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Young lava flows on the eastern flank of Ascraeus Mons:
Rheological properties derived from High Resolution Stereo Camera
(HRSC) images and Mars Orbiter Laser Altimeter (MOLA) data
H. Hiesinger,1,2 J. W. Head III,1 and G. Neukum3
Received 21 March 2006; revised 6 September 2006; accepted 30 November 2006; published 23 May 2007.
[1] We report on estimates of the rheological properties of late-stage lava flows on the
eastern flank of Ascraeus Mons, Mars. From previous studies it is known that the
dimensions of flows reflect rheological properties such as yield strength, effusion rates,
and viscosity. Our estimates are based on new high-resolution images obtained by the
High Resolution Stereo Camera (HRSC) on board the European Space Agency’s Mars
Express spacecraft in combination with Mars Orbiter Laser Altimeter (MOLA) data.
Compared to earlier studies, the high spatial resolution of the HRSC and MOLA data
allowed us to map 25 late-stage lava flows and to measure their dimensions, as well as
their morphological characteristics, in greater detail. Our estimates of the yield
strengths for these flows range from 2.0 102 Pa to 1.3 105 Pa, with an average of
2.1 104 Pa. These values are in good agreement with estimates for terrestrial basaltic
lava flows and are comparable to previous estimates derived for a small number of
lava flows on Ascraeus Mons. Our investigation indicates that the effusion rates for the
studied Ascraeus Mons flows are on average 185 m3 s1, ranging from 23 m3 s1 to
404 m3 s1. These results are higher than earlier findings that indicate effusion rates
of 18–60 m3 s1, with an average of 35 m3 s1. However, our effusion rates are similar to
terrestrial effusion rates of Kilauea and Mauna Loa and other Martian volcanoes. On
the basis of our estimates of the effusion rates and the measured dimensions of the flows,
we calculated that the time necessary to emplace the flows is on average 26 days.
Viscosities were estimated on the basis of yield strengths and effusion rates, yielding
average values of 4.1 106 Pa-s and ranging from 1.8 104 Pa-s to 4.2 107 Pa-s.
On the basis of newly available data sets (e.g., HRSC, MOLA) we are now able not
only to identify possible differences in eruptive behavior between Ascraeus Mons and
Elysium Mons but also to study such differences over time.
Citation: Hiesinger, H., J. W. Head III, and G. Neukum (2007), Young lava flows on the eastern flank of Ascraeus Mons:
Rheological properties derived from High Resolution Stereo Camera (HRSC) images and Mars Orbiter Laser Altimeter (MOLA) data,
J. Geophys. Res., 112, E05011, doi:10.1029/2006JE002717.
1. Introduction
1.1. Geological Context
[2] The Tharsis Montes, Arsia Mons, Pavonis Mons, and
Ascraeus Mons, are large volcanic constructs that are part of
the Tharsis bulge. The Tharsis bulge is commonly interpreted to be the result of a long-lasting large mantle diapir
that due to the absence of plate tectonics on Mars, had
enough time to significantly uplift the lithosphere and
initiate tectonic faulting and volcanism [e.g., Solomon and
1
Department of Geological Sciences, Brown University, Providence,
Rhode Island, USA.
2
Institut für Planetologie, Westfälische Wilhelms-Universität, Münster,
Germany.
3
Institut für Geologische Wissenschaften, Freie Universität Berlin,
Berlin, Germany.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2006JE002717$09.00
Head, 1982; Banerdt et al., 1992; Breuer et al., 1996;
Harder, 1998; Smith et al., 1999a, 1999b; Zuber et al.,
2000; and references therein]. MOLA data indicate that the
Tharsis bulge is topographically separated from Olympus
Mons and Alba Patera and is located at the Martian
dichotomy boundary [e.g., Smith et al., 1999a, 1999b;
Zuber et al., 2000]. The Tharsis Montes are the locations
of some of the youngest volcanic deposits on Mars [Scott
and Tanaka, 1986; Neukum et al., 2004a] and also show
evidence for very recent glaciation [e.g., Head and Marchant,
2003; Head et al., 2003, 2005; Shean et al., 2004; Parsons
and Head, 2004; Neukum et al., 2004a]. As discussed below,
the Tharsis Montes are considered to be large shield
volcanoes [e.g., Pike, 1978; Scott and Tanaka, 1986;
Greeley and Crown, 1990], but evidence has been presented
that indicates that these volcanoes might actually be composite volcanoes [Head and Wilson, 1998a, 1998b; Head
et al., 1998b].
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Figure 1. Geologic map of the Tharsis Montes [Scott and Tanaka, 1986]. Ascraeus Mons is at the upper
right of the map.
1.2. Dimensions
[3] Ascraeus Mons is the northern most (11°N, 256°E)
of the Tharsis volcanoes (Figure 1) and has a base diameter
of 435 km and a caldera of about 55 km on average
[Hodges and Moore, 1994]. On the basis of Mariner and
Viking data, Hodges and Moore [1994] estimated the height
of Ascraeus Mons to be on the order of 26 km, but MOLA
data indicate that the summit of the volcano is about 18 km
high (Figure 2).
1.3. Age
[4] Ascraeus Mons was previously mapped by Scott et al.
[1981] as Hesperian to Amazonian in age (AHvu). Similarly, in the geologic map of Scott and Tanaka [1986],
Ascraeus Mons is mapped as member 3 (AHt3) of the
Tharsis Montes Formation, which is Hesperian to Amazonian in age (N(2) = 320– 440; N(5) = 50 –75). Crumpler
and Aubele [1978] counted craters on two Viking images,
located on the southeast flank (90A49) and the summit area
(90A50). Compared to other Martian volcanoes such as
Arsia and Pavonis Mons, they found low cumulative crater
size distribution slopes, which they interpreted as evidence
for recent obliteration of small craters by numerous lava
flows. Crater counts of Neukum and Hiller [1981] suggest a
model age of the central shield of Ascraeus Mons of
1.3 b.y. and a model age of the caldera fill of 0.4–
1.0 b.y. On the basis of a model developed by Soderblom et
al. [1974], Hodges and Moore [1994] published model ages
of 0.26 b.y. for the central shield and 0.1 – 0.25 b.y. for the
caldera fill. More recent crater counts on the basis of HRSC
data revealed very young model ages of 0.1 b.y. for the floor
of the main caldera and up to 0.8 b.y. for the older smaller
calderas [Neukum et al., 2004a]. Finally, Schaber et al.
[1978] counted craters on the surrounding plains immediately northwest, west, and southwest of Ascraeus Mons. For
their unit K they found 300– 500 craters larger than 1 km
per 106 km2 (N(1) = 300 – 500) and for their slightly older
unit M they counted 850 –1150 craters larger than 1 km per
106 km2 (N(1) = 850 – 1150). Assuming that the Martian
crater production rate is a factor of two greater than that of
the Moon, Schaber et al. [1978] calculated absolute ages of
0.2– 0.33 b.y. for unit K and 0.58– 0.78 b.y. for unit M.
Figure 3 is a compilation of stratigraphic systems [e.g.,
Neukum and Wise, 1976; Tanaka et al., 1992; Hartmann
and Neukum, 2001], crater density ranges for N(2), N(5),
and N(16) [Scott and Tanaka, 1986], and ages of Ascraeus
Mons volcanic deposits found in the literature [e.g., Schaber
et al., 1978; Neukum and Hiller, 1981; Scott and Tanaka,
1986; Hodges and Moore, 1994; Neukum et al., 2004a]. On
the basis of data shown in Figure 3, we conclude that the
shield itself formed at least 1 to 1.5 b.y. ago, and that
units M and K are slightly younger and probably contemporaneous with the covering of the caldera floors by lava
flows. From this discussion we further conclude that the
investigated flows are not only stratigraphically young (i.e.,
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Figure 2. MOLA topography and MOLA shaded relief map with superposed location of HRSC orbit
h0016, which was used for this analysis.
superposed on older flows), but are also very young in terms
of absolute model ages.
1.4. Structure
[5] Are the Tharsis Montes and Ascraeus Mons in particular, shield volcanoes or composite volcanoes? Bates and
Jackson [1984, p. 463] define a shield volcano as ‘‘a broad,
gently sloping volcanic cone of flat domical shape, usually
several tens or hundreds of square miles in extent, built
chiefly of overlapping and interfingering basaltic lava
flows. Typical examples are the volcanoes Mauna Loa
and Kilauea on the island of Hawaii.’’ A composite volcano
or stratovolcano is described as ‘‘a volcano that is constructed of alternating layers of lava and pyroclastic deposits, along with abundant dikes and sills. Viscous, acidic lava
may flow from fissures radiating from a central vent, from
which pyroclastics are ejected’’ [Bates and Jackson, 1984,
p. 495]. On the basis of early Mariner and Viking images,
the Tharsis Montes were often interpreted as shield volcanoes, primarily because of their shapes, abundant lava
flows, distinct shield-like caldera complexes, and the apparent distinctiveness from other edifices interpreted to
represent pyroclastic eruptions [e.g., Pike, 1978; Scott and
Tanaka, 1986; Greeley and Crown, 1990]. However, Head
and Wilson [1998a, 1998b] and Head et al. [1998b]
concluded that there is a strong theoretical and observational
basis for a reinterpretation of the Tharsis Montes as
composite volcanoes. Support for an interpretation of the
Tharsis Montes as stratovolcanoes includes observations of
edifice mantling material, flank fragmental deposits, lobeshaped features, smooth deposits, summit cinder cones and
constructs, near-summit pit craters, the andesitic nature of
some flows, similarities to other pyroclastic deposits, differences between flank vent and edifice eruptions, and the
edifice morphometry [Head and Wilson, 1998b]. Furthermore, calculations indicate that under Martian conditions
(i.e., atmospheric pressure and gravity) even magmas with
very low volatile contents of 0.03 wt% will be disrupted
into pyroclastics in order to produce hawaiian or even
plinian explosive eruptions [Head and Wilson, 1998a].
