Seventh International Conference on Mars 3123.pdf

TROPICAL MOUNTAIN ON MARS: -ICE INTERACTIONS AND INFLUENCES ON GLACIAL EVOLUTION. L. Wilson1 and J. W. Head2; 1Environmental Science Dept., Lancaster University, Lancaster LA1 4YQ, UK ([email protected]); 2Dept. Geological Sciences, Brown University, Providence, RI 02912, USA ([email protected]).

Introduction: Late Amazonian fan-shaped the Arsia deposit and are separated by about 25-30 km. deposits on the northwest flanks of the Tharsis Graben, pits and moberg-like ridges characterize the Montes volcanoes are interpreted to have formed as a eastern ridge. The western structure contains a lobe- result of cold-based tropical mountain glaciation [1- shaped plateau and crater near the edge of the hummocky 6] (Figs. 1, 2). Deposits and landforms interpreted to facies, and forms subaerial cones along its northern have formed during subglacial eruptions have been extension, between the inner and outer ridged facies, and documented beneath and within these deposits [4, 7- again outside the glacial deposits to the north. The lobe- 8]. These include low ridges interpreted to be dikes, shaped plateau (~9 km long and 6 km wide) extends lobate deposits interpreted to be steep-sided flows, downslope and the adjacent crater is ~4 km wide and linear mounds and low ridges interpreted to be ~100 m deep. The plateau is ~130-150 m high and moberg-like ridges and cones, and elongated extends from the base of a ridge that is ~350 m high. We depressions and trough-like features interpreted to be interpret the ridge, located along the strike of the linear the result of subglacial and englacial trend, to be a sub-glacial moberg-like ridge, and the phreatomagmatic eruptions. Phreatomagmatic elongate plateau to be a subglacial, sill-like flow eruptions could produce enough tephra and country extending from the vent. Superposed on the lobate rock to alter the behavior of the through direct plateau, and extending downslope and out onto the melting, albedo-induced effects, surface insulation, subjacent lava flows, is a sinuous ridge that is generally and influences on the rheology of ice. In some continuous for ~14 km; we interpret this to be an places, sufficient meltwater appears to have been draining subglacial eruption-induced meltwater. generated during the intrusion of steep-sided sill-like Adjacent to and downslope from the ridge/lobate plateau subglacial flows to produce subglacial lakes. The the configuration of the drop bows outward for presence of subglacial lakes could have led to local a distance of ~5-10 km. The most prominent drop wet-based conditions, local tongue-like wet based occurs at the distal (downslope) edge of this glacial surges, and to the release of meltwater from inner set of drop moraines and several fluvial channels the margin of the glacier in fluvial drainage channels emerge from its base and extend at least 5-7 km into the or jokulhlaups. In this contribution we examine surrounding terrain. several of these factors from an observational and We interpret this configuration to be the result of modeling point of view. subglacial and volcanically induced meltwater Arsia Mons Cold-Based Tropical Mountain generation and drainage. Sufficient meltwater appears to Glacier: Subglacial Eruptions, Polythermal have been generated to cause a local transition from Glaciation, and Distal Drainage of Meltwater: At cold-based to wet-based conditions, producing local wet- Arsia Mons, the tropical mountain glacier deposits based surging of a 30-40 km wide portion of the contain features and structures interpreted to otherwise cold-based glacier. In addition, drainage of the represent subglacial and englacial eruptions. Here we meltwater from beneath the glacier out into the describe a series of features that together are surrounding terrain formed local fluvial channels at the interpreted to represent a linear subglacial eruption glacier margin. that caused the production of sufficient meltwater to Subglacial Eruptions and Polythermal Glaciation form , a local wet-based glacial that on Mars: The Pavonis Mons Tropical Mountain formed anomalously lobate moraines, and a distal Glacier Steep-Sided Subglacial Flows: Subglacial and series of channels emerging from the edge of the englacial in contact with glacial ice can cause glacial deposit and flowing downslope into the melting and produce meltwater. Here we examine the surrounding terrain that is interpreted to represent heating associated with specific features interpreted to be subglacial drainage following the eruption. subglacial eruptions [4,7-8] (Fig. 2) in order to assess A series of NW-trending preglacial lava flows their relative ability to melt glacial ice and to produce extend down the flanks of Arsia and the fan-shaped sufficient meltwater to potentially cause a transition from glacial deposits are superposed. Facies in the NW cold-based to wet-based glacial conditions. We find that part of the Arsia deposit include concentric ridged among the range of subglacial eruption features, steep- deposits interpreted to be drop moraines and sided subglacial lava flows are the most efficient in hummocky deposits interpreted to be sublimation producing meltwater. Due to their sill-like nature and tills. Two generally parallel graben-ridge systems the continuing supply of overlying ice replacing draining trend for several hundred km across the NW part of meltwater, water volumes of up to 450 km3 can be Seventh International Conference on Mars 3123.pdf

