{{short description|Martian volcano}} {{Infobox feature on celestial object |name = Tharsis Tholus |image = [[File:Tharsis Tholus THEMIS day IR 100m v11.jpg|240px]] |caption = [[Thermal Emission Imaging System|THEMIS]] daytime IR mosaic of Tharsis Tholus. The western and eastern flanks of the volcano have been sites of large-scale flank collapse events.<ref name = "Platz2011">{{Cite journal | doi = 10.1016/j.epsl.2011.03.012| title = Vertical and lateral collapse of Tharsis Tholus, Mars| journal = Earth and Planetary Science Letters| volume = 305| issue = 3–4| pages = 445–455| date = 2011-05-15| last1 = Platz | first1 = T.| last2 = Münn | first2 = S.| last3 = Walter | first3 = T. R. | last4 = Procter | first4 = J. N. | last5 = McGuire | first5 = P. C. | last6 = Dumke | first6 = A.| last7 = Neukum | first7 = G.|bibcode = 2011E&PSL.305..445P }}</ref> An irregular caldera is present in the center. |location = [[Tharsis quadrangle]] |coordinates = {{coord|13.25|N|90.69|W|globe:mars_type:landmark|display=inline,title}} |coordinates_footnotes = <ref name="Nomenclature">Gazetteer of Planetary Nomenclature. https://planetarynames.wr.usgs.gov/Feature/5949</ref> |peak = 9 km above datum |discoverer = Mariner 9 |type = Shield volcano }}
'''Tharsis Tholus''' is an intermediate-sized [[shield volcano]] located in the eastern [[Tharsis]] region of the planet [[Mars]]. The volcano was discovered by the Mariner 9 spacecraft in 1972<ref>Carr, M.H. (1973). Volcanism on Mars.'' J. Geophys. Res.'' '''78'''(20), p. 4050, Fig. 1.</ref> and originally given the informal name Volcano 7.<ref>''National Geographic'' (1973) Mars Map Supplement; February, '''143'''(2), p. 255A.</ref> In 1973, the International Astronomical Union ([[IAU]]) officially designated it Tharsis Tholus.<ref name="Nomenclature" /> In [[planetary geology]], ''[[tholus]]'' (pl. ''tholi'') is the term for a small [[wikt:domical|domical]] [[mountain]], usually a volcano.<ref>Hartmann, W.K. (2003). ''A Traveler's Guide to Mars: The Mysterious Landscapes of the Red Planet;'' Workman: New York, p. 28.</ref>
==General Description== [[File:Tharsis Quadrangle map-la.svg|thumb|left|340px|Map of [[Tharsis quadrangle]]. Tharsis Tholus is on the far right.]] Tharsis Tholus lies on the eastern edge of the [[Tharsis quadrangle]] at 13.5°N, 91°W. It is about 800 km east-northeast of [[Ascraeus Mons]],<ref name="Platz">Platz, T. et al. (2009). Growth and Destruction Cycles and Eruption Styles at Tharsis Tholus, Mars. 40th Lunar and Planetary Science Conference; LPI: Houston, Abstract #1522. http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1522.pdf.</ref> the northernmost of the large [[Tharsis Montes]] volcanoes. Lava from the Tharsis Montes and other sources within the interior of Tharsis completely surrounds Tharsis Tholus, forming a broad volcanic plain at the volcano's base.{{refn | Plescia, 2003, p. 224<ref name="Plescia, J.B. 2003 pp. 223-241"/>| group = lower-alpha }} Tharsis Tholus measures 155 km x 125 km.<ref name="Platz" /> It is distinctly bulbous in appearance, and is unique among Martian volcanoes in the degree to which it has been modified by [[fault (geology)|faulting]].<ref name="Plescia">Plescia, J.B. (2001). Geology of Tharsis Tholus, Mars. 32nd Lunar and Planetary Science Conference, LPI: Houston, Abstract #1090, p. 1. http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1090.pdf.</ref> Large normal faults cut across and completely penetrate the volcano, dividing the edifice into several major blocks, or sectors.<ref name="Plescia, J.B. 2003 pp. 223-241">{{cite journal|last1=Plescia|first1=J.B|title=Tharsis Tholus: an unusual martian volcano|journal=Icarus| volume=165|issue=2| year=2003|pages=223–241| issn=0019-1035| doi=10.1016/S0019-1035(03)00199-4|bibcode = 2003Icar..165..223P |url=https://zenodo.org/record/1259603}}</ref> The volcano has an elongated central [[caldera]] (collapse crater) measuring 36.7 × 38.9 km and about 3 km deep.
