Lunar and Planetary Science XXXI 1346.pdf

MORPHOLOGY OF LARGE VOLCANO BUILDING FLOWS ON : EXAMPLES FROM SIF, GULA AND KUNAPIPI MONTES. J. E. Guest1 and E. R. Stofan1, 2, 1Department of Geological Sciences, Uni- versity College London, WC1E 6BT, UK ([email protected]), 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.

Large volcanoes with basal diameters greater than These flows are much more common on Gula Mons 100 km and heights normally between 2 and 3 km are than on the other two volcanoes. Flank eruption sites a characteristic form of . They are are less common at Gula Mons than at Sif or KP Mon- built of lava flow fields of a variety of forms and tex- tes. As at Sif, flank eruptions originate at pits, frac- tures. We have divided the flow fields at Sif, Gula and tures, and from small edifices. However, more flows at Kunapipi (KP) Montes into three general types based Gula appear to originate from the summit, unlike the primarily on planform shape in radar images: major fan flows seen at Sif. digitate, fan and sheet flow fields. Each of these types The main cone of KP is made up of shorter more has at least one sub-type. Most of the flows fields are numerous flows, compared with the outer apron which compound, consisting of more than one flow. Some consists of fewer more extensive flows. The far major- flows appear to be simple flows, but where flows of ity of flows at KP (82%) are sheet flows, with the re- similar radar backscatter properties are adjacent they are maining 18% of flows having digitate morphology. difficult to distinguish and may be confused as a sim- Most of the sheet flows have intermediate radar back- ple flow. scatter, and are complex. The complex flows are com- Sif is surrounded by about 47% by area of sheet pound flows, where flow fronts can be identified, but flows, with weakly defined interior flow fronts provid- individual flows are difficult to trace. ing evidence of broad overlapping flows. Sheet flows The three broad categories of flows (digitate, fan occur both on the steeper flanks of the cone and on the and sheet flows) are thus found in differing proportions outer apron of flows. Hummocky sheet flows to the at each volcano. KP Mons, dominated by sheet flows, southwest of the caldera on the steepest portions of the has the lowest slopes and largest volume of the three cone tend to have more mottled, irregular textures, volcanoes. Gula Mons, which has the most digitate similar to the radar signature of terraced pahoehoe flows flows, has the steepest slopes. , with its mix- at Mt. Etna, Sicily. Most of the rest of the flows at Sif ture of flow types, is intermediate between the other (45%) are digitate, where the aspect ratio (ratio of the two volcanoes. Fan flows are rare at KP and Gula length to width of the flow as seen in planimetric form) Montes, and most common on the eastern flank of Sif are high. The distal margins of some flows are topog- Mons. There is more diversity between types of flow raphically controlled by preexisting structures, for ex- units at Sif, in terms of radar backscatter and morphol- ample, flows captured by graben. An example of a ra- ogy. And, Sif and KP Montes have relatively circular dar-dark channel can be seen in one of the digitate shapes in planform, while Gula Mons is elliptical. flows, indicating that at least some of the digitate flows The overall morphology of flows is likely to be are channel-fed. The channel has a relatively constant governed by effusion rate, vent geometry, and eruption 3 km width and can be traced for about 60 km, termi- duration (which in turn is a function, in part, of the nating downslope in a fan of radar-bright flow materi- stress field), rather than by composition [1]. All three als. types of flows at the venusian volcanoes have enor- The majority of flows at Gula (65% by area) have mous volumes; examples of each type of flow can be digitate morphologies. Twenty percent of the flows are found that extend for over 100 km. While these flows sheet flows, with the remaining 15% fan flows. Some are likely to be compound flows, they are still likely to sheet-like flows at the summit change downslope into indicate very high volume eruptions, much larger than high aspect ratio digitate flows with dendritic flow typical terrestrial eruptions and more comparable to fronts; the change in morphology typically occurs near Mars. the break in slope of the main edifice. Some of the Flows with unusual texture, termed hummocky sheet flows have been captured by the linear structures flows, are found on all three of the venusian volcanoes. of Guor Linea. Mosaic sheet and digitate flows are These flows have a mottled appearance with arcuate seen on all three volcanoes, including Gula Mons. bright crescents pointing in a down-slope direction, These flows have radar-bright to intermediate polygo- similar to the radar signature of pahoehoe flows of nally-fractured surfaces surrounded by relatively darker 1614-1624 on Mt. Etna, Sicily. This terrestrial flow material. field consists of mega-tumuli and terraces with rough At Gula, one of the most typical types of digitate surfaces of tipped slabs [2]. The flow apparently de- flows have relatively radar-dark to intermediate, veloped by self-damming of tube-fed flows. The smoother centers and brighter, rougher margins. These hummocky flows of Venus do tend to be located on the bright-edged flows tend to be more common on the steeper portions of the volcano, and may indicate that southern flank of the volcano, and on its lower slopes. similar processes to those of the unusual Etnean flow Lunar and Planetary Science XXXI 1346.pdf

VENUS FLOW MORPHOLOGY: J.E. Guest and E.R. Stofan

fields also occurred on Venus. We can combine the overall morphology of each volcano with its flow types to make some generaliza- tions about each volcano’s evolution. The steeper flanks of Gula, its greater height and the dominance of digitate flows are consistent with lower volume or shorter duration eruptions, relative to the other two volcanoes. Its steeper flanks are also consistent with the fact that most of the eruptions appear to originate from the summit rather than from flank vents. At Sif Mons, more flank eruptions have occurred with varying morphologies, probably indicating a range of eruption durations, effusion rates and vent geometries. KP Mons is volumetrically larger, has lower flank slopes and more sheet flows, indicating higher volume, probably longer duration eruptions.

References: [1] Hughes J. W. et al. (1990), In Magma Transport and Storage, Ed Ryan M P, Wiley, 385 - 406. [2] Guest, J. E. et al. (1984) Bull. Vol- canol, 47, 635 - 648.