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Figure 3. Compilation of ages of volcanic deposits associated with Ascraeus Mons (black bars) in
relationship to stratigraphies of Tanaka et al. [1992] and Neukum and Wise [1976] and Neukum and
Hartmann. The stratigraphies of Neukum and Hartmann are published in a joint paper, Hartmann and
Neukum [2001]. Also shown are crater densities of N(2), N(5), and N(16) [Scott and Tanaka, 1986].
Numbers 1 to 5 in the upper right indicate the following references: (1) Schaber et al. [1978]; (2) Scott
and Tanaka [1986]; (3) Neukum and Hiller [1981]; (4) Hodges and Moore [1994]; (5) Neukum et al.
[2004a]. AHt3 is the unit of the geologic map of Scott and Tanaka [1986], SH stands for the age of the
entire shield, CF indicates the age of the caldera fill, and K and M are units defined by Schaber et al.
[1978].
Hynek et al. [2003] reported on evidence for explosive
volcanism in the Tharsis region and argued that under
Martian atmospheric conditions, the ashes of such explosive
volcanic eruption will be widespread and that current
Martian winds would preferentially transport them from
Tharsis to the east and west depending on the season. This
is consistent with several far-field deposits that were interpreted as pyroclastic deposits of the Tharsis Montes, such as
northwest of Biblis Patera, west of Arsia Mons, the
‘‘Stealth’’ area, the ‘‘Greater Stealth’’ area, and the Medusae
Fossae Formation [e.g., Scott and Tanaka, 1982; Muhleman
et al., 1991; Edgett, 1997; Edgett et al., 1997; Head et al.,
1998b; Hynek et al., 2003].
[6] The calderas and their implications for the evolution
of Martian volcanoes have been the subject of numerous
studies [e.g., Crumpler et al., 1996; Head et al., 1998a;
Scott and Wilson, 2000; and references therein]. Crumpler
et al. [1996] defined two types of calderas, the Olympustype and the Arsia-type, with the Olympus-type being
characterized by distinct fault-related boundary walls and
nested and overlapping collapse craters. Because the caldera
of Ascraeus Mons shares these basic characteristics with the
caldera of Olympus Mons, Crumpler et al. [1996] considered the Ascraeus caldera as an Olympus-type caldera. The
new HRSC data are consistent with this definition, because
they show evidence for numerous graben and normal faults
along the caldera margins, steep caldera walls, and at least
8 nested and overlapping collapse craters, as previously
described by Zimbelman and McAllister [1985]. The complex summit caldera of Ascraeus Mons indicates multiple
stages of magma ascent and withdrawal and the large depth
and diameter of the last caldera might be related to volu-
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minous eruptions elsewhere on the flanks, especially in the
SSW and NNE of the volcano [Crumpler et al., 1996].
Nested caldera sequences, pits on the volcano flanks, largescale slumping and terracing, as well as the sector structure
were interpreted as evidence for dike emplacement and
fissure eruptions outward from shallow magma chambers
[Crumpler et al., 1996].
1.5. Composition
[7] High-resolution imaging data show that Ascraeus
Mons was built by a very large number of individual lava
flows, many of which show lava channels, signaling an
eruption style similar to basaltic or basaltic-andesitic
hawaiian shield volcanoes [e.g., Greeley, 1973; Zimbelman,
1985; Greeley et al., 2000; Head et al., 2001]. In situ
analyses of rocks at the two Viking, the Pathfinder, and the
two MER landing sites indicate a basaltic to andesitic
composition of a large number of rocks [e.g., Rieder et
al., 1997; McSween et al., 1999, 2004; Greeley et al., 2005].
McSween et al. [1999] argued that the andesitic composition
of sulfur-free rock at the Pathfinder landing site resembles
terrestrial anorogenic icelandites, which formed by fractionation of tholeiitic basalt magmas. Analyses of McSween et
al. [1999] indicate that the rocks of the Pathfinder landing
site have Si02 contents of 52 –62 wt%, hence fall within the
basalt-basaltic andesite-andesite fields of the SiO 2 Na2O + K2O diagram of Le Bas et al. [1986]. At least
three rocks (Adirondack, Humphrey, Mazatzal) in Gusev
crater, analyzed by the Spirit rover, are basaltic in composition with relatively low SiO2 content (45 –46 wt%),
hence plot along the left margin of the basalt field in the Le
Bas diagram [McSween et al., 2004]. A comparison of the
compositions of Adirondack, Humphrey and Mazatzal with
compositions of dust-free Pathfinder rocks, MGS-TES surface types and Martian meteorites indicates that these three
rocks have the lowest SiO2 abundances among the analyzed
rocks [McSween et al., 2004]. SNC meteorites, especially
the shergottites, also plot within the basalt field of the Le Bas
et al. [1986] alkali-silica diagram [e.g., McSween, 1985,
1994; Banin et al., 1992]. However, not all SNC meteorites
are basaltic in composition as chassignites are olivine-rich
dunites and nakhlites are clinopyroxenites/wehrlites
[McSween, 1994]. On the basis of TES data, Bandfield et
al. [2000] identified two global spectral end-members that
they interpreted as two distinct lithologies, i.e., basalt and
andesite. Whereas surface type 1 is consistently interpreted
as basalt, surface type 2 is either interpreted as andesite
[Bandfield et al., 2000; Hamilton et al., 2001] or partly
altered basalt [Wyatt and McSween, 2002; Morris et al.,
2003; Ruff, 2004; Wyatt et al., 2004]. As Wyatt et al. [2004]
pointed out, this ambiguity arises because a spectral component of surface type 2 can be interpreted as volcanic
siliceous glass, common for andesite, or as secondary
phases (e.g., smectite, palagonite, silica coatings, zeolite)
common in altered basalt.
[8] In summary, on the basis of morphology, in situ
sample analysis, remote sensing data, and SNC compositions, most Martian lava flows are thought to have compositions that range from basaltic to andesitic [e.g., Greeley
and Spudis, 1981; McSween, 1985, 1994; Banin et al.,
1992; Mouginis-Mark et al., 1992; Greeley et al., 2000;
Bandfield et al., 2000; Hamilton et al., 2001; Wyatt and
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McSween, 2002; Morris et al., 2003; McSween et al., 2004;
Ruff, 2004; Wyatt et al., 2004].
1.6. Lava Flows
[9] In the HRSC images we observe several lava flows
with well-defined leveed channels on the eastern flanks of
Ascraeus Mons, some of which are truncated by the
collapse of the calderas and extend for tens of kilometers
downslope. On the basis of morphologic similarities
between terrains on Ascraeus Mons and terrestrial shield
volcanoes, Zimbelman and McAllister [1985] proposed that
individual prominent flows on Ascraeus Mons are a’a flows
and the planar areas adjacent to the flows are pahoehoe
flows. Figure 4 illustrates the similarities in dimensions and
flow outlines between terrestrial basaltic flows on Mauna
Loa and flows on Ascraeus Mons.
[10] Wilson et al. [1993] reported that if no other factors
intervene, thermal constraints will be the limiting factor for
the maximum length of a flow fed by a given volume or
mass effusion rate. They classified lava flows into several
categories, including (1) cooling-limited flows, (2) volumelimited flows, (3) accidentally breached flows, (4) break-out
flows, (5) captured flows, and (6) tube-fed flows. A coolinglimited flow is characterized by a flow front that stops due to
cooling and a central channel that did not drain. If the vent
remains active, a breakout flow will form at some point on
the margin of the initial flow. In volume-limited flows the
flow front stops when the effusion from the vent stops. In
this case the channel may drain, but there are normally no
breakout flows associated with volume-limited flows.
According to work by Wilson et al. [1993], volume-limited
flows are shorter than cooling-limited flows. If the central
channels are blocked, a breakout flow will form from a
point upstream of the blockage and consequently the
accidentally breached flow will be shorter than if it had
not been breached. Breakout flows form on the sides or
fronts of cooling-limited flows when the effusion continues
after the flow front of the initial flow stopped due to cooling
or blockage. Such a breakout flow itself may become
cooling-limited or volume-limited. If the pre-existing
topography confines a flow to a width that is narrower
than the flow would have adopted on a flat, inclined plane,
a so-called captured flow will form. Tube-fed flows are
flows that are characterized by a roofed-over tube system.
Wilson et al. [1993] argued that although lava cools only
slowly within the tube system, tube-fed lavas are overall
slightly cooler than lavas erupting from a primary vent.
Therefore tube-fed flows will be shorter compared to flows
that originated from a primary vent at the same effusion rate
[Wilson et al., 1993].
[11] On the basis of the new HRSC data we mapped
25 lava flows, which we named A through R. In cases
where the flow split into several flow lobes, we labeled each
individual lobe of the flow with numbers, e.g., E1, E2, and
E3. We find that our flow N shares numerous characteristics
of a volume-limited flow such as short length, a drained
channel, and no breakout flows. However, the majority of
flows appear to be more akin to cooling-limited flows or
breakout flows. Flows E1 E3 might have formed as
accidentally breached flows or breakout flows. We did not
find evidence for captured flows and evidence for tube-fed
flows remains at best ambiguous.
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Figure 4. Comparison of lava flows on (left) Mauna Loa, Hawaii [Lockwood et al., 1988] and (right)
Ascraeus Mons at the same scale. The map of Mauna Loa shows lava flows of different ages, i.e.,
historical flows erupted since 1843 (red), group IV basalts (0.75– 0.107 ka, orange), group III basalts
(1.5 – 0.75 ka, purple), group II basalts (4.0 – 1.5 ka, blue), and group I basalts (older than 4.0 ka, green).
Similarly, colors of the Ascraeus map indicate relative ages based on superposition. The youngest flows
are shown in yellow with orange, red, and purple colors representing successively older flows. Larger
impact craters and fields of secondary craters are shown in blue. Several Ascraeus Mons flows discussed
in the text are shown, for example, flows B1, C1, E1-3, F1, G2, and H1. See Figure 5 for close-up views
of these flows. Note the general similarity of flow size and flow outlines, supporting the idea that the
Ascraeus Mons flows are probably also similar in composition.
