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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 Tatsumura Yoshioka Hawaii Institute of Geophysics and Planetology and Hawaii Center for Volcanology School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu
Abstract. Viking orbiter images are used to study the distribution and morphology of 59 lava flows in the Elysium Planitia region of Mars. Average widths for these flows range from 3 to 16 km, and many of the flows exceed 100 km in length. The aspect ratio (flow length to average width) is highly variable (>40:1 to <5:1), all flows that erupted <200 km from the summit are short (<70 km long), and the 11 longest flows all have vents >294 km from the summit of Elysium Mons. An unusual attribute of five flows is their segmented nature, with up to 13 individual segments on a single flow, which each have a surface area from 25 to 250 km 2. Photoclinometry is used to derive an average thickness of 40-60 m for these flows, indicating that individual flow volumes range from 17.7 to 68.1 km 3. Plausible values for the effusion rate (101-104 m 3 s -1) suggest that individual eruptions could have lasted for a few months to several decades and could have injected between 1.36 and 2.04 x 1012 kg of water vapor into the atmosphere, assuming a 1 wt % water content for the parental magma. A total volume for all 59 flows is estimated to be 817-1226 km 3, which would have released -2.04-3.01 x 1013 kg of water vapor into the atmosphere or -0.2% of the amount previously calculated [Plescia, 1993] as the amount released from the Cerberus flows in SE Elysium Planitia.
1. Introduction
The dimensions and spatial distribution of lava flows can provide valuable information on eruption and emplacement processes and on the large-scale structure of a volcano [e.g., Wadge, 1977; Rowland, 1996]. In particular, lava flows longer than -100 km provide constraints on local structure because their occurrence implies that large volumes of melt can be generated and erupted by the volcano at one time. We describe here a particularly good example of a field composed of many long lava flows located in Elysium Planitia, Mars, to the north and west of the volcano Elysium Mons (Figure 1). Nearly 60 easily identified lava flows >50 km in length can be found here.
Previous investigations have documented the geology and tectonics of Elysium Planitia, which contains the second largest young volcanic province on Mars [Mouginis-Mark et al., 1984; Hall et al., 1986; Tanaka et al., 1992]. The area is dominated by the volcanoes Elysium Mons, Albor Tholus, and Hecates Tholus, and there are numerous graben and mesas that appear to have been affected by ground ice [Mouginis-Mark, 1985]. Although older than the better known Tharsis volcanics, the Elysium lavas are interpreted to be quite young (early amazonian) [Greeley and Guest, 1987], with an absolute age between 3.2-3.9 Ga [Neukum and Hiller, 1981] and 0.8-1.5 Ga [Soderblom et al., 1974]. Relatively young (upper amazonian; 200-500 Ma) volcanic flows have also been identified in the southeastern portion of Elysium in the Cerberus region [Plescia, 1993]. Thus, while the Elysium Planitia flows described here are old by terrestrial standards, as planetary examples, they are very well preserved and are relatively young.
Regional topography is poorly known for Elysium Planitia, owing to the lack of global altimetry and because this area is too
Copyright 1998 by the American Geophysical Union.
Paper number 98JE01126. 0148-0227/98/98JE-01126509.00
far north to be visible to Earth-based radar ranging experiments [Esposito et al., 1992]. Topographic data derived from spacecraft occultations and stereogrammetric measurements from low- resolution spacecraft images [ U.S. Geological Survey, 1991] indicate that the summit of Elysium Mons rises -17 km above the mean Mars datum. The volcano has very shallow flanks in areas where the majority of the examined flows are located, with northward dipping slopes of <0.5 ø over horizontal distances of several hundred kilometers.
Images of the Elysium Planitia were collected by Viking Orbiter 1 in December 1977 (orbit 541) and April 1978 (orbit 651). These data were obtained during a period of low atmospheric dust, and each orbit had a uniform spatial resolution. Solar incidence angles for these observations varied from 65 ø to 75 ø for orbit 541 (150 m/pixel) and from 60 ø to 70 ø for orbit 651 (40 m/pixel).
