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Home , Ascraeus Mons, Beta Regio, Dali Chasma, Devana Chasma, Gula Mons, List of montes on Venus

Icarus 277 (2016) 433–441

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Icarus

journal homepage: www.elsevier.com/locate/icarus

Geomorphology and of Mons,

∗ Peter J. Mouginis-Mark

Hawaii Institute and Planetology, University of , Honolulu, HI 96822, USA

a r t i c l e i n f o a b s t r a c t

Article history: Full-resolution (FMIDR) radar backscatter images have been used to characterize the Received 23 December 2015 and volcanology of the on Venus. This volcano has often been identified by remote

Revised 8 April 2016 sensing techniques as one of the volcanoes on the that could have been recently active, and is the Accepted 13 May 2016 highest volcano on Venus with a relief of ∼9 km. The of Maat Mons is characterized by a Available online 4 June 2016 complex ∼26 ×30 km in diameter with at least six remnant pit craters ∼10 km in diameter preserved in

Keywords: the walls of the caldera, suggesting that multiple small volume ( < 16 km 3) collapse events formed the Venus surface caldera. Four flow types, described as “digitate flows”, “sheet flows”, “fan flows” and “filamentary flows”, can be identified on the flanks. Three rift zones can be identified from the distribution of 217 pit Geological processes craters > 1 km in diameter on the flanks. These pits appear to have formed by collapse with no effusive activity associated with their formation. No evidence for explosive volcanism can be identified, despite the (relatively) low atmospheric pressure ( ∼55 bar) near the summit. There is also a lack of evidence for lava channels, deformation features within the caldera, and thrust faults on the flanks, indicating that the physical volcanology of Maat Mons is simpler than that of typical and terrestrial volcanoes. Preservation of fine-scale (3–4 pixels) structures within the pit craters and summit pits is consistent with geologically recent activity, but no evidence for current activity can be identified. © 2016 Elsevier Inc. All rights reserved.

1. Introduction Detailed geomorphic mapping of the summit area and flanks of the volcano extending up to ∼100–120 km from the summit Over the past 30 years, several investigations have hinted that ( Fig. 2 ) has been conducted here to better characterize the styles Venus is volcanically active today, but none have been defini- of volcanism at Maat Mons. Despite the importance of Maat Mons tive. Episodic injection of dioxide into the atmosphere for investigating recent , the available data > ( Esposito et al., 1988 ), high radar emissivity at elevations 2.5 km sets for such analysis are scarce, even by comparison with else- above the 6051 km mean planetary radius ( Robinson and Wood, where on the planet. Only left-looking Magellan synthetic aperture 1993 ), visible and infrared emissivity measurements of the surface radar (SAR) data are available for the entire volcano (right-look ( Smrekar et al., 2010 ), and enhanced microwave thermal emission data are missing for the summit), and no stereo-derived topogra- ( Bondarenko et al., 2010 ) have all been proposed as indicators of phy ( Lerberl et al., 1992; Gleason et al., 2010 ) is available. FMIDR recent eruptions. Maat Mons (194 °E, 1 °N) ( Fig. 1 ) is possibly the Magellan radar backscatter images were used for this study; at best candidate for a recently active volcano on Venus, by virtue the latitude of Maat Mons, these data have a spatial resolution of of the spatial variability of radar emissivity values at the summit 108 m (cross track) and 110 m (along track) prior to projecting the ( Klose et al., 1992; Robinson and Wood, 1993; Campbell, 1994 ), data. Thus the intrinsic preserved resolution of the radar images > near-infrared spectra ( Shalygin et al., 2012 ), and high ( 8 km) to- is probably no better than ∼150 m x 150 m. Topographic data come pographic relief which suggests that the volcano is still being from the Magellan -looking radar altimeter that has mapped constructed. In addition, Magellan gravity data show that the Atla the surface at a horizontal resolution of 10–30 km ( Ford and Pet- ∼ Regio region ( 10°S to 25°N, 180° to 215°E), where Maat Mons is tengill, 1992 ), so this has resulted in poor knowledge of the sum- located, is one of the areas on Venus that could be situated over mit caldera geometry and the detailed shape of the upper slopes an active hot spot and thus is consistent with the hypothesis that of the volcano ( Fig. 3 ). Furthermore, Maat Mons is located on the Maat Mons could be active today ( Smrekar, 1994; Shalygin et al., Equator at 194 °E, so that the volcano is never visible from - 2012 ). based radar ( Campbell and Campbell, 1992 ) thereby precluding any multi-incidence angle radar studies of the texture of lava flows. ∗ Tel: + 18089566490. Magellan SAR data have an incidence angle of ∼45 ° over Maat E-mail address: [email protected] , [email protected] Mons. http://dx.doi.org/10.1016/j.icarus.2016.05.022 0019-1035/© 2016 Elsevier Inc. All rights reserved. 434 P.J. Mouginis-Mark / Icarus 277 (2016) 433–441

