Volume 109, Issue E3
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Morphometric properties of Martian volcanoes

J. B. Plescia

J. B. Plescia

U.S. Geological Survey, Flagstaff, Arizona, USA

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First published: 05 March 2004
Citations: 136

Abstract

[1] Mars Orbiter Laser Altimeter (MOLA) data have been used to construct Digital Elevation Models (DEM) of the Martian volcanoes in order to determine height, flank slope, caldera depth, and volumes. Summit elevations range from 21.1 km to −0.5 km, and relief varies from 1.0 km to almost 22 km. Average flank slopes are in the range of <1° to ∼10°, consistent with basaltic shield volcanism. The very low slopes of highland patera are also consistent with pyroclastic volcanism. Minimum volumes range from <1012 to 1015 m3. Estimates of the time required to build these volcanoes, on the basis of long-term terrestrial eruption rates, range from hundreds of thousands to tens of millions of years.

1. Introduction

[2] Morphometric properties of Martian volcanoes are critical to developing an understanding their geology, as interpretations often employ comparisons with terrestrial volcanoes. In addition, models of lava flow rheology and tectonic deformation require such information. Prior to the Mars Global Surveyor (MGS) Mars Observer Laser Altimeter (MOLA) data, heights and slopes of Martian volcanoes were derived from Viking stereogrammetric or photoclinometric data locally supplemented with Earth-based radar.

[3] For some volcanoes such as Olympus Mons a significant number of Viking stereo images were available such that a reasonable model of the topography could be established [e.g., Wu et al., 1984]. For other volcanoes, the topography was based on shadow measurements or photoclinometry [e.g., Robinson et al., 1993]. Mouginis-Mark et al. [1982b] examined broader scale relations by comparing Earth-based radar derived slopes with those indicated by lava flow directions in the Tharsis region. Although such data provided broad constraints, details of the flank slope, caldera depth, and edifice volume were largely unconstrained. MOLA data [Zuber et al., 1992], however, allow a more precise estimate of the various morphometric parameters and provide a data base for use in a wide spectrum of studies. The basic morphometric data for each of the Martian volcanoes is presented here.

[4] Several previous studies have estimated the altitude, flank slopes, relief and volume of different volcanoes, including Malin [1977], Pike [1978], Blasius and Cutts [1981], Robinson et al. [1993], Hodges and Moore [1994], Wu et al. [1984], and Zisk et al. [1992]. This work describes in more detail the results presented by Plescia [2002]. A similar analysis of Martian volcano height has been made by Mouginis-Mark and Kallianpur [2002].

2. Methodology

[5] The MOLA data base was queried for tracks covering each volcano and the surrounding area. Those data were imported into ArcView and gridded into a digital elevation model (DEM). Grid spacing was either 250 or 500 meters depending upon the dimension of the area to be gridded. Grids were produced using a spline algorithm. Initial DEMs were examined for bad tracks which were removed and the remaining data regridded. A few artifacts (linear or north-trending diamond-shaped artifacts) are observed in some DEMs where data gaps occur. Uncertainties associated with the development of the DEMs involve gaps in the data and the averaging to grid points. These effects may result in either missing or smoothing away the local maximum or minimum elevations.

[6] For each volcano a shaded relief image and an elevation contour map are presented. The illumination angle for the shaded relief was chosen to highlight the morphology. A summary of the morphometric attributes is presented in Table 1. Table 2 notes the differences between MOLA elevation and relief and that determined by stereogrammetry for the large Tharsis shields. Table 3 lists the flank slopes and roughness estimates.

Table 1. Morphometric Dataa
Volcano Edifice Dimension, km Summit Elev., km Relief, km Volume, m3 Flank Slope, deg Caldera Floor Elev., km Caldera Depth, km Caldera Complex Dimension, km
Alba Patera 1015 × 1150 6.8 5.8 1.8 × 1015 1.0 5.6 1.2 106 × 138
Albor Tholus 157 × 164 4.1 4.2 2.9 × 1013 5 0.6 3.5 36
Apollinaris Patera 189 × 278 3.2 5.4 7.3 × 1013 5 1.4 1.8 73 × 85
Arsia Mons 461 × 326 17.7 11.7 9.2 × 1014 5 16.2 1.5 108 × 138
Ascraeus Mons 375 × 870 18.1 14.9 1.1 × 1015 7 14.4 3.7 62 × 67
Biblis Patera 128 × 176 7.3 3.6 1.8 × 1013 5 2.7 4.6 53 × 59
Ceraunius Tholus 98 × 130 8.8 6.6 2.4 × 1013 9 6.6 2.2 25
Elysium Mons 375 14.1 12.6 2.0 × 1014 7 14.0 0.1 14
Hecates Tholus 177 × 187 4.8 6.6 6.7 × 1013 6 4.4 0.4 13
Jovis Tholus 52 × 58 3.1 1.0 8.7 × 1011 3 2.0 1.0 27 × 32
Olympus Monsa 840 × 640 21.1 21.9 2.4 x1015 5 17.9 3.2 72 × 91
Pavonis Mons 380 × 535 14.0 8.4 3.9 × 1014 4 9.2 4.8 91 × 96
Tharsis Tholus 131 × 158 9.1 7.4 3.1 × 1013 10 2.2 6.8 43 × 55
Ulysses Patera 100 5.8 1.5 2.9 × 1012 4 3.4 2.4 60
Uranius Patera 242 × 280 4.7 3.0 3.5 × 1013 3 2.3 2.4 88 × 115
Uranius Tholus 62 4.9 2.9 3.4 × 1012 8 4.6 0.3 20
Amphitrites Patera 600–700 2.0 0.5–1.5 nc 0.5 1.2 0.8 120
Peneus Patera 600–700 1.1 0.5–1.5 nc 0.5 0.3 0.8 120
Hadriaca Patera 330 × 550 −0.5 1.1 1.6 × 1013 0.6 −1.3 0.7 90
Tyrrhena Patera 215 × 350 3.2 1.5 2.1 × 1013 1.0 2.4 0.6 41 × 55
Syrtis Major Mereo Patera 1000 × 1400 2.2 4 nc 0.25–0.5 0.1 1.9 71
Syrtis Major Nili Patera 1000 × 1400 2.2 4 nc 0.25–0.5 0.2 1.3 52
  • a nc: not calculated.
Table 2. Comparison of USGS- and MOLA-Derived Topography
Volcano Summit Elevation, km Relief, km
MOLA USGS Δ MOLA USGS Δ
Alba Patera 6.8 6.5 0.3 5.8 3.5 2.3
Arsia Mons 17.7 20.0 −2.3 11.7 10.0 1.7
Pavonis Mons 14.0 18.0 −4.0 8.4 12.5 −4.1
Ascraeus Mons 18.1 26.0 −7.9 14.9 8.0 6.9
Olympus Mons 21.1 26.0 −4.9 21.9 24.0 −2.1
Table 3. Flank Slopes and Roughness
Volcano Average Slope,a deg Mean Slope,b deg Rms Slope,c deg Rms Heightd
Alba Patera
  Upper 0.9 0.9 1.0 292
  Lower 1.1 1.2 1.6 3385
Arsia Mons 5.1 5.2 7.7 2119
Pavonis Mons 4.3 4.3 5.1 1633
Ascraeus Mons 7.4 7.4 9.0 3649
Olympus Mons 5.2 5.2 6.4 3370
Uranius Patera 2.6 2.4 3.0 863
Uranius Tholus 7.7 7.7 7.6 526
Ceraunius Tholus 9.1 9.1 9.3 1456
Tharsis Tholus 9.8 10.3 10.3 1327
Biblis Patera 4.8 4.8 5.2 713
Ulysses Patera 3.6 3.6 4.0 307
Jovis Tholus 3.4 3.4 3.6 200
Elysium Mons 6.9 7.1 7.3 2506
Albor Tholus 5.4 5.6 6.6 912
Hecates Tholus 6.0 6.0 6.2 1780
Apollinaris Patera 5.3 5.4 6.8 1063
Hadriaca Patera 0.6 0.8 1.1 208
Tyrrhena Patera 1.0 1.3 1.6 148
  • a Average slope is calculated using the total elevation change and distance across the flank.
  • b Mean of the point to point slope calculations along the profile.
  • c RMS slope calculated using equation (3) of Shepard et al. [2001].
  • d RMS height calculated using equation (1) of Shepard et al. [2001].

[7] The critical parameter for determining the height and volume of a volcano is the selection of a reference surface. For the smaller volcanoes (e.g., Uranius Patera or Biblis Patera which are embayed by younger lavas) the reference surface is the contact between the flank and the surrounding plains. For the large Tharsis Montes shields and Elysium Mons, the “edge” of the volcano is harder to define. In these cases long lava flows extend for considerable distance away from the summit, covering the surrounding plains [e.g., Kallianpur and Mouginis-Mark, 2001]. The assumption is made that these flows are volumetrically minor [e.g., Plescia and Saunders, 1980] and so the diameter chosen is at the topographically lower change in slope. The volume quoted for each volcano is the volume derived from the DEM above the reference surface. Volumes quoted here should be considered a minimum since they consider only the region above the datum. Any volcanics hidden by the embayment by young plains or depressed due to loading would not be included. The volumes also do not include any material that has been eroded.