In profile, Tharsis Tholus is dome-shaped (convex upward),<ref>Carr, M.H. (2006). ''The Surface of Mars;'' Cambridge University Press: Cambridge, UK, p. 57.</ref> with flank slopes ranging from less than 1° near the summit to up to 16° at the base.<ref name="Platz" /> The average flank slope is 10°, making it one of the steepest volcanoes on Mars.<ref name="Plescia2004">{{cite journal|last1=Plescia|first1=J. B.|title=Morphometric properties of Martian volcanoes|journal=Journal of Geophysical Research|volume=109|issue=E03003|year=2004|page = Table 1|issn=0148-0227|doi=10.1029/2002JE002031|bibcode = 2004JGRE..109.3003P |doi-access=free}}</ref> The volcano rises to an elevation of about {{convert|9|km|mi|abbr=on}}. The base of the volcano is buried by young (Amazonian-aged) lava flows presumably from the Tharsis Montes, so the true size of the edifice cannot be accurately determined. Estimates of the thickness of lava burying the base of Tharsis Tholus range from 500 m<ref name="Platz" /> to 3.5 km.<ref>Robinson, M. (1993) PhD dissertation cited by Plescia (2003), p. 236.</ref>
Most of the volcano's surface is mantled with a thick layer of fine dust or ash,<ref name="Platz" /> giving the volcano's flanks a smooth to slightly [[hummock]]y appearance.<ref name="Plescia" /> The mantle material obscures the underlying bedrock except in areas with steep slopes, such as along the caldera's upper walls. [[Volcanic landslide|Landslide]] debris from the caldera's western and southeastern wall is present on the floor of the caldera.
==Geology== [[File:Tharsis Tholus JMars.jpg|thumb|right|380px|Oblique view of Tharsis Tholus looking northeast. Note the down-dropped block on western flank bounded by normal faults. A large debris avalanche deposit (DAD) is present at upper left. Narrow NE-trending grabens cross the volcano. Large lava flows from [[Tharsis]] are visible in lower left and upper right. Image is [[Thermal Emission Imaging System|THEMIS]] daytime [[infrared|IR]] mosaic overlain on [[Mars Orbiter Laser Altimeter|MOLA]].]] The volcano's relatively steep slopes and dome-like shape led some early researchers to conclude that the volcano is built from [[viscosity|viscous]], [[intermediate composition|silicic]] lavas or [[Pyroclastic rock|pyroclastics]], rather than fluid [[basalt]]s.<ref name="Plescia" /><ref>Greeley, R.; Spudis, P., 1981. Volcanism on Mars. ''Rev. Geophys.'' '''19,''' pp. 13–41. Cited by Plescia (2003), p. 223</ref><ref name="Robinson, M. 1993 p. 232">Robinson, M. (1993) PhD dissertation. Cited by Plescia (2003), p. 232.</ref> However, more recent studies based on data from the [[Mars Odyssey]], [[Mars Express]], and [[Mars Reconnaissance Orbiter]] (MRO) spacecraft indicate that Tharsis Tholus is probably a basaltic [[shield volcano|shield]]{{refn | Plescia, 2003, p. 223<ref name="Plescia, J.B. 2003 pp. 223-241"/>| group = lower-alpha }} or a shield with a [[stratocone]] component.<ref name="Platz" /> [[CRISM]] data from MRO show that lava exposed on the caldera scarp is composed of high- and low-calcium [[pyroxene]]s. [[Olivine]] is present in abundance on the west flank of the volcano, but it appears to be present only in the dust that mantles the volcano. No [[phyllosilicate]]s (clay minerals), [[sulfate]]s, or iron oxides have been detected.<ref name="Platz" />
Tharsis Tholus has a complex volcano-tectonic history. The edifice has been modified by at least four distinct episodes of structural deformation.<ref name="Platz" /> The most prominent area of deformation is the central caldera complex, which consists of an outer, older caldera and an inner, younger one.{{refn | Plescia, 2003, p. 229<ref name="Plescia, J.B. 2003 pp. 223-241"/>| group = lower-alpha }} The calderas are bordered by a well-preserved system of concentric [[normal fault]]s and peripheral ring faults.<ref name="Platz" /> A third, barely visible, caldera is present on the volcano's southern flank. Much of that caldera's rim is buried by ejecta from a young impact crater; however, the caldera's presence is outlined by an almost level circular area about 45 km in diameter.<ref name="lpi.usra.edu">Maciejak, F.; Lenat J.-F.; Provost, A. (1995). Volcano-Tectonic Evolution Of Tharsis Tholus, Mars. 26th Lunar and planetary Science Conference, LPI: Houston, Abstract #1441, pp. 881-882. http://www.lpi.usra.edu/meetings/lpsc1995/pdf/1441.pdf.</ref>