Fig. 1. Location map for Maat Mons. Study area of Maat Mons ( Fig. 2 ) and the NW lava flow field ( Fig. 8 ) are identified. “V1” and “V2” denote the and landing sites. Mosaic covers an area from 30 °N to 45 °S and 58 ° to 215 °E. Part of JPL image PIA00256.

Fig. 2. Magellan SAR mosaic of Maat Mons. Superposed contours from the Magellan radar altimeter are at 500 m intervals. White boxes show the locations of subsequent figures. Geographic area extends from 0.3 °S–2.2 °N, 193.4 °E–196.2 °E. Topographic data from Ford and Pettengill (1992) . Magellan image mg_0 024/f0 0n194. See Fig. 1 for location.

2. New mapping of the summit craters. Collectively, the mapping permits insights into several characteristics of the volcano, including: This study of Maat Mons has included an analysis of the distri- bution of pit craters on the flanks, the spatial distribution of lava (a) An investigation of the role of elevation on the degassing flow fields (radar-bright and radar-dark flows), and the morphology of on Venus. In particular, a search for evidence P.J. Mouginis-Mark / Icarus 277 (2016) 433–441 435

Fig. 4. Summit caldera of Maat Mons. There are numerous collapse features (iden- tified in the insert at top right) within the broad collapse feature that forms the Fig. 3. Pair of oblique views of Maat Mons derived from Magellan radar images ∼26 ×30 km diameter caldera. See Fig. 2 for location. Radar look-direction is to- and altimetry. The summit elevation of the volcano is 8860 m, and the lowland to wards the right. Magellan images mg_0 024/f0 0n194/ff21 and ff22. the NW is at ∼−100 m, so there is ∼9 km difference in elevation. These two views are from slightly different angles viewed from the north-east, but the flank profiles ( Senske et al., 1992; Stofan et al., 2001 ). Magellan radar backscat- and geometry of the summit appear to be quite different due to the low spatial resolution of the Magellan topographic data. Numbers identify the same features in ter and altimetry indicate a narrow rim to the west that served to each image. Top view is JPL image PIA00254 (vertical exaggeration is 22.5 ×) and contain the flows on the caldera floor, and this is consistent with lower view is JPL image PIA00106 (vertical exaggeration is 10 ×). the impression that flows from within the caldera have spilled out to the east and the north sides of the caldera. On Earth, it is recognized that are about the same size of at the lower-pressure high elevations, as the chambers beneath them ( Walker, 1988; Parfitt and which would constraints on the volatile content of the Wilson, 2008 ), and many large calderas may have evolved incre- magmas. mentally in response to a of moderate-sized eruptions (b) Consideration of the probable size and spatial migration of ( Walker, 1984 ). Collapse features within a caldera are inferred to the within the edifice of Maat Mons, based have formed by collapse due to evacuation of a shallow magma upon the distribution of collapse craters within the sum- chamber ( Walker, 1988 ), so that the spatial distribution of the six mit caldera. If there is evidence for different collapse crater