[8] Flank slopes were calculated using two methods. The first method is simply the slope determined by the difference in elevation and the length of the profile extending across all or most of the volcano flank and represents a single average for that flank. The second method uses the point to point slope calculated from the DEM and then presents the average of those values.

3. Volcano Topography

3.1. Tharsis

[9] Tharsis is the principal volcanic region on Mars with numerous constructs of differing age and morphology. The volcanoes are discussed in the following order: Alba Patera, the Tharsis Montes Group, Olympus Mons, the Uranius Group and the Western Group.

3.1.1. Alba Patera

[10] Alba Patera (Figure 1) is an enormous volcanic shield lying on the northern margin of Tharsis [Greeley and Spudis, 1981; Schneeberger and Pieri, 1991; Mouginis-Mark et al., 1988; Cattermole, 1990; McGovern et al., 2001]. It has long been recognized to be a wide and low-relief construct, although the vertical dimensions were uncertain. Alba is characterized by an extensive set of flanking graben (Tantalus and Alba Fossae) that extend from south of the volcano across the plains to the north. The summit region has a caldera complex, extensive lava flows and local dendritic valleys on the flanks that have been interpreted to indicate a pyroclastic phase of the eruptive history [Mouginis-Mark et al., 1988]. MOLA data for Alba Patera (Figure 1) indicate a more complicated morphology and geologic history than was suggested from Viking images. McGovern et al. [2001] have already presented preliminary MOLA topography of the shield showing its basic elements.

Details are in the caption following the image
Alba Patera. Shaded relief map (illumination from northeast) and contour map. Contour interval is 1 km, every fifth contour is highlighted. Features sp: summit plateau; us: upper shield; aw: western apron; ae: eastern apron are discussed in the text.

[11] The volcano (Figure 1) can be divided into a broad lower elevation construct (“lower shield”) having a wide summit plateau (referred to as the “bench” by McGovern et al. [2001]) on which is built a younger shield (“summit shield”). Alba Patera has an overall basal width of 1015 × 1150 km and a summit elevation of 6.8 km. Ivanov and Head [2002] suggested a basal width of 1350 × 1130 km and noted the marginal lava aprons and the lower and summit shields. The diameter is, as for several other large shields, somewhat arbitrary since lava flows extends well out onto the surrounding plains. The plateau and summit shield correspond to the upper member of the Alba Patera Formation mapped by Scott and Tanaka [1986]. However, the lower shield is far less extensive that the area mapped as the middle member of the Alba Patera Formation. On the east and west margins of Alba, aprons of lava flows radiate from the edge of the plateau. The eastern apron extends up to 400 km from a point on the edge of the plateau at 39.5°N, 104°W. A larger apron, 400–450 km long, extends down the western flank from a point ∼200 km west of the caldera near 41°N, 115°W.

[12] The lower shield reaches an altitude of ∼4 km (∼5 km above the northern plains) and exhibits a broad, relatively level plateau some 520 km across. Flank slopes are about 1.2°. On the southwest side of the plateau, the smaller, younger summit shield (∼350 km across and 2 km high) has been built with a summit caldera complex (Figures 1 and 3); it has flank slopes of <1° The plateau surface dips slightly from the faulted edges toward the summit shield. This deformation may reflect loading by the summit shield. McGovern et al. [2001] modeled the fault locations on Alba and concluded that surface loads alone can not account for the deformation and that a combination of intralithospheric sill complexes and regional stresses need to be considered to produce the observed pattern. Figure 2 illustrates north-south and east-west profiles across Alba Patera. Head et al. [1998a] and McGovern et al. [2001], using preliminary MOLA results, noted the topographic asymmetry and that the upper and lower flanks had different slopes.

Details are in the caption following the image
Shaded relief of the summit caldera complex of Alba Patera. The low shield that fills and overtops the northeast part of the caldera, essentially dividing it into an eastern and western half, is noted with an arrow. Calderas 1, 2, and 3 are discussed in the text.

[13] The summit region is composed of several overlapping calderas (Figures 2 and 3). The overall structure (190 × 110 km) has a subtle to well-defined margin and includes an isolated 25 km caldera (“1” in Figure 2) and the more obvious main caldera complex. The main caldera complex (“2” in Figure 2) is defined around most of its margin by a well-defined fault scarp and appears to represent at least two major episodes of caldera formation. The older episode appears to have formed a structure 165 × 100 km followed by the construction of a small shield volcano on the northeast part of the floor (Figure 2). The youngest event is the formation of the young 85 × 50 km caldera (“3” in Figure 2) in the southeast corner. Caldera floors have elevations of 5.5 to 5.8 km and are ≤500 m below the surrounding flank. The shield in the caldera has a summit elevation of ∼6.0 km, standing several hundred meters above the adjacent caldera floor. It is capped by a circular depression 12.5 km across which was noted by Mouginis-Mark et al. [1988], who suggested it as a possible vent for late pyroclastics. The morphology suggests this feature is the summit crater of a low shield rather than an explosive vent. A low shield [Greeley, 1982] is a term for a small constructional feature formed by the eruption of lava flows from a central vent; they are found on the Snake River Plain and in Hawaii (e.g., Mauna Ulu). In principal they are identical to a shield volcano except in scale.

Details are in the caption following the image
Topographic profiles across Alba Patera. (Upper panel) North-northeast to south-southwest. (Lower panel) West-northwest to east-southeast profile. Note the asymmetric profile in the NNE-SSW profile with steeper slopes on the north side compared with the more symmetric WNW-ESE profile. The fossae can be seen to occur at the break in slope on the flanks.

[14] The extensive network of graben that deform the flanks (Tantalus and Alba Fossae) occur on the upper edge of the lower shield. Tantalus Fossae cut up across the eastern apron and then down the southern flank. Alba Fossae, on north and west, extend up slope on the north flank and turn sharply southward along the western edge of the 4 km plateau and continue down the flank to the south. The graben occur at higher altitudes on the west side than the east (5.4 km vs. 4.3 km, respectively) because the western apron of lava stands about 1.5 km higher than the eastern apron. Fractures on the south side of Alba cut up the southern flank to a maximum altitude of about 5.4 km (lower altitudes to the east and west) where they appear to be buried by the younger summit shield. The fossae are positioned at and below the plateau margin on a slight (200 m) topographic rise.

[15] Overall, the morphology and shape of Alba Patera suggest it has had a complex geologic history. The lower shield is interpreted to be an older construct built to a height of ∼5 km. Presumably it was capped by a caldera complex that was subsequently filled, which might explain the present plateau. The summit shield volcano, with its own caldera, was then built with lava aprons on the east and west side burying the original flanks of the lower shield. The eastern and western aprons may have formed along marginal rift zones after the summit shield formed. Finally, the graben complex cutting the flanks of Alba were formed. Graben formation may have occurred in part concurrent with the building of the summit shield as the graben extending up the southern flank are partly buried. The presence of small valley networks in the summit region led Mouginis-Mark et al. [1982a] to suggest that a surface pyroclastic layer was present. However, the topographic data do not provide insight into whether such pyroclastics are present.

3.1.2. Tharsis Montes

[16] The Tharsis Montes include Arsia Mons, Pavonis Mons, and Ascraeus Mons, from southwest to northeast, respectively. These volcanoes have average surface ages which decrease to the northeast [Plescia and Saunders, 1979; Neukum and Hiller, 1981]. MOLA topography indicate the shields are morphologically more complex than suggested by Viking images and topography. Each has a lower summit elevation and thus less relief (Table 2) than indicated on the USGS topographic map [U.S. Geological Survey (USGS), 1989]. This difference was also noted by Smith et al. [2001], who presented estimates of the elevation, relief, and volume based on MOLA data collected at the beginning of the MGS mission. The greatest difference occurs at Ascraeus Mons where the USGS summit altitude is 26 km and the MOLA altitude is 18 km. The difference in summit altitude between the two data sets may in part be due to different datum, but since it varies from shield to shield, it must be a real discrepancy between the data sets. An interesting relation between the two data sets is that difference in summit elevations increases northward across the Tharsis Montes.

3.1.2.1. Arsia Mons

[17] Arsia Mons is the oldest of the group [Plescia and Saunders, 1979; Neukum and Hiller, 1981] having a summit altitude of 17.7 km on both the east and west sides of the caldera. This value is slightly greater than that originally reported by Head et al. [1998a] on the basis of preliminary data. Arsia's summit is characterized by a single, very large caldera bounded by concentric normal faults [Crumpler and Aubele, 1978; Scott and Zimbelman, 1995; Crumpler et al., 1996]. The volcano is composed of a central edifice and two aprons - one on the northeast and a second on the south-southwest side (Figure 4). Mouginis-Mark [2002] has recently suggested that ash deposits may be a widespread on the volcano.

Details are in the caption following the image
Arsia Mons. Shaded relief map (illumination from the northwest) and contour map. Contour interval is 1 km, every fifth contour highlighted. Note aprons on the northeast (an) and southwest flanks (as).

