Icarus 267 (2016) 68–85
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Icarus
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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 3750 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 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 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 _ 1 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 10 24 (s 1 Pa 3) (m s 2) 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) 1 9 Ud lateral flow speed of magma in dike (m s ) l shear modulus of crustal rocks, equal to 3 10 (Pa) 1 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 s 1) sionless) 3 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 m 3)
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. The horizontal extent, L, of the sill along the strike of the dike material around Galaxias Mons. that fed it was 21 km, with the sill extending laterally away from De Hon et al. (1999) identified a second landform to the SE of the dike for 5.4 ± 0.1 km on both sides. The dike protrudes above Galaxias Mons at 33.7°N, 216.3°W that we informally call here the present-day surface for 10 km along strike on either side of ‘‘Galaxias Mons 2” (Fig. 9), which also lies along the strike of a dike. Galaxias Mons. Three randomly-chosen cross-sections of the dike Unlike Galaxias Mons, this second example has a series of pits to the NW show a mean width of 209 ± 46 m and a mean height extending along the axis of the landform which we interpret to of 12 ± 4 m. Seven randomly-chosen cross-sections to the SE show be volcanic in origin. Like Galaxias Mons, the upper surface of a mean width of 271 ± 49 m and a mean height of 22 ± 3 m (Fig. 5). Galaxias Mons 2 is not flat. Galaxias Mons 2 also differs from The mean slopes flanking these dike segments of 7 ± 3° and 10 ± 3°, Galaxias Mons in that we can identify possible flows on both sides respectively, are very shallow and clearly the result of erosion and of the center-line of the landform. In addition to thinning away re-working of dike material. Longitudinal and transverse cross- from the location of the dike inferred to have fed their growth, both sections through Galaxias Mons (Fig. 6) itself reveal that the upper mounds show a pattern of topographic undulations with axes surface stands a maximum of 152 ± 23 m above the surrounding oriented approximately at right-angles to the strike of the dike surface, with the average height being 91 ± 20 m. A relatively (Figs. 2, 3 and 9). These are interpreted to be the consequences flat-topped ridge is present along the axis of the edifice, contiguous of spatial variations in the local volume flux of magma being with the dike exposures on either side, and stands 40 ± 7 m above injected into the sill. Variations along strike of the upward volume the main part of the structure; its width is 623 ± 34 m and the flow rate of magma are common in fissure eruptions on Earth, mean slope of its steep sides is 7 ± 1°. giving rise to the classic ‘‘curtain of fire” effect, and the same There are no images that show dike segments associated with pattern of flow deposits is seen in fissure eruption sites on Mars, Galaxias Mons at high enough resolution to make it possible to such as the fissure system near the volcano Jovis Tholus (Fig. 10). 70 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 1. Segment of Galaxias Quadrangle investigated in this study. (a) Daytime IR THEMIS mosaic, with the locations for Figs. 2, 7, 14 and 16 identified. (b) Key geomorphic features are identified, namely the flow from Hrad Vallis (interpreted as ‘‘fluted plains material” or unit ‘‘Apf” by De Hon et al., 1999) and the flood-plain material ‘‘Achp” on the eastern side of a new source identified here. Galaxias Mons and a structure we informally call ‘‘Galaxias Mons 2” are also shown, along with the most prominent fractures and exhumed dikes (black lines). Area of mosaic extends from 33.0 to 37.3°N, 215.0 to 218.0°W. Insert at top right of image at right shows the general location, with ‘‘EM” denoting the volcano Elysium Mons and ‘‘HT” the volcano Hecates Tholus.
The consequence of local variations in the elastic properties of host the sill where it joins the dike is ds, the total volume flux V entering rocks causing ‘‘pinch-and-swell” patterns in the dike width, with the sill is consequent variations in the magma flow rate, is just as likely to V ¼ 2d LU ð1Þ occur when dikes feed sills as when they feed surface eruptions. s s where L is the length of the sill along strike. In the present case the sill has axial symmetry and is longer than it is wide; its lateral 3. Modeling the sill emplacement and its consequences spreading can therefore be treated as two-dimensional, and we shall be concerned with quantities per unit distance along strike, 3.1. Elastic sill emplacement theory and so rewrite Eq. (1) as = To numerically model the formation of Galaxias Mons, we ðV LÞ¼2ds Us ð2Þ assume that each sill is fed by magma from the dike in such a The sill grows by the elastic deformation of the host materials, rock way that the inlet pressure, dictated by the difference in elevation below, ice above. Assume that the horizontal extent of the sill on between the dike top and the inlet level, is constant. This treatment either side of the dike after it has been growing for a time t is E, is similar to that adopted by Wilson and Head (2002a) in modeling and that the magma pressure at the point of injection is Ps. An elas- subglacial sills but differs in that they fixed the value of the tic response by the host materials implies that ds and E are related injected magma volume flux as well as the injection pressure, by (Pollard, 1987): whereas we keep the latter as a variable. If magma flows into the ds ¼½ð1 mÞ=l ðp=2ÞPs E ð3Þ sill on both sides of the dike at a speed Us, and the thickness of P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 71
Fig. 2. Galaxias Mons. Note the ridge running down the crest of the landform. Also shown are the locations of Figs. 4, 5 and 8. Part of CTX image G19_025816_2145.