1.7. Motivation
[12] In the past, rheological properties of Martian lava
flows have been studied in great detail [e.g., Hulme, 1976;
Moore et al., 1978; Zimbelman, 1985; Cattermole, 1987;
Mouginis-Mark and Yoshioka, 1998; Peitersen et al., 2001;
Warner and Gregg, 2003; Baloga et al., 2003; Glaze et al.,
2003a, 2003b]. Some of these studies were based on
Mariner and Viking imagery with spatial resolutions of tens
of meters. With HRSC and MOLA we can considerably
extend these studies because the new data offer the opportunity to investigate large areas at high spatial (10 m) and
vertical resolution.
[13] In this study we will address the following questions:
(1) What are the rheological properties (e.g., yield strength,
viscosity) of 25 individual lava flows on Ascraeus Mons?
(2) How do flows on Ascraeus Mons compare to other
Martian lava flows? (3) How do the rheological properties
of these flows compare to terrestrial and lunar basalt flows?
(4) What are the rates of emplacement and how do they
compare to terrestrial and lunar analogs?
2. Database
[14] For our study we used data from several space
missions, including data from the American Mars Global
Surveyor (MGS) and Mars Odyssey spacecraft and the
European Mars Express spacecraft. In particular we utilized
data from the Mars Orbiter Laser Altimeter (MOLA) and
the High-Resolution Stereo Camera (HRSC), but also
inspected images of the Mars Orbiter Camera (MOC) and
the Thermal Emission Imaging System (THEMIS). As most
of these instruments and data are described elsewhere, we
will only provide a brief introduction to the HRSC data
[e.g., Malin et al., 1992, 1998; Smith et al., 1998, 1999a,
1999b, 2001; Christensen et al., 1999, 2001; Malin and
Edgett, 2001].
[15] The concept of the High-Resolution Stereo Camera
(HRSC) was originally developed for the Russian Mars ’96
mission. After the failure of Mars ’96, the camera was
selected as payload for the European Mars Express Mission,
which was launched on June 2nd, 2003. The HRSC camera
is a linescan camera with 9 CCD lines (blue, green, red, IR,
3 stereo channels, 2 photometric channels) oriented perpendicular to the flight direction [Neukum et al., 2004b]. The
HRSC camera acquires images at spatial resolutions as high
as 10 m/pixel and is complemented by a Super Resolution
Channel (SRC) with a 1024 1032 pixel frame CCD,
which obtains images of about 2.3 m/pixel from an altitude
of 250 km at periapsis. The high-resolution images of the
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SRC are nested within the HRSC image, yielding very
detailed information for areas of special interest.
[16] Due to the large amount of data acquired by the
HRSC and SRC cameras during one periapsis pass, there is
a need for data reduction. The camera experiment utilizes
two methods: pixel summation and compression. The 1 1
summation keeps the full resolution of the nadir channel of
orbit h0016, which we used for this study. The second
method is a JPEG-based data compression. Depending on
the dynamic range of the image scene, the compression
factor can be varied between 4 and 10. For downlink, the
images of orbit h0016 have been compressed on board the
spacecraft by a factor of 7.6021.
[17] The nadir image of Ascraeus Mons taken during
orbit h0016 has a spatial resolution of 11.00961 14.84151 m (depending on the distance to the periapsis),
an incidence angle of 30 42.5°, an emission angle of
0.3 0.6°, and a phase angle of 30 42°. While taking the
image, the spacecraft was 275 371 km above the surface
of Mars. There are no Super Resolution Channel (SRC)
images of orbit h0016 available for the investigated flows.
3. Approach and Technique
[18] Lava flow morphologies are thought to reflect the
rheological characteristics of the lavas [e.g., Wilson and
Head, 1983]. Consistent with previous studies, we make a
few basic assumptions: (1) rheological properties can be
estimated from remotely sensed data, (2) flow dimensions
are related to the rheological properties of the flow, (3) lava
flows behave as Bingham fluids, (4) lava flows in laminar
fashion, (5) no inflation of lava flows has occurred, (6) the
densities of Martian volcanic rocks are on average
2,500 kg m3, (7) the Graetz number is 300, and (8) the
thermal diffusivity is on the order of 104 – 108 m2 s1
with an assumed value of 3 107 m2 s1.
[19] Lava flows are usually modeled as a Bingham plastic
controlled by two parameters, the yield strength and the
plastic viscosity [e.g., Wilson and Head, 1983]. The yield
strength t of lava flows (Pa) can be related to the flow
dimensions by the following equations [e.g., Moore et al.,
1978]
t ¼ r g sina h
ð1Þ
t ¼ r g h2 =w
ð2Þ
t ¼ r g sin2 a 2wl
ð3Þ
t ¼ r g sin2 aðw wc Þ
ð4Þ
where r is the density (kg m3), g is the gravitational
acceleration (m s2), a is the slope angle (degree), h is the
flow height (m), w is the flow width (m), wl is the total
levee width (m), and wc is defined as the width of a leveed
channel (m).
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[20] The effusion rates Q (m3/s) were calculated as
Q ¼ Gz k x w=h
ð5Þ
where Gz is the dimensionless Graetz number, k is the
thermal diffusivity (m2 s1), x is the flow length (m), and w
and h are defined as above [e.g., Wilson and Head, 1983;
Zimbelman, 1985].
[21] The mean flow velocity u of lava flows (m/s) is
related to the effusion rate Q by
Q¼whu
ð6Þ
[22] For the determination of the viscosities h (Pa-s) we
made use of the relationship given for example by Fink and
Griffiths [1990] and Warner and Gregg [2003]
h ¼ ðQ h=r gÞ1=4
ð7Þ
[23] Note that the equation of Fink and Griffiths [1990]
and Warner and Gregg [2003] assumes a Newtonian flow
behavior and is therefore a simplified approach as lava
flows have a Bingham rheology [e.g., Shaw et al., 1968;
Hulme, 1976].
[24] Jeffrey’s equation also relates the viscosity of a flow
to its effusion rate and its dimensions [e.g., Nichols, 1939;
Gregg and Fink, 1996; Gregg and Zimbelman, 2000].
h ¼ r g h3 w sina =nQ
ð8Þ
[25] In this equation n is a constant equal to 3 for broad
flows and 4 for narrow flows. Gregg and Fink [1996]
pointed out that although Jeffrey’s equation has been widely
used to derive lava flow characteristics, it requires the
simplified assumption that lava behaves as a Newtonian
fluid.
[26] Alternative methods to calculate viscosities are presented by Wilson and Head [1983] and Zimbelman [1985]
r ¼ wc =ðw wc Þ
h ¼ w3c t sin2 a=ð24QÞ
ð9Þ
for r < 1
h ¼ w11=4
t 5=4 sin6=4 a= Q g1=4 r1=4
c
for r 1
ð10Þ
ð11Þ
with all variables defined as above.
4. Measurements and Input Parameters
[27] In the following paragraphs we will discuss the input
parameters necessary for the determination of the rheology
of lava flows.
4.1. Dimensions
[28] We measured the dimensions of 25 lava flows,
located on the eastern flank of Ascraeus Mons in order to
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Figure 5. HRSC (orbit h0016) images of the investigated lava flows. White lines indicate the locations
of our measurements of the length and widths of these flows. The north arrows indicate the proper
orientation of each flow.
derive their rheological properties. Values for the slope a,
flow length l, flow width w, flow height h, total levee width
wl, and the width of the leveed flow channel wc can be
determined directly either from MOLA data or from the
HRSC imaging data. Length and width of these lava flows
can be readily measured on HRSC images, but the height of
these flows is close to or below the vertical resolution of
digital elevation models (DEM) that can be confidently
calculated from the HRSC data at this time. For this reason
we measured the flow height with two other techniques:
shadow measurements and individual MOLA profiles. For
the shadow measurements we used HRSC data with a
spatial resolution of 12.5 m/pixel because there are no
MOC images available for the investigated flows, and
Thermal Emission Imaging System (THEMIS) and VIKING data either cover only limited areas or are often of
insufficient spatial resolution.
8 of 24
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HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
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Figure 5. (continued)
[29] We measured the length of each flow along the
central white line in Figure 5 and performed about
184 measurements of the flow width, and 80 measurements
for the levee and channel widths. On the basis of our
measurements, we find the average flow length to be
19 km, ranging from 4.1 to 38.3 km. As some of
our flows are truncated by the Ascraeus caldera and some
flows are covered by younger flows, we consider our length
measurements to be minimum estimates. The average width
is 1.3 km, with a minimum of 573 m and a maximum of
2,031 m. Figure 5 shows the locations of our measurements for each individual flow. For lava flows that show
leveed channels, we measured the average width of the
levees and the channel, as well as the average width of
the leveed flow. We find the average leveed flow width to
be on the order of 991 m, ranging from 593 m to
2,267 m. The average levee width varies between 450 m
and 1,742 m, with an average of 707 m. The average
channel width is about 284 m, with a minimum of 142 m
and a maximum of 526 m.
[30] While the length and the width of the lava flows can
be readily measured, the estimation of the height of the
flows with shadow measurements on HRSC images is
complicated by the relatively high sun angle that results in
short shadow lengths, which are difficult to measure precisely at the resolution (12.5 m/pixel) of the HRSC data.
In a first attempt to constrain the flow height, we used
HRSC images to perform 224 individual shadow measurements along the 25 investigated flows. On the basis of these
shadow measurements, we find that these flows are on
9 of 24
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HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
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Figure 5. (continued)
average 39 m thick, varying from 24 to 88 m. In a
second attempt, we used individual MOLA profiles to
independently derive flow heights. These MOLA profiles
across the flows indicate an average thickness of 13 m,
ranging from 5 to 24 m. These values are considerably
smaller than for the shadow measurements and are at the
lower end of the published flow thicknesses [e.g., Schaber
et al., 1978; Zimbelman, 1985; Head et al., 1998b;
Peitersen et al., 2001; Warner and Gregg, 2003; Glaze et
al., 2003a, 2003b]. However, lava flows on Earth are
commonly only a few meters thick, and on the basis of
the discussion below, we conclude that MOLA measurements of the flow thickness are more reliable than our
shadow measurements.
