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The Long Lava Flows of Elysium Planita, Mars

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The Long Lava Flows of Elysium Planita, Mars JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E8, PAGES 19,389-19,400, AUGUST 25, 1998 The long lava flows of Elysium Planita, Mars Peter Mouginis-Mark and Michelle TatsumuraYoshioka Hawaii Instituteof Geophysicsand Planetologyand Hawaii Centerfor Volcanology Schoolof Oceanand Earth Scienceand Technology,University of Hawaii, Honolulu Abstract. Viking orbiterimages are usedto studythe distributionand morphology of 59 lava flows in the Elysium Planitia regionof Mars. Averagewidths for theseflows rangefrom 3 to 16 km, and many of the flows exceed100 km in length. The aspectratio (flow lengthto average width) is highly variable(>40:1 to <5:1), all flows that erupted<200 km from the summitare short (<70 km long), and the 11 longestflows all have vents>294 km from the summitof Elysium Mons. An unusualattribute of five flows is their segmentednature, with up to 13 individual segmentson a singleflow, which each have a surfacearea from 25 to 250km 2. Photoclinometry is usedto derive an averagethickness of 40-60 m for theseflows, indicatingthat individualflow volumesrange from 17.7 to 68.1km 3. Plausiblevalues for the effusion rate (101-104 m 3 s-1) suggestthat individual eruptions could have lasted for a few monthsto severaldecades and could haveinjected between 1.36 and 2.04 x 1012kg of watervapor into the atmosphere, assuming a 1 wt % water contentfor the parentalmagma. A total volumefor all 59 flows is estimatedto be 817-1226km 3, whichwould have released -2.04-3.01 x 1013kg of watervapor into the atmosphereor -0.2% of the amountpreviously calculated [Plescia, 1993] as the amountreleased from the Cerberusflows in SE Elysium Planitia. 1. Introduction far northto be visible to Earth-basedradar ranging experiments [Espositoet al., 1992]. Topographicdata derived from spacecraft The dimensionsand spatial distribution of lava flows can occultations and stereogrammetric measurementsfrom low- provide valuable information on eruption and emplacement resolution spacecraftimages [ U.S. Geological Survey, 1991] processesand on the large-scale structure of a volcano [e.g., indicatethat the summitof ElysiumMons rises-17 km abovethe Wadge, 1977; Rowland, 1996]. In particular,lava flows longer meanMars datum. The volcanohas very shallowflanks in areas than -100 km provide constraintson local structurebecause their where the majority of the examinedflows are located,with occurrenceimplies that large volumesof melt can be generated northwarddipping slopesof <0.5 ø over horizontal distancesof and erupted by the volcano at one time. We describe here a several hundred kilometers. particularlygood example of a field composedof many long lava Images of the Elysium Planitia were collected by Viking flows locatedin Elysium Planitia, Mars, to the north and west of Orbiter 1 in December 1977 (orbit 541) and April 1978 (orbit the volcanoElysium Mons (Figure 1). Nearly 60 easilyidentified 651). These data were obtained during a period of low lava flows >50 km in lengthcan be foundhere. atmosphericdust, and each orbit had a uniform spatialresolution. Previous investigations have documentedthe geology and Solar incidenceangles for theseobservations varied from 65ø to tectonicsof Elysium Planitia, which containsthe secondlargest 75ø for orbit 541 (150 m/pixel) andfrom 60ø to 70ø for orbit 651 young volcanic province on Mars [Mouginis-Mark et al., 1984; (40 m/pixel). Hall et al., 1986; Tanakaet al., 1992]. The areais dominatedby the volcanoesElysium Mons, Albor Tholus, and HecatesTholus, 2. Distribution of Flows andthere are numerous graben and mesas that appearto havebeen One problemcommon to manystudies of planetarylava flows affectedby groundice [Mouginis-Mark,1985]. Althougholder is thegreat difficulty in identifyingtheir source vents. By analogy than the better known Tharsis volcanics,the Elysium lavas are to the Earthit is likely thatthe Martianlava flowswere erupted interpretedto be quite young (early amazonian)[Greeley and from