2. Distribution of Flows One problem common to many studies of planetary lava flows
is the great difficulty in identifying their source vents. By analogy to the Earth it is likely that the Martian lava flows were erupted from fissures or circular vents that were only a few meters wide [Wilson and Head, 1994], and that the near-vent constructional topography was quite limited even when the flows were pristine. Given the image resolution and the fact that the Elysium flows are sufficiently old for the vent areas to have experienced degradation by small meteorite impacts, we cannot identify the proximal ends of the flows, and many of the flow lengths reported here are probably underestimated. We therefore recognize that our flow length measurements are most likely minima and are limited by the point at which the flows have a sufficient thickness (at least -10 m) to be seen in the Viking images (which have a solar incidence angle of-60ø). For brevity we use the term "vent" to denote this proximal point on the flow in the following discussion.
We have identified 59 lava flows in the region between 26 ø and 34øN, and between 207 ø and 224øW (Figure 2). No strong preferential vent distribution is evident for these flows and the
19,389
19,390 MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS
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Figure 1. Location map for the Elysium Planitia region of Mars (17ø-37øN, 205ø-230øW). Boxes denote the locations of Figures 2, 3, and 11. From the U.S. Geological Survey digital image of Mars.
flows exhibit a wide variety of morphologies (Figure 3). Some flows appear to have been emplaced over relatively featureless terrain, while others form complex flow fields where individual lobes partially bury older units. In general, the shapes of these flows can be characterized as "simple" (short (<40 km) and narrow (<3 km)), "lobate" (several lobes along the flow length cause variations in flow width), "long linear" (flows exceed 100 km in length but few, if any, have pronounced lobes), and "ponded" (where the flow width abruptly increases, probably as a result of a local depression being infilled). Representative examples of the flow shapes are given in Figure 3c.
We have measured length, average width, surface area, and the distance of the proximal end of the flow (the vent) from the summit of Elysium Mons for all 59 flows (Figure 4 and Table 1). A single-length measurement was made along the central axis of
each flow. Average flow width was determined using width measurements of each flow at 10 km intervals along, and perpendicular to, this centerline, beginning at the proximal end of the flow. Surface area was measured by tracing the flow outline (at a scale of 1:500,000) onto graph paper and by counting the number of squares (each square was equivalent to 2.89 km 2) on the paper. The geometric center (24.88øN, 213.24øW) of the Elysium Mons caldera, which is -14.0 km in diameter and almost circular, was used for the measurement of the distance to the
proximal end of each flow. Several general characteristics of the flows can be identified from these data:
1. All flows that erupted <200 km from the summit are short (<70 km), narrow (<3 km), and have surface areas <200 km 2. A large number of flows with their proximal ends at radial distances between 200 and 550 km also have these attributes.
MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS 19,391
Figure 2. Distribution of the 59 lava flows identified in this analysis. Arrows show the length of the lava flows. Ball at proximal end of each flow may mark the location of the vent, although the image resolution (40-150 m/pixel) is insufficient to positively identify the sources. Base map is a mosaic of the digital image models prepared by the U.S. Geological Survey. See Figure 1 for location.
2. Other flows, characterized by average widths between 6 and 10 km, have their proximal ends at radial distances >250 km from the summit. These flows have a wide variety of lengths, from 23 to 200 km. Surface areas range from 110-1500 km 2.
3. The aspect ratio (flow length versus average width) for the flows is highly variable and ranges from >40:1 to <5:1. No
correlation appears to exist between flow aspect ratio and the distance of the proximal end of the flow from the summit.
4. Eleven flows exceed 100 km in length and have proximal ends located between 294 and 486 km from the summit. Aspect ratios range from 12.1:1 to 33.8:1, and surface areas range from 427 to 4110 km 2. The four longest flows (152-246 km long) can first be identified at radial distances of 224, 292, 310, and 510 km from the summit.
5. Two flows have unusually large average widths (> 16.0 km). One also has the greatest observed length (246 km) and area (4110 km 2) of any flow in the region, while the other (to the east of Hecates Tholus) is 73 km long, has an area of 1043 km 2, but can only be identified at its distal end owing to poor image resolution closer to the summit.
3. Flow Morphology
The 40 m/pixel Viking images for the area to the west of Hecates Tholus, where six large lava flows can be identified (Figure 5), provide new insights into Martian lava flow morphology. The proximal ends for these flows are -300 km from the summit of Elysium Mons. The top surface of each flow is generally flat, and there are no festoon ridges comparable to those that have been identified in the Tharsis region of Mars by Theilig and Greeley [1986]. On the basis of measurements of the fractal dimensions of the lobe margins, Bruno et al. [1992] interpreted these Elysium flows to be a'a. Unlike at Alba Patera, Mars, where very long flows are interpreted to be associated with
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Figure 3a. Image mosaic of compound lava flow field -230 km north of the summit of Elysium Mons. Many overlapping flow units can be seen at an image resolution of- 150 m/pixel, including the proximal portions of flows 9, 10, 12, and 14 (Table 1). Illumination direction is from the left. See Figure 1 for location. Viking orbiter image 541A30.
