Alba Mons
Template:Short description Template:Infobox feature on celestial object
Alba Mons (formerly and still occasionally known as Alba Patera, a term that has since been restricted to the volcano's summit caldera;<ref name = "USGS2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> also initially known as the Arcadia ring<ref name=watters1995>Template:Cite journal</ref>) is a volcano located in the northern Tharsis region of the planet Mars. It is the biggest volcano on Mars in terms of surface area, with volcanic flow fields that extend for at least Template:Convert from its summit.<ref name=Cattermole01p85>Cattermole, 2001, p. 85.</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Although the volcano has a span comparable to that of the United States, it reaches an elevation of only Template:Convert at its highest point.<ref name=Carr06p54>Carr, 2006, p. 54.</ref> This is about one-third the height of Olympus Mons, the tallest volcano on the planet.<ref name=Plescia04>Template:Cite journal</ref> The flanks of Alba Mons have very gentle slopes. The average slope along the volcano's northern (and steepest) flank is 0.5°, which is over five times lower than the slopes on the other large Tharsis volcanoes.<ref name=Carr06p54 /><ref>Boyce, 2008, p. 104.</ref> In broad profile, Alba Mons resembles a vast but barely raised welt on the planet's surface.<ref>See Carr, 2006, p. 54, Fig. 3.10 for MOLA profile of Alba Mons compared to Olympus Mons. The difference in relief is striking.</ref> It is a unique volcanic structure with no counterpart on Earth or elsewhere on Mars.<ref name=Carr06p54 />
In addition to its great size and low relief, Alba Mons has a number of other distinguishing features. The central portion of the volcano is surrounded by an incomplete ring of faults (graben) and fractures, called Alba Fossae on the volcano's western flank and Tantalus Fossae on the eastern flank. The volcano also has very long, well preserved lava flows that form a radiating pattern from the volcano's central region. The enormous lengths of some individual flows (>Template:Convert) implies that the lavas were very fluid (low viscosity) and of high volume.<ref name=GreeleySpudis>Template:Cite journal</ref> Many of the flows have distinctive morphologies, consisting of long, sinuous ridges with discontinuous central lava channels. The low areas between the ridges (particularly along the volcano's northern flank) show a branching pattern of shallow gullies and channels (valley networks) that likely formed by water runoff.<ref name=Gulick90>Template:Cite journal</ref>
Alba Mons has some of the oldest extensively exposed volcanic deposits in the Tharsis region. Geologic evidence indicates that significant volcanic activity ended much earlier at Alba Mons than at Olympus Mons and the Tharsis Montes volcanoes. Volcanic deposits from Alba Mons range in age from Hesperian to early Amazonian<ref name=Ivanov06>Template:Cite journal</ref> (approximately 3.6<ref>Template:Cite journal</ref> to 3.2 billion years old<ref name=Hartmann05>Template:Cite journal</ref>).
Name originEdit
For years the volcano's formal name was Alba Patera. Patera (pl. paterae) is Latin for a shallow drinking bowl or saucer. The term was applied to certain ill-defined, scalloped-edged craters that appeared in early spacecraft images to be volcanic (or non-impact) in origin.<ref>Russell, J.F.; Snyder, C.W.; Kieffer, H.H. (1992). Origin and Use of Martian Nomenclature in Mars, H.H. Kieffer et al., Eds.; University of Arizona Press: Tucson, AZ, p. 1312.</ref> In September 2007, the International Astronomical Union (IAU) renamed the volcano Alba Mons (Alba Mountain), reserving the term Alba Patera for the volcano's two central depressions (calderas).<ref name=GPN /> Nevertheless, the entire volcano is still commonly called Alba Patera in the planetary science literature.<ref>A Google Scholar search of the astronomy and planetary science literature from 2007 to 2011 reveals 106 uses of Alba Patera versus 5 for Alba Mons (accessed May 7, 2011).</ref>
The term Alba is from the Latin word for white and refers to the clouds frequently seen over the region from Earth-based telescopes.<ref>Hartmann, 2003, p. 308</ref> The volcano was discovered by the Mariner 9 spacecraft in 1972 and was initially known as the Alba volcanic feature<ref>Template:Cite journal</ref> or the Arcadia Ring<ref>Template:Cite journal</ref> (in reference to the partial ring of fractures around the volcano). The IAU named the volcano Alba Patera in 1973.<ref name=GPN /> The volcano is often simply called Alba when the context is understood.