[18] The main edifice has a west-northwest/east-southeast width of 430 km and a similar dimension probably existed in the north-northeast/south-southwest direction prior to formation of the aprons. Slopes on the northwest and southeast flanks are ∼5°. The summit caldera is 130 km across, has a typical floor elevation of about 16.3 km and is 1.3 km deep with respect to the rim. A line of nine low shields having relief of 150 m crosses the floor along the same trend as the axis of the south-southwestern apron (N19°E). Head et al. [1998a, 1998b], using early MOLA results, noted the caldera floor lay about 1 km below the rim. They also discussed details of the bounding fault blocks noting steep scarps, inward-tilted fault blocks, and narrow troughs suggesting extension.

[19] The aprons, mapped as a separate unit from the flank by Scott and Zimbelman [1995], are younger than the main edifice, bury the main flanks and are not symmetrically positioned. The northeast apron is oriented N38°E whereas the southwest apron has an azimuth of S21°W°. The aprons are formed by lava flows extending from alcoves on the lower flanks of the main shield, a relation that was noted in the Viking images [Crumpler and Aubele, 1978]. The northeast apron begins at an elevation of 13 km and can be seen in the topography to extend for 275 km at slopes of 1–4° into the saddle between Arsia and Pavonis Mons. The south-southwestern apron begins at an elevation of 12–13 km and extends about 300 km with slopes of <1–2°. It is difficult to define a limit of the southern apron as lava flows extend all the way to the heavily cratered terrain of Memnonia.

[20] The geologic history of Arsia Mons appears to have had several major events: building of the main edifice, the development of the flanking aprons, filling of the caldera and finally construction of the low shields on the caldera floor.

3.1.2.2. Pavonis Mons

[21] Of the three aligned Tharsis Montes, Pavonis Mons (Figure 5) has the lowest summit altitude, 14 km, but still has as much as 10 km relief with respect to the surrounding plains. It also has very complex embayment relations with Arsia and Ascraeus Mons. Pavonis has a minor asymmetry in that the eastern flank has a slightly steeper slope than the western flank (4.6° vs. 4.1°). A series of short concentric arcuate graben, 500–1500 m deep, cut across the lower north flank. Aprons of lava, younger than the main edifice, extend from the northeast and southwest flanks. These aprons were mapped as separate units by Scott et al. [1998]. The northeast apron trends N43°E and is rather short, extending only about 100–110 km from its source at 7 km elevation; slopes on the apron surface are <1°.

Details are in the caption following the image
Pavonis Mons. Shaded relief map (illumination from the northeast) and contour map. Contour interval is 1 km, every fifth contour highlighted. Note the summit caldera complex (large diameter sag and deep young caldera). Features an: northeast apron, as1: symmetric southern apron; as2: linear southern apron; ls: low shields; sf: smooth facies; and d: debris apron are discussed in the text.

[22] The southwest apron is larger, more complex, and the lavas that are part of this apron extend all the way around the eastern margin of the volcano and overlie lavas from the northeast apron. The southwest apron is composed of two overlapping segments; an older segment trending southwest (S31°W) and a younger linear segment trending south-southwest (S15-18°W). The larger older segment begins at an altitude of about 8.5 km, the younger linear segment begins at a slightly lower altitude of ∼7 km. The surface of the older one is characterized by lava flows radiating from the summit of the apron. The younger one is characterized by lava flows extending from the apron axis and a cluster of low shields. Scott et al. [1998] map the older southern segment as part of the main shield (At4) and map the lavas of the younger segment as part of the youngest lava flows associated with the shield (At6).

[23] The summit region of Pavonis Mons includes a broad shallow depression with a young caldera inset at the southern end. In light of the MOLA topography, the geology is more easily understood than it was based only on Viking images. The broad shallow depression is about 90 km across and appears to be an older caldera now filled with lava and sagging; elevations vary from 12.6 to 11.9 km. The younger caldera is ∼48 km across, has a floor elevation of 9.2–9.3 km and thus it is almost 5 km deep with respect to its southern rim. Morphologically, it appears to have been formed a simple piston-like depression of the older materials.

[24] The large debris apron (“da” in Figure 5) extending down the northwest flank (the knobby and ridged facies of the fan-shaped surficial deposits of Scott et al. [1998]) is embayed at its margins. On the southwest side it is covered by lavas from Arsia Mons; the northern edge is buried by lavas from Ascraeus Mons. The smooth facies (“sf” in Figure 5) was interpreted by Scott et al. as occurring in topographically low areas. That material is, however, topographically high, standing as much as 800 m above the surrounding area, extends 165 km east-west and as far as 135 km from the volcano flank. Along its northern margin, the smooth facies appears to erode into a knobby facies. The arcuate ridges of Pavonis Sulci are 50–100 m high.

[25] After the initial shield-building phase ended at Pavonis Mons, the debris apron of the northwest flank was formed. This was followed by the development of the volcanic aprons on the flanks. Eruptions associated with the southwestern apron either developed later or persisted longer (or both) such that it ultimately buried the northeastern apron.

3.1.2.3. Ascraeus Mons

[26] Ascraeus Mons is the tallest (18.2 km) of the Tharsis Montes (Figure 6). Similar to Pavonis Mons, Ascraeus has a main edifice and marginal lava aprons extending from the northeast and southwest flanks. The flanks of Ascraeus Mons have average slopes of 7.4° and exhibit terraces as do the flanks of Olympus Mons. Flank terraces have been suggested to be both thrust faults [Thomas et al., 1990] and normal faults [Montesi, 2001; Cipa et al., 1996]. The summit has at least six coalesced calderas [Mouginis-Mark, 1981; Crumpler et al., 1996; Scott and Wilson, 2000]. Remnants of the several caldera are perched around the margin of the central caldera with age relations among these calderas having different interpretations [cf. Mouginis-Mark, 1981; Scott and Wilson, 2000]. Elevations around the caldera rims range from 18.2 km in the north to 17.4 km in the southeast.

Details are in the caption following the image
Ascraeus Mons shaded relief map (illumination from the southwest) and contour map. Contour interval is 1 km, every fifth contour highlighted. Note the aprons on northeast (an) and southwest flanks (as1, as2).

[27] Details of the calderas are discussed by Mouginis-Mark [1981]. The caldera on the northwest side is about 24 km across with a floor elevation of 17.4–17.2 km (sloping toward the main caldera). The caldera floor lies some 800–1000 m below the high point on the rim. The southwest caldera is an arcuate segment 35 km across (although the original caldera may have been larger) with a floor elevation of 16.7 to 16.9 km; the floor lies about 700 m below the local rim. On the southeast side, the caldera is about 20 km across (originally it may have been larger), has a floor elevation of 16.1 to 15.9 km and lies 1.5 km below the rim. The northeast caldera is some 32 km across and quite complex with at least two levels. Expanses of caldera floor occur at 17.6 and 16.5 km elevation. The main, youngest caldera is ∼36 km across and has a floor elevation of 14.4–14.6 km, 3.7 km below the summit elevation.

[28] The apron extending from the northeast flank is enormous, beginning at an elevation of 6.0 km lavas flows extend well out on the plains to the north and northeast. The axis of the apron trends N38°E and slopes are 1–1.5°. It is characterized lava flows and several low shield vents just above the contact with the eastern plains. A second apron on the southern side is also quite large consisting of two segments that begin near 8.5 km elevation–a symmetric southwest-trending (S25°W) segment (“as” in Figure 6) and a younger linear south-trending (S4°W) segment (“al” in Figure 6). Slopes on the older segment are about 1° and only 0.25° perpendicular to the long axis of the younger segment. Lava flows extend down the older segment turning west and north to northeast covering the plains west of Ascraeus Mons. The younger linear apron begins several km below the level where the older apron originates. Lava flows from the younger apron extend down slope and along the eastern margin of the volcano burying the distal eastern flows of the northern apron. The linear apron and the plains to the east are also populated by numerous low shields and vents; only a few low shields are observed on the plains west of the volcano.

[29] Similar to Pavonis Mons, Ascraeus Mons has a long complex volcanic history with the development of the shield and multiple summit calderas. The flank eruptions produced enormous volumes of aprons of lava that covered wide areas with a corresponding development of numerous scattered low shields.

3.1.2.4. Lava Aprons and Low Shields

[30] Lava aprons on Arsia Mons begin at elevations of 12–13 km; those on Pavonis at 7 km and those on Ascraeus Mons begin at 8.5 and 6 km. Although the elevations of the points at which the lava aprons begin are different among the three volcanoes, the elevations of the apron on opposite sides of a single volcano are similar. This suggests that on each volcano, a single magma source fed the aprons on both sides. The similarity of the elevations on Pavonis and Ascraeus Mons at which the aprons begin suggest the source region under both volcanoes might have been similar; the higher elevations for Arsia Mons' aprons suggest its source may have been be deeper.