19,392 MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS
Figure 3b. Individual lava flows to the NW of Elysium Mons exceed 100 km in length and yet may be as narrow as 2 km. Notice the diversity of morphologies: Flow 45 (see Table 1) has a central channel, flow 46 has two very narrow (<1 km) constrictions, and flow 47 has a narrow flow segment extending from a broad flow field. As is true for all of the flows identified in this analysis, no vents or constructs that might be pyroclastic cones can be identified. Viking orbiter images 541A26 and 28; image resolution is -150 m/pixel.
flows that were originally channel fed and that then had their channels roofed over [Carr et al., 1977; Cattermole, 1987], there is no morphological evidence to suggest that this roofing over took place on these Elysium flows (Figure 6). No examples of
20 km
32
13
49
Figure 3c. Representative flow outlines are shown for the different types of flows observed in the Elysium region Flow numbers correspond to those given in Table 1. Flows 13 and 46 are "lobate" and have several lobes along the flow length; flows 8 and 49 are "long linear" flows; flow 32 is interpreted to be a "ponded flow"; and flow 5, 17, 19, 21, 23, and 29 are "simple flows." The proximal end of each flow is to the right.
aligned collapse pits, which would be indicative of lava tubes, can be found.
An intriguing observation is that three of the Elysium flows seen at -40 m/pixel resolution are divided into discrete segments (Figure 7) of similar size, which suggests that their emplacement took place as a series of pulses [Mouginis-Mark, 1992]. This segmentation always appears at the downstream side of the flow rather than at the side of the flow, it appears to have been a one- time event at each location (there are no instances of multiple segmentation at a lobe boundary), and each segment has essentially the same width. Three complete segments can be identified for flow A (part of flow 12, in Table 1), with a total surface area of 220 km 2. Extrapolating the shape of this flow beneath the more recent impact crater adds an estimated additional 135 km 2 to the area. Flow B (part of flow 9, in Table 1) has 13 segments and a total area of 1362 km 2, and flow C (part of flow 13, in Table 1) has 10 segments and a total area of 1038 km 2. No surface deformation can be seen on the upslope lobe segment (Figure 8), nor does the flow thicken at this segment boundary (see section 4 ). Figure 9 summarizes the dimensions of these flow segments, which range in length from 9 to 41 km and which range in area from 26 to 253 km 2.
Each breakout of a new flow segment occurred at the distal end of the earlier (upslope) lobe (Figure 10). No examples of downslope lobes emerging from the side of an earlier lobe have
MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS 19,393
300'
E 250. v
i- 200.
z uJ 150,
0 lOO, _1
u- 50 ! :1
0• 0
A)
100 200 300 400 500 600
VENT DISTANCE FROM SUMMIT (km)
300'
E 250' v
t- 200' (5 Z uJ 15o,
• 10o. o
u. 50
t ø 0.•
B)
0;o oooo
FLOW AREA (km^2)
300
E 250.
i- 200.
z u.i 15o,
•: ' 100' O
u. 50'
0 0
•' ß ß ß 5:1
5 10 1'5 20 AVERAGE FLOW WIDTH (kin)
Figure 4. Morphometric relationships for the 59 lava flows identified in this study, including lengths of lava flows versus vent distances from the center of the summit caldera of Elysium Mons, lengths of lava flows versus flow surface area, and lengths of lava flows versus average flow widths, see text for a discussion of the measurement methods. Lines denote constant aspect ratio (length versus width).
been found, so that some (unknown) limit imposed by the rheology of the flow most likely prevented the lateral growth of the flows. The bland morphologies and relatively flat surfaces of the flow surfaces indicate that these breakouts were not associated
with the inflation of the earlier part of the flow (Figure 10). Inspection of all 59 flows studied here shows that only 5 of the
lava flows >50 km long (flows 9, 12, 13, 30, and 32; in Table 1) appear to possess individual flow segments and that none of the shorter flows are segmented. This observation may, however, be a function of image resolution, since much of the area was only imaged at -150 m/pixel, so subtle morphologic features could have been missed. Nevertheless, even where image resolution is -40 m/pixel, segmentation is rarely observed on other flanks of Elysium Mons. Flow segmentation of the type seen in Elysium Planitia has also been reported by Gregg and Fink [ 1997] for their laboratory experiments that simulate lava flow eruption under
plausible planetary conditions, under conditions that they call the "rifting regime," where the lava simulant temporarily had a sufficiently high hydraulic pressure beneath the solid flow carapace to rupture the surface crust.
