1982 and 1986].
Volcanic plains cover large, mostly lowland areas separated by mountains or ridge belts; they extend over 85% of the planetary surface (Figure 9-1). The plains range in elevation from about 1.5 km below to 2 km above the mean planetary radius of 6051.84 km [Guest et al., 1992]. They consist of extensive sheets of flood lavas hundreds of kilometers in width and mostly 100 to 700 km in length, and they show a range of radar backscatter characteristics. Wide sheets of flood lavas with uniform radar responses may represent complexes composed of multiple flow units. In the absence of defining radar characteristics, however, it may not be possible to discriminate individual flow units in the image data.
Lava flows form important radar mapping units in Magellan images. The outlines and large-scale dimensions of lava flows together with such small-scale features as the smoothness or blockiness of the surfaces, the presence or absence of channels, levees, and pressure ridges, and the characteristics of the flow margins, provide important information about the mechanisms of flow emplacement. Preferred flow directions may reveal topographic and/or structural control. The age relations of adjacent flows can be determined where superposition is evident, particularly where older lavas are transected by fractures that do not disrupt younger, overlying lavas.
Variations in image brightness between flows or in different portions of a single flow are commonly thought to denote changes in surface roughness at the scale of the radar wavelength (12.6 cm). Flood lavas may be strongly contrasted, or they may show mottling over large areas. A radar mosaic of plains in the Navka Planitia region (Figure 9-2) shows bright and dark patches that represent....
....overlapping flows. However, the sources of the flood lavas are not generally perceptible. The significance of the brightness changes may relate to such variables as the rheology of the flows at the time of emplacement or to differences in preservation states caused by postdepositional surface processes (see Chapter 6) [Arvidson et al., 1992]. Many flows contain small volcanic shields and cones.
Other physical parameters, such as dielectric properties or the radar viewing geometry, may also affect the radar brightness of flow surfaces. At different incidence angles, the sensitivity of backscatter to variations in roughness and dielectric constant may differ [Plaut and Arvidson, 1992]. Corresponding images of lava flows taken in successive imaging cycles at different incidence angles indicate that some brightness variations are caused by such intrinsic reflectivity differences. A flow field south of Ozza Mons in Atla Regio (Figure 9-3) consists of numerous adjacent and overlapping flows that show varying backscatter strengths. A simple interpretation of this image, which was taken at 43.7 deg incidence (Cycle 1), would relate image brightness directly to wavelength-scale surface roughness. However, the image taken at 24.9 deg incidence (Cycle 2, Figure 9-4) provides different contrast relations. In particular, one of a number of bright flows in the Cycle 1 image appears in the Cycle 2 image as the brightest flow in the field. A plot of backscatter values as a function of incidence angle (Figure 9-5) shows that this bright flow and a more typical flow to the east differ greatly in backscatter at smaller angles, but converge to nearly identical values at larger angles.
The scattering function of the flow to the east is not strongly dependent on incidence angle, a condition consistent with a very rough surface. The bright flow, however, shows a strong dependence on incidence angle, with a scattering function slope similar to that of a plains surface to the south and to the average Venus scattering function itself. The bright flow may have a relatively smooth upper surface that, because of a high dielectric constant, appears bright at both incidence angles. This interpretation is supported by measurements (Figure 9-6) that show the bright flow has lower emissivity, and thus higher dielectric constants, than any of the other flows that appear bright in the Cycle 1 image. The higher dielectric constants of the bright flow may be the result of a greater bulk density, unusual chemical composition, or a combination of both.
Mylitta Fluctus is a complex of six flow fields in the southern hemisphere of Venus [Roberts et al., 1992]. The area is similar in size to the Columbia River flood basalt province on Earth. Each flow field is composed of numerous individual flow episodes identified by continuities in flow boundaries and central channels and by similarities in radar backscatter and surface texture. Many of the flows are long....
....(hundreds of kilometers), comparatively narrow (tens of kilometers), and have uniform radar textures that show no pronounced transverse ridges (Figure 9-7). Combined with measurements of relief, radar backscatter, and dielectric constant, these landform and surface characteristics are thought to indicate a generally basaltic composition [Campbell and Campbell, 1992].
