Tharsis Tholus
Encyclopedia
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 and originally given the informal name Volcano 7. In 1973, the International Astronomical Union (IAU
) officially designated it Tharsis Tholus. In planetary geology
, tholus (pl. tholi) is the term used to describe a small domical mountain
, usually a volcano.
at 13.5°N, 91°W. It is about 800 km east-northeast of Ascraeus Mons
, 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.
Tharsis Tholus measures 155 km x 125 km. It is distinctly bulbous in appearance, and is unique among Martian volcanoes in the degree to which it has been modified by faulting. Large normal faults cut across and completely penetrate the volcano, dividing the edifice into several major blocks, or sectors. 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), with flank slopes ranging from less than 1° near the summit to up to 16° at the base. The average flank slope is 10°, making it one of the steepest volcanoes on Mars. The volcano rises to an elevation of about 9 km (5.6 mi). 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 to 3.5 km.
Most of the volcano's surface is mantled with a thick layer of fine dust or ash, giving the volcano's flanks a smooth to slightly hummock
y appearance. The mantle material obscures the underlying bedrock except in areas with steep slopes, such as along the caldera's upper walls. Landslide debris from the caldera's western and southeastern wall is present on the floor of the caldera.
, silicic
lavas or pyroclastics, rather than fluid basalt
s. However, more recent studies based on data from the Mars Odyssey, Mars Express
, and Mars Reconnaisance Orbiter (MRO) spacecraft indicate that Tharsis Tholus is probably a basaltic shield
or a shield with a stratocone component. 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 phyllosilicates (clay minerals), sulfate
s, or iron oxides have been detected.
Tharsis Tholus has a complex volcano-tectonic history. The edifice has been modified by at least four distinct episodes of structural deformation. The most prominent area of deformation is the central caldera complex, which consists of an outer, older caldera and an inner, younger one. The calderas are bordered by a well-preserved system of concentric normal faults and peripheral ring faults. 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.
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. 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 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, catastropic detachment of a volcano's flank caused by lateral spreading of the volcano. Another suggested terrestrial analog
for Tharsis Tholus is the Maderas
Volcano in Nicaragua
.
In addition to the nomal faults, a system of narrow, parallel graben
s cut across large parts of the volcano. The grabens 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. 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 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.
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.
Tholus
In planetary geology, tholus is the term used to describe a small domical mountain or hill. The word is from the Greek θόλος, which means a circular building with a conical or vaulted roof. The Romans transliterated the word into the Latin tholus, which means cupola or dome...
is an intermediate-sized shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
located in the eastern Tharsis
Tharsis
The Tharsis region on Mars is a vast volcanic plateau centered near the equator in Mars’ western hemisphere. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as...
region of the planet Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
. The volcano was discovered by the Mariner 9 spacecraft in 1972 and originally given the informal name Volcano 7. In 1973, the International Astronomical Union (IAU
IAU
IAU may refer to:*International Astronomical Union*International American University*International American University College of Medicine*International Association of Universities*International Association of Ultrarunners...
) officially designated it Tharsis Tholus. In planetary geology
Planetary geology
Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites...
, tholus (pl. tholi) is the term used to describe a small domical mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...
, usually a volcano.
General Description
Tharsis Tholus lies on the eastern edge of the Tharsis quadrangleTharsis quadrangle
The Tharsis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey Astrogeology Research Program. The Tharsis quadrangle is also referred to as MC-9 ....
at 13.5°N, 91°W. It is about 800 km east-northeast of Ascraeus Mons
Ascraeus Mons
Ascraeus Mons is a large shield volcano located in the Tharsis region of the planet Mars. It is the northernmost and tallest of three shield volcanoes collectively known as the Tharsis Montes. The volcano's location corresponds to the classical albedo feature Ascraeus Lacus.Ascraeus Mons was...
, the northernmost of the large Tharsis Montes
Tharsis Montes
The Tharsis Montes are three large shield volcanoes in the Tharsis region of the planet Mars. From north to south, the volcanoes are Ascraeus Mons, Pavonis Mons and Arsia Mons. Mons is the Latin word for mountain...