2 where m and l are the host material’s Poisson’s ratio and shear ðV=LÞ¼ðp =2Þ½ð1 mÞ=l Ps EdE=dt ð6Þ modulus, respectively; the elastic properties of ice (Davidson, 1983) are not grossly different from those of silicate rock (Pollard, On the assumption for the moment that the flow of magma into the 1987) and we use average values of m = 0.3 and l = 4 GPa. The shape sill is laminar, we use the standard relationship between mean flow of the growing sill can be approximated by an ellipse with semi- speed, Us, sill thickness (represented by the inlet value, ds), average major axis E and semi-minor axis d /2; the total cross-sectional magma pressure gradient (Ps/E), and magma viscosity, g: s nohi area, A, of the sill (i.e. its volume per unit distance along strike) is = 2 = = Us ¼ð1 4Þ ds ðPs EÞ ½6g ð7Þ A ¼ 0:5pds E ð4Þ The factor (1/4) is introduced in Eq. (7) to allow for the fact that, and substituting for ds from Eq. (2), though dimensionally correct, it would overestimate the mean flow 2 speed by a factor of 4(Rubin, 1995) because it treats the sill thick- A ¼ 0:5p½ð1 mÞ=l ðp=2ÞPs E ð5Þ ness and pressure gradient as constants whereas in fact they vary
By definition, the volume flux per unit distance along strike is equal with position along the sill. Substituting for ds from Eq. (3) into to the rate of growth of A with time, i.e., (V/L)=dA/dt, so from Eq. (5): Eq. (7): 72 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 3. Oblique image (looking north) of Galaxias Mons, illustrating the ridges that extend perpendicular to the strike of the dike. White arrows point to segments of the exposed dike. Vertical exaggeration = 11.7 . Base image is CTX frame G19_025816_2145.
Fig. 4. HiRISE view of the summit of Galaxias Mons, showing a veneer (dark toned material with shallow slopes surrounding depressions) over the summit. Note also the lack of evidence for lava flows on this feature. Illumination is from the left. See Fig. 2 for location. Part of HiRISE image PSP_006670_2150. P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 73
Fig. 5. Locations of topographic profiles up-rift (south east) of Galaxias Mons showing the height (at left) and the maximum width (at right), both in meters, of the exposed dike. See Fig. 2 for location. Part of CTX image G19_025816_2145.
no = = 2 2 3 = A complication can arise if the sill becomes so thick that the Us ¼ð1 4Þð½ 1 mÞ l p Ps E ð24gÞð8Þ magma flow into it becomes turbulent. In that case the appropriate formula for the flow speed is Finally, substituting Eq. (8) for Us into Eq. (2) and equating the resulting expression for (V/L) with Eq. (6), we find, after some 1=2 U ¼ð1=4Þf½d ðP =EÞ =½f q g ð12Þ algebra, s s s m no where f is a wall friction factor with a value close to 0.02 and qm is = = = 2 3 = 3 dE dt ¼ð1 4Þð½ 1 mÞ l pPs E ð12gÞð9Þ the magma density, 3000 kg m . Using this expression instead of Eq. (7) and carrying out the equivalent substitutions and simplifica- Using the boundary condition that at time t = 0 the sill has some tions we find very small length E , and introducing the timescale given by 0 s = = 2 = 1 2 E ¼ Et þfð2½ð1 mÞ l Ps Þ ðpf qmÞg ðt t0Þð13Þ = = 2 3 s ¼ð48gÞ ½ ð1 mÞ l pPs ð10Þ where Et is the distance that the sill has intruded when magma flow changes from laminar to turbulent at time t . Thus the sill growth is Eq. (9) can be integrated to give 0 exponential while flow is laminar but becomes linear if turbulence
E ¼ E0 expðt=sÞð11Þ sets in. In addition to the emplacement of the sill itself, we are also con- The above equations allow the evaluation of the variation with time cerned with the way that the sill growth induces strain and even- of the lateral sill extent, the sill thickness at its inlet, the magma tual failure of the overlying mud and ice layers. Since the sill has a flow speed into the sill, and the corresponding magma volume flux much greater horizontal extent than vertical thickness during its per unit length along strike. early growth, the length of the upper semi-perimeter of the ellipse 74 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 6. Cross sections through Galaxias Mons to show topography of the ridges that run perpendicular to the long-axis of the mound. Data derived from digital elevation model created from CTX images. Insert with each profile shows location across Galaxias Mons. Vertical exaggeration is 10.37 for each profile.