A volcano on the plains between Artemis Chasma and Imdr Regio displays a sheet of thick radar-bright flows and broad flow lobes. The lobes and flows show prominent transverse ridges that have an average spacing of about 750 m. The flow features are associated with a complex comical structure about 100 km across and I km in relief (Figure 9-8). They are surrounded at a lower elevation by plains surfaces that expose radar-bright volcanic deposits [Moore et al., 1992]. These materials extend some 360 to 400 km from the volcano. They appear to overlie the radar-dark, lowland plains that dominate this region of the surface.
This volcano and the surrounding radar-dark plains were imaged from the left at about 27.5 deg incidence (Cycle 1) and 15.6 deg incidence (Cycle 3), and from the right at about 25.3 deg incidence (Cycle 2). Contrasts in backscatter intensities combined with stereo observations of fine-scale topography in the image data provided the basis for discriminating eight radar mapping units (Figure 9-9). Stratigraphic ages determined from contact relations indicate that the plains units are older than the mesa units and that backscatter increases systematically with decrease in relative age of the units.
Local relief of the scarps and lobes in the mesa units was computed from geometric distortions (parallax) of corresponding features observed in the left- and right-side viewing images. Where appropriate, the measurements were compared with calculations based on an assumed symmetry of features parallel to the illumination vector (details of these methods are given in Chapters 4 and 7). The measurements.....
....indicate that in the ridged mesas the scarps have relief up to 205 m; in the lobate mesas, the relief is from 133 to 723 m.
The contrasted distribution, form, and thickness of the sheet flows and lobes with the deposits on the adjacent radar bright plains are thought to have resulted from the differentiation of a primary magma source [Moore et al., 1992]. By analogy with volcanism on Earth, it is postulated that an initial, low-viscosity, mafic eruptive phase could have produced the extensive, thin, bright-plains deposits. A more viscous, silicic secondary phase resulted in the emplacement of the much thicker, more areally restricted sheet flows and broad flow lobes.
Lava channels hundreds to thousands of kilometers in length are conspicuous on the volcanic plains of Venus. Simple channels typically show little or no branching. They include long sinuous forms, termed "canali," and sinuous rifles. However, Venusian channels are not as tightly sinuous as terrestrial rivers. Complex channels show anastomosing or braided patterns. Integrated valleys that may have formed by sapping processes have also been observed in Magellan images [Baker et al., 1992].
Canali are best preserved in regions of subdued relief. They have a high width-to-depth ratio and maintain a remarkably constant width over very long distances. Images reveal the presence of meanders, point bars, cut banks, and abandoned channel segments. Canali that are transverse to the radar illumination may show bright radar response from the far-range banks of some meander segments (Figure 9-10). This may indicate localized steepening of the cross channel slope due to bank cutting.
Both the source and the distal ends of many canali are buried or extensively subdued by lava flows younger than those that formed the channels. Measurements have shown considerable relief in longitudinal channel profiles, implying significant tectonic deformation of the plains since the channels formed [Parker et al., 1992]. Wrinkle ridges and ridge belts commonly transect canal). Vertical displacements of hundreds of meters over horizontal distances of a few kilometers are common at ridge crossings.
Sinuous rifles emanate from roughly circular to elongate depressions and become progressively narrower and more shallow in the downstream direction (Figure 9-11). They are typically 1 to 2 km wide and tens to hundreds of kilometers in length. Channel walls form a distinct boundary between the channel floor and the surrounding terrain. Channel material is similar to that of the surrounding terrain and may be either radar dark or radar bright.
An example of complex flow channels occurs in the proximal region of Mylitta Fluctus (Figure 9-7, area 6). Channels in the medial and distal regions are wider, broadly sinuous, relatively constant in width, and generally radar dark. A radar-dark channel that extends for about 100 km in the medial region of Mylitta Fluctus (Figure 9-7 north of area 4) shows infilling of one segment by a younger lava flow.
A channel over 1200 km long that displays both simple and complex reaches is located in the northern Lada Terra region (Figures 9-12 and 9-13). The channel originates in a collapse structure on the southwest flank of the volcano Ammavaru. The main channel (about 5.5 km wide) flows south-southeast in a linear trough for about 300 km. This channel is too narrow to be resolved in Magellan altimetry data.