Location and sizeEdit
Alba Mons is centered at Template:Coord in the Arcadia quadrangle (MC-3). Much of the volcano's western flank is located in the adjacent Diacria quadrangle (MC-2).<ref name=GPN /> Flows from the volcano can be found as far north as 61°N and as far south as 26°N (in the northern Tharsis quadrangle). If one takes the outer margin of the flows as the volcano's base, then Alba Mons has north–south dimensions of about Template:Convert and a maximum width of Template:Convert.<ref name=Carr06p54 /> It covers an area of at least 5.7 million km2<ref name=Cattermole89>Template:Cite journal</ref> and has a volume of about 2.5 million km3.<ref name=Ivanov06 /> The volcano dominates the northern portion of the Tharsis bulge and is so large and geologically distinct that it can almost be treated as an entire volcanic province unto itself.<ref>Frankel, 2005, p. 134.</ref><ref name=Tanaka90>Template:Cite journal</ref>
Although Alba Mons reaches a maximum elevation of Template:Convert above Mars’ datum, the elevation difference between its summit and surrounding terrain (relief) is much greater on the north side of the volcano (about Template:Convert) compared to the south side (about Template:Convert). The reason for this asymmetry is that Alba straddles the dichotomy boundary between the cratered uplands in the south and the lowlands to the north. The plains underlying the volcano slope northward<ref>Jager, K. M.; Head, J. W.; Thomson, B.; McGovern, P. J.; Solomon, S. C. (1999). Alba Patera, Mars: Characterization Using Mars Orbiter Laser Altimeter (MOLA) Data and Comparison with Other Volcanic Edifices. 30th Lunar and Planetary Science Conference, Abstract #1915. http://www.lpi.usra.edu/meetings/LPSC99/pdf/1915.pdf.</ref> toward the Vastitas Borealis, which has an average surface elevation of Template:Convert below datum (-Template:Convert). The southern part of Alba Mons is built on a broad, north–south topographic ridge that corresponds to the fractured, Noachian-aged terrain of Ceraunius Fossae<ref name=Ivanov06 /> (pictured left).
Physical descriptionEdit
Alba's size and low profile makes it a difficult structure to study visually, as much of the volcano's relief is indiscernible in orbital photographs. However, between 1997 and 2001, the Mars Orbital Laser Altimeter (MOLA) instrument of the Mars Global Surveyor spacecraft took over 670 million<ref>MOLA Shot Counter. MIT MOLA Website. http://sebago.mit.edu/shots// (accessed May 23, 2011).</ref> precise elevation measurements across the planet. Using MOLA data, planetary scientists are able to study subtle details of the volcano's shape and topography that were invisible in images from earlier spacecraft such as Viking.<ref name=Ivanov06 />
The volcano consists of two, roughly concentric components: 1) an oval-shaped central body with approximate dimensions of Template:Convert by Template:Convert across surrounded by 2) a vast, nearly level apron of lava flows that extends an additional Template:Convert or so outward. The central body is the main topographic edifice of the volcano, marked by pronounced break in slope at the inner boundary of the apron. Extending east and west from the central edifice are two broad fan-shaped lobes (or shoulders), which give the volcano its elongation in the east–west direction.<ref name=Ivanov06 /><ref name=Ivanov02>Ivanov, M.A.; Head, J.W. (2002). Alba Patera, Mars: Assessment of its Evolution with MOLA and MOC Data. 33rd Lunar and Planetary Science Conference. LPI: Houston, TX, Abstract #1349. http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1349.pdf.</ref> The central edifice has the steepest slopes on the volcano, although they are still only 1°.<ref name=Carr06p54 /> The crest and upper flanks of the edifice are cut by a partial ring of graben that are part of the Alba and Tantalus Fossae fracture system. Inside the ring of graben is an annulus of very low and in places reversed slopes<ref name=Carr06p54 /> that forms a plateau on top of which lies a central dome Template:Convert across capped by a nested caldera complex.<ref name=Ivanov02 /> Thus, the central edifice of Alba Mons resembles a partially collapsed shield volcano with a smaller, summit dome sitting on top (pictured right). The summit dome has a distinct tilt to the east.
The caldera complex consists of a large caldera about Template:Convert by Template:Convert across at the center of the summit dome. A smaller, kidney-shaped caldera (about Template:Convert by Template:Convert) lies in the southern half of the larger one. Both calderas are relatively shallow,<ref name=Cattermole01p85 /> reaching a maximum depth of only Template:Convert.<ref name=Plescia04 />
The larger caldera is bounded at the westernmost end by a steep, semicircular wall Template:Convert tall. This wall disappears at the northern and southern sides of the caldera, where it is buried by volcanic flows originating from the younger, smaller caldera.<ref name=Cattermole01p85 /> The smaller caldera is outlined everywhere by a steep wall that varies in height over a range of a few hundred meters. The walls of both calderas are scalloped, suggesting multiple episodes of subsidence and/or mass wasting.<ref name=Ivanov06 /> Two small shields or domes, several hundred meters high, occur within and adjacent to the large caldera. The shield within the large caldera is about Template:Convert across. It is capped by a peculiar concentric circular feature Template:Convert in diameter<ref name=Ivanov06 /><ref name=Ivanov02 /> (pictured left).