fissuresor circularvents that were only a few meterswide Guest, 1987], with an absoluteage between3.2-3.9 Ga [Neukum [Wilson and Head, 1994], and that the near-vent constructional and Hiller, 1981] and 0.8-1.5 Ga [Soderblom et al., 1974]. topographywas quite limited even when the flowswere pristine. Relatively young (upper amazonian;200-500 Ma) volcanicflows Giventhe image resolution and the fact that the Elysium flows are havealso been identified in the southeasternportion of Elysiumin sufficientlyold for thevent areas to haveexperienced degradation the Cerberusregion [Plescia, 1993]. Thus, while the Elysium by smallmeteorite impacts, we cannotidentify the proximal ends Planitia flows describedhere are old by terrestrialstandards, as of the flows, and many of the flow lengthsreported here are planetary examples, they are very well preserved and are probablyunderestimated. We thereforerecognize that our flow relativelyyoung. lengthmeasurements are mostlikely minimaand are limitedby Regional topographyis poorly known for Elysium Planitia, the point at whichthe flows have a sufficientthickness (at least owing to the lack of global altimetry and becausethis area is too -10 m) to be seen in the Viking images (which have a solar incidenceangle of-60ø). For brevity we usethe term "vent"to denotethis proximal point on theflow in thefollowing discussion. Copyright1998 by theAmerican Geophysical Union. We haveidentified 59 lava flowsin theregion between 26 ø and Papernumber 98JE01126. 34øN, and between207 ø and 224øW (Figure 2). No strong 0148-0227/98/98JE-01126509.00 preferential vent distribution is evident for these flows and the 19,389 19,390 MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS •:. .. ß•;:.'-;....;.:::.::;.:..:;;.:.:.::•.•.:•::.• ................L.:.:.:.:.:................:.;............ ;:.:.;:...;:.::;:.:.:.:.:..4 - .':-.'- ß ................ '............ ..... --• "' '::..... ß:-- '- . .... '-- ........... •:::•:;:-:•:.:-:;:!:;ii' .... •::;;;::; ::::?.!.;i ;;•:::::::i; i i'i' -'-<- ........................'.......................................... •::::::::::...... •-•'• • ::•-•:: , •':• •.,.• ........................ 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'........ ::•::::::]:7'.•.... :•::•:::::•::•• • •:•::::•::• ....... •....•::::•::•::• .........•... •:::•:••.:•....... •::•:•:::.•?•:::::•?•:::::.•:::::?•}•:•::•::• •:•:•::::•'g:J•::'D• ........•':.... •::• ..................•*•:::•::*•: ••:•::: :.4•'•;• • •:*:• ß:.•:•::•..:•::•:•:::•?•.•:•J•:•:::.•::...•:•:•:•:::(•:::•:•::•:•:::::: :::•:•:::::::•:•::• Figure 1. Location map for the Elysium Planitia region of Mars (17ø-37øN, 205ø-230øW). Boxes denote the locationsof Figures2, 3, and 11. From the U.S. GeologicalSurvey digital imageof Mars. flows exhibit a wide variety of morphologies(Figure 3). Some each flow. Average flow width was determined using width flows appear to have been emplacedover relatively featureless measurements of each flow at 10 km intervals along, and terrain, while others form complex flow fields where individual perpendicularto, this centerline,beginning at the proximal end of lobes partially bury older units. In general, the shapesof these the flow. Surface area was measuredby tracing the flow outline flows can be characterized as "simple" (short (<40 km) and (at a scale of 1:500,000) onto graph paper and by countingthe narrow (<3 km)), "lobate" (several lobes along the flow length numberof squares(each square was equivalent to 2.89km 2) on causevariations in flow width), "long linear" (flows exceed 100 the paper. The geometric center (24.88øN, 213.24øW) of the km in length but few, if any, have pronounced lobes), and Elysium Mons caldera,which is -14.0 km in diameterand almost "ponded"(where the flow width abruptly increases,probably as a circular, was used for the measurement of the distance to the result of a local depression being infilled). Representative proximal end of each flow. Severalgeneral characteristics of the examplesof the flow shapesare given in Figure 3c. flows can be identified from these data: We have measuredlength,
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