[31] Along the southern and eastern flanks of both Pavonis and Ascraeus Mons are numerous low shields and localized vents (Figure 7). The low shields are typically 200 m high and of the order of tens of kilometers wide. Many are located on the south-trending aprons that extend from the flanks of both volcanoes. In addition, low shields occur on the plains east of the Pavonis Mons and north of Noctis Labyrinthus. There does not appear to be any particular structural control of the location or orientation of the low shields on the aprons. However, the low shields on the plains do have a preferred northeast orientation. The graben of Noctis Labyrinthus, which presumably lie at depth below the younger volcanic plains, have north-northeast trends. The low shields have a northeast structural control suggesting that the older tectonic features are no longer exerting influence.

Details are in the caption following the image
a. Low shield field northeast of Arsia Mons and north of Noctis Labyrinthus. Lava flows in many cases diverge around the lows shields suggesting the flows from the aprons post date the small shields. Note also that the shields have a northeast orientation compared with the north-northeast orientation of the Noctis Labyrinthus graben. Shading is from the south. b. THEMIS image V03712001 (resolution 18 m/pixel) showing the western margin of the low shield noted by the arrow in “a”.

3.1.2.5. Olympus Mons

[32] Olympus Mons (Figure 8) is the largest and most prominent shield volcano on Mars [Morris and Tanaka, 1994] having a basal width of 840 × 640 km. The dimension is somewhat arbitrary as it depends upon the morphologic feature chosen to represent the edge (e.g., the upper edge of the scarp vs. the distal edge of the scarp-draping flows). In this case the bottom of the basal scarp or the location of the change in slope of the scarp-draping flows was chosen. Clearly the shield extended farther than the basal scarp but the flank has been truncated by an unknown amount.

Details are in the caption following the image
Olympus Mons. Shaded relief map (illumination from the southwest) and contour map. Contour interval is 1 km, every fifth contour is highlighted. Feature df: draping flows–flows extending over and burying the basal scarp.

[33] The edifice rises to an altitude of 21 km and stands almost 22 km above the surrounding plains. It has a symmetric shape and slopes are relatively uniform (typically ∼5.2°). Slopes on scarp-draping lavas on the east and southwest margins are 2–6°. There is no correlation between slope and elevation, although variations in the slope are associated with terracing of the flanks (Figure 8); slopes become steeper over the terraces. Steep slopes (up to 20–30°) are observed on portions of the basal scarp that are not covered with lavas and on the caldera walls. The summit caldera complex is composed of six nested calderas of varying age and dimension [Mouginis-Mark and Robinson, 1992; Zuber and Mouginis-Mark, 1992]. Different parts of the caldera have different elevations. The northeast and southwest caldera and center of the main caldera all have minimum elevations that are similar, about 17.7 to 17.9 km. The larger part of the main caldera floor slopes toward the center with the highest elevations around the edge and the lowest in the middle.

[34] The total exposed volume of Olympus Mons is 2.4 × 1015 m3. This value represents the volume above the average elevation of the surrounding plains and does not include any volume that lies below this level, as might occur if there were loading-induced subsidence. The volume is similar to Alba Patera and to that of the entire Hawaii-Emperor chain [Bargar and Jackson, 1974], but an order-of-magnitude greater than the other Tharsis Montes shields.

3.2. Eastern Tharsis

[35] The geology of the Uranius Group (Uranius Patera, Uranius Tholus and Ceraunius Tholus) in northeast Tharsis was summarized by Plescia [2000], who also included estimates of the relief and volumes. South of this group and east of the Tharsis Montes lies Tharsis Tholus [Plescia, 2003]. In most cases the relief, volume and slopes estimated here are similar to earlier estimates, some of which were in part based on early MOLA data.

3.2.1. Uranius Patera

[36] Uranius Patera (Figure 9) has 3.0 km of relief and a summit altitude of 4.7 km. Early MOLA results [Head et al., 1998a] suggested at least 1.3 km relief. Flank slopes are asymmetric with a shallow northeast flank (0.8°) and steeper slopes on the other flanks (2.5°). The main caldera floor lies 2.4 km below the rim at an elevation of 2.3 km with slightly higher floor elevations in the northeast section. Slopes on the caldera walls are steep (16–25°). Similar to the impression from Viking images, the flank has a radial texture suggesting the presence of lava flows, but the flows are not resolved in the DEM. In Viking images, the contact along the eastern margin of the volcano flank and the adjacent faulted terrain was poorly resolved; MOLA data clearly define this contact.

Details are in the caption following the image
Uranius Patera. Shaded relief map (illumination from the southwest) and contour map. Contour interval is 500 m, every other contour highlighted. Note the lava aprons extending down the west (aw) and east flanks (ae).

[37] The shape of Uranius Patera indicates its development was more complex than effusion from a single point. Several fan-shaped segments emanating from points on the caldera edge make up the flank with the most prominent fans occurring on the western, northwestern and northeastern flanks. The fan on the western flank (trend N61°W) was recognized in Viking images, but the fan on the east side (trend N60°E) was not. Uranius Patera appears to have experienced multiple stages of growth and changes in shape. The oldest feature appears to be a shield with its long axis oriented N24°E. This was followed by the expansion of the caldera to the northeast and the development of the eastern apron and then an episode that built the western apron. The asymmetric topography may indicate that the regional slope to the northeast predates the construction of the volcano.

3.2.2. Uranius Tholus

[38] Uranius Tholus is a small volcano (Figure 10) standing 3.2 km above the surrounding plains with a summit altitude of 4.7 km. The structure is fairly equant, with flank slopes of ∼7.7°. The summit region is defined by a caldera 22 × 19 km across having a relatively flat floor with a smaller inset caldera and pit on the east side. A 200-m-high rim defines the northern margin of the caldera. The caldera floor dips southward, from 4.7 km at the northern margin to 4.4 km at the southern edge and was interpreted to result from the filling of a caldera and overflow down the south flank [Plescia, 2000]. The inset caldera floor is about 200 m below the main caldera floor and the pit is 300–400 m deep with respect to the main caldera level.

Details are in the caption following the image
Uranius Tholus. Shaded relief map (illumination from the south) and contour map. Contour interval is 500 m, every other contour highlighted.

3.2.3. Ceraunius Tholus

[39] Ceraunius Tholus (Figure 11) is intermediate in size among the Uranius Group. Its most prominent morphologic characteristic is the set of troughs extending down its flanks [Gulick and Baker, 1990; Reimers and Komar, 1979] and the large elliptical crater, Rahe, which occurs on the plains immediately north and into which the largest of the flank troughs debouch. Within Rahe, at the end of the trough, is a lobe of material presumably representing the material eroded from the trough and deposited on the crater floor.

Details are in the caption following the image
Ceraunius Tholus. Shaded relief map (illumination from the northwest) and contour map. Contour interval is 500 m with every fourth contour highlighted. Note deposit on the floor of crater Rahe that lies at mouth of the trough extending down the flank. Linear features east of the caldera are artifacts.

[40] The edifice has dimensions of 133 (WNW) × 99 km (ENE) with a maximum elevation of 8.8 km on the east side of the caldera; it stands 6.4 km above the surrounding plains. Slopes are 9° on the flank except on the western side where a younger lava apron has shallower slopes of ∼5°. Troughs on the west flank are about 100–200 m deep; the largest on the north flank is about 200–300 m deep. Trough depths increase down slope. The summit caldera is 26 × 27 km across and 2.2 km deep; caldera walls are steep (22–25°). Along the northern margin of the main caldera wall is a sliver of an older caldera. The floor of this older caldera lies 200 m below the rim and 1 km above the main caldera floor. A set of low hills some tens of meters high occur on the northern part of the caldera floor.

3.2.4. Tharsis Tholus

[41] Tharsis Tholus (Figure 12) lies on the eastern flank of Tharsis, south of the Uranius Group [Plescia, 2003]. It is unique among the Martian volcanoes in that the flanks are broken into blocks by large normal faults [Robinson and Rowland, 1994]. The embayed flanks of the volcano are clearly illustrated on the DEM which shows lavas flowing east across the plains around the obstacle of the volcano.

Details are in the caption following the image
Tharsis Tholus. Shaded relief (illumination from the southwest) and contour map. Contour interval is 1 km with every other contour highlighted. Note the divergence of flows around the edifice as they move to the northeast from the central Tharsis region. Flows also appear to have ponded against the southwest flank.

[42] The volcano is 131 (NE) × 158 km (NW) and rises 7.4 km above the surrounding plains to a summit altitude of 9.0 km. The highest elevation occurs on the west side of the caldera; the flank on the east side of the caldera is considerably lower at 4.8 km. The large faults that cut the flank have offsets of as much as 1.5 km, increasing from the summit toward the base of the volcano. At the base of the northwest flank are a group of fault-bounded blocks (up to 1.4 km tall) whose relation to the volcano is unclear.

[43] In profile, the volcano has a distinct convex shape, consistent with its bulbous appearance. The distribution of slopes around the flank is quite variable; typical slopes are of the order 7.5°. They range from 17.1° on the lower southwest flank to 4.8° on the northeast flank. The southwest and northwest flanks have steeper slopes on the lower part of the flank (∼17°) and more shallow slopes on the upper part of the flank (∼10°). The caldera floor is slightly tilted and lies at an altitude of about 2.2 km, 1 km above the surrounding plains.