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.
Tharsis Tholus measures 155 km x 125 km. It is distinctly bulbous in appearance, and is unique among Martian volcanoes in the degree to which it has been modified by faulting. Large normal faults cut across and completely penetrate the volcano, dividing the edifice into several major blocks, or sectors. The volcano has an elongated central caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
(collapse crater) measuring 36.7 × 38.9 km and about 3 km deep.
In profile, Tharsis Tholus is dome-shaped (convex upward), with flank slopes ranging from less than 1° near the summit to up to 16° at the base. The average flank slope is 10°, making it one of the steepest volcanoes on Mars. The volcano rises to an elevation of about 9 km (5.6 mi). 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 to 3.5 km.
Most of the volcano's surface is mantled with a thick layer of fine dust or ash, giving the volcano's flanks a smooth to slightly hummock
Hummock
A hummock is a boss or rounded knoll of ice rising above the general level of an ice-field, making sledge travelling in the Arctic and Antarctic region extremely difficult and unpleasant....
y appearance. The mantle material obscures the underlying bedrock except in areas with steep slopes, such as along the caldera's upper walls. Landslide debris from the caldera's western and southeastern wall is present on the floor of the caldera.
Geology
The volcano's relatively steep slopes and dome-like shape led some early researchers to conclude that the volcano is built from viscousViscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, silicic
Intermediate composition
In igneous petrology an intermediate composition refers to the chemical composition of a rock that has 52-63 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite, dacite and trachyandesite among volcanic rocks and diorite and granodiorite...
lavas or pyroclastics, rather than fluid basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s. However, more recent studies based on data from the Mars Odyssey, Mars Express
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...
, and Mars Reconnaisance Orbiter (MRO) spacecraft indicate that Tharsis Tholus is probably a basaltic shield
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
or a shield with a stratocone component. CRISM
CRISM
The Compact Reconnaissance Imaging Spectrometer for Mars is a visible-infrared spectrometer aboard the Mars Reconnaissance Orbiter searching for mineralogic indications of past water on Mars. The CRISM instrument team comprises scientists from over ten universities and led by principal...
data from MRO show that lava exposed on the caldera scarp is composed of high- and low-calcium pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
s. Olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
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 phyllosilicates (clay minerals), sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
s, or iron oxides have been detected.
Tharsis Tholus has a complex volcano-tectonic history. The edifice has been modified by at least four distinct episodes of structural deformation. The most prominent area of deformation is the central caldera complex, which consists of an outer, older caldera and an inner, younger one. The calderas are bordered by a well-preserved system of concentric normal faults and peripheral ring faults. 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.
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. 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 have compared the deformation on the flanks of Tharsis Tholus to "sector collapse" observed at Mauna Loa
Mauna Loa
Mauna Loa is one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean, and the largest on Earth in terms of volume and area covered. It is an active shield volcano, with a volume estimated at approximately , although its peak is about lower than that...
in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
and other terrestrial volcanoes. Sector collapse is a sudden, catastropic detachment of a volcano's flank caused by lateral spreading of the volcano. Another suggested terrestrial analog
Analogy
Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...
for Tharsis Tholus is the Maderas
Maderas
With a height of 1,394 m, Maderas is the smaller of the two volcanoes which make up the island of Ometepe, situated in Lake Nicaragua in Nicaragua, Central America. Unlike Concepción, the other volcano on the island, Maderas has not been active in historical times...
Volcano in Nicaragua
Nicaragua
Nicaragua is the largest country in the Central American American isthmus, bordered by Honduras to the north and Costa Rica to the south. The country is situated between 11 and 14 degrees north of the Equator in the Northern Hemisphere, which places it entirely within the tropics. The Pacific Ocean...
.
In addition to the nomal faults, a system of narrow, parallel graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
s cut across large parts of the volcano. The grabens strike
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
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. 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 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.
Age
Crater counting shows that the oldest parts of Tharsis Tholus formed around 3.82 billion years ago (Gya) in the late NoachianNoachian
The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the presence of abundant surface water...
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.