smaller pits around the perimeter of the Maat Mons caldera there- sizes, then this might provide information on the size of the fore suggests that the magma chamber within the edifice could magma chamber over time. have migrated with time, comparable to the collapse features iden- (c) An interpretation of the distribution of pit craters and frac- tified within the calderas of Olympus and Ascraeus Montes on tures on the flanks. The spacing and orientation of pit ( Mouginis-Mark, 1981; Mouginis-Mark and Rowland, 2001 ). craters may be used as potential indicators of rift zones Head and Wilson (1992) predicted that large volcanic edifices on within the volcano, placing constraints on the internal Venus should have both a deep and shallow magma reservoir. The “plumbing system”. Specifically, the widths, lengths, and size and distribution of intra-caldera pits may therefore allow in- depths of dikes are important for the identification of the in- ferences to be made on the geometry of the shallow magma cham- ternal structure of the volcano ( Head and Wilson, 1992 ). The ber. Working on the assumption that all of the collapse events at occurrence of graben and faults would provide indicators of the summit of Maat Mons involved collapse events similar in size the tectonic regime under which the volcano grew. to the observed pits, it would have taken ∼20 episodes of col- (d) A comparison of the spatial distribution of radar-bright lava lapse (with none of these events overlapping an earlier collapse) flows with radar-dark lava flows with patterns common on to produce the observed larger caldera. With only seven discrete terrestrial shield volcanoes (e.g., Rowland, 1996 ). pits visible today, this would imply that about two thirds of these hypothesized pits have subsequently been buried by intra-caldera 3. Caldera and shield morphology lava flows. With only one exception (the broken central pit on the southern portion of the caldera floor), there is no clear indication The summit of Maat Mons ( Fig. 4 ) has a caldera complex of the sequence by which the preserved pits within the summit ∼26 ×30 km in diameter. At least six remnant pit craters (the caldera formed. largest of which is ∼10 km in diameter) are preserved in the walls Relatively radar-dark units are common on the floor of the of the caldera, as well as one complete pit in the middle of the caldera, although there is at least one radar-bright flow ∼12 km caldera. Only landforms with a clear radar-bright scarp are identi- in length which erupted from the single complete pit in the mid- fied here as collapse pits, although several other quasi-circular dark dle of the caldera floor and flowed to the east. This is in contrast features of similar size occur within the caldera and could there- to the caldera of Sapas Mons and on Venus ( Senske fore be remnants of earlier pits. The summit caldera of Maat Mons et al., 1992 ), where no intra-caldera flows can be identified. is therefore similar in size and structure to the summit of Mons Relatively radar-bright flows are common on much of the caldera 436 P.J. Mouginis-Mark / Icarus 277 (2016) 433–441