3.3. Western Tharsis

[44] Three small volcanoes occur along the western flank of Tharsis including Biblis Patera, Ulysses Patera and Jovis Tholus. All three volcanoes are older than the Tharsis Montes shields and embayed by younger lavas from those shields [Plescia, 1994].

3.3.1. Biblis Patera

[45] Biblis Patera (Figure 13) is an elongate edifice measuring 128 × 176 km; the long axis is oriented downslope to the northwest. The surrounding plains dip to the northwest and range in elevation from ∼4.3 km southeast of the volcano to ∼2.6 km to the northwest. Biblis has a large, deep caldera bounded by concentric faults that drop pieces of the flank into the caldera; the flank is also cut by northwest-trending graben and fractures.

Details are in the caption following the image
Biblis Patera. Shaded relief (illumination from the southwest) and contour map. Contour interval is 500 m with every fourth contour highlighted. North-south trending feature west of the caldera is an artifact.

[46] Biblis Patera has a summit elevation of 7.3 km and relief of ∼3.6 km. Slopes are asymmetric with steeper slopes on the eastern side (2.5°) compared with the northwest flank (4.8°). This asymmetry suggests the volcano is younger than development of the regional slope. Fractures which cut the southern flank trend N26°W and are about 200 m deep. The caldera is somewhat conical in shape, about 53 × 59 km wide and has a minimum floor elevation of about 3 km (Figure 14), which is 1–1.5 km below the elevation of the surrounding plains. The southeast margin is composed of numerous blocks of the flank dropped down along concentric faults. Along the northwestern wall is a bench, presumably an older piece of the caldera wall.

Details are in the caption following the image
Southeast-northwest topographic profiles. Upper panel shows the Biblis Patera profile; lower panel shows a profile for Ulysses Patera. Both plots are at the same scale. Note caldera floors lie below the elevation of the surrounding plains.

3.3.2. Ulysses Patera

[47] Ulysses Patera (Figure 15) has a unique morphology in terms of the relatively large caldera and the two large impact craters on the flank. Its topography suggests a simple circular volcano with relatively uniform flank slopes of about 3.6°. The caldera is 60 km across, some 60% of the diameter of the entire edifice. Its rim stands 1.5 km above the surrounding plains reaching a summit altitude of 5.8 km. The elevation and relief are considerably less than that suggested by Plescia [1994], whose estimate was based on photoclinometry. Lying at an altitude of 3.4 km, 2.4 km below the rim, the caldera floor is almost 1 km below the surrounding plains (Figure 14). Debris flows, extending off the north and eastern walls onto the caldera floor from the impact craters on the flanks, are about 100 m thick.

Details are in the caption following the image
Ulysses Patera shaded relief (illumination is from the southwest) and contour map. Contour interval is 500 m with the 5 km contour highlighted.

3.3.3. Jovis Tholus

[48] Jovis Tholus (Figure 16) is a small shield volcano in the northwestern part of Tharsis. It stands only about 1 km above the surrounding plains, reaching a maximum altitude of 3.1 km. Early MOLA data [Head et al., 1998a] suggested about 900 m of relief and a summit elevation of 2.9 km. Plescia [1994] had suggested relief of 2 km. Slopes are uniform around the flank (average 3.4°), although the summit area has shallower slopes than the lower flanks. The caldera complex (27 × 32 km) covers much of the western portion of the shield and has a flat floor with an elevation of 2.0 km, only about 50 m above the plains to the west and 1.1 km below the summit. On the east side of the caldera is a topographic bench at 2.4 km which represents a piece of perched older caldera floor. The western edge of the caldera is separated from the plains by only a small septa of flank material standing ∼200 m above the level of the caldera floor and the surrounding plains. An interesting aspect of the local geology revealed by the MOLA data is the presence of a large, low shield volcano to the southeast (Figure 16). This shield is about 60 km across but only 250 m high; it was recognized in Viking images [Plescia, 1994] but its relief could not be determined. Its slopes are very shallow, about 0.5° and its lavas embay those of Jovis Tholus.

Details are in the caption following the image
Jovis Tholus shaded relief (illumination is from the south) and contour map. Contour interval is 250 m, the 2 and 3 km contours highlighted. Note low shield to the southeast that embays the construct.

3.4. Elysium Region

[49] Three volcanoes occur in the central Elysium region (Elysium Mons, Hecates Tholus, and Albor Tholus) and a fourth, Apollinaris Patera, lies to the south [Tanaka et al., 1992; Mouginis-Mark et al., 1984]. Malin [1977] reviewed the general attributes of the region and the three volcanoes based largely on Mariner 9 data and Mouginis-Mark et al. [1984] considered the regional geology in light of Viking data.

[50] MOLA data (Figure 17) indicate the presence a broad regional high ∼1600 km wide with a relatively flat summit plateau ∼500 km wide at an altitude of ∼1.3 km. This plateau stands some 5 km above the plains to the north and 3–4 km above the Cerberus Plains to the southeast. The overall regional high is asymmetric having steeper slopes on the northern to western and southwestern margins (0.6–0.9°) compared with those on the eastern margin (0.1–0.4°). The regional topography of Elysium is similar to that of Alba Patera with a broad high capped by younger volcanic constructs. The relief and dimensions of the Elysium region, based on preliminary MOLA data, were also discussed by Head et al. [1998a]; results presented here are similar to those.

Details are in the caption following the image
Elysium region shaded relief and topography. Shading is from the northwest highlighting the regional uplift and summit plateau. Contour interval is 1 km with every fifth contour highlighted. H: Hecates Tholus; EM: Elysium Mons; A: Albor Tholus; p: summit plateau discussed in the text.

[51] The volcanoes are not centrally located with respect to the regional high. Elysium Mons lies along the northeast margin of the plateau; Hecates Tholus lies at the northeastern base of the topography and Albor Tholus lies on the southeast margin. Because they are embayed by younger lavas, the diameters of Hecates Tholus and Albor Tholus are easily defined, although they represent minimum values. The diameter of Elysium Mons is somewhat subjective as lavas associated with the volcano extend up to ∼1000 km from the summit region.

[52] Many of the radial and concentric troughs that characterize the Elysium region appear to be controlled by the regional topography rather than by the topography of Elysium Mons. Troughs begin at or below the elevation of the regional summit (1.4 km) suggesting the source for the volatiles that formed the troughs is the regional high, rather than Elysium Mons. Similarly, the location of the arcuate graben correlate with the regional topography. Many are concentric about the regional topography rather than being concentric about Elysium Mons. If the graben represent loading phenomena, then the correlation between their location and orientation and the regional topography suggests it is the regional load rather than the load of Elysium Mons that is responsible for their formation.

3.4.1. Elysium Mons

[53] Elysium Mons (Figure 18) is the largest of the group; it rises to a summit altitude of 14.1 km, having relief of 12.6 km with respect to the surrounding plains. The relief is similar to that estimated by Malin [1977] although the summit altitude is considerably less than the 20 km he suggested. The summit elevation is also less than that indicated on the USGS topographic map, but the relief is similar. Slopes around the flank of Elysium Mons are fairly uniform and there is a correlation between elevation and slope; steeper slopes at higher altitudes (Figure 19). Average flank slopes are about 7°, although they range from ∼1° to 10° across the flank. The summit caldera is 14 km across but only about 100 m deep with respect to the rim, considerably less than the 500–1000 m suggested by Malin [1977].

Details are in the caption following the image
Elysium Mons shaded relief (illumination from the northwest) and contour map. Contour interval 1 km, every fifth contour highlighted. Note the level plateau southwest and western of the shield at an elevation of 2 km. p: summit plateau discussed in the text.
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Elysium Mons elevation and flank slope. Slopes are steepest at high altitudes. The low slopes at the summit reflect the caldera interior.

[54] For purposes of estimating the volume, the base of Elysium Mons was defined as the altitude of the regional high (1.3 km) since volcano slopes increase significantly above this altitude. Using this contour, Elysium Mons has a diameter of ∼375 km. Lava flows associated with Elysium Mons extend considerable distances down the regional slope [e.g., Mouginis-Mark and Tatsumura-Yoshioka, 1998]. If these lavas are considered, then the dimensions of the volcano would increase by hundreds of kilometers; the volume would probably not increase by as large a factor as the lavas are probably relative thin.

3.4.2. Albor Tholus

[55] Albor Tholus (Figure 20) has 5.6 km of relief with respect to the surrounding plains. The edifice appears to have a surrounding moat, best defined on the north and west sides, that is 30–50 km wide and filled with lava. The volcano flank is convex; slopes are steepest near the base and shallowest at the summit (average slope 5.5°). The highest elevation (3.9 km) occurs on the south side of the caldera. Two calderas characterize the summit region; an older large caldera and a small, younger caldera inset on the north side of the older one. The larger caldera is ∼32 km across; its floor lies at an altitude of 0.5–1.0 km and slopes northward. On the north side is the young caldera ∼10 km across with a floor having an elevation of ∼300 m, thus lying ∼300 m below the main caldera floor. Slopes on the caldera walls range from 16–27°. Albor Fossae, a set of linear troughs, cut across the southern flank of the volcano. The fossae are a few hundred meters deep and are concentric about the regional high.