[31] Reasons for the observed differences in flow thicknesses between shadow measurements and MOLA profiles
include the illumination geometry, the image resolution and
the different orientations of the lava flows with respect to
the incoming sun light. The illumination of the scene results
in relatively short shadows, which are difficult to measure
precisely at 10– 15 m/pixel image resolution. For example, at the given illumination, if the measurements of the
shadow length were off by only ±1 pixel in the images, this
would result in an error in the calculated flow height of
±15 m. In addition, flows oriented parallel to the sunlight
only cast a few shadows that can be measured. Due to the
decreased accuracy of the determination of flow heights
with HRSC shadow measurements, we decided to only use
flow thicknesses extracted from individual MOLA profiles
for our calculations of the rheological properties. On the
basis of our study, we agree with Glaze et al. [2003b] that at
this time co-registered high-resolution imaging data with
10 of 24
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HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
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point-to-point slopes. We find that the average slopes of the
studied flows range from 1.5° to 6.7°, with most flows
being emplaced on slopes of 3.6°. For the calculation of
these average slopes 10– 80 individual MOLA data points
were used, depending on the length of the flow. Figure 6 is
an example of a profile along flow E1 and its point-to-point
slopes. We used the average point-to-point slopes as input
for equations (3), (4), (8), (10), and (11). While thickening
along the length of the flow might influence our slope
measurements, such an effect is negligible [e.g., MouginisMark and Yoshioka, 1998; Glaze et al., 2003a; Baloga et
al., 2003].
Figure 6. Example of a profile and point-to-point slopes
along flow E1 based on MOLA data.
individual MOLA data points/profiles is probably the best
way to derive flow heights with maximum accuracy. In
summary, all calculations in this paper are based on flow
thicknesses extracted from individual MOLA profiles; due
to the difficulties outlined above, we did not use thicknesses
based on shadow measurements in HRSC images.
[32] In order to constrain the reasonable range of Martian
flow thicknesses for comparison with our measurements, we
performed an extensive literature search. Schaber et al.
[1978] reported that the calculated heights of flow scarps
range from 5 to 65 m, with the small values (<5 m to 20 m)
representing narrow, channeled flows associated with the
steeper slopes (0.5° to 4.5°) of the large shield constructs.
Large flow heights (20 m to 65 m) were measured on broad,
flat flows commonly found on the less steep slopes of the
lower terrain [Schaber et al., 1978]. On the basis of shadow
measurements of lava flows on Ascraeus Mons, Zimbelman
[1985] derived average flow heights of 30 m. Peitersen et
al. [2001] used MOC image SP2-39605 to measure flow
thicknesses of 13– 33 m, averaging 19 m. Using MOLA
topographic data, Head et al. [1998b] measured the thickness of lava flows on Arsia Mons, Alba Patera, Elysium
Mons, and Syrtis Major. They found flow thicknesses from
25– 220 m. A thickness of 20 m was published by Glaze
et al. [2003a] on the basis of a MOLA profile across a flow
on Ascraeus Mons. Individual MOLA profiles were used by
Glaze et al. [2003b] to measure thicknesses of up to 37 m
for a flow northwest of Elysium Mons. Using MOLA data,
Warner and Gregg [2003] estimated average flow heights to
be on the order of 65 ± 20 m. From this comparison, we
conclude that our MOLA-based measurements are consistent with flow thicknesses found in the literature.
4.2. Slopes
[33] For slope measurements we relied on Mars Orbiter
Laser Altimeter (MOLA) gridded topography data with a
resolution of 128 pixel/degree. From this data set we
extracted elevations along the same white lines that we
used for the length measurement (Figure 5) and calculated
4.3. Rock Densities
[34] Due to the lack of Martian samples, the densities of
rocks that make up Martian lava flows are poorly constrained. Moore et al. [1978] assumed in their calculations
densities that range from 2,500 to 2,900 kg m3, Cattermole
[1987] used a density of 2,600 kg m3, and Zimbelman
[1985] and Warner and Gregg [2003] used 2,500 kg m3.
However, Wilson and Head [1994] showed that, depending
on the porosities of volcanic rocks at the surface (ranging
between 25 and 75%), densities can vary between 725 to
2,175 kg m3. On the basis of SNC meteorites, Longhi
[1990] suggested a more ultramafic composition with melt
densities of 2,750 – 2,960 kg m3. Due to these widely
ranging numbers, we need to discuss the influence of
density variations on our calculations. On the basis of our
literature review we chose to apply a density of 2,500 kg
m3 for our nominal calculation. However, being aware of
the large density differences found in the literature, we also
performed calculation in which we varied the density by
±10%, ±30%, and ±50%, resulting in densities of 1,250,
1,750, 2,250, 2,750, 3,250, and 3,750 kg m3.
4.4. Thermal Properties
[35] The Graetz number relates the rate of heat loss from
a flow to the rate of heat advection within a flow along its
length [e.g., Gregg and Fink, 1996]. Because there is
theoretical and observational evidence that suggests that
terrestrial lavas cease to flow when the Graetz number
decreases to about 300 [e.g., Wilson and Head, 1983, and
references therein], we assume such a value for our calculations. Such a value for the Graetz number was also used
for example by Zimbelman [1985], Warner and Gregg
[2003], and Gregg and Fink [1996]. Thermal diffusivity
appears to be less well known and consequently we found
values that range over three orders of magnitude. For
example, Gregg and Fink [1996] used a thermal diffusivity
of 7.2 104 m2 s1, whereas Warner and Gregg [2003]
used 3.0 107 m2 s1, a value similar to the one of
Zimbelman [1985], i.e., 7.0 107 m2 s1. In their
Table 9.1, Gregg and Zimbelman [2000] list thermal diffusivities for basalt (5.0 107 m2 s1), andesite (3.0 107 m2 s1), dacite (2.0 107 m2 s1), and rhyolite
(1.4 106 m2 s1). On the basis of these references, we
chose to use a thermal diffusivity 3.0 107 m2 s1.
5. Results
[36] For this study we selected 25 young lava flows, that
were erupted late in the history of Ascraeus Mons, are
11 of 24
12 of 24
22.84
22.09
16.17
16.92
12.04
9.06
12.8
21.54
15.98
15.29
10.78
9.72
5.8
10.25
10.9
24.27
8.98
10.44
11.28
11.8
7.95
19.17
4.53
13
7.82
1,245
1,949
1,786
2,031
1,132
1,173
964
1,504
1,326
1,414
975
1,321
1,388
1,869
645
573
1,085
1,155
1,324
1,212
1,381
1,936
1,084
1,917
963
1,334
Flow
Width,
m
5.05
6.77
4.87
4.48
4.88
3.52
2.41
2.28
3.97
2.96
4.29
3.15
2.13
1.98
6.09
4.06
1.97
2.05
3.35
3.46
3.57
3.08
2.98
3.77
1.57
3.55
Slope,
deg
250
208
298
252
249
158
388
526
360
284
574
429
496
610
1,742
688
707
464
257
775
920
745
743
536
142
259
262
237
253
269
Channel
Width,
m
450
743
727
571
565
697
Levee
Width,
m
991
1,049
826
678
654
998
2,267
995
950
834
1,239
1,177
593
1,002
989
809
818
966
Leveed
Flow
Width,
m
1.81E
1.61E
1.28E
9.69E
4.71E
3.36E
8.26E
1.04E
1.11E
7.82E
3.73E
3.14E
5.74E
6.76E
3.49E
8.09E
4.89E
5.87E
6.55E
5.90E
3.85E
1.18E
1.16E
7.49E
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
04
04
04
03
03
03
03
04
04
03
03
03
03
03
03
03
03
03
03
03
03
04
03
03
6.41E + 03
Yield
Strength 1,
Pa
2.72E
2.24E
2.15E
2.27E
1.40E
5.09E
1.15E
3.06E
2.44E
1.65E
7.80E
4.71E
4.86E
1.71E
1.02E
4.75E
5.68E
8.38E
8.59E
6.70E
5.43E
1.79E
1.99E
1.45E
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
03
03
03
03
03
02
03
03
03
03
02
02
02
03
03
03
02
02
02
02
02
03
02
03
4.58E + 02
Yield
Strength 2,
Pa
+
+
+
+
+
04
04
04
04
04
+
+
+
+
+
04
04
04
04
05
5.11E + 04
5.55E + 04
1.26E
1.02E
3.15E
4.15E
1.26E
4.18E + 04
1.90E + 04
1.20E + 04
6.92E + 04
4.58E + 04
6.06E
8.46E
9.81E
4.02E
1.85E
1.01E + 05
Yield
Strength 3.
Pa
+
+
+
+
+
04
04
04
04
03
+
+
+
+
+
03
03
04
04
04
2.55E + 04
2.78E + 04
6.30E
5.12E
1.58E
2.07E
6.30E
2.09E + 04
9.51E + 03
5.99E + 03
3.46E + 04
2.29E + 04
3.03E
4.23E
4.91E
2.01E
9.27E
5.03E + 04
Yield
Strength 4,
Pa
2.79E
3.63E
4.05E
1.81E
8.48E
1.93E
2.83E
2.05E
6.78E
1.81E
8.26E
5.39E
3.11E
4.23E
5.86E
7.05E
1.32E
1.72E
4.91E
3.29E
2.20E
2.42E
6.77E
2.14E
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
04
04
04
04
03
03
04
04
03
04
03
03
03
03
03
03
04
04
04
03
03
04
02
04
3.94E + 04
Ave. Yield
Strength (1 – 4),
Pa
Yield strength 1 was calculated applying equation (1), yield strength 2 was calculated using equation (2), yield strength 3 was derived from equation (3), and yield strength 4 was calculated making use of equation (4)
as described in the text. See text for details.
