Volcanism on Mars
Encyclopedia
Volcanic activity, or volcanism
, has played a significant role in the geologic evolution of Mars
. Scientists have known since the Mariner 9
mission in 1972 that volcanic features cover large portions of the Martian surface. These features include extensive lava
flows, vast volcanic plains, and the largest known volcanoes in the Solar System
. Martian volcanic features range in age from Noachian
(>3.7 billion years) to late Amazonian (< 500 million years), indicating that the planet has been volcanically active throughout its history and probably still is so today.
planets built from similar chondritic materials. Many of the same magmatic processes that occur on Earth also occur on Mars, and both planets are similar enough compositionally that the same names can be applied to their igneous rock
s and mineral
s.
Volcanism is a process in which magma
from a planet’s interior rises through the crust
and erupts on the surface. The erupted materials consist of molten rock (lava
), hot fragmental debris (tephra
or ash), and gases
. Volcanism is a principal way that planets release their internal heat. Volcanic eruptions produce distinctive landform
s, rock
types, and terrain
s that provide a window on the chemical composition, thermal state, and history of a planet's interior.
Magma is a complex, high-temperature mixture of molten silicates
, suspended crystals, and dissolved gases. Magma on Mars likely ascends in a similar manner to that on Earth. It rises through the lower crust in diapir
ic bodies that are less dense than the surrounding material. As the magma rises, it eventually reaches regions of lower density. When the magma density matches that of the host rock, buoyancy
is neutralized and the magma body stalls. At this point, it may form a magma chamber
and spread out laterally into a network of dikes
and sills
. Subsequently, the magma may cool and solidify to form intrusive igneous bodies (plutons). Geologists estimate that about 80% of the magma generated on Earth stalls in the crust and never reaches the surface.
As magma rises and cools, it undergoes many complex and dynamic compositional changes. Heavier minerals may crystallize and settle to the bottom of the magma chamber. The magma may also assimilate portions of host rock or mix with other batches of magma. These processes alter the composition of the remaining melt, so that any magma reaching the surface may be chemically quite different from its parent melt. Magmas that have been so altered are said to be "evolved" to distinquish them from "primitive" magmas that more closely resemble the composition of their mantle
source. (See igneous differentiation
and fractional crystallization
.) More highly evolved magmas are usually felsic
, that is enriched in silica, volatiles
, and other light elements compared to iron- and magnesium-rich (mafic
) primitive magmas. The degree and extent to which magmas evolve over time is an indication of a planet's level of internal heat and tectonic activity. The Earth's continental crust is made up of evolved granitic
rocks that developed through many episodes of magmatic reprocessing. Evolved igneous rocks are much less common on cold, dead bodies such as the Moon. Mars, being intermediate in size between the Earth and the Moon, is thought to be intermediate is its level of magmatic activity.
At shallower depths in the crust, the lithostatic pressure on the magma body decreases. The reduced pressure can cause gases (volatiles
), such as carbon dioxide and water vapor, to exsolve from the melt into a froth of gas bubbles. The nucleation
of bubbles causes a rapid expansion and cooling of the surrounding melt, producing glassy shards that may erupt explosively as tephra
(also called pyroclastics). Fine-grained tephra is commonly referred to as volcanic ash
. Whether a volcano erupts explosively or effusively as fluid lava depends on the composition of the melt. Felsic magmas of andesitic
and rhyolitic
composition tend to erupt explosively. They are very viscous
(thick and sticky) and rich in dissolved gases. Mafic magmas, on the other hand, are low in volatiles and commonly erupt effusively as basalt
ic lava flows. However, these are only generalizations. For example, magma that comes into sudden contact with groundwater or surface water may erupt violently in steam explosions called hydromagmatic (phreatomagmatic or phreatic) eruptions. Also, erupting magmas may behave differently on planets with different interior compositions, atmospheres, and gravity fields.
s are extrusive igneous rocks derived from the partial melting of the upper mantle. They are rich in iron and magnesium (mafic
) mineral
s and commonly dark gray in color. The principal type of volcanism on Mars is almost certainly basaltic too. On Earth, basaltic magmas commonly erupt as highly fluid flows, which either emerge directly from vents or form by the coalescence of molten clots at the base of fire fountains
(Hawaiian eruption
). These styles are also common on Mars, but the lower gravity and atmospheric pressure on Mars allow nucleation of gas bubbles (see above) to occur more readily and at greater depths than on Earth. As a consequence, Martian basaltic volcanoes are also capable of erupting large quantities of ash in Plinian-style
eruptions. In a Plinian eruption, hot ash is incorporated into the atmosphere, forming a huge convective column (cloud). If insufficient atmosphere is incorporated, the column may collapse to form pyroclastic flow
s. Plinian eruptions are rare in basaltic volcanoes on Earth where such eruptions are most commonly associated with silica-rich andesitic
or rhyolitic
magmas (e.g., Mount St. Helens
).