4. Flow Thickness and Volume
Photoclinometry, a technique for determining shape from shading, allows the estimation of topographic relief on a planetary surface when used with calibrated images [Davis and Soderblom, 1984]. We have collected 20 photometric profiles (Figure 11) across four lava flows in Elysium Planitia (imaged on orbit 651A at 40 m/pixel) using the Planetary Image Cartography System (PICS) software, which was developed by the Astrogeology Branch of the U.S. Geological Survey. Radiometric calibration was performed on the digital data, and level I processing was carried out to remove internal geometric distortions, reseau marks, and radiometric errors. The resulting 16-bit digital image has a known and constant viewing geometry with the raw data numbers converted to radiometrically defined units. Fortunately, the Elysium Planitia images have a solar azimuth that is almost perpendicular to the regional slope, so that the cross sections for the lava flows reported here were made almost parallel (within -20 ø ) to the illumination direction.
Certain assumptions need to be made before thickness estimates can be derived for the flows. First, the surface is taken
to be flat and horizontal in the direction in which the topographic profiles are made. While this is a reasonable assumption for most flows, it is possible that the preexisting topography was not flat
+ 34øN,
Flow//13
!Fig. 7
34øN, 210øW
Flow//12
-t- 30øN, 215øW
Flow #6
Flow
30øN, . 50 km 210ow '1-
Figure 5. Sketch map showing the location of six flows to the north of Elysium Mons. Topographic contours were obtained from the U.S. Geological Survey [1991]. Boxes mark the locations of Figures 6a, 6b, and 7. Circular features with barbed symbols are meteorite impact craters. Flow numbers are the same as those listed in Table 1.
19,394 MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS
Table 1. Dimensions of Lava Flows Identified in This Study
Length, Average Width, Area, Flow Latitude Longitude km km km 3
Vent Distance, km
1 30.6 207.4 50 8.7 413.3
2 30.5 207.9 73 16.7 1043.3
3 29.1 210.9 67 8.7 722.5
4 30.2 211.5 23 9.3 141.6
5 29.8 211.9 34 2.2 66.5
6 31.2 211.6 124 10.2 1101.1
7 30.5 212.6 29 3.5 130.1
8 29.9 212.8 113 3.6 427.7
9 29.8 213.7 246 16.5 4109.6
10 30.4 214.7 108 7.3 783.2
11 30.9 214.9 53 7.9 404.6
12 30.2 214.2 200 7.6 1543.3
13 31.8 213.3 116 7.6 1083.8
14 30.5 215.0 30 3.8 138.7
15 32.2 215.2 29 3.8 118.5
16 32.4 215.4 32 10.0 349.7
17 32.7 216.4 31 2.3 86.7
18 32.1 216.0 18 5.7 78.0
19 32.6 216.6 30 2.0 49.1
20 32.6 216.9 18 2.0 49.1
21 32.3 217.1 29 2.2 72.3
22 32.2 217.3 63 4.8 442.2
23 31.9 217.3 28 1.8 54.9
24 31.5 217.1 14 2.0 31.8
25 31.5 216.6 25 2.9 80.9
26 31.5 216.5 46 2.9 141.6
27 30.1 216.8 39 9.8 274.6
28 32.1 217.9 60 2.9 182.1
29 32.9 218.5 36 1.8 75.1
30 32.1 218.6 185 7.3 1320.7
31 31.7 218.9 39 6.2 202.3
32 32.7 219.5 100 9.1 965.3
33 31.9 220.2 21 1.5 40.5
34 29.8 222.2 25 1.3 40.5
35 29.1 221.2 122 3.6 445.1
36 29.2 221.1 126 5.0 543.3
37 29.2 220.8 53 2.3 109.8
38 29.2 219.2 20 1.3 34.7
39 29.3 218.5 32 2.9 86.7
40 29.9 219.7 25 2.2 52.0
41 30.3 219.6 27 1.3 43.4
42 30.3 219.5 16 1.7 31.8
43 30.3 219.2 12 1.3 23.1
44 30.2 218.8 23 3.1 69.4
45 29.1 217.2 59 2.1 86.7
46 28.0 215.8 152 5.1 803.4
47 28.6 216.3 89 4.4 326.6
48 29.2 216.2 25 1.8 54.9
49 28.3 215.4 83 1.8 159.0
50 28.0 215.0 52 2.7 170.5
463
446
246
331
300
365
338
294
292
372
386
310
416
339
443
460
473
448
484
494
482
478
467
443
431
426
352
490
532
510
494
554
548
534
486
484
467
392
369
427
467
478
454
445
309
224
263
293
221
203
MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS 19,395
Table 1. (continued)
Length, Average Width, Area, Vent Distance, Flow Latitude Longitude km km km 3 km
