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46th Lunar and Planetary Science Conference (2015)
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THE DISTRIBUTION AND IMPLICATIONS OF FLANK VENTS ON OLYMPUS MONS. S. I. Peters1 and
P. R. Christensen1, 1Mars Space Flight Facility, School of Earth and Space Exploration – Arizona State University,
201 E. Orange Mall, Tempe, AZ 85287-6305, USA, [email protected].
Introduction: Flank vents are common features on
polygenetic volcanoes, indicating that magma has propagated away from the main conduit and/or magma
chamber [1]. The distribution of volcanic vents about a
central caldera can provide information on dike propagation and stress orientation in a volcano [1-3]. In addition, the morphology and morphometry of flank vents
and associated deposits provide a proxy for eruption
style, effusion rate and magmatic properties such as gas
content [2, 4]. They have been documented extensively
on terrestrial volcanoes such as Mt. Etna and Fernandina
volcano (Galapagos Islands) [2, 3]. Similar features are
observed on Martian volcanoes. Olympus Mons, the
largest shield volcano on Mars at ~23 km, represents ~3
Ga of volcanic activity [5-7]. Located west of the volcano-tectonic Tharsis Province, Olympus Mons is
shrouded in spectrally thick dust cover preventing detailed compositional analysis [8]. Consequently, morphology and spatial relationships offer the best chances
at understanding the volcanic history of Olympus Mons.
The first vent on Olympus Mons was identified using
THEMIS VIS (18 m/pix) [9]. A variety of volcanic features and flow morphologies have been identified and
mapped using HRSC, providing evidence of effusive
eruptions during the Amazonian [4]. Higher resolution
imagery has allowed for the identification and characterization of smaller volcanic features.
Methods: We mosaicked 159 CTX images (~5
m/pix) covering Olympus Mons in order to identify and
map the distribution of vents. We initially identified 22
possible flank vents. Thanks to a new dataset of small
volcanic vents in the Tharsis Province, we were able to
incorporate an additional 18 vents into the current study
[10]. We used MOLA data to determine the dimensions
of the vents, the elevation of the vents above the surrounding terrain and the average distance from the center of the volcano to each vent. Using ArcGIS, we performed an average nearest neighbor analysis to determine the spatial distribution of vents.
Figure 1: Inset of vent taken from CTX mosaic. Illumination is from
the left and north is up. Red arrows represent regional flow pattern.
White arrows represent flow from vent. Red arrows illustrate the diversion of flow around vent indicating emplacement subsequent to
vent formation. Subsequent flows have not buried vent indicating the
vent formed later in Olympus Mons history.
Observations: We observed and mapped 40 interpreted flank vents on the slopes of Olympus Mons. The
vents are identified as topographically high features
with flows emanating from their summits (Fig. 1). In
most cases, the vents have diverted subsequent flows
around them. The vents average ~1 km across. They
range in elevation from 1.5 – 17 km, with an average of
9 km (Fig. 2). The distance between the center of the
caldera and the flank vents ranges from ~160 – 340 km,
with an average of ~220 km. Average nearest neighbor
analysis reveals the vents are clustered. Vents are considered clustered, as opposed to dispersed, if the observed average distance between vents is less than the
expected distance between randomly spaced vents.
Vents are sometimes associated with linear depressions
roughly tangential to contours of the volcano. These
linear features are usually a few hundred meters across
and several kilometers long. In some instances, material
appears to have emanated from these features. Further
analyses is needed to understand whether these features
represent tectonism, volcanism or a combination of
both.
46th Lunar and Planetary Science Conference (2015)
Elevation (km)
Vent Elevation with Distance from Center
of Caldera
20
15
Flank
Vents
10
5
0
0
200
400
Distance from Caldera (km)
Figure 2: Distribution of vents as a function of elevation and distance
from the caldera center. Note the cluster of vents between ~5 – 15 km
at ~200 km from the center of the volcano. The occurrence of vents
on the lower flanks of Olympus Mons supports the idea of vent migration downslope [5, 7, 11]. Since the observed vents have not been
buried, they are the youngest expression of flank volcanism.