a
A1
A2
B1
C1
D1
E1
E2
E3
E4
F1
G1
G2
H1
J1
K1
K2
L1
M1
M2
N1
O1
P1
P2
Q1
R1
Average
Flow
Number
Flow
Height,
m
Table 1. Calculated Yield Strengths of the Investigated Lava Flows on Ascraeus Monsa
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
E05011
E05011
Table 2. Calculated Effusion Rates and Eruption Durations of the Investigated Lava Flows on Ascraeus Monsa
Flow
Number
Flow
Height,
m
Flow
Width,
m
Flow
Length,
m
Slope,
deg
7.82
1,245
1,949
1,786
2,031
1,132
1,173
964
1,504
1,326
1,414
975
1,321
1,388
1,869
645
573
1,085
1,155
1,324
1,212
1,381
1,936
1,084
1,917
963
1,334
23,032
14,614
29,397
38,394
17,882
36,158
18,377
16,347
17,315
21,969
4,145
27,811
20,650
10,679
11,324
6,563
16,747
17,951
30,463
6,607
29,051
22,243
13,272
18,284
4,565
18,954
5.05
6.77
4.87
4.48
4.88
3.52
2.41
2.28
3.97
2.96
4.29
3.15
2.13
1.98
6.09
4.06
1.97
2.05
3.35
3.46
3.57
3.08
2.98
3.77
1.57
3.55
A1
A2
B1
C1
D1
E1
E2
E3
E4
F1
G1
G2
H1
J1
K1
K2
L1
M1
M2
N1
O1
P1
P2
Q1
R1
Average
22.84
22.09
16.17
16.92
12.04
9.06
12.8
21.54
15.98
15.29
10.78
9.72
5.8
10.25
10.9
24.27
8.98
10.44
11.28
11.8
7.95
19.17
4.53
13
Effusion
Rate,
m3 s1
Eruption
Duration 1,
days
Eruption
Duration 2,
days
330
8
9
207
318
113
226
132
244
161
130
23
216
239
185
113
33
150
77
404
69
320
328
163
165
87
185
67
63
34
37
19
11
21
60
33
30
15
12
4
14
15
76
10
14
16
18
8
47
3
26
80
57
27
31
14
5
17
49
28
29
17
12
3
12
2
73
11
12
17
21
9
55
2
25
a
Effusion rates are based on equation (5); eruption duration 1 was calculated by dividing the flow length by the calculated mean flow velocity. Eruption
duration 2 was derived from dividing the flow volume by the effusion rate. See text for details.
clearly superposed on older flows, show a range of flow
morphologies that can easily be measured, and are illuminated in a way that allows shadow measurements or are
crossed by individual MOLA profiles in order to estimate
their heights. In the following section we will discuss our
results for the determinations of yield strengths, effusion
rates, eruption durations, and viscosities.
Table 3. Calculated Viscosities of the Investigated Lava Flows on Ascraeus Monsa
Flow
Number
A1
A2
B1
C1
D1
E1
E2
E3
E4
F1
G1
G2
H1
J1
K1
K2
L1
M1
M2
N1
O1
P1
P2
Q1
R1
Average
Flow
Height,
m
Flow
Width,
m
Slope,
deg
7.82
1,245
1,949
1,786
2,031
1,132
1,173
964
1,504
1,326
1,414
975
1,321
1,388
1,869
645
573
1,085
1,155
1,324
1,212
1,381
1,936
1,084
1,917
963
1,334
5.05
6.77
4.87
4.48
4.88
3.52
2.41
2.28
3.97
2.96
4.29
3.15
2.13
1.98
6.09
4.06
1.97
2.05
3.35
3.46
3.57
3.08
2.98
3.77
1.57
3.55
22.84
22.09
16.17
16.92
12.04
9.06
12.8
21.54
15.98
15.29
10.78
9.72
5.8
10.25
10.9
24.27
8.98
10.44
11.28
11.8
7.95
19.17
4.53
13
Channel
Width,
m
Leveed
Flow
Width,
m
269
966
142
259
262
237
253
593
1,002
989
809
818
464
257
1,239
1,177
250
208
298
995
950
834
252
249
158
388
526
826
678
654
998
2,267
360
1,049
284
991
Viscosity 1, Pa s
Viscosity 2,
Pa s
Viscosity 3,
Pa s
Ave. Viscosity
(1 – 3),
Pa s
1.06E + 05
7.50E + 05
3.70E + 05
4.09E + 05
1.23E
6.98E
5.66E
3.39E
1.48E
2.57E
1.55E
1.55E
2.67E
2.36E
5.26E
4.50E
9.31E
3.12E
8.77E
4.21E
1.50E
1.60E
4.71E
5.50E
2.29E
7.65E
4.49E
5.58E
1.16E
5.05E
1.95E
1.68E
7.61E
2.04E
1.25E
8.42E
3.61E
1.24E
4.24E
2.78E
1.31E
3.04E
2.79E
6.28E
7.49E
2.75E
3.08E
7.49E
1.79E
3.23E
2.81E
8.99E
1.21E
4.05E
1.26E
6.54E
5.74E
1.09E
6.68E
5.34E
2.39E
9.32E
3.41E
2.61E
9.62E
2.86E
1.77E
3.91E
4.13E
1.84E
3.10E
5.52E
2.00E
1.64E
2.21E
1.66E
8.80E
3.17E
7.16E
5.52E
4.06E
a
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
07
06
06
06
06
05
06
07
07
06
05
05
04
06
05
07
05
06
05
05
05
06
04
06
+
+
+
+
+
05
05
06
05
04
3.50E + 06
2.99E + 05
1.64E + 05
1.78E + 04
3.86E + 04
3.07E
7.55E
1.83E
2.21E
3.61E
+
+
+
+
+
04
04
04
06
06
1.24E + 06
8.69E + 05
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
07
07
06
06
06
05
06
07
07
06
05
05
05
06
05
07
05
06
05
06
05
07
04
06
Viscosity 1 is based on equation (7), viscosity 2 made use of equation (10), and viscosity 3 utilized equation (8). See text for details.
13 of 24
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
07
06
06
06
05
05
06
06
07
06
05
05
05
06
05
07
05
06
06
05
05
06
04
06
14 of 24
Artemis Festoon Lobe 1
Ovda Festoon Plains
Atalanta Festoon
Artemis Festoon Lobe 2
4.12
2.07
1.22
1.32
104
105
105
105
1.5 102
4.2 102
2 102
7.7 14.2 104
5.3 13.1 104
1.3 104
1.94 104
2.25 104
2.41 104
102
3.5 102 7.2 103
<7 103
70 8 103
9.4 103
4 103 2 104
103 5 104
1.5 103 5 104
0.4 104
0.23 1.1 104
1.5 105
1.2 3.3 105
4.99 104 1.57 106
Makaopuhi, Hawaii
Mauna Loa, Hawaii
Columbia River, N. Am.
Makaopuhi, Hawaii
Mount Etna, Italy
Makaopuhi, Hawaii
Mount Etna, Italy
Kilauea, Hawaii
Mauna Loa, Hawaii
Hawaii
Mount St. Helens, N. Am.
Mono Craters, N. Am.
Sabancaya, Peru
Oldoinyo Lengai, Tanz.
Columbia River, N. Am.
Mauna Loa, Hawaii
Paricutin, Mexico
Arenal, Costa Rica
Arenal, Costa Rica
Teide, Tenerife
Kilauea, Hawaii
Mauna Loa, Hawaii
Mare Imbrium
Mare Imbrium
Mare Imbrium
Gruithuisen Domes
Mairan Domes
Aristarchus
Aristarchus
Necho
King
Yield Strength, Pa
Location
7.12 9.28 2.34 7.31 106
109
109
109
3.2 13.9 108
1.3 11.5 108
4.4 107
7.26 109 1.64 1013
10 – 100
5.0 4 103
1.7 105
3.6 106
1.0 107
7 102 4.5 103
9.4 103
1.4 102 5.6 106
Viscosity, Pa s
Venus
Moon
Earth
Table 4. Comparison of Rheologic Properties of Lava Flows on Earth, the Moon, Venus, and Mars
1.02
2.4
9.52
2.54
104
102
102
103
5.5 119.3
48.0 51.5
2 400
8 9,292
0.33
1 13
417 556
Effusion Rate, m3/s
basalt
basalt
basalt
trachyte
andesite
rhyolite
trachyte/andesite
carbonatite
basalt
basalt
andesite
basaltic andesite
andesite
phonolite
basalt
basalt
basalt
basalt
basalt
basalt
basalt
Lava Type
McColley
McColley
McColley
McColley
and
and
and
and
Head
Head
Head
Head
[2004]
[2004]
[2004]
[2004]
Moore and Schaber [1975]
Hulme and Fielder [1977]
Booth and Self [1973]
Wilson and Head [2003]
Wilson and Head [2003]
Hulme and Fielder [1977]
Moore et al. [1978]
Moore et al. [1978]
Moore et al. [1978]
Shaw et al. [1968]
Moore [1987]
McBirney and Murase [1984]
Cigolini et al. [1984]
Pinkerton and Sparks [1976]
Moore et al. [1978]
Kilburn [1985]
Fink and Zimbelman [1986]
Moore et al. [1978]
Moore et al. [1978]
Moore et al. [1978]
Moore et al. [1978]
Warner and Gregg [2003]
Dawson et al. [1990]
Murase and McBirney [1973]
Hulme [1976]
Hulme [1976]
Cigolini et al. [1984]
Pinkerton and Wilson [1994]
Hulme [1976]
Rowland and Walker [1990]
Rowland and Walker [1990]
Source
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
20
105
23 404
18 60
E05011
5.1. Yield Strength
[37] For the estimation of yield strengths of each
individual investigated lava flow on Ascraeus Mons, we
used equations (1)– (4), in which gravity g is known as
3.7278 m s2 and density r was chosen to be 2,500 kg m3.
Other input parameters, e.g., the flow height and width and
the slope angle, which are necessary for the calculation of
the yield strength, were derived from either measurements
in HRSC images or MOLA topographic data. Depending
on the equation used, we find a minimum yield strength of
2.0 102 and a maximum yield strength of 1.3 105
Pa for individual flows (Table 1). If we average the yield
strengths derived from equations (1) – (4) respectively, this
results in yield strengths that range from 1.4 103 Pa to
5.1 104 Pa. Calculating the average of all derived yield
strengths, we find a value of 2.1 104 Pa; a yield
strength basically identical to that published by Zimbelman
[1985], i.e., 2.1 104 Pa.