Calderas form by collapse following withdrawal and depletion of a magma chamber after an eruption. Caldera dimensions allow scientists to infer the geometry and depth of the magma chamber beneath the summit of the volcano.<ref>Mouginis-Mark, P.J.; Harris, A.J.; Rowland, S.K. (2008). Terrestrial Analogs to the Calderas of the Tharsis Volcanoes on Mars in The Geology of Mars: Evidence from Earth-Based Analogs, M. Chapman, Ed.; Cambridge University Press: Cambridge, UK, p. 71.</ref> The shallowness of Alba's calderas compared to those seen on Olympus Mons and most of the other Tharsis volcanoes implies that Alba's magma reservoir was wider and shallower than those of its neighbors.<ref name=Cattermole01p86>Cattermole, 2001, p. 86.</ref>
Surface characteristicsEdit
Most of the central edifice of Alba Mons is mantled with a layer of dust approximately Template:Convert thick.<ref name=Christensen86>Template:Cite journal</ref><ref>Ruff, S. W.; Christensen, P. R. (2001). A Spectrally-based Global Dust Cover Index for Mars from Thermal Emission Spectrometer Data. First Landing Site Workshop for the 2003 Mars Exploration Rovers, Abstract #9026. http://www.lpi.usra.edu/meetings/mer2003/pdf/9026.pdf.</ref> The dust layer is visible in high resolution images of the summit (pictured right). In places, the dust has been carved into streamlined shapes by the wind and is cut by small landslides. However, some isolated patches of dust appear smooth and undisturbed by the wind.<ref>Keszthelyi, L.P. (2006). Dusty Top of Alba Patera Volcano. University of Arizona HiRISE Website. http://hirise.lpl.arizona.edu/PSP_001510_2195. (accessed May 18, 2011).</ref>
Heavy dust cover is also indicated by the high albedo (reflectivity) and low thermal inertia of the region. Martian dust is visually bright (albedo > 0.27) and has a low thermal inertia because of its small grain size (<Template:Convert).<ref name=Christensen86 /><ref name=Putzig05>Putzig, N.E. et al. (2005). Global Thermal Inertia and Surface Properties of Mars from the MGS Mapping Mission. Icarus, 173 Tbl. 1, Fig. 5, p. 331.</ref> (See the Martian surface.) However, the thermal inertia is high and albedo lower on the northern flanks of the volcano and in the apron area farther to the north. This suggests that the northern portions of Alba's surface may contain a higher abundance of duricrusts, sand, and rocks compared to the rest of the volcano.<ref name=Putzig05 />
High thermal inertia can also indicate the presence of exposed water ice. Theoretical models of water-equivalent hydrogen (WEH) from epithermal neutrons detected by the Mars Odyssey Neutron Spectrometer (MONS) instrument suggest that the regolith just below the surface on Alba's northern flank may contain 7.6% WEH by mass.<ref>Feldman, W.C.; Mellon, M.T.; Gasnault, O.; Maurice, S.; Prettyman, T.H. (2008). Volatiles on Mars: Scientific Results from the Mars Odyssey Neutron Spectrometer in The Martian Surface: Composition, Mineralogy, and Physical Properties, J.F. Bell III, Ed.; Cambridge University Press: Cambridge, UK, p. 135 and Fig. 6.8. Template:ISBN.</ref> This concentration could indicate water present as remnant ice or in hydrated minerals.<ref>Barlow, N.G. (2008). Mars: An Introduction to its interior, Surface, and Atmosphere; Cambridge University Press: Cambridge, UK, p. 202. Template:ISBN.</ref> Alba Mons is one of several areas on the planet that may contain thick deposits of near-surface ice preserved from an earlier epoch (1 to 10 million years ago), when Mars’ axial tilt (obliquity) was higher and mountain glaciers existed at mid-latitudes and tropics. Water ice is unstable at these locations under present conditions and will tend to sublimate into the atmosphere.