generated from a typical 100 km long, 15 km wide, sources of sediment include global airborne dust, 500 m thick subglacial flow emplaced at a plausible volcano summit and flank explosive tephra-forming effusion rate of ~10,000 m3/s over a 2.5 year period. eruptions, country rocks from phreatomagmatic This volume of meltwater is equivalent to a layer eruptions, and rocky debris derived from alcoves and averaging 4.5 m deep below an area equivalent to the scarps near the accumulation area. Abundant evidence maximum extent of the Pavonis tropical mountain suggests that the glaciers were cold-based and thus glacier but more importantly is equivalent to an incorporation of subglacial debris into the glacial ice was average water layer thickness of 45 m beneath the minimal. specific zone between the flows and the distal part of Subglacial volcanic eruptions occurring during the the lobe. time of the presence of the glaciers show evidence for This analysis suggests that there may have been dike, sill, subglacial flow, moberg ridge, and cone-like some predictable influences from the generation of activity. Cone-like activity may represent eruptions that this volume of meltwater. First, thermal calculations produce local to regional tephra blankets that could be a show that a transient should form substantial source of glacial debris and serve as agents of down-slope of the advancing lava flow. Shallow direct heating and melting (hot airfall deposits) or lakes would rapidly refreeze, but the volumes albedo-induced melting (blanketing dark tephra). We generated here could lead to additional consequences. model a hypothetical set of cones based on composite For example, volumetrically significant local observations. We locate the cones along a 40 km long subglacial magmatic melting (such as calculated ridge beneath the Tharsis Montes tropical mountain here) could produce sufficient meltwater to cause a glacier ridged facies, interpreted to be drop moraines at local transition from cold-based to wet-based the outer, thinner part of the glacial system. The ridge is conditions, producing local wet-based surging of a interpreted to be the subglacial manifestation of a dike; portion of the otherwise cold-based glacier. the cones are interpreted to represent eruptive centers Evidence for such activity might be manifested in forming along wide places in the dike. We model the anomalous arcuate lobes of moraines downslope of cones as occupying a 6 km long segment of the ridge and the subglacial flows. In addition, drainage of the ranging from 600-1200 m in width, 600-1800 m in meltwater from beneath the glacier out into the length, and 150-280 m in height. Such cones are surrounding terrain could form local fluvial zones interpreted to represent eruptions into a melted cavity in ranging in scale from small channels up to the scale the ice or onto the glacier surface. Widths of such cones of jokulhlaups. Rapid drainage would be consistent are related to the ranges of ejected pyroclasts and imply with the survival of the released water against rapid speeds of ~ 40 m/s and magma water freezing in the low-temperature, low-atmospheric contents of ~0.1 wt %. If they were emplaced from dikes pressure martian environment. releasing magma at 1-10 cubic meters per second per In summary, we find that local sill-like subglacial meter along strike (a common rate on Earth), eruptions can produce sufficient meltwater to cause emplacement times would have been a few hours to 1-2 localized transitions from cold-based to wet-based days. Away from the cones, the ridge is modeled as ~ 60 glacial behavior, and produce related, and perhaps m high and ~250 m wide; here effusion rates would be catastrophic, drainage of meltwater. Studies are much smaller, ~0.004 cubic meters per second per meter underway to assess the relationships between the from a dike ~0.1 m wide. These values and this specific steep-sided lobate features interpreted to be difference are well within the range observed for narrow subglacial sill-like lava flows, local arcuate and wider places in basaltic dikes feeding eruptions perturbations in patterns of moraines, and fluvial along the East rift zone of Kilauea. features located along the distal margin of the glacier. This treatment can be used to investigate the potential Tephra Deposition on Glaciers and Ice Sheets contribution of a layer of tephra deposits to: 1) the on Mars: Implications for Glacial Debris Content sediment load in the glacier, 2) the amount of melting and Glacial Melting: Facies associated with Late that might have occurred due to hot tephra emplacement, Amazonian cold-based tropical mountain glacier