Discussion: Our results suggest that the formation
of observed flank vents is related to preferential magmatic transport and depth of the magma chamber.
Dike and sill evolution. The tectonic stresses present
at the time of flank vent formation likely influenced
magmatic transport. This allowed dikes and sills to
propagate outward hundreds of kilometers. A radial orientation of the vents would suggests the magma responded to a radial σ3 (minimum stress) stress field[2].
The stress field on Olympus Mons is probably a result
of compression due to gravitational relaxation of the edifice consistent with the observation of thrust faulting in
the upper flanks [11-13]. Clustering of the vents implies
formation via a common source or mechanism. This
may imply that vents are forming primarily along a few
preferential pathways. Furthermore, this suggests the
effects of regional tectonism on the formation of the observed vents is limited.
Depth of magma chamber. The depth of the Olympus Mons magma chamber has been modeled at 13 to
16 km below the summit of the volcano or approximately 7 to 10 km above the surrounding terrain[13, 14].
We found that 29 of the 40 flank vents occur at elevations between 8 – 14 km. The elevation of the vents
combined with the modelled depth of the magma chamber suggests nearly lateral transport of magma, perhaps
produced by sill and dike propagation away from the
magma chamber.
Tectonism or Volcanism. The linear depressions associated with some of the flank vents may represent extensional features, volcanic fissures or some combination of both. Some models have predicted the greatest
extensional stresses would occur on the lower flanks
[11]. If these linear depressions are a result of faulting,
it could support the existence of a filled magma chamber
[14]. They could also represent a stress field due to the
flexure of lithosphere beneath the volcano [11, 13]. A
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more detailed analysis of the morphology and morphometry of these linear features and their relationship
to flank vents is necessary to ascertain their origin and
implications.
Ongoing and Future Work: The current project
aims to continue incorporating vents into our study for
spatial analyses [10]. This will include updated average
nearest neighbor analyses and the distribution of vents
as a function of elevation. We will attempt spectral
analysis using the THEMIS DCS technique in order to
obtain compositional information. THEMIS DCS imagery highlights a suite of compositional and thermophysical properties associated with surfaces. Therefore,
despite an optically thick dust cover, spectral variations
indicate true differences in surface properties. In addition, we plan to use digital elevation models (DEMs)
constructed from CTX and HiRISE images to calculate
volume estimates and characterize flow morphologies.
Acknowledgments: Special thanks to PhD student
Jonathon Hill (ASU) for providing insight on the mosaicking procedure and post-doc Mark Salvatore (ASU)
for helpful feedback on the abstact. We are also very
grateful to PhD student Jacob Richardson (USF) for the
introduction of a very helpful dataset.
References: [1] Nakamura, K. (1976) J. Volcanology and Geothermal Res., 2, p. 1-16. [2] Rowland, S.
K. (1996) J. Geophysical Res., 101, p. 27,657 – 27, 672.
[3] Corazzato, C. and Tibaldi, A. (2006) J. Volcanology
and Geotherm. Res., 158, p. 117 – 194. [4] Bleacher,
J.E. et al. (2007) JGR, 112. [5] Car, M.H. (1973) JGR,
78, p. 4049 – 4062. [6] Blasius, K.R. and Cutts, J.A.
(1981) Icarus, 45, p. 87 – 112. [7] Carr, M.H. et al.
(1977) JGR, 82, p. 3985 – 4015. [8] Christensen, P.R.
(1986) JGR, 91, p. 3533 – 3545. [9] Mouginis-Mark,
P.J. and Christensen, P.R. (2005) JGR, 110. [10]
Bleacher, J.E. et al. (2010) LPSC XLI, Abstract #1615.
[11] McGovern, P.J. and Solomon, S.C. (1993) JGR, 98,
p. 23,553-23,579. [12] Byrne, P.K. et al. (2009) Earth
and Planetary Science Letters, 281, p. 1 – 13. [13]
Thomas, P.J. et al. (1990) JGR, 95, p. 14,345 – 14, 355.
[14] Zuber, M.T. and Mouginis-Mark, P.J. (1992) JGR,
97, p. 18, 295 – 18, 307.