Lahars in the Elysium Region of Mars

Lahars in the Elysium region of Mars

Eric H. Christiansen Department of Geology, Brigham Young University, Provo, Utah 84602

ABSTRACT Photogeological studies of the Elysium volcanic province, Mars, show that its sinuous channels are part of a large deposit that was probably emplaced as a series of huge lahars. Some flows extend 1000 km from their sources. The deposits are thought to be lahars on the basis of evidence that they were (1) gravity-driven mass-flow deposits (lobate outlines, steep snouts, smooth medial channels, and rough lateral deposits; deposits narrow and widen in accord with topography, and extend downslope); (2) wet (channeled surfaces, draining features); and (3) associated with volcanism (the deposits and channels extend from a system of fractures which also fed lava flows). Heat associated with magmatism probably melted ground ice below the Elysium volcanoes and formed a muddy slurry that issued out of regional fractures and spread over the adjoining plain. The identification of these lahars adds to the evidence that Mars has a substantial volatile-element endowment.

INTRODUCTION from these fractures and from three shield vol- (1 977), Mouginis-Mark et al. (1984), and Gree- Perhaps the most fascinating result of the canoes centered on the fracture system. Elysium ley and Guest (1987). spacecraft missions to Mars was the discovery of Mons is the largest of the shields and is closely numerous sinuous channels that resemble those related to the channeled deposits described here; WHAT ARE LAHARS? produced by fluvial erosion on Earth (Baker, it rises 13 km above the dome and is about 600 Lahar is the Indonesian name for a volcanic 1982; Mars Channel Working Group, 1983). km in diameter. Details about the geology of the debris flow. Summarizing from Fisher and The presence of the channels is taken as evi- Elysium volcanic province are given by Malin Schmincke (1984), lahars are (1) mass-flow dence that some fluid, generally believed to be water, flowed across the surface of Mars and formed the valleys. The genesis of the channels by flowing water is perplexing in that liquid water is not stable at the present surface of Mars. Various explanations for the occurrence of liquid water at the surface include climate change, the outbreak of aquifers confined to depths at which liquid water is stable (greater than about 1 km; Carr, 1979), and the melting of ground ice by magmatic heat (Masursky et al., 1977). In this paper I outline the geomorphic evidence that lobate channeled deposits in the Ely- sium volcanic province of Mars are lahars, as initially suggested by Christiansen and Greeley (1981). The apparent source of the lahars lies beneath the Elysium volcanic province. This conclusion bears upon the "volcanic" origin of some channels and adds to the evidence for extensive subsurface reservoirs of volatiles on Mars (Carr, 1986).

GEOLOGIC SETTING The reputed lahars extend 1000 km from their sources, cover an area of 1000000 km2, and probably have a cumulative volume exceed- ing 100000 km3 (Fig. 1). The deposits occur on the steep northwestern slopes of a flexural dome in the Martian lithosphere (Hall et al., 1986). The topographic dome is about 2000 km across Figure 1. Geologic map of Elysium region of Mars showing distribution of lahars (I) and and stands 3 to 5 km above the surrounding major volcanoes. Numbered boxes show locations of other figures. Fine lines in lahar areas plains of the northern hemisphere. The dome is are channels; line and ball symbol indicates open fractures and grabens. Geographic features: AT = Albor Tholus, EM = Elysium Mons, GV = Granicus Valles, HT = Hecates transected by northwest-trending fractures and Tholus, HV = Hrad Valles, VL2 =site of Viking Lander 2. Summit craters (circles) and outlines grabens and is capped by lavas that erupted of shield volcanoes are shown. Scale is for lat 20°N to 50°N.

GEOLOGY, v. 17, p. 203-206, March 1989 203 deposits, (2) wet (water acts as a mobilizing agent), and (3) generated as the direct or indirect result of volcanism. Using these criteria, the Ely- sium channels and associated deposits are interpreted as lahars. The evidences against alterna- tive modes of origin (stream erosion, debris avalanche, pyroclastic or lava flow) of these channeled, lobate deposits are also considered in this context.