<ref>Template:Cite journal</ref> Theoretical calculations indicate that remnant ice can be preserved below depths of 1 m if it is blanketed by a high-albedo and low-thermal-inertia material, such as dust.<ref>Feldman, W. C.; Prettyman, T. H.; Maurice, S.; Lawrence, D. J.; Pathare, A.; Milliken, R. E.; Travis B. J. (2011). Search for Remnant Water Ice from Past Glacial Climates on Mars: The Mars Odyssey Neutron Spectrometer. 42nd Lunar and Planetary Science Conference, Abstract #2420. http://www.lpi.usra.edu/meetings/lpsc2011/pdf/2420.pdf.</ref>
The mineral composition of rocks making up Alba Mons is difficult to determine from orbital reflectance spectrometry because of the predominance of surface dust throughout the region. However, global-scale surface composition can be inferred from the Mars Odyssey gamma-ray spectrometer (GRS). This instrument has allowed scientists to determine the distribution of hydrogen (H), silicon (Si), iron (Fe), chlorine (Cl), thorium (Th) and potassium (K) in the shallow subsurface. Multivariate analysis of GRS data indicates that Alba Mons and the rest of the Tharsis region belongs to a chemically distinct province characterized by relatively low Si (19 wt%), Th (0.58 pppm), and K (0.29 wt%) content, but with Cl abundance (0.56 wt%) higher than Mars' surface average.<ref>Gasnault, O. (2006). Unsupervised Definition of Chemically Distinct Provinces at Mars. 37th Lunar and Planetary Science Conference, Abstract #2328. http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2328.pdf.</ref> Low silicon content is indicative of mafic and ultramafic igneous rocks, such as basalt and dunite.
Alba Mons is an unlikely target for unmanned landers in the near future. The thick mantle of dust obscures the underlying bedrock, probably making in situ rock samples hard to come by and thus reducing the site's scientific value. The dust layer would also likely cause severe maneuvering problems for rovers. Ironically, the summit region was originally considered a prime backup landing site for the Viking 2 lander because the area appeared so smooth in Mariner 9 images taken in the early 1970s.<ref name=Carretal77>Template:Cite journal</ref>
GeologyEdit
Much of the geologic work on Alba Mons has focused on the morphology of its lava flows and the geometry of the faults cutting its flanks. Surface features of the volcano, such as gullies and valley networks, have also been extensively studied. These efforts have the overall goal of deciphering the geologic history of the volcano and the volcano-tectonic processes involved in its formation. Such understanding can shed light on the nature and evolution of the Martian interior and the planet's climate history.
Lava flowsEdit
Alba Mons is notable for the remarkable length, diversity, and crisp appearance of its lava flows.<ref name=Carretal77 /> Many of the flows radiate from the summit, but others appear to originate from vents and fissures on the lower flanks of the volcano.<ref name=Cattermole87>Template:Cite journal</ref> Individual flows may exceed Template:Convert in length.<ref name=Pieri88>Pieri, D.; Schneeberger, D. (1988). Morphology of Lava Flows at Alba Patera. 19th Lunar and Planetary Science Conference, Abstract #1471. http://www.lpi.usra.edu/meetings/lpsc1988/pdf/1471.pdf.</ref> Lava flows near the summit calderas appear to be significantly shorter and narrower than those on more distal parts of the volcano.<ref>Schneeberger and Pieri, 1991, cited by McGovern et al., 2001.</ref> The two most common types of volcanic flows on Alba Mons are sheet flows and tube-and-channel fed flows.
