Late-Stage Summit Activity of Martian Volcanoes. P. J. Mouginis-Mark, Depart- Ment of Geological Sciences, Brown University, Providence, R
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Late-Stage Summit Activity of Martian Volcanoes. P. J. Mouginis-Mark, Depart- ment of Geological Sciences, Brown University, Providence, R. I. 0291 2. The formation of the large Tharsis and Elysium volcanoes apparently dm- inated the most recent periods of volcanism on Mars (1,2). A1 though many such constructs are superficially similar to intra-plate shield volcanoes on Earth (notably Kilauea and Mauna Loa, Hawaii; refs. 3,4), volcanoes on Mars illus- trate a wide diversity in their probable eruptive styles (5-7) and final morphometry (8,9). An appreciation of the diversity of martian volcanoes, and the spatial relationships of flank activity to the summit craters, consequently becomes an integral component of the analysis of surface geology and, indi - rectly, 1i thospheric evolution. In particular, the identification of explo- sive vs. effusive activity holds important ramifications for magma evolution (6,10), volcanoe morphology (11,12), and the areal distribution of volcanic materials (13). The acqui sition of high- (15-30 m/pixel ) and medium-resol ution (50-1 50 m/pixel) Viking orbiter images of the summits of Olympus Mons (Fig. I), Ascraeus Mons (Figs. 2,3), Hecates Tholus (Fig. 4) and Elysium Mons (Fig. 5) has permitted the eruption characteristics of these volcanoes to be identi- fied. Table 1 summarizes the salient features of the sumit areas, from which the following differences in activity are evident between these volca- noes and their terrestri a1 counterparts: 1) A1 though Olympus Mons, Mons and Hecates Tholus a1 1 possess nested caldera, the size of successive craters decreases for Olympus Mons, increases for Ascraeus Mons, and remains almost constant for Hecates Tholus. Unlike terrestrial volcanoes such as Teide, Tenerife (14) and Piton da la Fournaise, Reunion Island (1 5), in no instance is a subsequent cone constructed within a martian caldera. Because caldera collapse is believed to be associated with the reduction of magma chamber pressure (usually due to an eruption)(l6), the size variations of the martian caldera indicate that the long-term lava eruption rates on Mars were highly variable and non-cyclic in character. 2) Unlike terrestrial shield volcanoes like Kilauea (3), Piton de la Fournaise (15) and Pinzon, Galapagos (17), the floors of the Martian volca- noes 1ack individual recognizable lava flows. Typically, each caldera floor is relatively smooth with superposed wrinkle ridges (Fig. 1) that suggest that the last eruption created a lava lake over the entire crater floor. Lava terraces in Ascraeus Mons, possibly produced by the withdrawal of magma from a lava lake, support this inter~retation. Fig.1: Mosaic of the emit area of Olympus Mons, showing the nested caldera and the complex system of ridges on the relatively old portions of the caldera floor. Note that subsequent collapse e isodes of the wall material failed to exploit pre-existing fracture systems fn the older caldera floor. Part of cPLmosaic 211-5930. Scale bar is 10 km. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Martian Volcanoes: P. J .Mouginis-Mark 3) Lateral activity (near-summit flank eruptions; ref, 18) was highly variable on the four volcanoes investigated. Ascraeus Mons possesses 1 ava flows and channels that extend completely to the caldera rim and have been dissected by subsequent wall collapse (Fig. 2). Olympus Mons has no recog- nizable flows closer than $10 km to the present caldera rim. Subtermi nal activity possibly similar to that observed on Mount Etna (19) characterizes the flanks of Elysium Mons (Fig. 5), while a hummocky surface possibly com- posed of ash deposits (6) surrounds the summit, Hecates Tholus (Fig. 4) displays numerous channels originating from the caldera rim and flanks that are probably lava channels (6). No lava flows can be recognized on Hecates Tholus, but the lower flanks display numerous sinuous channels that may have been produced by volcanic debris flows (12). 4) Volcano/tectonic features can be recognized on a1 1 four volcanoes. In the case of Olympus Mons, a very complex pattern of wrinkle ridges extends across the caldera floor (Fig. 1 ) , while Ascraeus Mons has we1 1-developed ridge elements on its southern flank close to the rim (Fig. 3). Hecates Tholus has a single down-dropped trough on its eastern flank (Fig. 4) and a single sinuous wrinkle ridge extends $150 km on the eastern side of Elysium Mons (Fig. 5). By analogy with lunar mare ridges (20), these martian ridges are interpreted here to be tectonic in origin, possibly associated with con- traction of the sumnit regions following tumescence (21). No collapse fea- tures comparable to the Valle del Bove, Etna (22) or Portillo-Tauce, Tenerife (14) are recognizable on the martian volcanoes. Further analyses of these and other martian volcanoes are currently in progress, and include interpretations of surface materials from impact crater morphology, the evolution of the ridges on the caldera floors, and the distri- bution of flank eruptions with respect to the summit caldera. Fig.2 (left): Multiple periods of caldera collapse have truncated parts of lava channels (arrowed) on the rim of Ascraeus Mons. Viklng frame 401B17. Fig.3 (right) c ~rink1.eridges (arrowed) characterize the flank of Ascraeus Mons close to the caldera rim. Note the striking differences that exist in the morphology of the two collapse craters. Viking frame 401B18. Scale bars are 5 km. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Martian Volcanoes: P .J .Rou~inis-!lark Table 1: Sunnary ~5 vnlc?noe nornhology OLYMPUS MONS ASCRAEUS MONS HECATES THOLUS ELYSIUM MONS Figure No. 1 2,3 4 5 ' Location 18N, 134W 11N, 114W 31N, 210W 25N, 214W Caldera Dia. (km) 80 55 12 14 No. Sumit Craters 6 4 5 1 Size Progression Large-small Small-large Constant -- - Caldera Flows NO No ? No Caldera Ridges Yes Yes No No Flank Ridges No Yes No Yes Lateral Flows Yes Yes No N o Subterminal flows Yes Yes No Yes Impact craters Ballistic Ballistic Fluidized Fluidized Fig.4 (left): Numerous r,adial lava channels and multiple episodes of caldera collapse are evident on Hecakes Tholus. Viking frame 651A19, scale bar is 10 km. Fig.5 (right): The summit area of Elysium Mons ("A") is character- ized by subdued, hummocky topography that may be ash deposits. 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