Profiles from stereo measurements indicate relief up to 600 m across the collapsed source area [Parker et al., 1992]. Because the linear trough is oriented transverse to the illumination, the distal wall appears very bright and the proximal wall is dark in the images (Figures 9-12 and 9-14).
The southern, topographically lower end of the trough was flooded with lavas that appear radar-dark in the image (Figure 9-12). At 51.5°S, 25.5°E (Figure 9-13), the flood spreads into a broad anastomosing reach (Figure 9-15). East of the highlands at this locality the channel branches into a distributary reach for about 130 km. Three radar-dark distributaries change to radar-bright with dark margins about midway along this reach (Figure 9-12). 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 km. 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.
 Small Volcanoes
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 km are considered small. Small features occur typically on the plains but they are found also on the flanks of large volcanoes and in association with coronae and arachnoids; they consist mostly of small shields and cones. Some domes are included in this size category.
Small shields have outlines that range from circular to elongate. Generally they display very shallow slopes and are not associated with flow deposits. In cases where the slope angle is low (less than about 5 deg), it is not possible to determine the height or slope of the features from single images. Nevertheless, the height of such features can be measured by stereo techniques provided corresponding coverage is available at different incidence angles (see Chapter 4). Many small shields have been discriminated by their smooth circular outlines and image tones that are darker than the surrounding plains. The outlines may also be diffuse. However, in cases where there is no clear backscatter contrast, the shields may be distinguished by the presence of a centrally located, circular summit pit about 1 km or less in diameter. With respect to the radar illumination vector, the distal wall of the pit is bright and the proximal wall is dark.
Clusters of small shields (~ 10 km in diameter) are widely distributed in association with linear fracture belts on the plains, mostly at intermediate elevations (Figure 9-2). Though many shields in a cluster show circular outlines, some are elongate with the long axis parallel to the strike of the associated fractures. In this type of setting, individual flows may be perceptible in association with the small shields. Possible terrestrial analogues include plains volcanism in the Snake River Plain of southern Idaho [Greeley, 1982] and clusters of central vent edifices that have been identified on the Mid-Atlantic Ridge [Smith and Cann, 1990].
Cones are circular features with steep slopes and a centrally located summit pit. Slopes from 12 to 23 deg and heights from 200 to 1700 m have been measured [Guest et al.. 1992]. Because of their steepness, foreslopes appear compressed and bright in images; correspondingly, backslopes are elongated and dark (Figure 9-16). Individual flows are mostly not 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, while other cones or shields in the cluster are superposed on the fractures and thus are younger.
Comparison of image pairs obtained at significantly different incidence angles demonstrates that the discrimination of small, low-relief features on the volcanic plains may be strongly governed by the radar incidence angle. In the Aino Planitia region, a group of small domes with diameters of 6 km or less and relief up to 140 m was imaged from approximately the same illumination direction at incidence angles of 28.5 deg in Cycle I and 15.6 deg in Cycle 2 (Figure 9-17).
At the smaller incidence angle (Cycle-2 image), the radar backscatter from the steep slopes that face toward the illumination is dominantly specular. Small changes in slope give large changes in backscatter. This provides bright radar responses that enhance the perception of the domes in the image. At the larger incidence angle (Cycle-1 image), backscatter is much less sensitive to topographic variations. Thus, the ability to discriminate these comparatively small scale features on the surrounding plains is much reduced.
The perception of small-scale roughness differences on the plains also varies with the incidence angle. At the higher incidence angle (Cycle-1 image), the radar scattering is dominantly diffuse for rough surfaces and dominantly specular (forward scattering) for smooth surfaces. This results in strongly contrasted levels of radar brightness that enable the discrimination of rough (bright) and smooth (dark) surfaces in the images. At the lower incidence angle (Cycle-2 image), the backscatter from the plains surfaces is controlled primarily by topographic variations. As a consequence, the contrasts in radar image brightness that provide surface roughness discrimination in the Cycle-1 image are mostly not perceptible in the Cycle-2 image.
Intermediate volcanoes are defined as centers between 20 to 100 km 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 km to about 100 km with a mean of about 24 km and in height from 70 to 2000 m with a mean of about 700 m....