representing the sill, S, can be approximated by Michon’s (2015) being appropriate for mafic magmas that appear to dominate on addition to the second of the formulae given by Ramanujan Mars (Bandfield et al., 2000; Hamilton et al., 2001). An estimate
(1914) for very elongate ellipses: of Ps is obtained by considering the stress environment of the hinohiupper part of a dike coming to rest. If the dike has ceased to prop- : = : = 1=2 S ¼ 0 5p½2 ð7pÞ 22 ð0 5ds þ EÞ 1 þð3hÞ 10 þ ðÞ4 3h agate vertically, which we assume to be the case at the time the sill ð14Þ considered here was injected, the pressure Pt in the magma at the upper tip of the dike is not likely to be greater than the ambient where h is given by lithostatic load by an amount equal to the tensile strength of the 2 host material or less than the lithostatic load by an amount equal h ¼ððÞ2E dsÞ=ð2E þ dsÞ ð15Þ to the compressive strength of the host. Stress differences greater The total strain, e, at any moment in the overlying ice and mud lay- than the tensile strength would cause failure of the host and con- ers is therefore given by tinued upward dike propagation; differences less than the com- e ¼ðS 2EÞ=ð2EÞð16Þ pressive strength could cause crushing and implosion of the material around the dike tip and eventual formation of a collapse We assume that the indurated mud layer is relatively weak but depression at the surface. We estimated above that the upper tip that the overlying ice behaves in a brittle fashion if strained of the dike was 71 m below the ice surface. Thus the ambient sufficiently rapidly, and therefore keep track of the incremental external lithostatic load at the tip of the dike would have been strain rate, i.e. the change in strain between any two time steps 0.24 MPa. The tensile and compressive strengths of ice are 3 _ in the calculation, denoted e. The depth, dc, of surface crevasses and 30 MPa, respectively, over the 230–270 K temperature range induced in the overlying ice is then obtained from Nye’s (1957) relevant to near-surface crustal layers on Mars (Petrovic, 2003). formula Since Pt cannot be negative, the above requirements imply that its value lay in the range from zero up to 3.24 MPa. Finally, since d ¼ ðÞq g 1ðe_=KÞð17Þ c i we have found that the top of the dike was at a height of 3 where qi is the ice density, 917 kg m , g is the acceleration due to 260 m above the level of the sill injection, the pressure Ps at that gravity, 3.71 m s 2, and K is a constant equal to 6.8 10 24 s 1 Pa 3 level would have been the pressure in the dike tip plus the weight (Paterson, 1994). If the crevasse depth exceeds the ice thickness the of a 260 m high column of magma. To allow for the likelihood of ice layer ceases to deform elastically, and we shall show shortly that the presence of some exsolved volatile bubbles in the upper part it was under these inelastic conditions that most of the emplace- of the dike a density of 2000 kg m 3 is assumed for the magma, ment of Galaxias Mons took place. implying an extra pressure of 1.9 MPa. The range of values of Ps To use the above relationships we need to assume a magma vis- to be considered is therefore 1.9–5.1 MPa, and in the subsequent cosity and we require an estimate of Ps. We take g = 100 Pa s as illustrations values of 2, 3.5 and 5 MPa are used. P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 75
Fig. 7. High-resolution image of dike similar to that feeding sill forming Galaxias Mons. Insert illustrates the raised portion of the dike, which is 100 m at its maximum width. See Fig. 1 for location. Illumination is from the left. Parts of HiRISE image ESP_036866_2135.