Lahars Are Mass-Flow Deposits Many types of genetically diverse mass-flow deposits exist (mud and debris flows, lava flows, pyroclastic flows, landslides, and glaciers). All are produced by the gravity-driven flow of vis- cous, non-Newtonian fluids that have significant yield strengths. As a consequence, their deposits are morphologically similar; steeply sloping lobate snouts and distinctly elevated margins re- flect their generally non-Newtonian behavior. Debris flows have marked lateral deposits that flank medial deposits created by pluglike flow of the central part of a debris tongue-. (Johnson, Figure 2. Southern margin of lahars associated with Granicus Valles is composed of broad 1970). In plan view, most mass-flow deposits multiple lobes like thoseshown here, with rough, pitted lateral deposits (I) and smooth medial consistof <iply digitate lobes of material that deposits (m). Viking orbiter frames 612A48 and 612A50 show area about 90 km across. See came to rest at slightly different times. In Figure 1 for orientation of figures. general, where slopes are steep (and velocities high) the deposits left by all types of mass flow are relatively thin, or absent, and become thicker and broader on the gentle slopes of unconfined plains. The surfaces of lahars are smooth and have gentle undulations resulting from differen- tial compaction of the debris (Fisher and Schmincke, 1984; Siebert, 1984). The smooth- ness of lahar surfaces is important in distinguish- ing them from volcanic debris avalanches formed by sector collapse, which have hum- mocky surfaces and numerous hills (as niuch as 2 km across and 200 m high) and small mounds (Siebert, 1984). Lahars, like other gravity-driven mass flows, originate upslope, course down preexisting valleys or troughs, and terminate downslope. The passage of the debris stream is marked by a relatively thin accumulation of debris with a well- defined snout, smooth medial deposits, and rough coarse-grained lateral deposits. Terrestrial lahars are generally less than 5 m thick but range from less than 1 m to more than 200 m thick (Fisher and Schmincke, 1984). The simple observation that the Elysium landforms described here are gravity-driven mass- flow deposits and not fluvial erosion channels can be made from Viking orbiter images. Figure 2 shows a remarkable example of the morphol- ogy and definition of the deposit and its margins revealing its similarity to some types of mass- flow deposits. Snouts or flow fronts are well Figure 3. Sinuous channels of Granicus Valles cross surface of lahar near its source trough (A). defined; medial deposits (where not cut by later Channels are broadly sinuous, have streamlined features on their floors, and form anastomosing water flow) are developed inboard and are distributary pattern. Valleys become broader and shallower to northwest before they merge with smooth surface of sedimentary deposits. Northern margin (B) of lahar buries older lava flows generally smooth, like debris flows but unlike erupted from fissureltrough source (C). Viking images 541A20,541A22, 541A345, and 541A37. volcanic debris avalanches. Also like typical la- Area shown is 250 km across. hars, multiple, overlapping lobes are prominent

GEOLOGY, March 1989 in Figure 2. Shadow measurements here indicate that the flow front is less than 100 m high. The flow deposits trend down the western flank of the Elysium dome, dropping approximately 5000 m over the 1000 km course. Much of the drop occurs in the first 100 km (Downs et al., 1982). Where the slopes are steep, narrow chutes with no deposits developed; at the break in slope marking the base of the dome, the fan- like deposits less than 200 m thick (preexisting impact craters protrude) bury older lava plains (Fig. 3).