51 27.6 214.4 58 1.5 101.2 160
52 27.5 213.2 44 1.7 72.3 155
53 27.3 213.0 22 2.3 37.6 144
54 27.1 213.0 15 2.3 34.7 126
55 26.7 213.7 35 2.8 95.4 106
56 26.4 214.0 35 1.8 86.7 93
57 26.5 215.0 12 2.0 20.2 135
58 28.1 212.0 30 2.0 60.7 198
59 28.8 212.0 42 3.0 124.3 231
See text for description of measurement procedure.
Figure 6. High-resolution (40 m/pixel) views of individual flow lobes to the west of Hecates Tholus (see Figure 5 for locations), including (a) flows 9 and 12, Viking orbiter image 651A10, and (b) flow 6, image 651A14. Note that the flow at the bottom left of Figure 6b has a fractal dimension believed to be a pahoehoe tlow [Bruno et al., 1992] and that the flow is not included in this analysis.
19,396 MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS
Flow "C" ....,•........:• :•:!•:&i::,[[':'-.-'-:..--'.. -'/.. -..-"•
.:-,•.. •g: ':}:
Figure 7. Three lava flows to the west of Hecates Tholus illustrate a segmented structure, wherein discrete breaks in the flow can be identified. These flow segments are identified here for flows shown in Figure 5. Shading has been applied only to discriminate individual flow segments. Flows A, B, and C correspond to the distal parts of flows 12, 9, and 13 in Table 1, respectively. The flows were mapped from Viking orbiter images 651A08-12, which have a spatial resolution of 40 m/pixel.
where one flow lies on top of an earlier flow. In these instances, no photoclinometric measurements were made. Second, the albedo of the lava flows is uniform across the areas being measured. Photoclinometry assumes that a certain radiance in the image corresponds to a flat surface, which is then used for the entire profile. While it is possible that intrinsic differences in the albedo of the flows could have existed at their time of eruption, we assume that their age (possibly 1 or 2 billion years old) [Tanaka et al., 1992] would allow wind-blown dust to mantle the flows and produce a uniform albedo. Third, to collect usable thickness measurements, the solar incidence angle should typically be greater than -60 ø so that the brightness values of slopes facing toward and away from the Sun have measurable differences; all of our high-resolution images meet this criterion (incidence angle varies from 61.2 ø to 64.0 ø for frames 651A08- 651A15).
Our photoclinometric profiles were constructed across the flows over horizontal distances between 3.3 and 10.0 km (Figure 12). These profiles illustrate two flow attributes. First, while maximum thicknesses vary from 40 to 125 m, the flows are typically -40-60 m thick. These flows are therefore thicker than the ones to the southeast in the Cerberus region; Plqscia [1993] estimated (also using photoclinometry) individual flow fronts to be -10 m thick. Comparing our multiple profiles across a single
Figure 8. Details of flow segments in flow B (flow 9, in Table 1). See Figure 7 for location. Arrows identify the edge of each lobe segment. Notice the lack of surface disruption to the flows and that the new flow segments have almost the same width as that of the upslope segment (Viking orbiter image 651 A10, 40 m/pixel).
flow indicates a slight trend for thickness to increase toward the distal end (best shown in Figure 12 by profiles 16-20 but also observed in profiles 1-3). Second, the profiles display considerable diversity in the cross-sectional shape of the flows. Flows may have almost flat upper surfaces (e.g., profiles 4 and 12), while others have a central crest and shallow sloping sides (profiles 5 and 9). Several profiles illustrate the presence of central depressions, which appear to be channels (profiles 6, 13, 14, and 15), and many flows have topographic variations at the scale of 10-20 m.