3.2. Modeling results growth time depends weakly on the value assumed for the initial sill extent: changing this by an order of magnitude results in only Table 2 summarizes the results of the computations, assuming a 20% change in the growth time. an initial sill extent of 0.1 m at zero time in all cases. The horizontal The sill thickness is extremely small in the early stages, and one extent, E, of the sill and its vertical thickness at its inlet, ds, initially might expect that heat losses from the thin sheet of magma defin- increase exponentially with time as expected in all cases. For the ing the embryonic sill would be so great at this stage that the two larger values of Ps, the magma flow in the sill becomes turbu- magma would cool excessively and the sill would not propagate. lent (so that Us becomes constant) before the final sill extent of This issue was treated by Rubin (1995) in a paper focussing on 5.4 km is reached and the growth becomes linear. The volume flux the survival of granite dikes leaving magma reservoirs but also giv- into the sill, the local strain rate in the overlying ice, and the poten- ing results appropriate to basaltic magma intrusions leaving source tial downward extent of brittle fractures in the ice all increase with regions. In the present case the feeder dike plays the role of a time. Elastic sill emplacement times vary inversely with Ps and are magma reservoir and the thin sill is the equivalent of a horizontal 19, 3.5 and 1.25 h for Ps = 2, 3.5 and 5 MPa, respectively. The dike. Several factors may aid sill initiation. The feeder dike may 76 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Table 1 the ice fractured was 7 h, the sill inlet thickness was 4.1 m, the Topographic measurements on exposed dike segments. Reconstructed height is marginally turbulent magma inflow speed at that time was measured cross-sectional area divided by inferred 73 m width. 2.75 m s 1, and the magma volume flux per unit distance along Profile Measured Measured Average Cross-section Reconstructed strike was V/L = 23 m3 s 1 m 1. 2 number base width (m) height (m) slope (°) area (m ) height (m) This value of V/L deserves comment. Some values of this quan- (a) Dike segment NW of Galaxias Mons tity observed in, or deduced for, subaerial eruptions on Earth 1 246 8 4 984 14 include 3m3 s 1 m 1 for the 1961 fissure eruption at Askja, Ice- 2 158 12 9 948 13 land (Thorarinsson and Sigvaldason, 1962), 0.6 m3 s 1 m 1 for 3 224 16 8 1792 25 the 1783 Lakagigar eruption in Iceland (Thorarinsson, 1969), Mean 209 12 7 1240 17 7m3 s 1 m 1 for the July, 1974 summit eruption of Kilauea, ±1r ±46 ±4 ±3 ±480 ±7 Hawai’i (Heslop et al., 1989) and 12 m3 s 1 m 1 for the Yakima (b) Dike remnant on top of Galaxias Mons member of the Columbia River Basalt series (Swanson et al., 1 659 40 7 13,180 181 1975). These values include the entire range of tectonic environ- 2 579 30 6 8685 119 ments in the latter part of Earth’s history, including eruptions from 3 639 46 8 14,697 201 4 614 42 8 12,901 177 shallow magma chambers in hot-spot shield volcanoes, from mid- ocean ridge reservoirs, and from the base of the crust. Despite the Mean 623 40 7 12,370 169 ±1r ±34 ±7 ±1 ±2580 ±35 lack of plate tectonics on Mars, both shallow (Day et al., 2006) and deep (Wilson and Head, 2002b) reservoirs are implied, and there is (c) Dike segment SE of Galaxias Mons no reason to expect a dramatically different range of eruption 1 273 20 8 2730 37 parameters (Wilson and Head, 1994) from those on Earth in similar 2 197 27 15 2660 36 environments. However, the one type of terrestrial volcanic event 3 251 21 9 2636 36 4 360 18 6 3240 44 for which there are no direct estimates of conditions is surface 5 262 20 9 2620 36 eruption from the laterally emplaced giant dikes forming dike 6 262 24 10 3144 43 swarms in the archean. Theoretical estimates of the lateral magma 7 295 24 9 3540 48 flow conditions in these systems were made by Fialko and Rubin Mean 271 22 10 2940 40 (1999) and Wilson and Head (2002b), but surface eruptions were ±1r ±49 ±3 ±3 ±370 ±5 not treated. The (at least) 600 km long dike generating Galaxias Mons, along with many other dike exposures in the same region (Pedersen et al., 2010; Pedersen, 2013), probably belongs in this have been in place for some time before the sill was initiated, category, along with the more than 3000 km long dikes inferred allowing heat to propagate into the dike surroundings. These to have produced major graben systems like those radiating from would have been the ice sheet for the top part of the dike, the Tharsis (Wilson and Head, 2002b). Both Fialko and Rubin (1999) mud layer for the next 30 m below that, and finally the older bed- and Wilson and Head (2002b) estimated volume flow rates of rock beneath the mud layer. Heating of the mud layer and the bed- 109 m3 s 1 in the largest of such dikes. In the present case we rock would have partially offset heat loss from the sill tip. Any ice deduced the width of dikes in this area to be 70 m. Wilson and in the mud layer would have been melted and, very close to the Head (2002b) discuss the range of lateral pressure gradients, dP/ dike edge, boiled into pressurized steam. Water from the dike-ice dz, likely to be involved in the emplacement of giant radial dikes contact may have percolated down into this region and increased on Mars and find values in the range 100–10 Pa m 1 in the proxi- the vapor content of this region. Transient pressure spikes as water mal to distal parts of these systems. We expect that the part of flashed to vapor may have temporarily increased the driving pres- the dike responsible for forming Galaxias Mons was distal rather sure forcing magma injection and helped to prop open the inlet. than proximal to a source in the Elysium volcanic complex and Thus it is quite possible that the equilibrium sill growth modeled use the lower value for illustration to obtain a conservative esti- in Table 2 was preceded by a series of transient events guarantee- mate. For giant dikes with widths in the range 50–100 m the ing the subsequent thermal viability of the sill. This issue of the ini- magma flow will always be turbulent (Wilson and Head, 2002b) tial stages of growth applies to all sill formation events, on Earth as and so Eq. (12) can be used, substituting the dike width W for ds, well as Mars, and sills certainly do exist on Earth. to estimate the magma flow speed in the dike, Ud. Inserting the