Fig. 5. (a) A pair of flank shields north of the summit area of Maat Mons (see Fig. 2 for location). Magellan images mg_0 024/f0 0n194/ff05 and ff13. (b) Interpretative sketch of these satellite shields, including a partially buried structure on the northern flank of the southern shield. Arrows indicate flow paths of prominent lava flows that moved around these topographic highs, but as is obvious in left image, there are many other flows with a less certain relationship with the shields. (c) Details of the northern shield (see “a” for location). White arrows identify pits on these shields that may be due to the collapse of a shallow magma source. (d) Details of the southern shield (see “a” for location). Black arrows point to structural features. Radar look-direction is towards the right in all images. rim of Maat Mons, suggesting that at one stage the caldera was ways to large, deep, magma bodies most likely existed. In contrast, full and that flows spilled over the caldera rim, analogous to pre- small volume flows may have been sourced from a shallow, small- historic activity at Mauna Loa, Hawaii ( Lockwood and Lipman, volume, magma chamber. Four lava flow types can be identified 1987 ), but in the case Maat Mons erupted flows with high radar on Maat Mons, three of which are based upon the earlier subdivi- backscatter (Mauna Loa produced copious amounts of pahoehoe sion of flows on Sif and Gula Montes by Stofan et al. (2001) . The when the caldera was full). flows have a range of lengths and widths, but in general they are The summit is not the only center of activity at Maat Mons. < 100 km in length and < 25 km in width. Maat Mons flow types Robinson and Wood (1993) identified two small shields on the ( Fig. 6 ) are here described as “digitate flows”, “sheet flows”, “fan northern flank of Maat Mons ( Fig. 5 ). These shields are ∼32 km and flows” and “filamentary flows”. Stofan et al. (2001) described digi- ∼110 km from the rim of the caldera and are, respectively, tate flows as “significantly longer than they are wide, with vents ∼38 km and ∼24 km in diameter. The upper shield lies at an eleva- sometimes definable”, and fan flows as “distinct fan-shaped [in] tion ∼300 m below the summit, while the lower shield is a further appearance in planform, individual flows within fans often have 3 km lower down the northern flank. There is also a break of slope digitate appearance and the vent is definable”. In this analysis, north of the upper shield which indicates that the main shield is sheet flows are defined to be relatively dark in Magellan radar constructed upon a partially buried earlier cone. Both these shields backscatter images, are extensive and rarely have identifiable vent have kilometer-scale summit craters and several structural features regions. Filamentary flows are lobate, radar-bright, have identifi- (scarps and lineaments) which may mark the boundaries of larger able vents and are narrower than they are long. A further two sur- collapse structures. Individual radar-bright flows can be seen on faces are identified here, which are believed to be the consequence the of these shields and both shields have their lower of topographic effects: surfaces that are very radar-bright because flanks embayed by more recent flows from Maat Mons, indicating they are on steep slopes facing the radar, and a radar-dark sur- that they do pre-date the last activity further up-slope. on slopes facing away from the radar. Thus these topography- influenced flows are not inherently different flow types. Fig. 7 il-

4. Distribution of flows on the flanks lustrates the spatial distribution of the flows. Essentially each type of flow is found on all flanks of the volcano, with the exception

The diversity of lava flow types on any basaltic volcano can that the filamentary flows only occur on the northern and eastern provide useful insights into the subsurface structure and spa- flanks. Fan flows are absent within the summit caldera. tial distribution in magma production rates (e.g., Rowland, 1996 ). It is not possible to uniquely identify the type of lava flow (a‘a

Where high-volume flows are common, efficient subsurface path- or pahoehoe) for each of the flows identified. As demonstrated P.J. Mouginis-Mark / Icarus 277 (2016) 433–441 437

exists in rheology akin to the pahoehoe to a‘a transition described for terrestrial by Peterson and Tilling (1980) . Not included in this analysis are the very long lava flows which extend beyond the map area. There are three major flow fields to the north and west of the volcano that extend 425–500 km from the summit caldera. These flows are all radar-bright, have lobate outlines and have filamentary outlines ( Fig. 8 ). No vents on the lower flanks can be confidently correlated with these flows, but they are most likely associated with magma sources that are not located on the main edifice of Maat Mons. Head and Wilson (1992) predicted that the depth to the neutral buoyancy zone (where the magma chamber was located) should be at a shallower depth than on Earth. Greater volcano height should favor the existence of a larger shallow magma chamber, which is predicted to produce larger volume lava flows ( Head and Wilson, 1992 ). Although there are insufficient data to confidently deter- mine the thickness of individual lava flows on Maat Mons, there are no bright, radar-facing, edges to any of the flows at an image resolution of ∼150 m, which would indicate a flow thickness less than ten to twenty meters. Thus the volume of each lava flow on Maat Mons appears to be relatively small (of the order of ∼25 km 3 ) if one assumes an average thickness of 10 m ( Keddie and Head, 1994 ). The volume for individual lava flows on Maat Mons there- fore appears to be comparable to the probable volume of each of the collapse pits within the summits caldera ( Fig. 4 ); assuming that the long-axis of a summit pit is ∼10 km, and width and height ∼5 km, the collapse volume would be ∼16 km 3 and suggests that individual flank eruptions may have been responsible for the for- mation of a discrete collapse pit within the caldera.