Taking 40-60 m as the average thickness, the volumes of the individual flow segments shown in Figure 9 are estimated to be 1- 15 km 3. Assuming that all of the flows identified in this study (i.e., Table 1) have a range of thicknesses comparable to the one shown in Figure 12 implies a range of flow volumes from -0.8-
50
• 35 • 30 • 25 • 20
m 5 0
0 50 100 150 200 250 300
SEGMENT AREA (square km)
Figure 9. Characteristic dimensions of the flow segments. Lines of constant flow width between 4 and 8 km are shown as a guide to the equivalent average flow width (i.e., the segments of flow A all have an average width of -5 km). See Figure 7 for the location of the three different flows.
MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS 19,397
a. OBSERVED
--" 1. FLOW SEGMENTS IN SINGLE LINE. ?- BREAK-OUT ALWAYS AT LEADING EDGE
3. LOBE SEGMENTS ARE ABOUT SAME SIZE
1. LOBE SEGMENTS CONSTANT THICKNESS
2. NO UPRAISED EDGES BETWEEN SEGMENTS
1. MULTIPLE LOBES FROM ONE SEGMENT 2. LOBES FORMED FROM SEGMENT SIDE
NOT SEEN d.
DIRECTION OF FLOW •
1. UP-SLOPE SEGMENTS HAVE GREATER THICKNESS THAN DOWN-SLOPE SEGMENTS
2. DOWN-SLOPE SEGMENTS UNDERLIE UP-SLOPE SEGMENTS
Figure 10. Conceptual models of the planimetric and cross- sectional appearances of the lava flow segments. Figures 10a and 10b show features that are observed in the Viking orbiter images, while Figures 10c and 10d show features that are not observed. The interpretation is that new flow segments break out at the distal end of the existing flow lobe but that there is insufficient uplift to increase the overall thickness of the flow.
164 km 3 (40 m average thickness) to 1.2-246 km 3 (60 m average thickness). Such values compare with 0.001-0.012 km 3 for individual Pu'u O'o flows in Hawaii [Wolfe et al., 1988], 0.025- 0.030 km 3 for Mt. Etna, Sicily [Kilburn and Guest, 1993], 4.3 km 3 for the Carrizozo flow in New Mexico [Keszthelyi and Pieri,
1993], and 1-30 km 3 for the long lava flows of North Queensland, Australia [Stephenson and Griffin, 1976]. The profiles also provide constraints on the mechanism of lobe segmentation. A comparison of Figures 7, 11, and 12 shows that several lobe segments have similar thicknesses (-70 m for profiles 4, 5, 8, and 9). Downslope thickening between segments is seen on flow B (Figure 7) between profiles 8 and 7, whereas upslope thickening between lobe segments is observed in profiles 9-10.
5. Discussion
The morphological characteristics of the Elysium Planitia lava flows raise several questions. Are they unique, or do other segmented lava flows exist on Mars? Can we infer anything unusual about the eruption duration that produced these flows? How did the Elysium eruptions affect the Martian atmosphere, and was this activity particularly significant? Can the Elysium flows be used to learn anything about the mode of formation of very long terrestrial lava flows?
There are numerous different types of lava flow fields on Mars [e.g., Greeley and Spudis, 1981; Wilson and Head, 1994], but few display the segmented flow morphology that characterizes the flows shown in Figure 7. Segmented flows have been found to the NW of the summit caldera of Alba Patera, where multiple flow lobes, called "M-type flows" by Lopes and Kilburn [ 1990], occur. These Alba Patera flows appear to be topographically constrained, so that individual flows may have been forced by adjacent high points to override earlier flow lobes. However, image resolution
.•:• ........ •.•.-•.•:•..:•:•.:• • ....... •.
.......... 5:: • :'•::•:•-::::• :•'•'•:•..•.•..•.• •.• •?•. :::•.:•: •:: • • ........... 5• .........
• :::•:•:: .... ,.'-'-•.:. • :• • :•' •.'• •.•::-•' ...... •Z•-
..... :..•:•.•....:...•::: •..;.•::• . :::.'•:..:: :•:• ...... ..• ¾'•.• .............. :•:::?.•:.• -.:•:•:•.: .... : ...... •::• •.•: -..-.-.•-.:•:•.:• • ....