rel- 1 Once the thermally unrestricted growth of a sill in an infinite evant numerical values implies Ud = 3–4 m s . Multiplying these elastic medium is established, there is no limitation on the lateral speeds by the corresponding 70 m dike width and a plausible distance to which the sill can be injected other than relaxation of vertical height of a dike extending through the martian litho- the inlet pressure or cessation of the available magma supply. sphere, 50 km, gives volume fluxes in the range 1.1 107 to However, in the present case the sill was overlain by a 30 m thick 1.4 107 m3 s 1. The total volume flux from the dike feeding the mud layer above which was an ice layer that we estimated earlier sill can be estimated by multiplying its length along strike, to have been 345 m thick. If the strain rate in the ice induced frac- 21 km, by our estimated value of (V/L) = 23 m3 s 1 m 1. The tures that extended down through the full thickness of the ice layer resulting total volume flux into the sill, 4.8 105 m3 s 1, is one then the response would no longer have been elastic. The easiest and a half orders of magnitude less than the magma flux flowing response of the system to ongoing magma injection is inferred to in the main dike. Thus emplacement of the sill, whenever it have been uplift of the ice and mud layer as the sill inflated with occurred during the emplacement of the feeder dike, would have a now fixed lateral extent. Fig. 11 shows how the vertical extent had a negligible influence on the propagation of the dike itself. of crevasses varies with magma injection pressure Ps when The mean thickness of Galaxias Mons is 90 m (Fig. 6), and as E = 5.4 km. For crevasses to have extended down the 345 m to much as 30 m of the height may be the overlying mud layer, the base of the ice, the pressure must have been at least 2.8 MPa. implying that during the inelastic inflation of the sill its inlet thick- Furthermore, if the pressure had been significantly greater than ness increased from 4m to 60 m. However, the sill intrusion this value, Table 2 demonstrates that the ice would have fractured will have deformed the bedrock substrate elastically while magma before the sill had grown laterally to 5.4 km. Adopting 2.8 MPa as injection was taking place and it is possible that not all of this the best estimate of Ps, interpolation of the other parameters in deformation was relaxed subsequently, so that the base of the sill the table when E = 5.4 km implies that the emplacement time until may be some small distance below the present ground surface. P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 77
Fig. 8. Oblique image of the northern edge of Galaxias Mons, looking south showing parallel ridges in adjacent surface unit. Width of image in foreground is 3.6 km. See Fig. 2 for location. Part of HiRISE image PSP_007883_2155.
To make an allowance for this we assume that the sill thickness at strike, (V/L) = 23 m3 s 1 m 1, shows that the time required for the the magma inlet from the dike was da = 70 m after inflation. We inelastic inflation was a little more than 7 h. This calculation continue to model the shape of the inflating intrusion as an ellipse; assumes that both the magma inlet pressure and the available its semi-major axis is now fixed at E = 5.4 km and its semi-minor magma volume flux remained constant, whereas it is more likely axis has grown to one half of the 70 m just assumed. Its vertical that they were declining during this period. A major decrease in cross-sectional area is given by the standard formula for an ellipse, one or the other of these factors must have terminated the inflation 5 2 [pE(da/2)] = 5.9 10 m , and dividing this area by the rate of of the sill, and so the inflation time is probably a factor of a few growth of area, equal to the volume flux per unit length along times 7 h, as much as one or two days. 78 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 9. Details of Galaxias 2. Image at top is part of CTX image D21_035310_2145, with an interpreted units map at bottom. ‘‘a”, ‘‘b” and ‘‘c” identify three apparent episodes of mound construction. The flows could either be sub-glacially erupted lava flows or mud flows. The ‘‘caldron” is an area of collapse along the axis of the dike. On both sides of the main mound there are hummocky ridges. The pit craters (black dots) at the crest of the mound suggest that the eruption broke through the ice sheet.
While inflation was taking place, the length of the upper edge of layer that will have facilitated its ability to slide along the inclined the sill, S, will have increased steadily in accordance with Eqs. (14) upper surface of the sill. This may be relevant to the folded terrain and (15) with da substituted for ds – these are still good approxima- present immediately adjacent to Galaxias Mons, especially along tions because, even after inflating to 70 m thickness, the 5400 m its north-east edge. Using four topographic profiles across this half-width sill was still very elongate. On each side of the feeder region (Fig. 12) we have measured the along-surface length of dike the length increase will have been (0.5S E). With the folds and find a range of values from 22 to 62 m, with an da = 70 m, (0.5S) = 5433.5 m, making the increase in length of the average of 36 ± 15 m. curved upper sill surface on each side of the dike 33.5 m. This This is extremely similar to the calculated 33.5 m extension of increase in length will be equal to the total width of all of the cre- the sill surface, and suggests that the folded terrain may represent vasses that have been created in the ice layer. Presumably there the result of part of the mud layer sliding off the sill surface. This will also have been disruptions to the mud layer trapped between numerical match may be fortuitous, however. An alternative and the sill and the ice. Heat conducted from the sill margin into the more plausible assessment involves estimating the cross- mud layer will have begun to melt any ice component within the sectional area of the material forming the folds. The average, based P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 79
Fig. 10. A fissure system near the volcano Jovis Tholus, located at 18°100N, 114°40W, illustrates the morphology which we expect to be associated with a subaerial fissure eruption on Mars. The mode of formation of this fissure was discussed by Wilson et al. (2009). CTX image B02_010358_1983.