Lahars Are Wet By definition, lahars are intimate mixtures of liquid water and solids. For example, the ma- trices of 1980 Mount St. Helens lahars con- tained 2 to 36 vol% water (Pierson, 1985). Although most lahars move by nonerosive lam- inar flow, those on steep slopes of volcanoes and with high proportions of water may erode narrow channels (Fisher and Schmincke, 1984). On Earth, water separates from the granu- lar matrix of debris flows by downward infiltra- tion, by upward expulsion, and by evaporation (Lawson, 1982). Such dewatering may cause surface ponding of water and consequent Figure 4. Broad valleys with stubby tributaries (A) reappear about 450 km northwest of area smooth accumulations of sediment, branching shown in Figure 3. These probably represent seepage valleys formed as liquid from wet sapping valleys, and even collapse pits (Singe- lahars drained to surface of lahar. Viking images 612A34,612A36, and 612A55. Area is 80 km wald, 1928; Lawson, 1982; Kochel et al., 1985; across. and Higgins, 1984). These sapping and piping features are the best geomorphic evidence that debris flows are emplaced with a significant proportion of water. In addition, after the em- placement of many lahars, relatively under- loaded flood waters (issuing from the same Figure 5. Margins of distal sources that created the debris flows) erode ear- lobes of Elysium lahars lier deposits to form runoff (in contrast to seep- are dissected by closely age) valleys (Johnson and Rodine, 1984). spaced valleys (V) like Several lines of evidence demonstrate that the those in northern part of this image. Valleys are Elysium flow deposits were wet. The most ob- interpreted to represent vious indications of water are their channeled seepage of water from sed- surfaces. Several investigators have concluded iments and consequent that the channels were caused by fluvial erosion erosion. Dark smooth de- (Malin, Mouginis-Mark et a]., 1984); posits (D) at bottom are in- 1976; terpreted as intrachannel however, their intimate association with the debris flows. Viking frame flow deposits (Fig. 1) suggests a genetic connec- 10893 is of area 90 km tion with the deposits. The largest channels at across. Granicus Valles (Fig. 3) are near the source of the debris flows and appear to have been cut during a late water-rich phase of the development of the deposits. These channels are gently sinuous, have streamlined features on their floors, and have anastomosing distributary pat- terns. These valleys have analogs in the runoff channels associated with debris flows on Earth. To the northwest, the sinuous channels disap- waters were expelled to the surface; the presence hars (Fig. 5). The pitted nature of the channeled pear by merging with smooth medial deposits. of tributaries is inconsistent with an explanation landforms (Fig. 2) may be the result of dewater- About 75 km farther "downstream," stubby involving incomplete burial of channels by ing of coarse-grained marginal deposits-either tributaries start up and merge to form broad younger deposits. Short reticulate systems of by more active piping associated with down- smooth-surfaced channels (Fig. 4). The reap- sinuous valleys (individually less than about 5 ward draining, or as a result of enhanced pearance of these branched channels suggests km long) cut the distal parts of the deposits and evaporation of the included water. Distal chan- that they are seepage valleys formed as pore are similar to seepage channels on terrestrial la- nels are filled by dark flow materials resembling

