Sif Mons

False-color computer-generated view of Sif Mons, with height exaggerated
Feature type	Shield volcano
Location	Eistla Regio, Venus
Coordinates	22°00′N 7°36′W / 22.0°N 7.6°W / 22.0; -7.6[1]
Diameter	210 km (main structure) 1 200 km (lava flow apron)[2]: 77
Peak	2 km (base-to-peak)[2]: 77
Eponym	Sif

Sif Mons is a large shield volcano located in western Eistla Regio, a highland region on Venus. It is composed of a main volcanic structure with a 40-kilometre (25 mi) wide summit caldera surrounded by an outer apron of lava flows 1,200 kilometres (750 mi) in diameter. It rises about 2 kilometres (1.2 mi) from its base. It is one of a few extraterrestrial volcanoes where signs of ongoing volcanic activity have been observed, with candidate eruptions in 1990 and 1992. It is named after the Norse goddess Sif; the name Sif Mons was officially adopted by the International Astronomical Union (IAU) in 1982.^[1]

Geography and morphology

Sif Mons lies in western Eistla Regio,^[2]: 77 a roughly 2,500-by-1,500-kilometre (1,550 mi × 930 mi) highland in Venus's northern hemisphere that rises 0.5–1.0 kilometre (0.31–0.62 mi) high.^[3]: 16036 Sif Mons lies 750 kilometres (470 mi) to the west of the larger Gula Mons,^[4]: 177 and together the two mountains dominate the center of western Eistla Regio.^[5]: 1353

Sif Mons is a broad shield volcano^[6]: 975 rising 2 kilometres (1.2 mi) from its base. It is composed of a central main structure 210 kilometres (130 mi) in diameter, surrounded by an outer apron of lava flows extending up to 600 kilometres (370 mi) from the center. The main structure has slopes of 1–3°, which drop to 0.15–0.25° in the outer flow apron; the main structure has a volume of about 1,600 cubic kilometres (380 cu mi).^[2]: 77 Multiple figures have been given for the total diameter of the mountain, from several hundreds of kilometres^{[4]: 177 [7]: 13397} to a thousand kilometres.^[5]: 1353 Its 40-kilometre (25 mi) diameter summit caldera is nearly circular, with a relatively flat and smooth floor. Near its center, a linear trench extends radially in a northwest–southeast direction. The caldera's southern rim is well-defined, but its lower northern rim is less distinct and appears as a series of lineaments, partly covered by caldera lava flows. The northern rim lies 200 metres (660 ft) below the southern rim, and the caldera itself is no more than several hundred metres deep.^{[7]: 13404 [2]: 77} Nested groups of smaller calderas overlap the main caldera's northern and northeastern rims. The smaller calderas range from 1.5 to 6.5 kilometres (0.93 to 4.04 mi) in size, with the northeastern group partially flooded by lava from the main caldera. Chains of smaller pits 0.4–1.5 kilometres (0.25–0.93 mi) in size extend away from the southern main caldera rim to the southeast. Over 50 such pits have been identified on Sif Mons's summit.^{[7]: 13404–05}

Flank eruption sites are common around Sif Mons, which form small secondary structures on its slopes. Many of these flank structures are surrounded by flow deposits of their own; one northern cone's lava flow extends over 200 kilometres (120 mi) to the north into Sedna Planitia.^{[2]: 77 [5]: 1354} A system of fractures oriented radially from its center occupy most of its flanks; they are less prominent than the ones on Gula Mons. The fractures are best developed on Sif Mons's northwest flank, while being absent from the northeast.^[3]: 16040 These fractures cross older lava flows, and some younger lava flows have been channelized through the fractures.^{[7]: 13402, 13404} To the east, lava flows from Sif Mons and Gula Mons meet. At this point, flows from Sif Mons divert to the northwest by the elevated Idem-Kuva and Nissaba coronae.^[7]: 13406

Geology

Western Eistla Regio is a dome-shaped highland dominated by Sif and Gula Montes and their associated volcanic fields and plains.^[8]: 13496 The highland also hosts several smaller volcanic structures and two coronae on its northern flank.^[9]: 23319 According to the crater counting method, the age of the highland is similar to the 0.5–1.0 billion year age calcuated for Venus's surface as a whole.^[7]: 13410

Western Eistla Regio is associated with regional extensional tectonics,^[4]: 197 which may represent an extension of a tectonic zone involving Aphrodite Terra, Atla Regio, and Beta Regio.^[7]: 13410 Faulting in western Eistla Regio predates and postdates volcanic activity.^[9]: 23319 Gravity anomaly and topography data suggest the presence of a mantle plume below.^[9]: 23325 Using the principle of cross-cutting relationships and embayment, volcanic activity in the western Eistla Regio area first formed the radar-dark volcanic plains.^[4]: 198 The region is then uplifted, after which Sif and Gula Montes formed.^[4]: 198

References