Table 2 Results of elastic intrusion model. As a function of time, t, in hours after the onset of intrusion values are given for the lateral extent of the sill, E, in meters on either side of the feeder dike; the thickness, ds, of the sill at its inlet; the mean flow speed, Us, of the magma entering the sill inlet; the magma volume flux per unit distance along strike of the 3 1 1 _ feeder dike, V/L,inm s m ; the strain rate in reciprocal seconds, e, induced in the overlying ice layer; and the depth, dc, in meters of brittle crevasses in the ice. _ tE ds Us V/L e dc
Ps = 2 MPa 0.3 0.1 65 lm30lm/s 3.8 10 9 2.5 10 9 6.4 2.8 0.5 0.27 mm 0.13 mm/s 6.8 10 8 5.9 10 9 8.6 5.6 2.5 1.36 mm 0.62 mm/s 1.7 10 6 2.8 10 8 14.4 8.3 12.3 6.8 mm 3.1 mm/s 4.12 10 5 1.9 10 7 27.3 11.1 61 34 mm 15.4 mm/s 1.0 10 3 9.5 10 7 46.6 13.9 303 0.17 m 76 mm/s 0.025 4.7 10 6 79.5 16.7 1508 0.83 m 0.38 m/s 0.63 2.3 10 5 136 18.1 3362 1.85 m 0.85 m/s 3.13 5.2 10 5 177 18.9 5400 2.97 m 1.36 m/s 8.08 9.7 10 5 218
Ps = 3.5 MPa 0.28 0.2 0.2 mm 0.3 mm/s 1.5 10 7 3.5 10 8 15.5 0.83 1.3 1.3 mm 1.8 mm/s 4.5 10 6 1.3 10 7 24.2 1.39 7.3 7.1 mm 9.9 mm/s 1.4 10 4 7.4 10 7 43.0 1.94 41 39 mm 55 mm/s 4.4 10 3 5.8 10 6 85.3 2.50 229 0.22 m 0.31 m/s 0.14 3.3 10 5 151 3.06 1275 1.23 m 1.72 m/s 4.22 1.8 10 4 268 3.40 3733 3.59 m 3.75 m/s 26.9 7.1 10 4 423 3.47 4627 4.45 m 3.75 m/s 33.4 8.8 10 4 455 3.52 5400 5.19 m 3.75 m/s 38.9 1.1 10 3 484
Ps = 5 MPa 0.28 1.2 1.7 mm 4.8 mm/s 1.6 10 5 4.3 10 7 36 0.42 4.3 5.9 mm 17 mm/s 2.0 10 4 2.2 10 6 61 0.56 15 21 mm 59 mm/s 2.4 10 3 7.6 10 6 93 0.69 53 72 mm 0.21 m/s 3.0 10 2 2.6 10 5 141 0.76 98 0.14 m 0.39 m/s 0.10 6.5 10 5 190 0.97 643 0.88 m 2.53 m/s 4.48 3.2 10 4 326 1.15 3080 4.23 m 5.35 m/s 45.3 2.3 10 3 629 1.18 4240 5.79 m 5.35 m/s 62.0 3.2 10 3 698 1.21 5400 7.42 m 5.35 m/s 79.4 4.2 10 3 767 80 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 11. Uplift of the ice and mud layer as the sill inflated. Solid line shows the variation of the vertical extent of crevasses (dc) with magma pressure (Ps). Dashed line corresponds to the inferred base of the ice, which is taken to be at a depth of 345 m. on the 4 profiles shown in Fig. 12,is 11,000 m2. If this material through the surface of the ice sheet (Fig. 9). Assuming the simplest was derived from the wet part of the mud layer extending the full case, that the ice had a horizontal surface, this would imply that 5400 m half width of the sill, an approximately 2 m thickness of the summit of Galaxias Mons lay 200 m beneath the ice surface. the 30 m thick mud layer would have had to be involved. The However, there are several geomorphologic features of the time taken to melt 2 m into an overlying ice layer can be obtained Galaxias Quadrangle for which we have no good model of the for- from Table 4 in Wilson and Head (2002a) and is found to be mation, but which may nevertheless be easier to explain given the between 4 and 5 h. In the present case the relevant material is existence of an overlying ice sheet. Inspection of the surface of mud, not pure ice, but the latent heat of fusion of the ice compo- Galaxias Mons at HiRISE resolution reveals a prominent, layered, nent is the dominant thermal parameter, and if the frozen mud veneer (Fig. 4). This veneer is breaking up so that numerous ‘‘win- contained 20–30% ice the timescale would have been 20 h, com- dows” into the underlying unit can be seen. Numerous 100 m-scale parable to the total emplacement time of the sill. As ice melted to impact craters that formed within the veneer show that the veneer water the resulting mobilized mud will have attempted to drain has less strength than the underlying unit because of the shallow along the upper face of the sill but this will have been restricted slope exhibited by the rim material. The origin of this mantle of as long as the response of the overlying ice was elastic. The fact material on Galaxias Mons is not obvious, but an option could be that water has a smaller volume than the same mass of ice will that it represents an earlier mud flow from Hrad Vallis that was have meant that the time to elastic failure of the ice will have been subsequently uplifted as the sill intrusion was emplaced. Alterna- somewhat longer than calculated above. We infer that movement tively, the layers could have formed as a lag-deposit during the of the mobilized mud occurred after the overlying layers fractured. retreat of the ice, but this explanation seems less likely because Subsequent sublimation of the ice has left the topography now there is no evidence of a lag-deposit on the surfaces surrounding visible. Galaxias Mons. Unfortunately there are no HiRISE images for Galaxias 2, so that we are unable to say if Galaxias Mons is unique in possessing this veneer. 