Sheet flows (also called tabular flows<ref name=Pieri88 />) form multiple, overlapping lobes with steep margins. The flows typically lack central channels. They are flat-topped and generally about Template:Convert wide on the upper flanks of the volcano but become much wider and lobate toward their downstream (distal) ends.<ref name=Cattermole87 /> Most appear to originate near the Alba and Tantalus Fossae fracture ring, but the actual vents for the sheet flows are not visible and may have been buried by their own products.<ref name=GreeleySpudis /> Flow thicknesses have been measured for a number of sheet flows based on MOLA data. The flows range from Template:Convert to Template:Convert thick and are generally thickest at their distal margins.<ref>Shockey, K.M.; Glaze, L.S.; Baloga, S.M. (2004). Analysis of Alba Patera Flows: A Comparison of Similarities and Differences. 35th Lunar and Planetary Science Conference, Abstract #1154. http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1154.pdf.</ref>
The second major type of lava flows on the flanks of Alba Mons are called tube- and channel-fed flows, or crested flows.<ref name=Pieri88 /> They form long, sinuous ridges that radiate outward from the central region of the volcano. They are typically Template:Convert-Template:Convert wide. An individual ridge may have a discontinuous channel or line of pits that run along its crest. Tube- and channel-fed flows are particularly prominent on the western flank of the volcano where individual ridges can be traced for several hundred kilometers. The origin of the ridges is uncertain. They may form by successive buildup of solidified lava at the mouth of a channel or tube, with each pulse of flowing lava adding to the length of the ridge.<ref name=Carr06p55>Carr, 2006, pp. 55–56.</ref>
In addition to the two main types of flows, numerous undifferentiated flows are present around Alba Mons that are either too degraded to characterize or have hybrid characteristics. Flat-topped ridges with indistinct margins and rugged surfaces,<ref name=GreeleySpudis /><ref name=Carretal77 /> interpreted as lava flows, are common along Alba's lower flanks and become less sharp in appearance with increasing distance from the edifice.<ref name=Ivanov06 /> In high resolution images, many of the flows on the volcano's upper flanks originally characterized as sheet flows have central channels with levee-like ridges.<ref name=Riedel>Riedel, S. J.; Sakimoto, S. E. H. (2002). MOLA Topographic Constraints on Lava Tube Effusion Rates for Alba Patera, Mars. 33rd Lunar and Planetary Science Conference, Abstract #1410. http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1410.pdf.</ref>
The morphology of lava flows can indicate properties of the lava when molten, such as its rheology and flow volume. Together, these properties can provide clues to the lava's composition and eruption rates.<ref name=Carretal77 /> For example, lava tubes on Earth only form in lavas of basaltic composition. Silica-rich lavas such as andesite are too viscous for tubes to form.<ref name=GreeleySpudis /> Early quantitative analysis of Alba's lava flows<ref name=Cattermole87 /> indicated that the lavas had low yield strength and viscosity and were erupted at very high rates. Alba's unusually low profile suggested to some that extremely fluid lavas were involved in the volcano's construction, perhaps komatiites, which are primitive ultramafic lavas that form at very high temperatures.<ref name=Cattermole01p85 /> However, more recent work on the tube- and channel-fed flows indicates lava viscosities within the range of typical basalts (between 100 and 1 million Pa s−1).<ref>Sakimoto, S.; Crisp, J.; Baloga, S.M. (1997). Eruption constraints on Tube-Fed Planetary Lava Flows. J. Geophys. Res., 102 6597–6614. Cited in Cattermole, 2001, p. 85.</ref> Calculated flow rates are also lower than originally thought, ranging from 10 to 1.3 million m3 per second. The lower range of eruption rates for Alba Mons is within the range of the highest terrestrial volcanic flows, such as the 1984 Mauna Loa, North Queensland (McBride Province), and the Columbia River basalts. The highest range is several orders of magnitude higher than the effusive rates for any terrestrial volcano.<ref name=Riedel />
Since the late 1980s, some researchers have suspected that Alba Mons eruptions included a significant amount of pyroclastics (and therefore explosive activity) during early phases of its development. The evidence was based on the presence of numerous valley networks on the volcano's northern flanks that appeared to be carved by running water (see below). This evidence combined with thermal inertia data, which indicated a surface dominated by fine-grained materials, suggested an easily erodible material, such as volcanic ash, was present. The volcano's extremely low profile is also more easily explained if the edifice were built largely from pyroclastic flow deposits (ignimbrites).<ref>Mouginis-Mark, P.J.; Zimbelman, J.R. (1987). Channels on Alba Patera, Mars: Evidence for Polygenic Eruptions. 18th Lunar and Planetary Science Conference, Abstract #1346. http://www.lpi.usra.edu/meetings/lpsc1987/pdf/1346.pdf.</ref><ref>Template:Cite journal</ref><ref>Mouginis-Mark, P.J.; Wilson, L.; Zuber, M.T. (1992). Physical Volcanology in Mars, H.H. Kieffer et al., Eds.; University of Arizona Press: Tucson, AZ, pp. 247-248, and Fig. 6.</ref>