and 3) the effect of a dispersed tephra layer on albedo- deposits along the northwest flanks of the Tharsis induced melting. We find that fine-grained (sub-mm Montes (Fig. 1) [1-6] include concentric ridged sized) tephra would have been dispersed in a ~20 km deposits (drop moraines), hummocky deposits high plinian-like cloud under a normal range of eruption (sublimation tills), and lobate deposits (debris- conditions. If this were deposited only over an ~100,000 covered glacier remnants). These facies represent the square km area of the glacial deposits it would produce a long-term behavior of an extensive , layer of cooled pyroclasts ~3-4 mm deep. More likely including its advance, retreat, collapse and the ambient winds would have dispersed it over a 1-2 reactivation, and consist of sediment that is million square km area to a mean depth of ~200 microns. influenced by subsequent eolian activity. Candidate This could not have provided a significant contribution Seventh International Conference on Mars 3123.pdf

to the drop moraines, but would have greatly and glacial modification of the trough walls. influenced the thermal stability of the ice surface by Stratigraphic relationships suggest that the troughs dramatically changing its albedo. Thus, there may be formed during the emplacement of the Arsia TMG. The links between subglacial eruptions, surface tephra occurrence in the western distal part of the Arsia TMG of emplacement and ice sheet behavior. dike-related subglacial volcanism strengthens the Intrusion of Dikes into a Late Amazonian likelihood that subglacial eruptions might also have Tropical Mountain Glacier: Evidence for a occurred in the thicker proximal portion of the ice sheet. Phreatomagmatic Origin of the Aganippe Fossae As overlying ice becomes thicker (estimates range up to System, Western Arsia Mons, Mars: Synglacial 2-3 km), dikes will intrude further into the ice sheet volcanism associated with the Late Amazonian cold- causing melting of marginal ice and the production of based tropical mountain glaciers on the western meltwater both in the crustal cryosphere and the glacier flanks of the Tharsis Montes (themselves involving itself. We have modeled the intrusion of dikes into this contemporaneous effusive and explosive eruptions) configuration and find that 1) sufficient meltwater is appears to be largely related to the emplacement of produced to create hydromagmatic interaction and dikes into the shallow crust that predominantly explosive phreatomagmatic eruptions, and 2) that further produce narrow graben across the surface in non- explosiveness occurs when the glacial ice is breached glacial areas, but that intrude into glacial ice when and exsolved magmatic gas undergoes catastrophic they encounter the TMGs. Dike-related synglacial decompression to ambient conditions. For example, a eruptions produce dike-like ridges, sill-like typical 40 m wide dike emplaced into this configuration intrusions, steep-sided flows, tephra cones, and can create a that will form an moberg-like ridges. Here we document a new type of elongate depression ~2.7 km wide and ~2000 m deep. synglacial TMG subglacial eruption: linear troughs These phreatomagmatic eruptions would have produced produced by dike-related phreatomagmatic eruptions. ejecta composed of a combination of chilled fragmental The Aganippe Fossae system consists of two major magma, fragmented country rock, and aqueously altered troughs that extend in a NNW direction across the tephra. Estimates of the initial volumes of the troughs northwestern flank of Arsia Mons and are very suggest that as much as 600 km3 of country rock ejecta closely associated with TMG deposits. The fossae might have been produced, forming a layer averaging ~4 typically range from ~2-5 km in width and are meters thickness over the entire TMG deposit, discontinuous in nature, having been modified by potentially influencing glacial dynamics and stability. portions of the fan-shaped deposit. At the southern Subsequent glacial and mass-wasting activity has altered margin of the fan-shaped deposit, the wide western the fossae to their present configuration. trough abruptly changes its morphology into a narrow The Effect of Tephra Deposition on Equatorial graben, which is eventually covered along strike by Glacial and Polar Ice on Mars: The deposition of later lava flows. Although the fossae have clearly tephra from explosive volcanic eruptions on Mars can been modified by subsequent events, the widest and influence the stability of ice by: 1) direct melting from most distinctive structures generally occur in the hot tephra, 2) changing thermal inertia, 3) changing interior of the fan-shaped deposits. In this analysis surface albedo, 4) changing atmosphere temperature, and we addressed the questions of why the fossae are 5) modifying rheological properties. The Tharsis Montes, confined to the fan-shaped deposit, why their themselves formed of a mixture of products of effusive morphology changes in relation to proximity to the and explosive eruptions, contain cold-based glacial deposit