4. Discussion and interpretations The ‘‘ribs” of Galaxias Mons (Fig. 2), which give the feature its characteristic unique appearance, might be explained by the spa- We can gain some understanding of the potential thickness of tial variation of the volume flux along the strike of the dike. Such the ice sheet from the relative elevations of the mounds. Fig. 13 along-strike variability in eruption rate has been documented for illustrates a topographic profile that crosses through both mounds, fissures on Mars (Wilson et al., 2009) where an effusive eruption and shows that Galaxias Mons 2 lies at an elevation 200 m above produced lava flows on the surface (Fig. 10). In the case of Galaxias that of Galaxias Mons. Of course, it is important to note that the Mons, the ribs would be the analogous manifestations of higher current topography may be different from that which existed dur- intrusion rates. We note that there is a similar, but less well ing the formation of Galaxias Mons due possible flexure of Utopia formed, structure to Galaxias Mons 2, but that in this case the Planitia and/or vertical movement of the distal flanks of Elysium locations of presumed vents are not symmetric with the structure. Mons. There are numerous summit pits and flow features on This same spatial variation in the volume flux along the strike may Galaxias Mons 2, suggesting that the eruptions may have broken also explain why there are exposed segments of dikes (e.g., the P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 81
Fig. 12. Four topographic profiles across the crevasses on the northern side of Galaxias Mons, image at lower right provides the profile locations. Data were derived from a digital elevation model created from HiRISE images. Vertical exaggeration is 14.8 and is the same for each profile.
Fig. 13. Topographic profile showing that Galaxias Mons 2 is at a higher elevation than Galaxias Mons. Light tone indicates a potential surface for the proposed ice sheet or, perhaps, a frozen ocean, that once covered the Hrad Vallis area. It is also possible that, if it existed, this surface may have more closely mirrored the topographic profile. Elevations are in meters relative to the MOLA datum. Note that the SE end of the profile is well-constrained by observations of Galaxias 2, but at the NW end the thickness of the proposed ice is poorly defined. 82 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 14. Broader context view to show that Galaxias Mons 2 produced significant volumes of melt water that escaped to the east. See Fig. 1 for location. White boxes indicate the locations of Figs. 9 and 15. Mosaic of CTX images D21_035310_2145 and P22_009465_2144. examples shown in Figs. 5 and 7) which did not produce large flows) extend from the mound to the western side of the catch- ridge-like features such as Galaxias Mons. The observed dikes ment pond that fed the delta. An ice-sheet covering much of Galax- may have been intruded under conditions that did not initiate sur- ias Mons 2 at the time of the eruption could have readily provided face eruptions, perhaps because these dikes have different ages and sufficient melt water to have formed the delta and the outflow so the ice level may have fluctuated so that lithostatic loading pre- channel that extends toward the north (Fig. 1). However, we note vented the dike tip reaching close to the surface. that the units immediately to the east of Galaxias Mons 2 lack The perimeter of Galaxias Mons also displays some unusual fea- the braided morphology one would expect to be the signature of tures. To the NE of the mountain there are several ridges within the extensive melt water release associated with a sill intrusion into surrounding unit that may be genetically related to Galaxias Mons an ice sheet. Thus the mechanism of meltwater escape may have (Fig. 12). These ridges parallel the side of the mountain, have a cen- been supraglacial rather than subglacial. If water release was tral depression running down the long-axis, and have sloping sides. supraglacial, runoff could have taken place beyond the ice margin, None of the ridges connect to Galaxias Mons, nor do they display and as such would imply that the ice mass had a limited spatial any along-strike variation in width or height. Although similar extent. ridges can be found as rings around some of the larger craters in Adjacent to Galaxias Mons, there are several domes (Fig. 16) the Hrad Vallis flow, they seemed to have formed only in areas that that are 20–25 m high and <1 km in diameter, as well as numerous could have experienced strain during their formation. Thus one anomalous craters first identified by Morris and Mouginis-Mark possible mode of formation of the ridges could