5. Flank pits and inferences on the existence of rift zones Fig. 6. Four different types of lava flow field can be identified on Maat Mons, which are the same types of flows identified by Stofan et al. (2001) . Arrows indicate in- On Earth, pit craters are typically elliptical in plan form, with ferred flow directions, and “∗”marks the probable vents. Radar look-direction is overhanging, steep, or talus-covered walls ( Macdonald et al., 1990; to the right in all images. At left are the Magellan FMIDR images and at right are Okubo and Martel, 1998 ). Some of the best known pit craters in interpretive sketches showing the specific example of the flow type. (a) “Digitate

flows”, with radar-dark material separating individual flow units ∼1 km in width. Hawaii occur along the East and Southwest Rift Zones of Kilauea Roberts et al. (1992) described these flows as “mottled flows”, which were inter- volcano, and are believed to have formed through stoping above preted to be the result of overlapping flows of shorter length produced during the large subsurface rift zone fractures ( Okubo and Martel, 1998 ). - waning stages of an eruption. Magellan image mg_0 024/f0 0n194/ff12. (b) Exten- merous kilometer-sized pits can be found on the flanks of Maat sive, relatively radar-dark “sheet flows” with a few flow boundaries faintly recog- Mons, with the largest pits showing a distinct radial orientation nizable and no vent obvious. Magellan sub-scene mg_0 024/f0 0n194/ff20. (c) Nar- row (1.5–4.0 km), very radar-bright flows with multiple “filamentary” or “braided” with respect to the summit (Fig. 9). A total of 217 pits larger than lobes at the margins. Clear indication of vents is often possible due to narrow width 1 km in diameter have been identified on the flanks and extend of flows. Magellan image mg_0 024/f0 0n194/ff15. (d) Massive, fan-like radar-bright out to a radial distance of ∼120 km from the rim of the caldera

“fan flows” that appears to have originated from a single up-slope vent with the ( Fig. 10 ). The largest of these pits is 4.6 ×7.7 km in diameter and, flow spreading rapidly to both sides of the downslope direction. Dashed line shows approximate boundary. Magellan images mg_0 024/f0 0n194/ff30 and ff31. where they are elongated, all pits have their maximum dimension in the down-slope direction. Because of the large size of these pits, by Campbell and Campbell (1992) , the designation of radar-bright it is believed that they are not skylights into partially collapsed and radar-dark flows does not necessarily have the same implica- lava tubes, but rather represent the surface trace of a dike. These tion for lava flow texture (e.g., a‘a or pahoehoe) or effusion rate pits are interpreted to lie along the strike of rift zones and denote ( Rowland and Walker, 1990 ) as is the case on Earth; the backscat- the shallow (top few kilometers) structure of the volcano. ter characteristics of almost all lava flows on Venus are most sim- Several of the pits larger than 2 km wide show signs of incre- ilar to terrestrial pahoehoe flows ( Campbell and Campbell, 1992 ). mental collapse ( Fig. 9 ). Benches within the walls of the pits can Campbell and Campbell (1992) suggested three possible explana- be seen and some pits truncate earlier examples. No positive to- tions for this: (1) many flows may have been emplaced as low- pographic relief can be identified around the rims of any of these effusion rate (presumably tube-fed) pahoehoe flows; (2) the flows pit craters at Magellan radar resolution, nor does it appear that were originally emplaced as a‘a but have since been weathered to any lava flows were erupted from the pits. These two observations a smoother surface texture; and (3) a combination of atmospheric support the idea that none of the pits were eruptive centers. A few and magma compositional effects combined to inhibit a‘a forma- of the larger pits appear to be aggregates of several over-lapping tion even at high volume eruptions rates. However, Bruno et al. smaller pits, so that reactivation and growth due to successive dike (1992) and Bruno and Taylor (1995) investigated the fractal dimen- intrusions is possible. Magellan radar data lack the spatial resolu- sions of flows on Venus, and found that the edges of radar-bright tion to determine if all of the lava flows surrounding the pits pre- flows are consistent with terrestrial a‘a lava flows. Some of the date these collapse events, or whether there are flows that spilled radar-bright flows on the flanks of Maat Mons are most likely a‘a into the pit and partially infilled the depression, thus the timing flows, implying higher effusion rates ( Rowland and Walker, 1990 ). of pit formation relative to the youngest flows emplaced in this However, there are no clear examples of flows transitioning from area cannot be confidently determined. Despite the occurrence of radar-dark to radar-bright flows, so that no along-flow transition the numerous pits, there are no fault scarps or graben that inter- 438 P.J. Mouginis-Mark / Icarus 277 (2016) 433–441

