Icarus 267 (2016) 68–85 Contents lists available at ScienceDirect Icarus journal homepage: www.journals.elsevier.com/icarus Possible sub-glacial eruptions in the Galaxias Quadrangle, Mars ⇑ Peter J. Mouginis-Mark a, , Lionel Wilson a,b a Hawaii Institute Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA b Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, England, United Kingdom article info abstract Article history: We have identified several landforms in the Galaxias Quadrangle of Mars (MTM 35217), 33.0–35.5°N, Received 28 July 2015 216.0–218.0°W which are consistent with this area having been covered by an ancient ice sheet concur- Revised 17 November 2015 rent with volcanic eruptions. Volcanic activity was initiated by the intrusion of several large dikes mea- Accepted 21 November 2015 suring 50–100 m wide and protruding up to 35 m above the present-day surface. These dikes appear Available online 14 December 2015 to have originated from Elysium Planitia 600 km to the SE. In one instance, a dike (at an elevation of À3750 m) appears to have produced a subglacial mound (referred to here as ‘‘Galaxias Mons 2”) that Keywords: evolved into an extrusive eruption and produced copious volumes of melt water that carved an outflow Mars, surface channel that extends almost 300 km to the north. At a lower elevation ( 3980 m), a second putative dike Volcanism À Geological processes may have failed to break the surface of the ice sheet and formed Galaxias Mons as a laccolithic intrusion. We numerically model the formation of Galaxias Mons and find that at least 200 m of ice may once have existed at this latitude at the time of the dike intrusions. Such a conclusion supports the idea that enig- matic small domes in the area may be pingoes. Collectively, these observations suggest that the previous interpretations for the origin of near-by Hrad Vallis as a sub-aerial eruption may need to be revised. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction 21 Â 11 km in size, located nearly symmetrically atop the trace of a dike (Fig. 3). This exposed dike is well-preserved, and stereo The Galaxias Quadrangle of Mars (MTM 35217), 33.0–35.5°N, Context Imager (CTX) images allow the geometry of the subaerial 216.0–218.0°W, has been investigated for more than three decades portion of the dike to be investigated. Both to the north-west and as a potential site where there have been volcano-ground ice inter- the south-east of Galaxias Mons, a dike protrudes above the actions (Mouginis-Mark, 1985; De Hon et al., 1999; Wilson and present-day surface (Figs. 2 and 3). Two interpretations of this sit- Mouginis-Mark, 2003; Morris and Mouginis-Mark, 2006). Hrad uation are possible: (1) the dike intruded into rock layers that have Vallis, a prominent feature within this Quadrangle, could have subsequently been eroded, and (2) the dike intruded into an over- formed from the intrusion into the cryosphere of a sill that caused lying less massive layer (either eolian materials or ice) that has an explosive eruption (Wilson and Mouginis-Mark, 2003), and subsequently been removed. We find no morphologic evidence Chapman (1994) and Chapman et al. (2000) drew attention to for widespread erosion of material in this region (i.e., the lack of Galaxias Mons, noting that it resembled hyaloclastic ridges in wind-scoured highland remnants or eolian dunes surrounding Iceland and so may have had a sub-glacial origin. Igneous activity the Galaxias Quadrangle), and so assume that the second of these has also been postulated by virtue of numerous exposed dikes in possibilities is the correct one; the present-day surface once lay this region (Pedersen et al., 2010; Pedersen, 2013). Here we focus beneath an ice sheet that subsequently sublimed, with no melting on the specific consequences of the intrusion of these putative taking place. The implication is that the top of a dike extending dikes in the eastern portion of Galaxias Quadrangle (Fig. 1)to down the center-line of Galaxias Mons originally intruded into further interpret the stratigraphy of the area, as well as make the base of the ice layer, a mode of magma–ice interaction sug- new inferences about the likely availability of ice. gested as a theoretical possibility by Wilson and Head (2002a) Prominent within the Galaxias Quadrangle, is Galaxias Mons and inferred for the 1996 eruption of Gjálp volcano, Iceland by (Fig. 2), which is a positive relief feature 100 m high, Gudmundsson et al. (2004). Surrounding Galaxias Mons is a geo- logic unit (Fig. 1) that was interpreted by Wilson and Mouginis- Mark (2003) to be mud that originated from