--. • ::'• .. :::.: .:•:•.•.. .•"[•::•::•
•:• ':•' '• •? ........
Figure 11. High-resolution (40 m/pixel) mosaic of four flows (flows 6, 9, 12, and 13) west of Hecates Tholus, s'howing the locations of the 20 topographic profiles that were taken using photoclinometry. Mosaic is of Viking orbiter images 651A08-14, with the illumination from the left. See Figure 1 for location.
19,398 MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS
•8o] • distance aloa• profile (lcm) 0, • •: • "•'""', •'•, 0 5 10
Figure 12. Topographic profiles across the lava flows identified in Figure 11. Vertical exaggeration is -14.6 for all profiles. Notice the great diversity in flow cross-sectional shape. All profiles were created on the assumption that the underlying surface was horizontal, which appears to be reasonable for this area on the basis of the smooth topography on either side of the flows.
for the Alba Patera flows is -80 m/pixel, so that a direct photoclinometric comparison of flow thicknesses with the Elysium Planitia flow field is not possible. While Viking orbiter image resolution varies over the planet, it appears that segmented lava flows are rare and may indicate unusual eruption and/or emplacement conditions within Elysium Planitia. If this flow segmentation is indeed limited to the five flows to the west of Hecates Tholus, it is possible that the segmentation may have been caused by unusually flat preexisting terrain, unusual basal conditions such as abundant permafrost [Mouginis-Mark, 1985], or rare upstream conditions that promoted pulsing of the flow [e.g., periodic breaching of a perched lava lake].
Because a flow volume >10 km 3 is large by terrestrial standards, it is pertinent to consider how long the eruptions that generated the Martian flows may have lasted, particularly because such large-volume eruptions may have had a significant impact on the local climate [cf., Plescia, 1993; Robinson e! al., 1993]. High effusion rates, perhaps as great as 107 m 3 s -I , have been suggested as an explanation for the long lava flows on Mars [Wilson and Head, 1994], although Sakimoto et al. [1997] infer that a rate of 105 m 3 s -1 would be sufficient to produce the long tube-fed flows at Alba Patera. An effusion rate of 107 m 3 s-! would have
produced the individual flow segments mapped in Figure 7 in just -2-25 min if they each have a volume of 1-15 km 3. Total time to form the entire flow would be -24-36 min (flow A), 91-136 minutes (flow B), and 69-104 min (flow C). These are considered to be unrealistically short times for the formation of the segments and flows because we can observe that the distal margin of a lobe stopped (and is inferred to have partially cooled) prior to subsequent flow reactivation and advance at the leading edge. At the opposite extreme, Keszthelyi [1995] calculates that Martian lava flows the length of the Elysium examples could have been
flow in 22.4-129.1 years. It is, however, worth noting that this low effusion rate is inconsistent with the study of Bruno et al. [1992], who concluded that the Elysium flows have margins suggestive of a'a flows. Effusion rates <20 m 3 s -1 have been shown to produce pahoehoe flows in Hawaii [Rowland and Walker, 1990].
While there is no way to be certain which effusion rates are applicable for Elysium Planitia, eruption durations of a year to a few decades appear reasonable for the individual flows. These large-volume, long-duration eruptions may therefore have had a significant effect on the Martian atmosphere. While the exact composition of the erupted materials that formed Elysium Mons is not known, a large body of evidence suggests that most Martian lavas are mafic, most likely basalt [cf. McSween, 1985; Banin et al., 1992; Soderblom, 1992]. For example, if one assumes a value of I wt % for the released water content of the parent magma [Greeley, 1987; McSween and Harvey, 1993], a mean density of 2.5 x 103 kg m -3, and a volume of 54.5-81.7 km 3 for the flow B (Figure 7), then 1.36-2.04 x 10 ]2 kg of water vapor would have been released into the atmosphere during flow emplacement. If we pick intermediate values of 102-104 m 3 s -] for the effusion rate, then the eruption duration would have been 63 days to 25.9 years and the rate of volatile release would have been in the range of 1.4 x 108 to 3.2 x 10 ]ø kg d -l. The relationship between eruption duration and effusion rate for the largest and smallest flow segments and for flows A, B, and C, is shown in Figure 13.
This estimated amount of water vapor released from flow B is a small portion of the total flow Elysium Planitia flow field and is significantly lower (by a factor of 104) than the amount calculated by Plescia [ 1993] for the Cerberus flows. Taking the total area of all 59 flows identified in this study (20438 km 2, in Table 1) and
10 5
•' 104_
Z
•0 103.