GEOLOGY, March 1989 material remobilized by draining of the deposit point, but it may have generated abundant fine loading models for the formation of tectonic (Fig. 5). The geomorphic evidence for the pres- palagonitic particles as well as superheated features: Journal of Geophysical Research, v. 91, ence of liquid water in the deposits argues steam (Wohletz, 1986). The intersection of a p. 11,377-1 1,392. Higgins, C.G., 1984, Piping and sapping: Develop- against an interpretation of the lobate features as mixture of fine particles and water with the re- ment of landforms by ground-water outflow, dry lava or ash flows. gional fracture system may have led to the rapid in LaFleur, R.G., ed., Ground water as a and perhaps explosive expulsion of debris flows geomorphic agent: London, Allen and Unwin, Lahars Are Associated with Volcanism down the western slope of the province. Where p. 18-58. Johnson, A.M., 1970, Physical processes in geology: Lahars may be related to eruptions directly slopes were steep near the source, erosional San Francisco, Freeman and Cooper, 577 p. (by incorporation of ice, snow, ground water, or troughs developed; where the slope was gentler Johnson, A.M., and Rodine, J.R., 1984, Debris flow, stream water into pyroclastic flows or surges) or on the plains northwest of Elysium, the vast lo- in Brundsen, D., and Prior, D.B., eds., Slope in- indirectly (by mobilization of volcanic materials bate deposits formed and were later cut by con- stability: Chichester, England, John Wiley & on the slopes of volcanoes by torrential rains, by tinued draining of water from the fractures and Sons, p. 257-361. Kochel, R.C., Howard, A.D., and McLane, C., 1985, rapid melting of snow or ice, or by draining of a from the deposits. Channel networks developed by groundwater crater lake). sapping in fine-grained sediments: Analogs to Debris flows of the Elysium region are appar- CONCLUSIONS some Martian valleys, in Woldenberg, M., ed., ently linked with volcanism. Regional mapping Lobate deposits with well-defined snouts, Models in geomorphology: Boston, Allen and Unwin, p. 313-341. shows that lavas and debris flows formed late in medial channels, and lateral deposits issue from Lawson, D.E., 1982, Mobilization, movement, and the evolution of the volcanic province. The a northwest-trending system of fractures that cut deposition of active subaerial sediment flows, source of the Granicus lahars and channels (Fig. the Elysium dome. They extend 1000 km down Matanuska Glacier, Alaska: Journal of Geology, 3) is an elongate trough formed on the steep the regional slope to the northwest and cover V. 90, p. 279-300. western flanks of the Elysium dome. Adjacent 1000000 km2. Geomorphic evidence that these Malin, M.C., 1976, Age of Martian channels: Journal of Geophysical Research, v. 81, p. 4825-4845. troughs to the north fed lava flows (with no channeled deposits were debris flows and not -1977, Comparison of volcanic features of Ely- evidence of associated channels or surface the results of stream erosion, dry avalanches, sium (Mars) and Tibesti (Earth): Geological So- drainage features) and lahars (Figs. 1 and 3). lava flows, or pyroclastic flows includes the ciety of America Bulletin, v. 88, p. 908-919. The lava sources and the lahar sources all trend presence of (1) discernible deposits with appar- Mars Channel Working Group, 1983, Channels and valleys on Mars: Geological Society of America west-northwest and are part of the regional frac- ently fluvial runoff valleys on their surfaces; Bulletin, v. 94, p. 1035-1054. ture system that transects the Elysium dome and (2) seepage valleys; and (3) numerous irregular Masursky, H., Boyce, J.M., Dial, A.L., Schaber, G.G., apparently localized the volcanoes (Fig. 1). Al- depressions representing dewatering of the de- and Strobel, M.E., 1977, Classification and time though most of the lahars are younger than the posits. The debris flows issued from the same set of formation of Martian channels based on Vi- voluminous fissure-fed lavas, locally they are of fractures that fed extensive flank eruutions of king data: Journal of Geophysical Research, v. 82, p. 40164038. older (cf. Mouginis-Mark, 1985). These obser- Elysium Mons, suggesting their association with Mouginis-Mark, P.J., 1985, Volcano/ground ice in- vations show the close spatial and temporal as- volcanism. On the basis of this evidence, these teractions in Elysium Planitia, Mars: Icarus, v. 64, sociation of volcanism and lahar generation. channeled deposits are best interpreted as lahars p. 265-284. The estimated volume of the lahars is 10 to resulting from the interaction of volcanism with Mouginis-Mark, P.J., Wilson, L., Head, J.W., Brown, S.H., Hall, J.L., and Sullivan, K.D., 1984, Ely- 100 times greater than that of troughs from a reservoir of volatiles (probably water ice) bur- sium Planitia, Mars: Regional geology, volcanol- which they emanate. In contrast, for volcanic ied in the Martian crust. ogy, and evidence for volcano-ground ice avalanche deposits, source scar and deposit vol- interactions: Earth, Moon and Planets, v. 30, umes are equal. 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Christiansen, E.H., and Hopler, J.A., 1986, Geo- Squyres, S.W., Wilhelms, D.E., and Moosman, A.C., and Christiansen and Hopler (1986) have pre- morphic evidence for subsurface volatile reser- 1987, Large-scale volcano-ground ice interac- sented independent evidence that volatile-rich voirs in the Elysium region of Mars [abs.]: tions on Mars: Icarus, v. 70, p. 385-408. deposits lie beneath at least the western part of Houston, Lunar and Planetary Institute, Lunar Wohletz, K.H., 1986, Explosive magma-water interac- the Elysium province. Elevated heat flow related and Planetary Science XVII, p. 125-126. tions: Thermodynamics, explosion mechanisms, Downs, G.S., Mouginis-Mark, P.J., Zik, S.H., and and field studies: Bulletin of Volcanology, v. 48, to the development of the fmure-fed flank erup- Thompson, T.W., 1982, New radarderived p. 245-264. tions may have melted ground ice and mobilized topography for the northern hemisphere of subsurface materials. Squyres et al. (1987) Mars: Journal of Geophysical Research, v. 87, ACKNOWLEDGMENTS showed that intrusions into ice-rich permafrost p. 9747-9754. Supported by National Aeronautics and Space should produce large amounts of liquid water; Fisher, R.V., and Schmincke, H.U., 1984, Pyroclastic Administration Grant NAGW-537. Discussions with rocks: Berlin, Springer-Verlag, 472 p. R. Greeley and M. Ryan were helpful in formulating they calculated that the thickness of a liquid Greeley, R., and Guest, J.E., 1987, Geologic map of the ideas presented here. All the Viking images of the water layer produced by melting related to a sill the eastern equatorial region of Mars: U.S. Geo- surface of Mars were obtained from the National intruded in permafrost with 25% ground ice logical Survey Miscellaneous Investigations Map Space Science Data Center. would be comparable to or greater than the I-1802-B, scale 1:5,000,000. Manuscript received March 21, 1988 Hall, J.L., Solomon, S.C., and Head, J.W., 1986, Revised manuscript received October 12, 1988 thickness of the sill itself. Direct contact between Elysium region, Mars: Tests of lithospheric Manuscript accepted October 25, 1988 magma and water is difficult to prove at this

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