A system of large, arcuate normal faults lie radial to the volcano's center. The normal faults have large offsets and transect the volcano into at least four sectors.<ref name="Plescia, J.B. 2003 pp. 223-241"/> The north and south sectors have smooth, convex upward profiles. The western and eastern sectors of the volcano consist of huge, down-dropped blocks. A large debris avalanche deposit (DAD) is present at the northwestern edge of the western block. The DAD appears to be made up of coherent slabs of flank material originating from the down-dropped western flank. Some researchers<ref name="Platz" /><ref name="Plescia" /><ref name="Robinson, M. 1993 p. 232"/> have compared the deformation on the flanks of Tharsis Tholus to "sector collapse" observed at [[Mauna Loa]] in [[Hawaii]] and other terrestrial volcanoes. Sector collapse is a sudden, catastrophic detachment of a volcano's flank caused by lateral spreading of the volcano.<ref>Hawaiian Volcano Observatory (2008). Volcanic Spreading Induces Flank Collapse at Mauna Loa. USGS. https://hvo.wr.usgs.gov/volcanowatch/2008/08_10_16.html.</ref> Another suggested terrestrial [[Analogy|analog]] for Tharsis Tholus is the [[Maderas]] Volcano in [[Nicaragua]].{{refn | Plescia, 2003, p. 235<ref name="Plescia, J.B. 2003 pp. 223-241"/>| group = lower-alpha }}
In addition to the normal faults, a system of narrow, parallel [[graben]]s cut across large parts of the volcano. The grabens [[strike and dip|strike]] to the northeast and range in width from 0.5 to 2.8 km. Chains of pits craters associated with the grabens are also observed.<ref name="Plescia" />{{refn | Plescia, 2003, p. 228<ref name="Plescia, J.B. 2003 pp. 223-241"/>| group = lower-alpha }} The age relationship between the grabens and normal faults is complex. In places, the grabens appear to crosscut the normal faults, indicating that the grabens are younger. However, the strikes of the normal faults locally appear to have a strong northeast influence, suggesting that the stresses that formed the grabens predate the normal faults. It is likely that the grabens developed repeatedly over an extended period of time{{refn | Plescia, 2003, p. 232<ref name="Plescia, J.B. 2003 pp. 223-241"/>| group = lower-alpha }} and may be partly contemporaneous with the normal faults.
The presence of two calderas suggests that two separate magma chambers underlie the volcano. In addition, a fissure-type eruption that formed a small [[stratocone]] is present near the south flank of the main volcano. Thus, Tharsis Tholus may be the result of two volcanoes growing together, with the main one taking up the northern two thirds of the structure.<ref name="Platz" /><ref name="lpi.usra.edu"/>
==Age== Crater counting shows that the oldest parts of Tharsis Tholus formed around 3.82 billion years ago (Gya) in the late [[Noachian]] Period. The western flank seems to be about 3.73 Gya, and the east flank shows an age of about 1.08 Gya (Middle Amazonian Period). A fissure eruption on the south flank produced a lava flow around 196 million years ago (Late Amazonian Period). Volcanic activity thus covers more than 3.6 billion years of Mars history. The fissure-type eruption at the south flank of Tharsis Tholus is relatively young, so it may indicate that future volcanic activity is still possible at the mountain.<ref name="Platz" />
==Gallery== <gallery class="center" widths="190px" heights="180px" > Tharsis Tholus f225a13 f225a15.jpg|[[Viking program|Viking 1 orbiter]] image of Tharsis Tholus (1977). Tharsis Tholus summit f090a63.jpg|Viking image of summit caldera of Tharsis Tholus. Tharsis Tholus Caldera Wall ESP 012612 1940.jpg|[[HiRISE]] image of the northwestern wall of Tharsis Tholus caldera. Note the outcrops of bedrock at top. Tharsis Tholus Lava Plains PIA09133.jpg|[[Thermal Emission Imaging System|THEMIS]] image of lava flow plains northeast of Tharsis Tholus. The volcano is surrounded by these Amazonian-aged lava flows from the interior of the [[Tharsis]] region to the west. Tharsis Tholus block.JPG|[[HiRISE]] image of large slump block or debris avalanche deposit (DAD) at northwest edge of Tharsis Tholus (just off to right). Note the dark slope streak located in the middle left of this picture. Pit Chain Tharsis Tholus PIA08469.jpg|[[Mars Global Surveyor]] image of pit crater chain northwest of Tharsis Tholus. </gallery>
==Notes== {{reflist| group = lower-alpha}}
== References == {{reflist}}
==External links== *[http://www.esa.int/esaMI/Mars_Express/SEMIU8TWLUG_0.html Images of Tharsis Tholus from ESA's Mars Express] {{commons category|Tharsis Tholus}} {{Mars}} {{Portal bar|Solar System}}
[[Category:Tharsis quadrangle]] [[Category:Volcanoes of Mars]] [[Category:Shield volcanoes]]