....above the surrounding terrain. Mostly, they are surrounded by a steep perimeter and have a relatively flat top. Images reveal that these features are remarkably circular in outline. Measurements indicate that the surfaces are slightly rough and have a slightly lower reflectivity and correspondingly higher emissivity than the surrounding terrain. The combination of radial symmetry, a steeply sloping perimeter, and strong relief exhibited by many domes suggests that 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. Numerous domes show evidence of postemplacement alteration by processes that include gravitational collapse, slumping, tectonism, impact, and lava flooding.
Domes occur singly, in pairs, groups, or overlapping clusters. Many domes are associated with coronae (see Chapter 8), but the eruptive mechanism is not clearly understood. They are preferentially concentrated at elevations near or just below the mean planetary radius (~6052 km).
A cluster of four overlapping domes is shown in Figure 9-18. The large dome with a diameter of about 50 km in the upper center shows strongly contrasted radar brightness around the perimeter. Very bright returns in a narrow band around the northwest quadrant denote steep foreslopes in this sector. Dark tones from segments around the northeast quadrant denote weak backscatter from steep backslopes there. The outline of the standing walls shows that the perimeter of the dome has collapsed in this quadrant. Outflow toward the northwest is indicated by the nature of the surface in the collapsed area between the existing periphery of the dome and isolated remnants of its formerly circular perimeter walls.
In the southern half, the perimeter is topographically less pronounced and it intersects the equally less pronounced perimeters of two nearly equidimensional domes to the south. The absence of brightness from the foreslopes of these perimeter segments indicates subdued topography compared to the northwest quadrant of the central dome.
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 (see Chapter 6). In addition, scalloped-margin domes are often surrounded by concentric fractures. The dome in Figure 9-19 shows collapsed margins and associated landslide deposits in both the northwest and the northeast quadrants. Two pronounced slump scarps in the northwest provide very bright linear to curvilinear image responses. The landslide deposits show hummocky surfaces that extend up to 10 km away from the dome. In general, the scale of lava domes and collapse features on Venus is orders of magnitude larger than that on Earth.
Large volcanoes have diameters mostly in the 100- to 600-km 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. They do not occur in the linear concentration that characterizes subduction-related plate tectonic boundaries on Earth.
Sapas Mons is a large volcano approximately 400 km in diameter and 1.5 km high located on a topographic rise in Atla....
 .....Regio. The summit consists of two mesas with flat to slightly convex tops and smooth surfaces that appear radar-dark in the image (Figure 9-20). Surrounding radar-bright collapsed areas provide the mesas with plan view outlines that resemble those of some scalloped-margin domes (see Domes, above). The foreslopes of both mesas appear very bright and strongly compressed.
A discrepancy occurred between the positive topography of the summit mesas observed in the image and the negative topography indicated for the same area in bestfitting model echo templates derived from altimetry measurements (ARCDR data) [Plaut, 1992]. Discrepancies of this type have demonstrated that, under certain circumstances, derived altimetry data may contain spurious values. The details are outlined in Chapter 3.
The sides of the volcano show numerous bright overlapping flows that provide the edifice with a roughly radial outline. Many of the flows appear to be flank eruptions. Radial fractures clearly transect the flows to the east and south. Darker flows in the southeast quadrant are probably smoother than the bright flows closer to the eruptive center.
An impact crater with a diameter of 20 km 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.
Maat Mons is a large elliptical volcano (195 x 125 km) that rises 9.2 km in Atla Regio. A three-dimensional perspective obtained by combining radar altimetry data with the two-dimensional image data (Figure 9-21) shows lava flows that extend north for hundreds of kilometers across fractured plains in the foreground.
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.
Sacajawea Patera is an elliptical caldera (260 x 175 km) that forms a depression about 2 km deep (Figure 9-22). The depression is enclosed by a zone of concentric troughs that show radar-bright outlines extending from 60 to 130 km outward from the caldera floor. The floor is covered with smooth mottled plains. The brightest deposits occur around the periphery and near the center of the caldera floor where there is a ponded leveed flow. Linear to sinuous scarps show bright outlines that extend southeast from the eastern margin of the caldera. A small shield (12 km in diameter) is transected by one of these features.
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