5.2. Effusion Rates
[38] Effusion rates of the Ascraeus Mons flows are
based on equation (5). In analogy to terrestrial lava flows
we assumed a Graetz number of 300 and a thermal
diffusivity of 3 107 m2 s1. All other input parameters
were derived from measurements based on HRSC or
MOLA data. As a result we find that effusion rates range
from 23 to 404 m3 s1, averaging about 185 m3 s1
(Table 2). These values are considerably larger than
effusion rates of the Ascraeus Mons flows estimated by
Zimbelman [1985], i.e., 18– 60 m3 s1, with an average of
35 m3 s1.
1.7 10 4.2 10
7
4
5
2.0 10 1.3 10
2.5
1.9
3.3
8.8
2
9.7 105
1.7 105 1.9 106
6.4 105 2.1 108
2.3 105 6.9 106
3.1
3.9
2.8
8.3
4.5
5.3
0.39 103 103 103 103 1.8 Arsia Mons
Arsia Mons
Alba Patera
Ascraeus Mons
Olympus Mons
Olympus Mons
Elysium Mons
Alba Patera
Ascraeus Mons
Location
Table 4. (continued)
Yield Strength, Pa
103
103
104
104
104
104
Viscosity, Pa s
Mars
Effusion Rate, m3/s
5.6 103 4.3 104
Lava Type
basalt/basaltic andesite
Source
Moore et al. [1978]
Warner and Gregg [2003]
Cattermole [1987]
Zimbelman [1985]
Hulme [1976]
Moore et al. [1978]
Keszthelyi [1995]
Sakimoto et al. [1997]
this work
E05011
5.3. Eruption Duration
[39] One interesting question is how long did it take to
emplace the Ascraeus Mons flows. Eruption durations
were calculated in two ways. First, we used equation (6)
to calculate the mean flow velocity. Dividing the flow
length by the mean flow velocity, we obtained an
estimate of the eruption duration. Second, we divided
the flow volume by the effusion rate, which gives us the
time it took to emplace the calculated flow volume.
However, it has to be mentioned that these methods are
not completely independent from each other because both
are dependent on the flow volume, which is a component
of equations (5) and (6).
[40] On the basis of these calculations we find that the
minimum eruption duration of individual flows is on the
order of 2 days and the maximum eruption duration is
80 days. We find that method 1 yielded an average
eruption duration of 25 days, method 2 resulted in
slightly longer eruption durations of 26 days (Table 2).
It appears that both methods yield similar results that are
plausible in comparison to terrestrial flows of similar
length [e.g., Rowland and Walker, 1990; Keszthelyi and
Pieri, 1993]. These eruption durations of individual flows
are likely minimum estimates due to potential unmeasured
segments of the original flow length caused by the collapse
of the caldera or subsequent flows that covered the sources
of the studied flows. In addition, the calculated eruption
durations are for specific flows, and not for entire flow fields.
The actual eruption that formed a flow field, of which the
15 of 24
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
E05011
Table 5. Estimation of Errors for Each of the Calculated Yield
Strengths of Table 1 Based on a Variation of the Input Parameters
by +10, +30, +50, 10, 30, and 50%a
Parameter
and Assumed
Errors
Error YS 1,
%
Density: 2,500 kg m3
+10%
10
+30%
30
+50%
50
10%
10
30%
30
50%
50
Flow height: 13 m
+10%
10
+30%
30
+50%
50
10%
10
30%
30
50%
50
Flow width: 1334 m
+10%
n.a.
+30%
n.a.
+50%
n.a.
10%
n.a.
30%
n.a.
50%
n.a.
Levee width: 707 m
+10%
n.a.
+30%
n.a.
+50%
n.a.
10%
n.a.
30%
n.a.
50%
n.a.
Leveed flow width: 991 m
+10%
n.a.
+30%
n.a.
+50%
n.a.
10%
n.a.
30%
n.a.
50%
n.a.
Channel width: 284 m
+10%
n.a.
+30%
n.a.
+50%
n.a.
10%
n.a.
30%
n.a.
50%
n.a.
Slope: 3.5478°
+10%
10
+30%
30
+50%
50
10%
10
30%
30
50%
50
Totals
+10%
30
+30%
90
+50%
150
10%
30
30%
90
50%
150
Error YS 2,
%
Error YS 3,
%
Error YS 4,
%
10
30
50
10
30
50
10
30
50
10
30
50
10
30
50
10
30
50
21
69
125
19
51
75
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
9
23
33
11
43
100
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
10
30
50
10
30
50
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
14
42
70
14
42
70
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
4
12
20
4
12
20
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
21
69
125
19
51
75
21
69
125
19
51
75
22
76
142
18
38
25
41
129
225
39
111
175
41
129
225
39
111
175
E05011
order of 8.7 105 to 5.7 106 Pa-s (Table 3). Minimum
viscosities of individual flows are on the order of 1.8 104 Pa-s,
whereas maximum viscosities are about 4.2 107 Pa-s.
Table 4 shows a comparison of our results with viscosities
of lava flows on Arsia Mons, Alba Patera, Olympus
Mons, Elysium Mons, and Ascraeus Mons as derived by
various studies [e.g., Hulme, 1976; Zimbelman, 1985;
Cattermole, 1987; Warner and Gregg, 2003]. As all
of these previously published viscosities range from 1.7 105 to 2.1 108 Pa-s, we find an excellent agreement with
our results. Table 4 also indicates, that our results are
consistent with viscosities of terrestrial basalts and andesites, which are on the order of 1.4 102 to 1 107 Pa-s
[e.g., Murase and McBirney, 1973; Pinkerton and Sparks,
1976; Hulme, 1976; Cigolini et al., 1984; Moore, 1987].
5.5. Error Discussion and Theoretical Considerations
[42] Here we provide a brief discussion of the quality and
the effects of possible errors of the input data for our models
of the Ascraeus lava flows. Values for the slope a, flow
length l, flow width w, flow height h, total levee width wl,
and the width of the leveed flow channel wc were determined directly either from MOLA data or from the HRSC
imaging data. Because lava flows are not uniform constructs
but vary in their dimensions along the flow path, the
measurements are subject to errors. Additional errors
Table 6. Estimation of Errors for the Calculated Effusion Rates
and Each of the Calculated Eruption Durations of Table 2 Based on
a Variation of the Input Parameters by +10, +30, +50, 10, 30,
and 50%a
Parameter
and Assumed
Errors
a
Notes: ‘‘n.a.’’ indicates that a particular equation used to calculate the
yield strength is independent of this parameter.
investigated basalt flows are a part of, could have lasted
longer.
5.4. Viscosity
[41] Using equations (7), (8), and (10) we estimated the
average viscosity of the Ascraeus lava flows to be on the
Error ER 1,
%
Flow length: 18954 m
+10%
+30%
+50%
10%
30%
50%
Flow height: 13 m
+10%
+30%
+50%
10%
30%
50%
Flow width: 1334 m
+10%
+30%
+50%
10%
30%
50%
Totals
+10%
+30%
+50%
10%
30%
50%
Error DU 1,
%
Error DU 2,
%
10
30
50
10
30
50
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
9
23
33
11
43
100
9
23
33
11
43
100
21
69
125
19
51
75
10
30
50
10
30
50
10
30
50
10
30
50
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
9
23
33
11
43
100
11
37
67
9
17
0
21
69
125
19
51
75
8
16
17
12
56
150
a
Notes: ‘‘n.a.’’ indicates that a particular equation used to calculate the
eruption rate is independent of this parameter.
16 of 24
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
Table 7. Estimation of Errors for Each of the Calculated
Viscosities of Table 3 Based on a Variation of the Input Parameters
by +10, +30, +50, 10, 30, and 50%a
Parameter
and Assumed
Errors
Density: 2,500 kg m3
+10%
+30%
+50%
10%
30%
50%
Flow height: 13 m
+10%
+30%
+50%
10%
30%
50%
Flow width: 1334 m
+10%
+30%
+50%
10%
30%
50%
Channel width: 284 m
+10%
+30%
+50%
10%
30%
50%
Slope: 3.5478°
+10%
+30%
+50%
10%
30%
50%
Totals
+10%
+30%
+50%
10%
30%
50%
Error VI 1,
%
Error VI 2,
%
Error VI 3,
%
10
30
50
10
30
50
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
10
30
50
10
30
50
21
69
125
19
51
75
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
33
120
238
27
66
88
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
33
98
238
27
66
88
33
21
69
124
19
51
75
21
10
30
50
10
30
50
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
10
30
50
10
30
50
10
31
99
175
29
81
125
54
29
362
46
117
163
63
210
387
57
156
238
E05011
150% (Table 6). The total errors for viscosity in the
worst-case scenario are on the order of +390/240%
(Table 7). From this discussion it is clear that rheological
estimates are sensitive to variations in input parameters,
hence demonstrating the importance of an accurate determination of the flow dimensions with high-resolution data.
We conclude that at the present time, height and slope
measurements using MOLA data in combination with
length and width measurements on high-resolution HRSC
images provide the most accurate input for our models.
[43] Besides errors in the measurements of flow dimensions, there are physical and chemical factors that cannot be
derived directly from flow dimension measurements but
which have influence on the rheological properties of lava
flows. Therefore we will briefly discuss the effects, for
example, of temperature, composition and volatile content
on the viscosity.
[44] Figure 7 shows viscosity as a function of temperature
at 1 bar for volatile-free and crystal-free natural melts
ranging in composition from rhyolite to komatiite [Spera,
2000]. This figure illustrates several dependencies of the
viscosity of a given lava flow. First, there is the dependency
on temperature; higher temperatures result in lower viscosities. Second, viscosity is to a first order dependent on the
composition of the lava; higher silica contents result in
higher viscosities. Third, viscosities are dependent on the
water content of the lava; higher water contents result in
lower viscosities.