Fig. 7. Distribution of the four lava flow types identified in this analysis ( Fig. 6 ). Tonal variations due to steep radar-facing and radar-away-facing slopes are also indicated. Dashed lines denote approximate flow boundaries. Geographic area is the same as that shown in Fig. 2. connect the chains of pits. Indeed, there is a distinct lack of struc- tural features on the flanks of Maat Mons at Magellan resolution. The existence of rift zones due to dike intrusion is predicted to be easiest in the uppermost (youngest) layers of large shields on Venus ( McGovern and Solomon, 1998 ), and rift zones may pro- vide an indication of the most recent phase of activity at Maat Mons. The azimuthal distribution of pits ( Fig. 11 ) allows three po- tential rift zones to be identified on the SE, SW and W flanks of Maat Mons, and 41, 37 and 26 individual pits can be identified in these rift zones, respectively. The SE rift is ∼20 km wide, the W is 16 km wide, and the SW 13.5 km rift is wide. Hints of two rift zones also exist at greater radial distances from the summit on the NE (65 km) and S (70 km) sides of the volcano. There is an “exclu- sion zone” on both the eastern and northwestern flanks of Maat Mons where no pit craters can be identified. Only the western rift is non-radial to the summit caldera. Comparison of the pit distri- bution with the apparent flow direction of the lava flows (Fig. 7) Fig. 8. Lava flow field to the NW of Maat Mons. Here the radar-bright flows are shows that this “rift zone” may not follow the greatest topographic much narrower than those near the summit, but extend to ∼460 km from the gradient, as numerous lava flows cross the strike of the collapsed caldera rim. These flows have outlines similar to terrestrial a‘a flows ( Bruno et al., pits. However, it is possible that the mapped extent of the rift zone 1992; Bruno and Taylor, 1995 ). Direction of flow is towards the top left of the im- may be over-interpreted and may in fact be a combination of sev- age. Part of Magellan image mg_0025/f10n188. Radar look-direction is towards the right. See Fig. 1 for location. eral shorter alignments of pit craters. The average slopes of Maat Mons vary from ∼1.7 ° on the SW flank to ∼2.7 ° on the NW flank. This is in contrast to slopes of dates for geologically recent (perhaps even present-day) eruptions 3 °–5 ° on Mauna Loa and 3 ° for Kilauea volcano ( Rowland and Gar- on Venus. Thus the volcano structure, distribution of lava flows, beil, 20 0 0 ). It is apparent that the SW rift zone is associated with pit craters, and the morphology of the summit caldera may well the most shallow slopes on the flanks of the volcano. In contrast, provide one of the clearest views of the styles of constructional where chains of pits are absent, such as the NW flanks, the topo- volcanism on the planet. Certain small-scale features, such as the graphic slope is steeper. collapse structures within the summit caldera ( Fig. 4 ) and the nu- merous pit craters on the flanks ( Fig. 9 ), reveal fine structures 6. Conclusions and synthesis of volcanism which support the idea of recent activity on the volcano. It is clear that Maat Mons has experienced multiple eruptions at the summit, From various lines of evidence ( Esposito et al., 1988; - including at least seven small ( ∼16 km 3 ) episodes of col- son and Wood, 1993; Bondarenko et al., 2010; Smrekar et al., lapse. These collapse pits are potentially representative of the pro- 2010; Shalygin et al., 2012 ) Maat Mons is one of the best candi- cess which produced the large structure that now forms the sum- P.J. Mouginis-Mark / Icarus 277 (2016) 433–441 439