be deposition at (2006). These features lie at a lower elevation ( 3890 m) than the base of crevasses in the ice sheet that was deformed during Galaxias Mons ( 3800 m), and present-day topography hints the sill intrusion. If this were the case, however, it is still not clear that they lie in a local topographic low. Here we propose that these why the ridges are predominantly on the NE side of Galaxias Mons features could be pingoes and collapsed pingoes, respectively, (Fig. 2). which formed at a later time when the hypothesized ice sheet The supposition that the Hrad Vallis flow was once covered by started to recede. At this time, the melting of an overlying ice sheet an ice sheet provides a potential mode of formation for some of could have provided ample melt water to form a pingo, while the the surface features surrounding these mounds. Mouginis-Mark collapse may have been initiated by the subsequent loss of water (1985) identified a possible braided delta to the NE of Galaxias through sublimation. We also note that some of the collapsed pin- Mons 2 (Figs. 14 and 15), but with Viking Orbiter coverage that goes are surrounded by fold-like rings, potentially similar to the was available at that time he was unable to identify the source folds at the NE margin of Galaxias Mons, but we lack the confi- of the melt water that could have formed this delta. Our new map- dence to say whether or not these fold-like rings were also formed ping of Galaxias Mons 2 (Fig. 9) reveals that flows (most likely lava by material slumping off topographic highs. P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85 83
Fig. 15. Well-developed delta complex to the east of Galaxias 2, showing that large volumes of water travelled from lower left toward the top (North) of the image. See Fig. 14 for location. Part of CTX image P22_009465_2144.
5. Conclusions sheet (Chapman, 1994). The low elevation of Hrad Vallis ( 3900 m relative to the Mars datum) would certainly place Both the Galaxias Mons and Galaxias Mons 2 mounds appear to the landforms identified here within an elevation range where be good candidates for landforms formed by sub-glacial eruptions thick ice could once have existed in the form of a frozen ocean. on Mars. Earlier studies (Mouginis-Mark, 1985; Squyres et al., There are other landforms (e.g., unusual flow margins, flows that 1987; Chapman et al., 2000) had speculated about such activity, did not travel perpendicular to the local gradient) in this vicinity but it is only with the higher resolution image and topographic that support the idea of isolated blocks of ice at approximately data that are available today that we can numerically model the the same period of activity (Hamilton and Mouginis-Mark, 2015), effects of igneous intrusions that most likely formed these but the shape of the hypothesized ice sheet remains poorly landforms. In particular, the geometry of the mounds is consistent defined. with the thermal and dynamic effects that we model for the Our assertion that the mounds were formed as sub-glacial fea- emplacement of the observed dikes that most likely emanated tures raises questions about previous interpretations for the for- from Elysium Mons. mation of other landforms in the area, most notably the flow-like But a leading question is raised by our interpretation that the material that originated from the source of Hrad Vallis (34°N, mounds were produced by sub-glacial eruptions, namely, where 218°W). In an earlier paper (Wilson and Mouginis-Mark, 2003), did the ice come from? The presence of oceans on Mars has been we speculated that this feature was also formed by the intrusion speculated (Witbeck and Underwood, 1983; Lucchitta et al., of a sill from a dike sourced from Elysium Mons, but at that time 1986; Baker et al., 1991; Parker et al., 1993; Head et al., 1999), we only considered sub-aerial eruptions. The Hrad flow clearly an ancient lake was suggested to cover Utopia Planitia (Scott formed prior to the intrusion of the dikes described here because et al., 1992), and based on Galaxias Mons and other surrounding there are no flow features (ridges, depressions, or folds) within deposits this paleolake was interpreted to have been a frozen ice the flow as it encounters the dikes (Fig. 2). 84 P.J. Mouginis-Mark, L. Wilson / Icarus 267 (2016) 68–85
Fig. 16. Oblique image of enigmatic domes to the north of Galaxias Mons at 35°060N, 142°310W. The larger dome at top left is 25 m high and 850 m in diameter. We suggest that these features could be pingoes produced by freezing of melt water released from the overlying ice sheet. HiRISE image ESP_016256_2155 draped over digital elevation model from HiRISE images.
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