[45] On this plot (Figure 7), we superposed the minimum
and maximum values of viscosity derived from equations (7),
(8), and (10). Clearly, the calculated viscosities are higher
than the viscosities of the crystal-free melts. However,
natural magmas are not directly comparable to silicate
melts in the laboratory, in that they contain various
amounts of crystals and vesicles, which increase the
a
Notes: ‘‘n.a.’’ indicates that a particular equation used to calculate the
viscosity is independent of this parameter.
depend on the available data source, especially the illumination angles and the spatial resolution. For example, we
find that the determination of the flow height is particularly
difficult with the available imaging data. In order to
investigate the effects of such complications on our results,
we varied the input parameters for our calculations by ±10,
±30, and ±50%. Tables 5, 6, and 7 show the variations and
their effects on the yield strength, effusion rate, eruption
duration, and viscosity, as well as an estimate of the total
errors. As a result, we see that depending on the equations
used, variations in input parameters have different effects on
the results. According to our error analysis, in the worstcase scenario, the yield strength calculations can have errors
of up to +225% or 175% (Table 5), the effusion rate
can have errors of up to +67% or 17%, and the
eruption duration can have errors of up to +125% or
Figure 7. Diagram of Spera [2000] showing the dependency of the viscosity of volcanic rocks on temperature and
water content. Numbers indicate the wt% of dissolved water
(open markers). Superposed are the upper and lower
boundaries of the viscosities of the investigated basalts.
17 of 24
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
E05011
Figure 8. Diagram of Moore [1987] showing the relationship between the silica contents and the yield
strengths of lunar and terrestrial volcanic rocks. Superposed is the average yield strength of the Ascraeus
Mons flows with its upper and lower boundaries (solid gray; average is shown as box with star). On the
basis of this diagram, the yield strengths are consistent with a basaltic to andesitic composition of the
Ascraeus Mons flows.
viscosity. There is only limited data available, but Murase
and McBirney [1973] showed that the apparent viscosity of a
basalt liquid at 1128°C with 20 vol% of suspended crystals is
100 times greater than that of a basalt liquid of equivalent
chemical composition at the same temperature. Higher
amounts of crystals (40 vol%) increase the viscosity by
almost 5 orders of magnitude [Murase and McBirney, 1973].
Hess [1989, p. 64] proposed that high viscosities are the
results of the presence of rigid crystals, which impede the
flow of the lava. In addition, it is possible that surfacetension effects between crystals and liquids impart greater
cohesion to the suspension, an effect that can also be
produced by suspended gas bubbles [Hess, 1989, p. 64].
[46] Figure 7 also indicates that water content has a strong
influence on the viscosity of a lava flow. Under Martian
conditions it is very likely that melts are not completely
volatile-free but contain some water [Head and Wilson,
1998a]. Because hydrous melts are more depolymerized due
to the formation of nonbridging bonds and the networkmodifying characteristics of H20, the addition of water to a
melt will lower its viscosity [e.g., Hess, 1989, p. 63]. To
illustrate the effect of water contents on viscosity, in Figure 7
we superposed data of Spera [2000] that show the viscosities of melts with 1, 2, and 3 wt% of dissolved water
(shown as open markers). As a result we see that adding 2
or 3 wt% of water can decrease the viscosity by an order of
magnitude.
[47] In summary, we conclude that there are several
parameters which cannot be determined from the dimensions and the morphology of a lava flow but which have
dramatic effects on its viscosity. While one has to keep these
caveats in mind, our results still represent a valuable
contribution to our understanding of the general rheological
characteristics of Martian lava flows.
5.6. Terrestrial and Extraterrestrial Analogs
[48] Table 4 is a compilation of data on the yield
strengths, viscosities and effusion rates of lava flows on
Earth, the Moon, Mars, and Venus. Typical basalts on Earth
have yield strengths of 102 – 104 Pa, with more evolved
magmas having higher yield strengths of 104 – 106 Pa
[e.g., Shaw et al., 1968; Pinkerton and Sparks, 1976;
Hulme, 1976; Moore et al., 1978; McBirney and Murase,
1984; Cigolini et al., 1984; Kilburn, 1985; Fink and
Zimbelman, 1986; Moore, 1987]. Lunar mare basalts appear
to have yield strengths of about 102 Pa [e.g., Booth and Self,
1973; Moore and Schaber, 1975; Hulme and Fielder, 1977;
Moore et al., 1978], which is considerably less than the
yield strengths of 104 Pa calculated for the Gruithuisen
and Mairan domes [Wilson and Head, 2003]. The festoon
deposits on Venus, which were interpreted to represent
viscous lavas [Moore et al., 1992; Head and Hess, 1996;
McColley and Head, 2004], have yield strengths on the
order of 104 – 105 Pa. Yield strength estimates for Martian
lava flows of various volcanoes range from 103 – 104 Pa
[e.g., Hulme, 1976; Moore et al., 1978; Zimbelman, 1985;
Cattermole, 1987; Warner and Gregg, 2003].
[49] On the basis of our study we calculated the average
yield strength of the Ascraeus lava flows to be on the order
of 2.1 104 Pa, ranging from 2.0 102 to 1.3 105 Pa.
This result is basically identical with the yield strength of
some lava flows on Ascraeus Mons of 2.1 104 Pa derived
by Zimbelman [1985]. The result is also most consistent
with yield strengths of terrestrial basalt flows. Figure 8 plots
the yield strength of several terrestrial and lunar lavas versus
their silica content [Moore et al., 1978]. On the basis of this
diagram, our yield strength indicates that the investigated
Ascraeus lavas are basaltic to andesitic in composition.
Given that the data are not unambiguously interpreted, the
basaltic/andesitic composition of the Ascraeus Mons lavas
18 of 24
E05011
HIESINGER ET AL.: RHEOLOGY OF ASCRAEUS MONS LAVA FLOWS
Figure 9. Diagram of Mouginis-Mark and Yoshioka
[1998] showing the relationship between effusion rates
and eruption durations for lava flows on Elysium Mons.
Superposed are data for the Ascraeus Mons flows (solid
gray; average is shown as box with star). While there is
generally a good agreement, the Ascraeus Mons flows
appear to have been emplaced within slightly shorter
periods of time.
may be indirectly supported by data of the TES instrument,
which show a possible slight enhancement in the spectral
signal of basalt (surface type 1) in the interpretation of
Bandfield et al. [2000].
E05011
[50] It is well known that volcanoes on Earth exhibit a
wide range in effusion rates. For example, Rowland and
Walker [1990] estimated the effusion rates of Kilauea to
vary between 2 and 400 m3 s1 and that of Mauna Loa to
range from 8 to 9,292 m3 s1. Moore [1987] estimated the
effusion rate of the 1984 Mauna Loa eruption to be on the
order of 417– 556 m3 s1. The Gruithuisen and Mairan
domes on the Moon appear to have formed from eruptions
with effusion rates of 5.5 119.3 m3 s1 [Wilson and
Head, 2003]. Warner and Gregg [2003] found a significantly smaller effusion rate of 1 – 13 m3 s1 for the more
viscous lavas (trachyte/andesite) of Sabancaya volcano in
Peru. However, festoon deposits on Venus, which were also
interpreted to consist of more evolved lavas have larger
effusion rates of 102 – 104 m3 s1 [McColley and Head,
2004]. Similar effusion rates of 5.6 103 4.3 104 m3 s1 were derived for Martian volcanoes [Warner
and Gregg, 2003], but work by Zimbelman [1985] and
Keszthelyi [1995] indicates much lower effusion rates of
18– 60 m3 s1. On the basis of our calculations, we find
effusion rates on the order of 23– 404 m3 s1, hence being
in good agreement with terrestrial basaltic effusion rates and
some of the effusion rates derived for Martian volcanoes. In
Figure 9 we superposed our effusion rates and eruption
durations on a diagram by Mouginis-Mark and Yoshioka
[1998] for Elysium flows. Again, we generally see a good
agreement between the two data sets with the Ascraeus
Mons flows being emplaced within slightly shorter periods
of time. This could reflect differences in eruption behavior
between Ascraeus Mons and Elysium Mons. With the new
data, such as HRSC, MOLA, and THEMIS, we now have
the ability to analyze larger numbers of lava flows to better
understand possible differences between volcanic centers
and possibly differences over time.
Figure 10. Flow lengths and effusion rates of 84 Hawaiian lava flows shown as black squares [Malin,
1980]. Hatched lines are eruption durations. Tube-fed lava flows are characterized by long flow lengths at
relatively small effusion rates. Superposition of Martian average and minimum and maximum flow
lengths as well as calculated effusion rates indicate that the Ascraeus Mons flows (solid gray; average is
shown as box with star) are similar to the Hawaiian flows of Mauna Loa and Kilauea. If we correct for
lower gravity and higher effusion rates on Mars (dashed gray, average is shown as star), the Ascraeus
Mons flows are still very similar to Hawaiian basalt flows.
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Figure 11. Diagrams of Kilburn [2000] showing (a) the relationships between the maximum potential
length of a single a’a flow and the underlying slope and (b) the rate of discharge. Solid and dashed lines
indicate models for 2,000 and 2,200 kg m3 mean crustal densities. Superposed are Martian average and
minimum and maximum flow lengths as well as measured slopes and calculated effusion rates (solid
gray; average is shown as box with star). Corrected values for lower gravity and higher effusion rates on
Mars are shown as dashed gray lines, and the average is shown as a gray star. The Ascraeus Mons flows
are very similar in terms of slope and corresponding flow length to terrestrial a’a flows such as flows of
Etna, Kilauea, and Mauna Loa. In terms of discharge rate and corresponding flow length, the Ascraeus
Mons flows are more similar to Kilauea and Mauna Loa flows than to flows of Mt. Etna.
[51] Early work assumed that the flow length is mainly
controlled by the viscosity of the lava [e.g., Macdonald,
1972, pp. 66 – 67] but Walker [1973] showed that flow
length depends on the mean effusion rate and Malin
[1980] showed that it depends on the erupted volume.
Figure 10 is based on data from Malin [1980] for 84
Hawaiian flows and shows the relationship between eruption duration, flow length and mean effusion rate. Superposed is the average and lower and upper limits of flow
lengths and effusion rates of our investigated flows. Despite
the fact that flows on Mars are far longer than terrestrial
flows, mainly due to the lower gravity, we find an excellent
agreement with the Hawaiian flows. Head and Wilson
[1998c] reported that the lower gravity and higher effusion
rates cause cooling-limited lava flows to be 6 times longer
on Mars than on Earth. If we correct our data by this factor,
the Martian flows are still very similar to the Hawaiian
flows of Malin [1980] shown in Figure 10.