Fig. 9. Clusters of collapse pits to the SW and SE of the summit. The largest craters in “a” are 1.7 ×5.7 km and 4.6 ×7.7 km in size; in “b” 2.2 ×4.6 km; and in “c” 2.7 ×2.5 km and 2.6 ×4.6 km in size. These pits could well represent the surface expression of dikes, which in turn could define rift zones within the volcano. Note the fine-scale structure in some of the larger pits, which suggests that these features are quite young. Arrows point towards the summit caldera. See Fig. 2 for locations. Radar look-direction in each image is towards the right. Magellan FMIDR sub-scenes mg_0 024/f0 0n194/ff21, ff22, ff28, ff29.

Fig. 11. Azimuthal distribution of pits on the flanks of Maat Mons as a function of distance from the rim of the summit caldera. Plot excludes the pits at associated with the two satellite shields north of the volcano ( Fig. 5 ). Shaded areas correspond to the potential rift zones identified in Fig. 10.

Fig. 10. Distribution of all pit craters larger than 1 km in diameter within the Montes ( Stofan et al., 1989; Basilevsky and Head, 2007 ) and Gula mapped area. Shaded areas demarcate the inferred rift zones. Note that the western Mons ( Stofan et al., 2001 ), show clear signs of rifting associated rift is not radial to the summit caldera. Geographic area is the same as that shown with regional . In the case of Rhea Mons, this rifting was in Figs. 2 and 7. hypothesized to be due to the rise of a hot mantle diaper in- terpreted to be caused by a mantle plume ( Basilevsky and Head, 2007 ). The fact that the distribution of pit craters on Maat Mons mit caldera, suggesting that there is a shallow and a deep magma ( Fig. 9 ) does not mirror the regional tectonic pattern suggests that chamber ( Head and Wilson, 1992 ). There is good evidence, in the regional rifting has not been important during the evolution of the form of aligned pit craters, for rift zones on the southern flanks volcano. Maat Mons may therefore be sufficiently young that it has of the volcano, but the distribution of vents (as indicated by the been constructed on top of the now-stable regional tectonic fab- mapped distribution of flows) is not dominated by these rifts. At ric, and that the magma chamber is structurally-isolated from the least four different types of lava flows can be identified on the ba- mantle magma source ( Head and Wilson, 1992 ). sis of the shape of the flow units and their radar backscatter char- One of the most intriguing aspects of volcanism on Maat Mons acteristics ( Fig. 7 ). Neither local slope nor elevation on the volcano is the great range in elevation of the volcano and, thus, the vari- appear to control the spatial distribution of flow types. ation in the ambient atmospheric pressure. With a summit ele- In the broader regional context, Maat Mons is located at what vation of ∼8860 m, Maat Mons is the highest volcano on Venus amounts to be a triple junction of major rift zones ( Shalygin et al., as well as the volcano with the greatest amount of total re- 2012 ), with regional rift zones extending north beyond Ozza Mons lief. Thus atmospheric pressure will vary from ∼105 bar on the along Ghanis , southwest to , and east to Parga lower flanks to ∼55 bar at the summit. One might therefore ex- Chasma ( Fig. 1 ). Other volcanoes on Venus, such as Theia and Rhea pect to see pressure-dependent differences in the physical prop- 440 P.J. Mouginis-Mark / Icarus 277 (2016) 433–441 erties of volcanic deposits (i.e., vesicular lava flows and, poten- pyroclastic activity did not occur at any elevation on the tially, deposits) between the summit and lower flanks of flanks of Maat Mons. Magma volatile contents were most the volcano. Head and Wilson (1986) predicted that for magma likely low during the final stage of cone-building of the vesiculation to occur to the point of explosive disruption of the volcano. melt, the total dissolved volatile contend must exceed ∼1.5 wt% (c) Deformation features on the caldera floor, comparable to the