Details are in the caption following the image
Albor Tholus shaded relief (illumination from the southwest) and contour map. Contour interval is 500 m, every fifth contour highlighted. The filled moat on the north and west side of the edifice is denoted with arrows.

3.4.3. Hecates Tholus

[56] Hecates Tholus (Figure 21) lies along the northern base of the regional Elysium high and is embayed by flows extending down slope from Elysium Mons. The flanks of Hecates Tholus exhibit numerous troughs suggested to be of fluvial origin [Gulick and Baker, 1990] and the base of the western flank has a large circular reentrant. Hecates has been extensively studied by Mouginis-Mark et al. [1982a, 1984], who suggested that the summit is covered with a pyroclastic deposit. There is no obvious topographic expression associated with the smooth area on the summit of Hecates suggested to be a pyroclastic deposit.

Details are in the caption following the image
Hecates Tholus shaded relief (illumination from the southwest) and contour map. Contour interval is 500 m with even numbered contours highlighted.

[57] Hecates Tholus rises to an altitude of 4.8 km having an average relief of 7.2 km, and as much as 8.4 km with respect to the plains to the north. The edifice is 177 × 187 km and is capped by a small, 13 km diameter caldera that is 400 m deep. Flank slopes are variable; typically the slopes are ∼6°, although near the summit they shallow to ∼3°. The slopes define a convex profile and are low compared with the visual impression of steep slopes around the margins. An elliptical embayment on the western flank has a complicated morphology. The eastern floor of the embayment is flat at an elevation of about −2.0 km; the western floor slopes down to about −2.7 km at the exit. The origin of the feature is unclear. If it is erosional in origin, it must have been re-filled and then re-eroded to produce the present topography.

3.4.4. Apollinaris Patera

[58] Apollinaris Patera (Figure 22) lies to the south of main group of Elysium volcanoes, it is bordered on the north and northeast by the Medusae Fossae Formation and on the remaining sides by highly eroded knobby material. The edifice stands ∼5.4 km above the surrounding plains reaching an altitude of 3.2 km. Slopes on the flank of the main edifice are ∼5° whereas those on the apron to the south are lower, about 1.4°. The summit caldera is 73 × 85 km and has a smooth flat floor at an elevation of 1.3 km on the north side (Figure 22). The floor from the east around the south to the western side slopes down to the flat floor on the north side of the caldera at slopes of about 3° from elevations of 3.1 to 2.3 km. Morphologically this sloping floor is a broken up into numerous arcuate fault blocks (e.g., THEMIS images V01630003, V02741001, V03827002).

Details are in the caption following the image
Apollinaris Patera shaded relief (illumination from the northwest) and contour map. Contour interval is 1 km, every other contour highlighted. Note the apron extending from the southeast margin of the caldera. The shaded relief image suggests that the Medusae Fossae Formation overlies the shield along its eastern margin. The large crater at the southern end of the image is Gusev.

[59] On the basis of topography and morphology, the volcano can be divided into a circular edifice and a large apron on the southern and southeastern margins [Robinson et al., 1993; Robinson, 1993]. The circular portion of the volcano (190 km diameter) appears to be an older symmetric edifice. The apron to the south begins at an altitude of 1.8 km and extends at least 70 km to the east and 140 km to the south. East of the volcano, a surface having morphology similar to the apron is exposed from beneath the Medusae Fossae Formation suggesting that the apron could extend up to 180 km in that direction. The apron represents a younger unit in which materials presumably erupted at the summit and flowed down the southern and southeastern flanks. The morphology of the apron resembles that of the highland patera and may indicate that it is composed of friable pyroclastic material rather than lava flows.

[60] Robinson [1990, 1993] and Robinson et al. [1993] compiled topographic data for Apollinaris Patera on the basis of shadow and photoclinometric measurements. Those data suggested the volcano was at least 5.1 km tall with the caldera floor lying 700 m below the eastern rim, similar to the values derived from MOLA data. They also suggested slopes on the volcano flank were steeper on the western side compared with eastern side. MOLA data indicate that the slopes are similar on both sides, although the eastern flank exhibits local areas (benches) with lower slopes.

3.5. Highland Patera

[61] The highland patera have always been considered a distinct group of volcanoes because of their low relief and unique flank morphology vis-à-vis the shield volcanoes [Greeley and Spudis, 1981; Greeley and Crown, 1990; Crown and Greeley, 1993]. DEMs for these features confirm their low relief and relatively small volumes. Similar to some of the shield volcanoes, diameters can be somewhat subjective. Locally, the patera have well-defined margins (e.g., contacts with the cratered highlands); elsewhere the transition from the edifice to surrounding volcanic plains is gradual and the boundary is somewhat arbitrary [cf. Gregg et al., 2002].

3.5.1. Hadriaca Patera

[62] Hadriaca Patera (Figure 23) occurs on the northeastern margin of the Hellas Basin [Crown and Greeley, 1993; Greeley and Crown, 1990; D. A. Crown and R. Greeley, Geologic map of MTM-30262 and -30267 quadrangles, Hadriaca Patera region of Mars, submitted to U.S. Geological Survey, 2003] and is characterized by a large shallow caldera and is suggested to have formed by pyroclastic flows. The typical incised flank morphology is interpreted to represent erosion of those friable pyroclastic materials. Troughs extend for hundreds of kilometers to the southwest down the regional slope into the Hellas Basin. Along the southeast margin, the volcano has been cut by the complex trough system of Dao Vallis.

Details are in the caption following the image
Hadriaca Patera shaded relief map (illumination from the southwest) and contour map. Contour interval is 500 m, the 2 km contour line is highlighted.

[63] MOLA data indicate the volcano is ∼330 × 550 km across, although the southwest topographic margin is arbitrary as material associated with Hadriaca extend far down-slope into Hellas. The summit altitude is −0.5 km and there is 1.2 km of relief with respect to the surrounding plains. This amount of relief is small compared with knobs of cratered highlands material in the area that have greater relief. The caldera is 90 km across and 700 m deep. On the basis of USGS [1989] topography, Crown and Greeley [1993] suggested flank slopes of about 0.10 to 0.47°; MOLA data indicate somewhat greater average slopes of about 0.8°. The ridges and troughs on the flanks have 100–200 m of relief, similar to that noted by Crown and Greeley [1993] and Gregg et al. [2002].

[64] Along the eastern margin of Hadriaca Patera is the Dao Vallis channel system [Price, 1998]. There are three source areas at 29.76286°S, 264.49639°W; 31.96214°S, 263.49961°W; and 33.05570°S, 266.47257°W which converge down stream into a single channel. The channel-heads typically have floor elevations of about −2.3 km and are 1.2–2.4 km deep with respect to the surrounding plains. The similarity of the floor elevations, suggest a common source horizon at that depth. The gradient of Dao Vallis is <1° along several hundred kilometers of length.

3.5.2. Tyrrhena Patera

[65] Tyrrhena Patera (Figure 24) lies along the margin of the Hellas Basin within the ridged plains of Hesperia Planum. Greeley and Crown [1990] and Gregg et al. [1998] map several units in the area, including two primary units on the edifice (basal and summit shield material) along with a caldera unit. The shield materials are interpreted as pyroclastic. In addition, an effusive unit of lava flows is mapped as extending to the southwest down the regional slope. Details of the extent and morphometry of these flows have recently been examined with MOLA data [Gregg et al., 2002]. The volcano has a prominent topographic edifice 215 × 350 km but Gregg and Crown [2002] have speculated that material to the west of the edifice may, in fact, be part of the volcanic construct making it considerably larger. The summit stands 1.5 km above the surrounding plains, reaching an altitude of 3.2 km and has an irregular caldera 41 × 55 km across that has a floor elevation of about 2.4–2.5 km. Robinson [1990] estimated the caldera depth at 400 m, similar to the results obtained here. The volume cited in Table 1 includes only the central edifice and does not include peripheral lava flows or the flows that extend to the southwest.

Details are in the caption following the image
Tyrrhena Patera shaded relief map (illumination from the north) and contour map. Contour interval is 500 m with the 2 km contour highlighted.

[66] Tyrrhena's flanks have a morphology (wide shallow radial troughs) similar to that of Hadriaca. Troughs begin on the lower flank and cut upslope, the upper parts of the flank are less eroded. MOLA data indicate the troughs are 150 to 300 m deep; terrestrial radar data [Zisk et al., 1992] suggested similar depths of 200–300 m. Overall, flank slopes are about 1°, less than the 3.6° estimated by Robinson [1990] but similar to that estimated by Greeley and Crown [1990] of 0.09° to 0.37°. Topographic profiles show the volcano has a slightly convex shape. Beyond the topographic edifice, the plains of Hesperia Planum are essentially flat at an altitude of 1.4–1.7 km.