More recent data from Mars Global Surveyor and the Mars Odyssey spacecraft have shown no specific evidence that explosive eruptions ever occurred at Alba Mons. An alternative explanation for the valley networks on the north side of the volcano is that they were produced through sapping or melting of ice-rich dust deposited during a relatively recent, Amazonian-aged glacial epoch.<ref name=Ivanov06 /><ref>Carr, 2006, p. 56.</ref>
In summary, current geologic analysis of Alba Mons suggests that the volcano was built by lavas with rheological properties similar to basalts.<ref name=Schneeberger91>Template:Cite journal</ref> If early explosive activity happened at Alba Mons, the evidence (in the form of extensive ash deposits) is largely buried by younger basaltic lavas.<ref name=Ivanov06 />
Tectonic FeaturesEdit
The immense system of fractures surrounding Alba Mons is perhaps the most striking feature of the volcano.<ref name=Carr06p54 /> The fractures are tectonic features indicating stresses in the planet's lithosphere. They form when the stresses exceed the yield strength of rock, resulting in the deformation of surface materials. Typically, this deformation is manifested as slip on faults that are recognizable in images from orbit.<ref name=Banerdt92>Banerdt, W.B.; Golombek, M.P.; Tanaka, K.L. (1992). Stress and Tectonics on Mars in Mars, H.H. Kieffer et al., Eds.; University of Arizona Press: Tucson, AZ, pp. 248–297.</ref>
Alba's tectonic features are almost entirely extensional,<ref name=McGovern01>McGovern, P.J. et al. (2001). Extension and Uplift at Alba Patera, Mars: Insights from MOLA Observations and Loading Models. J. Geophys. Res., 106(E10), 23,769–23,809.</ref> consisting of normal faults, graben and tension cracks. The most common extensional features on Alba Mons (and Mars in general) are simple graben. Graben are long, narrow troughs bound by two inward-facing normal faults that enclose a downfaulted block of crust (pictured right). Alba has perhaps the clearest display of simple graben on the entire planet.<ref name=Carr06p86>Carr, 2006, pp. 86–87.</ref> Alba's graben are up to Template:Convert long, and have a width on the order of Template:Convert–Template:Convert, with depths of Template:Convert–Template:Convert.<ref name=Cailleauetal03>Template:Cite journal</ref>
Tension cracks (or joints) are extensional features produced when the crust is wrenched apart with no significant slippage between the separated rock masses. In theory they should appear as deep fissures with sharp V-shaped profiles, but in practice they are often difficult to distinguish from graben because their interiors rapidly fill with talus from the surrounding walls to produce relatively flat, graben-like floors.<ref name=Carr06p86 /> Pit crater chains (catenae), common within many graben on Alba's flanks, may be the surface manifestation of deep tension cracks into which surface material has drained.<ref name=Banerdt92 />
The graben and fractures around Alba Mons (hereafter simply called faults unless otherwise indicated) occur in swarms that go by different names depending on their location with respect to Alba's center.<ref name=Banerdt92 /> South of the volcano is a broad region of intensely fractured terrain called Ceraunius Fossae, which consists of roughly parallel arrays of narrow, north–south oriented faults. These faults diverge around the flanks of the volcano, forming an incomplete ring about Template:Convert in diameter.<ref name=Carr06p54 /> The set of faults on Alba's western flank is called Alba Fossae and the one on the eastern flank Tantalus Fossae. North of the volcano, the faults splay outward in a northeasterly directions for distances of many hundreds of kilometers. The pattern of faults curving around Alba's flanks has been likened in appearance to the grain of a piece of wood running past a knot.<ref>Morton, 2002, p.101-102.</ref> The entire Ceraunius-Alba-Tantalus fault system is at least Template:Convert long and Template:Convert–Template:Convert wide<ref>Template:Cite journal</ref>