Hrad Vallis and flowed ⇑ Corresponding author. across the surface from south to north, most likely prior to the E-mail addresses: [email protected] (P.J. Mouginis-Mark), L.Wilson@ emplacement of the ice layer (although the exact timing of the lancaster.ac.uk (L. Wilson). http://dx.doi.org/10.1016/j.icarus.2015.11.025 0019-1035/Ó 2015 Elsevier Inc. All rights reserved. P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 69 Notation 2 A vertical cross-sectional area of sill (m ) ds thickness of sill near feeder dike (m) E horizontal extent of sill on either side of feeder dike (m) e dimensionless strain in materials overlying sill _ À1 Et horizontal extent of sill when magma flow becomes tur- e strain rate in materials overlying sill (s ) bulent (m) f dimensionless wall friction factor for flow in fissure, E0 initial horizontal extent of sill (m) equal to 0.02 K constant in ice fracture relationship, equal to g acceleration due to gravity on Mars, equal to 3.71 6.8 Â 10À24 (sÀ1 PaÀ3) (m sÀ2) L horizontal extent of dike feeding sill (m) h dimensionless parameter defining sill elongation P pressure (Pa) t time (s) Ps magma pressure at sill inlet from feeder dike (Pa) tt time at which magma flow becomes turbulent (s) Pt pressure in dike tip (Pa) z horizontal distance (m) S perimeter length of upper half of sill (m) g viscosity of mafic magma, equal to 100 (Pa s) À1 9 Ud lateral flow speed of magma in dike (m s ) l shear modulus of crustal rocks, equal to 3 Â 10 (Pa) À1 US speed of magma flowing into sill (m s ) m Poisson’s ratio of crustal rocks, equal to 0.25 (dimen- V volume flux of magma entering sill (m3 sÀ1) sionless) À3 W mean width of dike (m) qi density of ice, equal to 917 (kg m ) da thickness of sill near feeder dike after inelastic inflation qm density of mafic magmatic liquid, equal to 3000 (m) (kg mÀ3) dc depth of crevasse extending down from ice surface (m) s time scale of elastic sill thickening (s) emplacement of the mud relative to the postulated ice cannot see if any part of it has survived erosion. However, measurements determined). We assume that this layer is now desiccated, indu- on High Resolution Imaging Science Experiment (HiRISE) images at rated and lithified, but refer to it as a mud layer for simplicity. Here 50 cm/pixel resolution of other dikes nearby (Fig. 7) suggest that we numerically model the formation of Galaxias Mons and explore the width, W, of the intact dike may have been 73 ± 11 m. Using the consequences of potential sub-glacial eruptions. these values, the measured cross-sectional areas of the eroded dike segments to the NW, on top of, and to the SE, of Galaxias Mons can 2. Morphology of Galaxias Mons and the dikes be used to reconstruct estimates of their original heights above the present level of their exposure, i.e., above the present ground sur- The upper surface of Galaxias Mons appears to be coated with a face in the case of the NW an SE segments and above the top of relatively smooth veneer of material that has been eroded in places Galaxias Mons (Table 1). All of the measured basal widths of these (Fig. 4). Layering is visible within this veneer, and there is a shal- eroded segments are several times greater than 73 m and so are lowing of slope around the margins of this layer, which suggests consistent with this procedure. The heights found are 17 ± 7 m, that it is less competent than the underlying materials. Chemical 169 ± 35 m, and 40 ± 5 m for the NW, summit, and SE segments, alteration between potentially warm mud and the ice may have respectively. Given that the Galaxias Mons edifice is itself produced this veneer. There is also a prominent series of ridges 91 ± 20 m thick, the dike top there would have been at a height perpendicular to the strike of the dike (Fig. 3). Rather than being of 260 m above the present surface. a hyaloclastic ridge that formed by explosive activity as magma A further unusual characteristic of Galaxias Mons is a series of interacted with ground ice (Chapman et al., 2000), we contend that parallel ridges that occur within a few kilometers from the bound- the simplest interpretation of these observations is that Galaxias ary between the mound and the surrounding materials (Fig. 8). Mons is a volcanic sill that was intruded at the interface between These ridges each have a central depression along the crest of the base of the mud deposit and the underlying bedrock, uplifting the ridge. Each ridge has a near-constant width (100 m) but the the mud layer and overlying ice sheet as it expanded into a laccol- ridges do not appear to have been formed due to lateral flow of ith.