[67] Rilles cut across the flank to the southwest, northwest and northeast from the caldera complex. The southwest rille trends S47°W and is ∼100 km long; it varies in depth from 500 m near the caldera to 150 m at the end. The northeast trough trends N30°E, is ∼55 km long and ranges in depth from 500 to 150 m. To the northwest, the rille cuts across the radial morphology of the volcano flank extending about 110 km with depths of 300 to 200 m. These rilles have been interpreted as lava channels [Gregg et al., 2002].

3.5.3. Amphitrites Complex

[68] Malea Planum (Figure 25) is an area of ridged plains on the southern margin of the Hellas Basin where two well-defined caldera, Amphitrites and Peneus Paterae, occur [Leonard and Tanaka, 2001; Peterson, 1977, 1978; Potter, 1976]. Overall, the regional topography is one of a broad, low-relief construct with the Peneus and Amphitrites calderas occurring on the high areas. Slopes on the plains extending into the Hellas Basin are 1.2–1.5°. The regional plains have linear to curvilinear wrinkle ridges with as much as several hundred meters of relief. Ridges are oriented in a strongly radial pattern around Amphitrites and less strongly around Peneus and the impact crater Barnard.

Details are in the caption following the image
Malea Planum. Shaded relief (illumination from the northeast) and contour map. Contour interval is 1 km with every fifth contour highlighted. Note the low relief of Amphitrites Patera (a) and Peneus Patera (p) which are not resolved with the 1 km contour interval. Features A and B are discussed in the text. bc: Barnard Crater; at: Australis Tholus.

[69] Potter [1976], in his geologic map of the eastern side of the Hellas Basin, mapped the area as “furrowed shield material” and interpreted it to be an old volcanic shield formed by low viscosity lavas. He identified four calderas: Amphitrites Patera, Barnard, an unnamed older caldera between and east of Amphitrites and Barnard, and a fourth to the north of Amphitrites. Peterson [1977] mapped the western side of the Hellas Basin including the western part of the Amphitrites complex. He mapped the area as “shield material” and defined two calderas: Peneus Patera and an unnamed feature immediately to the north. He also identified a dome referred to as Australis Tholus. More recently, Leonard and Tanaka [2001] map only Amphitrites and Peneus Patera as caldera.

[70] Amphitrites and Peneus Patera are well-defined circular depressions that can be interpreted as calderas (Figure 26). Barnard (61.65350°S, 298.58554°W) is an 120 km impact crater with a central peak, not a volcanic caldera. The crater floor lies ∼1.7 km below the surrounding plains, the rim stands ∼700–900 m high, except on the north side where a well-defined rim is absent; the central peak is 500–800 m tall. Australis Tholus (57.50273°S, 322.19621°W) is not a volcanic dome; rather it is an 11.7 km impact crater with an ejecta blanket 150–200 m thick. The feature north of Peneus (56.18323°S, 307.51078°W), interpreted as a caldera by Potter [1976], is defined by a circular ridge 122 km in diameter having a few hundred meters of relief. The eastern side of the ridge appears to be a piece of older terrain protruding through the plains; the western side is a formed by an arcuate wrinkle ridge. It is not a caldera, it may simply be a buried crater. The other unnamed features adjacent to Amphitrites that were suggested to be caldera are not observed in the topography.

Details are in the caption following the image
Close up of the Amphitrites (right) and Peneus (left) calderas. The large crater south of Amphitrites is Barnard. Note the concentric normal faults that bound the margins of Peneus and the ridge that bounds the Amphitrites caldera.

[71] Amphitrites Patera (59.01588°S, 298.90149°W) is 121 km in diameter and lies immediately north of the crater Barnard. Elevations around its margin are 1.5–1.8 km. The interior is bowl-shaped with a minimum elevation of 1.2 km, the lowest point being slightly offset to the west of the center. The surrounding plains have elevations of 100–500 m. Peneus Patera (58.12413°S, 307.45506°W) is 125 × 136 km (Figure 26). It has a relatively flat floor surrounded by a series of arcuate fault blocks that step down into the caldera; fault blocks have 100–200 m of offset. The surrounding plains ramp up toward the caldera edge across distances of 50 km. At the caldera rim, the elevations are highest in the southwest (1.1 km), about 700–800 m on the eastern and western margins, and 500 m on the northern margin. The floor is level at an elevation of 325 m, lying 150–800 m below the surrounding rim.

[72] MOLA data and Viking images indicate the presence of two additional depressions to the southwest of Amphitrites and Peneus Patera (Figure 27). The northern feature (Figure 27a) is located at 63.32852°S, 307.99209°W; it about 315 km in diameter and has a floor deformed by wrinkle ridges and patches of smooth material. The floor lies at an elevation of ∼0 km, 1–2 km below a regional high to the west and a few hundred meters below the plains to the east. The southern feature (Figure 27b) occurs at 67.26562°S and 322.36604°W and is 275 km across; it is surrounded by a well-defined sloping margin (1.6°) except on the eastern side where it opens onto the ridged plains. The floor lies 1.2–1.9 km below the surrounding plains at an elevation of ∼250 m. There is a smooth patch 90 km across and the eastern portion of the floor is very rough with massifs up to 800 m tall.

Details are in the caption following the image
Circular depressions on the southern part of Malea Planum. (a) Feature A in Figure 25 at 63.32852°S, 307.99209°W. (b) Feature B in Figure 25 at 67.26562°S and 322.36604°W.

[73] These two features appear to be simple sags. Perhaps they represent old impact craters filled with ridged plains material that sagged due to loading. There does not appear to be any obvious extrusive volcanism or vent structure associated with either feature. However, there are no obvious protrusions of old rim material through the plains either. This would suggest either the rims were heavily eroded prior to burial (typical of the oldest craters) or the material is sufficiently thick to bury any original rim topography. It is possible that the massifs in the southern feature represent an ancient central uplift.

[74] The morphology of the Malea Planum region suggests that it is of volcanic origin–the presence of what appear to be volcanic calderas at Amphitrites and Peneus and the ridges plains which clearly overlie the cratered highlands along the southern rim of the Hellas Basin. Overall the volcanic expression of Amphitrites Complex is more similar to that of Syrtis Major than of the other central vent volcanoes on Mars.

3.5.4. Syrtis Major

[75] Syrtis Major (Figure 28) lies on the western margin of the Isidis Basin [Meyer and Grolier, 1977; Schaber, 1982; Hiesinger and Head, 2002] and is characterized by two calderas (Mereo and Nili Patera) set in a larger elliptical depression. Hiesinger and Head [2002] measure the diameter of Syrtis Major as ∼1100 km with the highest elevations of 2.3 km occurring to the west of the calderas. Mereo and Nili Paterae lie at the northern end and the center of an elliptical depression 190 km (NE) by 400 km (NW) that is 0.8 to 1.3 km deep. The floor of the depression has an elevation of about 0.7 km (outside of the calderas); elevations on the western side are about 2.0 km and 1.7 km on the east side. The western margin of the depression has steeper slopes than the eastern margin (1.6° vs. 0.9°). Regional slopes on Syrtis Major Planum are relatively low (0.2°).

Details are in the caption following the image
Syrtis Major complex shaded relief and topography. Shading is from the south. Note the radial and concentric arrangement of wrinkle ridges and the scarp extending southeast from the calderas. Contour internal is 500 m with every other contour highlighted. N: Nili Patera; M: Meroe Patera.

[76] There are no significant tectonic features on the margins of the depression; only a few short, en echelon graben on the southwest edge. The depression appears to be accommodated by simple bending of the plains. Wrinkle ridges on Syrtis Major Planum form a rough rectilinear pattern, having either a radial or concentric orientation about the depression [Hiesinger and Head, 2002]. Ridges on the eastern side extending into the depression change orientation to a more radial pattern as they cross the margin and enter the depression. These relations suggest that ridge development post-dates the formation of the depression. Extending south-southeast from the depression is a scarp having several hundreds of meters to more than a kilometer of relief across it. The sense of offset is down to the east and the amount of offset decreases southward. The depression was not recognized by Schaber [1982], who used Earth-based radar to study the regional topography; not surprising given the relatively low spatial resolution of such data.

[77] Nili Patera (9.13683°N, 293.08849°W), the northern caldera, has a diameter of ∼70 km. Except for the southeast side, which opens into a broad irregular depression, the margin of Nili Patera is defined by concentric normal faults. The southern side of caldera is connected to the south-trending scarp mentioned above. The southeast part of floor is flat with an elevation of 0.1 km; the northwest part of the floor has a northeast trending ridge about 350 m. There is no obvious morphologic feature on the floor of Nili Patera to explain the relief (e.g., THEMIS I02207005 or V03318007). Mereo Patera (7.07614°N, 291.52978°W) is 71 km across and is bounded by normal faults except on the western side where it is open to the larger depression. The floor lies at an altitude of 0.2 km and is 1.2 km deep.

[78] The volume of the Syrtis Major volcanic feature is difficult to estimate since the background topography can not be resolved. However, Hiesinger and Head [2002] suggest the lava flows are 0.5–1.0 km thick and estimate the volume as ∼1.6 × 105 to 3.2 × 105 km3, which is probably a reasonable estimate.