) 102'
U j10 -
10ø1b o 10 6
,4• FLOW B
1 YEAR
• 3 MONTHS
•RGEST SEGMENT • • • ' ' i - i
EFFUSION •ATE (mas '•)
Figure 13. Range of implied eruption durations for flows and individual lobe segments identified in Figure 7, assuming an average flow thickness of 50 m. Lower two lines are for individual flow segments (with volumes of 1.3 km 3 and 12.6 km3). Upper three lines are for flow A (17.7 km3), flow B (68.1 km3), and flow C (51.9 km3). For plausible effusion rates, this suggests that
produced as tube-fed flows with an effusion rate as low as 20 m 3 individual flow segments may have formed in a few months, s -]. Such an effusion rate would have generated each lobe while an entire flow may have formed in a few years to a few segment in -1.6-24.0 years and would have generated each entire decades.
MOUGINIS-MARK AND YOSHIOKA: MARS LONG LAVA FLOWS 19,399
assuming an average thickness of 40-60 rn results in a total flow volume between 817 and 1226 km 3. The commensurate mass of
water vapor released from all of these flows (as above, assuming 1 wt %) would be 2.04-3.01 x 1013 kg or ~0.2% of the inferred amount released from the Cerberus flows.
Our observations indicate that the Elysium Planitia flows may illustrate an emplacement style not recognized for terrestrial flows. Because young lava flows >100 km long are very rare on Earth, they are commonly deeply eroded, and it is not possible to make an interpretation of flow emplacement based on surface morphology. Studies of pahoehoe lava flows in New Mexico [Keszthelyi and Pieri, 1993] and in Hawaii [Hon et al., 1994] have indicated that posternplacement inflation can be responsible for a thickening of a basaltic lava flow from an original depth of a few tens of centimeters to several meters. Such observations have led
Self et al. [1996] to reinvestigate the emplacement history of individual flow units in the vicinity of the Columbia River, Washington, United States. Here several units of these very large volume (>100-1000 km 3) flows indicate that the lavas may originally have been emplaced relatively slowly as thin flow units that were subsequently thickened by inflation. However, we do not see any morphologic evidence on the Elysium flows for lava pits or rises, which would be diagnostic of posternplacement inflation. Although the lower Martian gravity may have made it easier for the entire flow surface to have been raised (rather than at the discrete localities that are seen on terrestrial flows) [Walker, 1991 ], we see no morphologic evidence for this process within Elysium Planitia.
6. Conclusions
Many of the best preserved examples of very long lava flows on Mars are located to the north and west of the volcano Elysium Mons. The distribution of flows tens to hundreds of kilometers in
length over much of the NW flank of Elysium Planitia testifies to the existence of a well-developed late-stage magma supply system associated with the volcano, capable of erupting large volumes of lava more than 500 km from the volcano over extended periods of time (years to decades). The occurrence of flows in this area with numerous discrete segments is very unusual for Mars and may be due to unusual eruption conditions, very shallow local slopes, or to the influence of an underlying permafrost layer. Analysis of the mode of formation of these segmented flows should provide fruitful opportunities for future numerical modeling of flow emplacement, cooling rates, and lava rheology, as the geometry of these breakouts is remarkably consistent for all of the observed examples. These studies should be further enhanced as regional topographic data from the Mars Global Surveyor mission become available to constrain the regional slopes. While the volume of water vapor released into the atmosphere by the eruption of the Elysium flows was of the order of 1012 kg, this amount is small in comparison with the calculated amount released from young flows in the Cerberus region in SE Elysium Planitia identified by Plescia [ 1993].
Acknowledgments. We thank Mark Robinson for his skills in image processing that enabled us to collect the lava flow profile data presented here. Scott Rowland, Jon Fink, and G. Jeffrey Taylor provided many useful comments during the development of this work, which formed part of the Masters thesis of M.T.Y. Steve Baloga, David Crown, and Tracy Gregg provided many stimulating ideas via their formal reviews. This research was supported via NASA grant NAGW-437 from the Planetary Geology and Geophysics Program. This is Planetary Geosciences paper 992 and SOEST publication 4634.
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P. Mouginis-Mark and M. Tatsumura Yoshioka, Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822. (e-mail:[email protected])
(Received January 6, 1998; revised March 29, 1998; accepted April 2, 1998.)