at high elevations, and ∼3.5 wt% in the Venus lowlands. For the formation of ridges or graben within the mafic magmas expected to dominate on shield volcanoes on Venus caldera on Mars ( Zuber and Mouginis-Mark, 1992 ), cannot ( Hess and Head, 1990 ), it is probably adequate to assume that be identified. This would suggest that there has been no magmas disintegrate into a mixture of free gas and entrained pyro- large-volume evacuation of the shallow magma chamber at clasts at some specific volume fraction ( ∼0.7 to 0.8) of gas bubbles Maat Mons, so that the caldera floor remained essentially ( Papale, 1999 ). For explosive eruptions with large erupted mass flat and horizontal during the interval of time preserved fluxes (i.e., those leading to high convecting eruption plumes), the at the summit. This observation lends support to the idea models of Fagents and Wilson (1995) and Glaze (1999) can be used that the caldera was formed by multiple small-scale collapse to estimate likely plume rise heights and dispersal areas. Non- events rather than a single large eruption. buoyant plumes would collapse to form pyroclastic flows, one ex- (d) There are no convincing examples of young radar-dark (pa- ample of which has been possibly identified elsewhere on Venus hoehoe?) flows on Maat Mons. The dark flows that do exist at Scathach Fluctus (16 °S, 145 °E) ( Ghail and Wilson, 2015 ). In both are most likely stratigraphically older than the bright flows, the buoyant and non-buoyant cases, it is likely that any geologi- and could well have low radar brightness due to . cally recent explosive eruptions on Maat Mons would have resulted This is different from the flanks of ( Stofan et al., in widespread areas with uniform radar backscatter properties. The 2001 ) and the Mylitta Fluctus flow field ( Roberts et al., 1992 ) absence of evidence for explosive volcanism on Maat Mons, de- where there are several radar-dark, stratigraphically young, spite the lower atmospheric pressure compared to Scathach Fluc- flows. Higher effusion rates at the summit of Maat Mons tus (at an elevation of ∼1.5 km to 2.0 above datum), has implica- compared to these other localities is a possible explanation tions for the variation in volatile content of the parent magmas. for this observation. Ghail and Wilson (2015) contend that the magma for the Scathach (e) There is an absence of thrust faults on the flanks symmetric Fluctus explosive eruption must have come directly from the man- to the summit. Thrust faults have been attributed to struc- tle, which does not seem to be the case for magmas on Maat Mons. tural loading of the flanks of Ascraeus Mons ( Bryne et al., The lower magma volatile content at Maat Mons could be due to 2012 ) and Olympus Mons ( McGovern and Morgan, 2015 ) but the degassing of a shallow magma chamber prior to the eruption, Maat Mons does not appear to have experienced this type of or to regional differences in the volatile contents of the two pri- deformation. Thrust faults on martian volcanoes may be the mary melts. result of weak basal layers, so that the absence of such fea- Robinson et al. (1995) have proposed that Maat Mons may have tures on Maat Mons could indicate that competent material experienced a recent phase of explosive plinian-style volcanism, (i.e., lava flows instead of ash) comprise the entire edifice. and suggested that such eruptions could be responsible for un- usual spikes in atmospheric observed by the Pio- Acknowledgments neer Venus spacecraft ( Esposito et al., 1988 ). They identified a unit on the eastern flanks of Maat Mons (as well as a second on the This research was supported by the Hawaii Institute of Geo- northern flank beyond the area of this investigation) which could physics and Planetology, University of Hawaii at Manoa. I thank be a possible ash flow, suggesting pyroclastic deposits. Robinson Robert Herrick and an anonymous reviewer for their reviews that et al. 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