[52] Kilburn [2000] presented two figures that plot the
flow length of terrestrial lava flows versus their effusion rate
and their slope angle (Figure 11). Superposed on his figure
are the results for the Ascraeus Mons flows, indicating a
strong similarity of the Martian flows with Mauna Loa a’a
flows. However, in both diagrams, if one corrects for
Martian conditions, the Ascraeus Mons flows appear to be
more similar to the basaltic Kilauea a’a flows.
[53] Pinkerton and Wilson [1994] developed a nonisothermal Bingham model that allowed them to generate
empirical equations in order to relate flow length to rheological properties and other controlling factors (e.g., channel
width, thickness, gradient, effusion rate) of cooling-limited
flows. Figure 12 indicates the maximum calculated flow
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Figure 12. Diagram of Pinkerton and Wilson [1994] based on data from Walker [1973] showing the
maximum flow lengths predicted for basalts, andesites, and rhyolites. The flow lengths are based on
Graetz numbers of 300. All channel-fed basaltic flows lie below the basaltic line; the three basaltic flows
lying closest to the line are shown as black squares. Two rhyolites are represented by black circles; an
Arenal andesite flow is shown as a cross. Also shown are the different flow lengths that can be achieved
by two flows with initially the same effusion rate and eruption duration. A flow with constant effusion
rate would follow the line ABC, whereas a flow with decreasing effusion rate follows line ABD. Dashed
line represents the upper and lower limits of Walker [1973], which are discussed by Pinkerton and Wilson
[1994]. Superposed are the average and minimum and maximum flow lengths and calculated effusion
rates for our Ascraeus Mons flows (solid gray; average is shown as box with star). We conclude that the
Martian flows are very similar to terrestrial basalts and andesites in flow length and effusion rates. If we
correct for lower gravity and higher effusion rates on Mars (dashed gray; average is shown as star), the
Ascraeus Mons flows become even more akin to terrestrial basalt flows.
lengths of cooling-limited terrestrial flows with different
compositions, i.e., basalt, andesite, and rhyolite. If we
superpose the Martian flow data, we see that our flows
are very similar in their flow length and effusion rates to
terrestrial basalts and andesites. Again, if we correct for the
lower gravity and higher effusion rates on Mars, the lengths
of the Martian flows are on average well below the
maximum lengths calculated for terrestrial Pu’u O’o lavas
and even the most extreme flows are completely below the
maximum length of terrestrial basalts (Figure 12).
[54] On the basis of our literature search, viscosities of
terrestrial lava flows show variations of up to 12 orders of
magnitude (Table 4). For example, viscosities of hawaiian
basalts range from 1.4 102 to 5.6 106 Pa-s [e.g.,
Hulme, 1976; Cigolini et al., 1984; Moore, 1987]. Similar
viscosities were found for the Columbia River basalts and
Mt. Etna [e.g., Murase and McBirney, 1973; Pinkerton and
Sparks, 1976]. Hulme [1976] reported viscosities of 3.6 106 Pa-s for andesites of the Paracutin volcano in Mexico,
and of 4.4 107 Pa-s for phonolites of the Teide volcano in
Tenerife. These viscosities are similar to those of basaltic
andesites of the Arenal volcano in Costa Rica [Cigolini et
al., 1984]. More exotic lavas such as carbonatites have
viscosities as low as 10– 100 Pa-s [Dawson et al., 1990] and
trachytes/andesites of the Sabancaya flows were reported to
have viscosities up to 1.64 1013 Pa-s [Warner and Gregg,
2003]. Wilson and Head [2003] estimated viscosities of
1.3 13.9 108 Pa-s for the Gruithuisen and Mairan
domes on the Moon. Viscosities of the festoon deposits on
Venus were found to be on the order of 7.12 106 to 9.28 109 Pa-s, consistent with the interpretation that these deposits represent more evolved lavas [McColley and Head,
2004]. Similarly, Moore et al. [1992] estimated the viscosities of these festoon deposits to be on the order of 1 107
to 8 109 Pa-s. On the basis of previously published
studies, viscosities of Martian lava flows range from 1.7 105 to 2.1 108 Pa-s [e.g., Hulme, 1976; Zimbelman, 1985;
Cattermole, 1987; Warner and Gregg, 2003]. Our calculations for the Ascraeus Mons flows, which are based on a
much larger number of individual flows compared to
previous studies, yielded average viscosities of 8.7 105
to 5.7 106 Pa-s. We conclude that our viscosities are in
excellent agreement with previously published Martian
viscosities although they appear to be rather high compared
to terrestrial basalt flows. Nevertheless, Zimbelman [1985]
argued that such viscosities are most consistent with basaltic
or basaltic andesite lavas.
[55] In summary, we find our results for the yield
strength, effusion rate, eruption duration, and viscosity to
be in good agreement with previously published results. The
strength of our study is that we investigated a much larger
number of flows than in previous studies. Therefore our
study provides a more complete foundation of our understanding of Martian lava rheologies. Table 4 summarizes
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these results and compares them to results for lava flows on
Earth, the Moon, and Venus. From this comparison we
conclude that the Ascraeus Mons flows exhibit rheological
properties that are generally consistent with a basaltic/
andesitic composition for these flows.
6. Discussion
[56] On the basis of our investigation we conclude that
the investigated lava flows are likely basaltic to andesitic in
composition. To first order, our calculations and comparison
to terrestrial flows are consistent with these flows being a’a
flows. The MOLA pulse width data of Neumann et al.
[2003] can resolve surface roughnesses as small as 1 m
RMS. Unfortunately, the spatial resolution of the roughness
data of 1/4 degree is insufficient to resolve our flows in
order to distinguish between a’a and pahoehoe flows or
pyroclastic deposits. However, we expect that future photometric analyses using for example the capabilities of the
HRSC camera, will contribute to this question. Because
HRSC almost simultaneously takes images of a particular
surface feature under multiple viewing geometries, it is
unique among the camera experiments flown in Mars
orbit and its data can be used for detailed photometric
modeling [e.g., Hapke, 1984; Helfenstein and Veverka,
1987; Helfenstein, 1988]. For example, Hapke’s photometric equation contains a roughness parameter q, which
models the effects of spatially unresolved topographic relief
on the bidirectional reflectance. Consequently, by determining q from HRSC data [e.g., Pinet et al., 2006; Jehl et al.,
2006] it should be possible to characterize and quantify
differences in surface roughness of various flow types,
similar to the results of Helfenstein and Veverka [1987]
for lunar mare and highland regions. This in turn might help
to determine whether the investigated basalts are indeed a’a
flows or whether they are pahoehoe flows.
[57] As discussed above, there might be statistically
significant differences in eruption behavior between
Ascraeus Mons and Elysium Mons. On the basis of HRSC,
MOLA, and THEMIS data, it is now possible to investigate
larger numbers of lava flows to analyze such differences not
only between volcanic centers, but also possibly over time.
So far, we analyzed numerous flows from a single volcano
that were probably erupted within a geologically short
period of time. Interestingly, we found that, for example,
the range of viscosities of the young Ascraeus flows is fairly
narrow, compared to the overall range reported in Table 4.
Should this be confirmed by future observations, we might
be able to study changes in viscosities with eruption age, not
only for a single volcano, but also, in combination with
reliable age data, for all Martian volcanoes. Such investigations would greatly improve our understanding of the
volcanic history and evolution of Mars.
7. Conclusions
[58] Compared to previous studies we used data with
much higher resolution and we expanded our calculations to
a much larger number of flows than the earlier studies. The
results of our investigation of 25 lava flows on Ascraeus
Mons lead us to the following conclusions: (1) The investigated flows are on average 19 km long (4 – 38 km), and
E05011
1.4 km wide (0.5 – 2 km). On the basis of MOLA
profiles across individual lava flows we find an average
thickness of 13 m (5 – 24 m), which is significantly
smaller than the thickness derived from our shadow
measurements (39 m on average, 24 – 88 m range).
(2) The flows were emplaced on slopes of 1.5 – 6.7°.
(3) Average yield strengths of the studied basalt are on
the order of 2.1 104 Pa, ranging from 1.4 103 to
5.1 104 Pa. (4) Minimum and maximum yield strengths
of individual flows are on the order of 2.0 102 and
1.3 105 Pa. (5) Effusion rates of these flows range from
23– 404 m3 s1, averaging at 185 m3 s1. (6) The flows
were probably emplaced within less than a few days to
months. (7) Average viscosities of the Ascraeus Mons flows
range from 8.7 105 to 5.7 106 Pa-s with an overall
average of 4.1 106 Pa-s. (8) Minimum and maximum
viscosities of individual flows calculated in this study vary
from 1.8 104 to 4.2 107 Pa-s. (9) Ascraeus Mons
flows have rheological properties similar to flows elsewhere
on Mars. (10) Ascraeus Mons flows have rheological
characteristics, flow morphologies, and dimensions that
are similar to terrestrial basaltic/andesitic flows. (11) With
the available data, we are now able to investigate possible
differences in eruption behavior between volcanic centers as
well as over time.
[59] Acknowledgments. We greatfully acknowledge the superb work
of the HRSC design, engineering, image processing, and science team. The
authors wish to thank Lori Glaze and Jim Zimbelman for their excellent and
thorough reviews, which significantly helped improve the manuscript. We
also appreciate the comments by Patrick Pinet, David Williams, and Jake
Bleacher on an early version of the manuscript. Finally, we would like to
thank the HRSC teams at the Freie Universität Berlin and at the German
Aerospace Center (DLR) for their support and assistance with processing
the HRSC image. We thank NASA for supporting the participation of H.H.
and J.W.H. through the Planetary Geology and Geophysics Program.
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J. W. Head III and H. Hiesinger, Department of Geological Sciences,
Brown University, Box 1846, Providence, RI 02912, USA. ([email protected])
G. Neukum, Institut für Geologische Wissenschaften, Freie Universität
Berlin, D-12489 Berlin, Germany.
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