Several causes for the faults have been suggested, including regional stresses created by the Tharsis bulge, volcanic dikes, and crustal loading by Alba Mons itself.<ref name=Carr06p54 /> The faults of Ceraunius and Tantalus Fossae are roughly radial to the center of Tharsis and are likely a crustal response to the sagging weight of the Tharsis bulge. The faults ringing Alba's summit region may be due to a combination of loading from the Alba edifice and magma uplift or underplating from the underlying mantle.<ref name=McGovern01 /><ref name=Cailleauetal03 /> Some of the fractures are likely the surface expression of gigantic dike swarms radial to Tharsis.<ref>Template:Cite journal</ref><ref>Okubo, C. H.; Schultz, R.A. (2005). Evidence of Tharsis-Radial Dike Intrusion in Southeast Alba Patera from MOLA-based Topography of Pit Crater Chains. 36th Lunar and Planetary Science Conference, Abstract #1007. http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1007.pdf.</ref> An image from High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) shows a line of rimless pit craters in Cyane Fossae on the Alba's western flank (pictured right). The pits likely formed by the collapse of surface materials into open fractures created as magma intruded the subsurface rock to form dikes.<ref>University of Arizona HiRISE Website. http://hirise.lpl.arizona.edu/PSP_010345_2150.</ref>
Valleys and gulliesEdit
The northern slopes of Alba Mons contain numerous branching channel systems or valley networks that superficially resemble drainage features produced on Earth by rainfall. Alba's valley networks were identified in Mariner 9 and Viking images in the 1970s, and their origin has long been a topic of Mars research. Valley networks are most common in the ancient Noachian-aged southern highlands of Mars, but also occur on the flanks of some of the large volcanoes. The valley networks on Alba Mons are Amazonian in age and thus significantly younger than the majority of those in the southern highlands. This fact presents a problem for researchers who propose that valley networks were carved by rainfall runoff during an early, warm and wet period of Martian history.<ref>Template:Cite journal</ref> If the climate conditions changed billions of years ago into today's cold and dry Mars (where rainfall is impossible), how does one explain the younger valleys on Alba Mons? Did Alba's valley networks form differently from those in the highlands, and if so, how? Why do the valleys on Alba Mons occur mainly on the northern flanks of the volcano? These questions are still being debated.<ref>See Carr, M.H. (1996). Water on Mars; Oxford University Press: Oxford, UK, pp.90–92, for a more detailed discussion.</ref>
In Viking images, the resemblance of Alba's valley networks to terrestrial pluvial (rainfall) valleys is quite striking. The valley networks show a fine-textured, parallel to dendritic pattern with well-integrated tributary valleys and drainage densities comparable to those on Earth's Hawaiian volcanoes.<ref name=Gulick90 /><ref>Gulick, V.C. (2005). Revisiting Valley Development on Martian Volcanoes Using MGS and Odyssey Data. 36th Lunar and Planetary Science Conference, Abstract #2345. http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2345.pdf.</ref> However, stereoscopic images from the High Resolution Stereo Camera (HRSC) on the European Mars Express orbiter show that the valleys are relatively shallow (Template:Convert or less) and more closely resemble rills or gullies from intermittent runoff erosion than valleys formed from sustained erosion.<ref name=Ansan08>Ansan, V.; Mangold, N.; Masson, Ph.; Neukum, G. (2008). The Topography of Valley Networks on Mars: Comparison Between Valleys of Different Ages. 39th Lunar and Planetary Science Conference, Abstract #1585. http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1585.pdf.</ref> It seems likely that the valleys on Alba Mons formed as a result of transient erosional processes, possibly related to snow or ice deposits melting during volcanic activity,<ref name=Ansan08 /><ref>Template:Cite journal</ref> or to short-lived periods of global climate change.<ref name=Ivanov06 /> (See Surface characteristics, above.) Whether the eroded material is an ice-rich dust or friable volcanic ash is still uncertain.
Geologic historyEdit
Alba's well-preserved lava flows and faults provide an excellent photogeologic record of the volcano's evolution. Using crater counting and basic principles of stratigraphy, such as superposition and cross-cutting relationships, geologists have been able to reconstruct much of Alba's geologic and tectonic history. Most of the constructional volcanic activity at Alba is believed to have occurred within a relatively brief time interval (about 400 million years) of Mars history, spanning mostly the late Hesperian to very early Amazonian epochs. Faulting and graben formation in the region occurred in two early stages: one preceding and the other contemporaneous with the volcano's formation. Two late stages of graben formation occurred after volcanic activity had largely ended.<ref name=Tanaka90 />
Based on Viking Orbiter images, the volcanic materials related to the formation and evolution of the volcano have been grouped into the Alba Patera Formation, which consists of lower, middle, and upper members.<ref name=Ivanov06 /><ref name=ScottTanaka86>Scott, D.H.; Tanaka, K.L. (1986). Geologic Map of the Western Equatorial Region of Mars. USGS Miscellaneous Investigations Series Map I–1802–A.</ref> Members low in the stratigraphic sequence are older than those lying above, in accordance with Steno's law of superposition.