margins, their stratigraphic position in deposits on their NW flanks that imply ice cover over relation to the cold-based tropical mountain glacier areas of order 100,000 km2 to a depth of up to ~2-3 km. deposits, and candidate modes of origin. Summit magma reservoirs are estimated to have vertical Superposition relationships show that the fossae extents of up to 10 km and diameters of up to 120 km have been modified by the glacial processes (Arsia Mons). The volumes of typical eruptions associated with the Late Amazonian TMG. Smooth discharging ~1% of the reservoir magma could be up to facies interpreted to represent late-stage debris- ~1000 km3. If this volume were erupted exclusively as covered glaciers completely cover portions of the tephra and distributed uniformly over the volcano troughs and debris-covered glacial features on the surface, the deposit depth would be ~3 m, or ~6 m if trough floors clearly indicate that troughs have been distributed preferentially into the glacial deposit sector. modified and widened by glacial processes. In some A fall deposit from a summit eruption would consist places the troughs appear to be buried by the knobby entirely of clasts that had reached ambient temperature facies, thought to represent the collapse and vertical (~200 K) by the time that they landed, and would downwasting of the TMG, while in others the produce no direct thermal effects. A pyroclastic density margins of the troughs appear sharp, but are likely to current could have a temperature of ~700 K; about half have been modified by post-formation mass-wasting of the heat content would be transferred to the underlying Seventh International Conference on Mars 3123.pdf

ice, and could melt an ice thickness approximately melting in subglacial basaltic volcanic eruptions: equal to the thickness of the pyroclast layer, i.e. up to implications for volcanic deposit morphology and ~6 m, a very small fraction of the ~2-3 km estimated meltwater volumes. In Smellie, J. L. and M. G. Chapman, maximum thickness of cold-based glacial ice. A ed. Volcano-Ice interaction on Earth and Mars. London, subglacial phreatomagmatic eruption creating a Geological Society, 5-26. (Special Publications 202.), graben of the scale of Aganippe Fossa on the north- 2002; [10] L. Wilson and J. Head, Heat transfer in volcano- western flank of Arsia Mons could produce sufficient ice interactions: Synthesis and applications to processes country rock ejecta to form a layer ~4 m thick and landforms, Annals of , in press, 2006. averaged over the entire glacial deposit. A second consequence of volcanic mantling deposits is their effect on the long-term stability of the underlying ice. Both fall and deposits dominated by juvenile magma on Mars should consist of generally sub-mm sized clasts and have a low thermal inertia and high tortuosity, thus tending to protect underlying ice from sublimation; however, they would also have a lower albedo than the ice, thus reaching higher daytime surface temperatures. This effect would compete with the protective properties of the deposit. In contrast, phreatomagmatic country rock deposits should be coarse-grained, having higher thermal inertias that might encourage significant local melting. Furthermore, the deposition of extensive phreatomagmatic ejecta on top of the proximal/accumulation region of the tropical mountain glacier system could have major effects on the dynamics, stability, evolution and ultimate fate of the ice sheet. Calculations using the sub-mm grain size expected for juvenile pyroclasts on Mars show that fall deposits from the highest mass eruption rate events from these equatorial volcanoes can be dispersed Fig. 1. Tropical mountain glaciers on Mars on the planet-wide; 1000 km3 distributed uniformly over the northwest flanks of the Tharsis Montes and Olympus entire surface of Mars represents a layer ~1 cm deep. Mons. Such a layer over the polar ice deposits would have negligible protective ability but could significantly enhance sublimation, and would contribute to the lower-albedo layers seen in the polar layered terrain.

References: [1] D. Marchant and J. Head, LPSC 34, 1361, 2003; [2] D. Marchant and J. Head, LPSC 35, 1405, 2004; [3] J. Head and D. Marchant, Geology, 31, 641, 2003; [4] D. Shean et al., JGR, doi:10.1029/2004 JE002360, 2005; [5] D. Shean et al., JGR, doi: 2006JE002761, 2007; [6] S. Milkovich et al., Icarus, 181, 388-407, doi: 10.1016/j.icarus.2005.12.006, 2006; [7] J. Head, and L. Wilson, Heat Transfer in Volcano- Ice Interactions on Mars: Synthesis of Environments and Implications for Processes and Landforms. In press, Annals of Glaciology, MS 45A007, 2007; [8] J. Head and L. Wilson, Mars: a review and synthesis of general environments and geological settings of magma H20 interactions. In Smellie, J. L. and M. G. Chapman, ed. Volcano-Ice interaction on Earth and Mars. London, Fig. 2. Structures and units interpreted to be related to subglacial Geological Society, 27-58. (Special Publications 202.) volcanism at Pavonis Mons [4]. 2002; [9] L. Wilson, L. and J. Head, Heat transfer and