4. Discussions

4.1. Elevation and Relief

[79] Summit elevations of the volcanoes range from −0.6 to 21.1 km (Table 1). Smith et al. [2001] reported altitudes and volumes for several Martian volcanoes on the basis of early MOLA data; those estimates generally agree with the values reported here. However, their values for Alba Patera and Olympus Mons are slightly larger. Mouginis-Mark and Kallianpur [2002] also determined relief for several of the volcanoes and derived values similar to those reported here. There are discrepancies between all of the MOLA-derived values and those from Viking-era topography [Pike, 1978; Wu et al., 1988; USGS, 1989]. Smith et al. [2001] and references therein discuss these discrepancies and their origin.

[80] There is a general relation in that the tallest volcanoes are the youngest (Figure 29) which has long been recognized. A similar relation exists with respect to relief in that the younger volcanoes generally have greater relief (Figure 29). These changes may reflect a deepening of source depths or more prolonged eruptive histories over time (a concept originally proposed by Carr [1973]). MOLA data do not resolve the nature of the mechanism, but simply provide better morphometric constraints.

Details are in the caption following the image
Volcano relief and summit elevation as a function of time. The relative ages of the volcanoes are assigned on the basis of the number of craters ≥1 km/106 km2 [N(1)].

4.2. Slopes

[81] MOLA data allow a precise evaluation of the flank slopes for the first time. Slope information is important because it has implications for understanding the style volcanism and the composition of the lava. For terrestrial volcanoes, lower slopes are typically associated with mafic chemistries whereas relatively steep slopes are associated with more silicic chemistries and their associated pyroclastics.

[82] Figure 30 and Table 3 show the information on flank slopes: an average slope and a mean slope. The average slope is simply the difference in elevation between the beginning and end of the profile and the length of the profile. The mean slope is the mean of the calculated slopes over base lengths of 4 pixels (about 2 km). Also listed are estimates of the flank roughness calculated from the elevations for each pixel: RMS slope (expressed in degrees) and RMS height. The RMS slope and height were calculated using the equations of Shepard et al. [2001].

Details are in the caption following the image
Volumes of Martian volcanoes compared with terrestrial examples. Data for terrestrial examples are taken from Bargar and Jackson [1974] for Hawaii and the Hawaiian Emperor Chain and from Eldholm and Coffin [2000] for the other provinces.

[83] The mean slope and RMS slope and height could be calculated at different scales with a corresponding difference in the values. Variations in calculated roughness at different scales and for different Martian terrain types have been discussed by Kreslavsky and Head [2000]. However, the objective here is simply to provide representative values. As illustrated in some of the figures, there is considerable variation in the flank slope. In some cases the slope correlates with elevation, in other cases different portions of the flank have different slopes. These variations are important when the details of the geologic history of a volcano are examined.

[84] Average slopes of the volcanoes' flanks range from <1° to ∼10°; Tharsis Tholus has greater average slopes than the other volcanoes. Kallianpur and Mouginis-Mark [2001] also examined volcano slopes with MOLA data and found a similar large range. Overall, there is no obvious relation between slope and age of the volcano. The observed slopes are consistent with a mafic chemistry for the lava flows (when compared with the distribution of slopes for terrestrial basaltic shield volcanoes). Hodges and Moore [1994] concluded that all of the Martian volcanoes (excluding the highland patera) were basaltic shields and the slope information presented here is consistent with that interpretation. The observed values are also consistent with terrestrial examples. Mark and Moore [1987] note slopes on Kilauea of about 3° and 3–9° on the lower flanks of Mauna Kea and up to 14° on the upper slopes. Cullen et al. [1987] illustrate topographic profiles across the Galapagos shield volcanoes: Darwin, Fernandina and Wolf. Each has a convex profile and maximum slopes of 24°, 26°, and 23°, respectively.

4.3. Volumes

[85] Volumes of the Martian volcanoes span several orders of magnitude (Figure 30, Table 1). These volumes should be considered minimum values as many of the volcanoes are embayed by younger materials and any loading phenomena that would result in volcanic products being depressed below the surrounding level are not considered. There is no data, however, to suggest that the depth of burial of the flanks is significant. Wilson et al. [2001] cite volumes for Olympus Mons and the Tharsis Montes shields from Smith et al. [2001]. Those values are virtually identical to those cited here except for Arsia Mons, in which case their volume is about 1.6 times larger. The difference presumably results from the choice of base elevations.

[86] Calculated volumes range from 1015 m3 for Olympus Mons and Alba Patera to <1012 m3 for Jovis Tholus. Volumes fall into two groups: the large Tharsis shields (1015 m3) and the other volcanoes (1013 m3). Hadriaca and Tyrrhena Patera have relative small volumes (1013 m3), similar to the smaller shields, and similar to those estimated by Crown and Greeley [1993] and Greeley and Crown [1990], who estimated 1–2 × 105 km3.

[87] Individual Tharsis Montes shields have volumes similar to that of the entire Hawaiian-Emperor chain [Bargar and Jackson, 1974]; the smaller volcanoes have volumes slightly less than the island of Hawaii. As had been recognized early in the study of Martian volcanism, the older volcanoes have smaller volumes than the younger giant shields. When the volumes of the volcanoes are examined within the context of their age (as derived from impact crater frequencies) there is the general pattern of the older volcanoes being smaller and lower but otherwise there is no specific relation.

[88] The greater precision of the volume estimates should allow for a better understanding of the length of time necessary to build the individual volcanoes. However, such an analysis is hampered by the lack of data on long-term Martian eruption rates. Some insight into the problem can be gained by using terrestrial basaltic eruption rates. Figure 31 illustrates the amount of time necessary to build the volcanoes assuming the long-term eruption rate for the Hawaii-Emperor chain and the rate for the island of Hawaii over the last 5 My [Bargar and Jackson, 1974]. These data suggest the amount of time necessary to build the individual Martian volcanoes is short with respect to their age. For the large Tharsis shields, periods of a few tens of millions of years to slightly more than 180 My for Olympus Mons are required. For the smaller constructs the time required is only hundreds of thousands to a few million years. These values assume that the volcanoes were built during a single geologic event, aside from the geologically short periods of repose observed in terrestrial shield volcanoes and which are reflected in the long-term Hawaiian eruption rates.

Details are in the caption following the image
Amount of time necessary to build the Martian volcanoes using the long term eruption rate for the Hawaiian-Emperor chain and the rate for the island of Hawaii for the last 5 Ma [Bargar and Jackson, 1974].

[89] Alternatively, Wilson et al. [2001] make the argument that the presence of multiple, coalesced calderas implies episodic magma delivery and long periods of repose. They argue that each magma reservoir that is responsible for a caldera must solidify (or at least reach 25% crystal content) before a new reservoir can form such a new caldera develops. Further, they conclude that these volcanoes had lifetimes of the order 1 Gyr spending most of the time in repose, with repose periods of the order 200 Myr. This conclusion is based on modeling the cooling rate of magma chambers to produce the calderas and assumes magma flux rates from the mantle of <101 to 102 m3 sec.

[90] Clearly the presence of multiple calderas indicates some period of repose. Multiple calderas are observed on many terrestrial volcanoes (e.g., Mauna Loa) which have been built over geologically short periods of time with only brief (with respect to their age) period of repose. In addition, the calculations of Wilson et al. [2001] suggest a period of only a few million years are needed for a reservoir to solidify. Thus, even allowing for periods of repose of a few million years for each caldera to form, with no concurrent eruption and shield building, the shields could still be built on timescales considerably shorter than 1 Gy. Wilson et al. [2001] argue for average magma supply rates of 0.05 m3 sec which is about 0.1 to 0.03 of the rates for the Hawaiian hot spot [Bargar and Jackson, 1974]. The entire Hawaiian Emperor Seamount chain (from Hawaii to the Detroit Seamount) was erupted within the last 82 Ma or so [Sager, 2002; Keller et al., 1995]. It is unclear why Martian magma flux rates would be so low when compared to the Earth, given that mantle plumes were the likely mechanism for magma supply [e.g., Breuer et al., 1997; McKenzie and Nimmo, 1999].

5. Conclusions

[91] The MOLA topographic data allow a much more precise and accurate estimate of the topography, volume, and slopes of the Martian volcanoes than was previously available based on Viking-era data. These data have been used to compile morphometric data for all of the Martian volcanoes. Volcano relief varies from about 1.0 km to almost 22 km. Average slopes of the volcanoes are in the range of <1° to ∼10°. These values are consistent with basaltic shield volcanism, although the very low slopes of highland patera are also consistent with pyroclastic volcanism. Volumes range from <1012 to 1015 m3. The time required to built these volcanoes, on the basis of terrestrial long term eruption rates, ranges from hundreds of thousands to tens of millions of years.

Acknowledgments

[92] I very much appreciate the meticulous and thorough editorial comments of an anonymous reviewer and I am beholden to David Crown for his careful review of the manuscript and efficacious comments. This research was made possible by a grant from the NASA Planetary Geology and Geophysics Program; thank you NASA.