The oldest unit (lower member) corresponds to the broad lava apron surrounding the Alba Mons edifice. This unit is characterized by sets of low, flat-topped ridges that form a radial pattern extending for hundreds of kilometers to the west, north, and northeast of the main edifice. The ridges are interpreted to be lava flows,<ref name=ScottTanaka86 /> although the flow margins are now degraded and difficult to delineate. Broad lava flows with flat-topped ridges are characteristic features of lava flood provinces on Earth (e.g., Columbia River basalt) that were formed at high eruption rates.<ref>Hooper, P. R. (1988). The Columbia River Basalt, in Continental Flood Basalts, J. D. Macdougall, Ed.; Springer: New York, pp 1–33 and Self, S.; Thordarson, T.; Keszthelyi, L. (1997). Emplacement of Continental Flood Basalt Lava Flows, in Large Igneous Provinces, J. J. Mahoney and M. F. Coffin, Eds.; AGU, Monograph 100, pp. 381–410. Cited in Ivanov and Head (2006), p. 21.</ref> Thus, the earliest phase of volcanic activity at Alba Mons probably involved massive effusive eruptions of low viscosity lavas that formed the volcano's broad, flat apron. Lava flows of the apron unit straddle the early Hesperian-late Hesperian boundary, having erupted approximately 3700 to 3500 million years ago.<ref name=Ivanov06 /><ref name=Hartmann05 />
The middle unit, which is early Amazonian in age, makes up the flanks of the main Alba edifice and records a time of more focused effusive activity consisting of long tube- and channel-fed flows. Volcanic spreading occurred in a northward direction forming the two flanking lobes. (See Olympus Mons and Tharsis for a discussion of volcanic spreading on Mars.) Faulting and graben formation at Alba and Tantalus Fossae occurred penecontemporaneous with the lava flows. Any early explosive activity on the volcano may have occurred during the culmination of this middle phase of activity, which ended about 3400 million years ago.<ref name=Ivanov06 /><ref name=Hartmann05 /><ref name=Ivanov06fig32>Ivanov and Head (2006), Fig. 32.</ref>
The youngest unit, also early Amazonian, covers the summit plateau, dome, and caldera complex. This period of activity is characterized by relatively short-length sheet flows and construction of the summit dome and the large caldera. This phase ended with an eastward tilting of the summit dome, which may have initiated additional graben formation in Alba Fossae. The last volcanic features to form were the small shield and caldera at the summit. Much later, between about 1,000 and 500 million years ago, a final stage of faulting occurred that may have been related to dike emplacement and the formation of pit crater chains.<ref name=Ivanov06 /><ref name=Hartmann05 /><ref name=Ivanov06fig32 />
ClassificationEdit
The classification of the Alba Mons volcano is uncertain. Some workers describe it as a shield volcano,<ref name=Ivanov06 /><ref name="McGovern01"/> others as a lowland patera<ref>Cattermole, 2001, p. 72</ref> (in contrast to highland paterae, which are low-lying ancient volcanoes with furrowed ash deposits located in the southern Martian highlands), and still others consider it a one-of-a-kind volcanic structure unique to Mars.<ref name=Carr06p54 /><ref name=GreeleySpudis /> Some researchers have compared Alba Mons to coronae structures on the planet Venus.<ref>Barlow, N.G.; Zimbleman, J.R. (1988). Venusian Coronae: Comparisons to Alba Patera, Mars. 19th Lunar and Planetary Science Conference. Abstract #1019. http://www.lpi.usra.edu/meetings/lpsc1988/pdf/1019.pdf.</ref><ref>Template:Cite journal</ref> Alba Mons shares some characteristics with the Syrtis Major volcanic structure. (See Volcanism on Mars.) Both volcanoes are Hesperian in age, cover large areas, have very low relief, and large shallow calderas. Also like Alba, Syrtis Major displays ridged tube- and channel-fed lava flows.<ref>Woodcock, B. L.; Sakimoto, S. E. H. (2006). Lava Tube Flow: Constraints on Maximum Sustained Eruption Rates for Major Martian Volcanic Edifices. 37th Lunar and Planetary Science Conference, Abstract #1992. http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1992.pdf.</ref> Because Alba Mons lies antipodal to the Hellas impact basin, a few researchers have conjectured that the volcano's formation may have been related to crustal weakening from the Hellas impact, which produced strong seismic waves that focused on the opposite side of the planet.<ref name = "Peterson">Template:Cite journal</ref><ref name = "Williams">Template:Cite journal</ref><ref name = "Williams2">Template:Cite journal</ref>
See alsoEdit
ReferencesEdit
Further readingEdit
- Boyce, Joseph, M. (2008). The Smithsonian Book of Mars; Konecky & Konecky: Old Saybrook, CT, Template:ISBN
- Carr, Michael, H. (2006). The Surface of Mars; Cambridge University Press: Cambridge, UK, Template:ISBN.
- Cattermole, Peter, J. (2001). Mars: The Mystery Unfolds; Oxford University Press: Oxford, UK, Template:ISBN.
- Frankel, Charles (2005). Worlds on Fire: Volcanoes on the Earth, the Moon, Mars, Venus and Io; Cambridge University Press: Cambridge, UK, Template:ISBN.
- Hartmann, William, K. (2003). A Traveler’s Guide to Mars: The Mysterious Landscapes of the Red Planet; Workman: New York, Template:ISBN.
- Morton, Oliver (2003). Mapping Mars: Science, Imagination, and the Birth of a World; Picador: New York, Template:ISBN.
External linksEdit
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