Because the lower gravity of Mars generates less buoyancy
forces on magma rising through the crust, the magma chamber
s that feed volcanoes on Mars are thought to be deeper and much larger than those on Earth. If a magma body on Mars is to reach close enough to the surface to erupt before solidifying, it must be big. Consequently, eruptions on Mars are less frequent than on Earth, but are of enormous scale and eruptive rate when they do occur. Somewhat paradoxically, the lower gravity of Mars also allows for longer and more widespread lava flows. Lava eruptions on Mars may be unimaginably huge. A vast lava flow the size of the state of Oregon
has recently been described in western Elysium Planitia
. The flow is believed to have been emplaced turbulently over the span of several weeks and thought to be one of the youngest lava flows on Mars.
The tectonic
settings of volcanoes on Earth and Mars are very different. Most active volcanoes on Earth occur in long, linear chains along plate boundaries, either in zones where the lithosphere
is spreading apart (divergent boundaries
) or being subducted
back into the mantle (convergent boundaries
). Because Mars currently lacks plate tectonics
, volcanoes there do not show the same global pattern as on Earth. Martian volcanoes are more analogous to terrestrial mid-plate volcanoes, such as those in the Hawaiian Islands
, which are thought to have formed over a stationary mantle plume
. (See hot spot.)
The largest and most conspicuous volcanoes on Mars occur in Tharsis
and Elysium
regions. These volcanoes are strikingly similar to shield volcano
es on Earth. Both have shallow-sloping flanks and summit caldera
s. The main difference between Martian shield volcanoes and those on Earth is in size: Martian shield volcanoes are truly colossal. For example, the tallest volcano on Mars, Olympus Mons
, is 550 km across and 21 km high. It is nearly 100 times greater in volume than Mauna Loa
in Hawaii
, the largest shield volcano on Earth. Geologists think one of the reasons that volcanoes on Mars are able to grow so large is because Mars lacks plate tectonics. The Martian lithosphere does not slide over the upper mantle (asthenosphere
) as on Earth, so lava from a stationary hot spot is able to accumulate at one location on the surface for a billion years or longer.
region or the Tharsis bulge. This immense, elevated structure is thousands of kilometers in diameter and covers up to 25% of the planet’s surface. Averaging 7–10 km above datum (Martian “sea” level), Tharsis contains the highest elevations on the planet. Three enormous volcanoes, Ascraeus Mons
, Pavonis Mons
, and Arsia Mons
(collectively known as the Tharsis Montes
), sit aligned northeast–southwest along the crest of the bulge. The vast Alba Mons (formerly Alba Patera) occupies the northern part of the region. The huge shield volcano
Olympus Mons
lies off the main bulge, at the western edge of the province.
Built up by countless generations of lava flows and ash, the Tharsis bulge contains some of the youngest lava flows on Mars, but the bulge itself is believed to be very ancient. Geologic evidence indicates that most of the mass of Tharsis was in place by the end of the Noachian Period, about 3.7 billion years ago (Gya). Tharsis is so massive that it has placed tremendous stresses on the planet’s lithosphere
, generating immense extensional fractures (grabens and rift valley
s) that extend halfway around the planet. The mass of Tharsis may have even altered the orientation of Mars' rotational axis, causing climate changes.
es centered near the equator at longitude 247°E. All are several hundred kilometers in diameter and range in height from 14 to 18 km. Arsia Mons
, the southernmost of the group, has a large summit caldera that is 130 km across and 1.3 km deep. Pavonis Mons
, the middle volcano, has two nested calderas with the smaller one being almost 5 km deep. Ascraeus Mons
in the north, has a complex set of internested calderas and a long history of eruption that is believed to span most of Mars' history.
The three Tharsis Montes are about 700 km apart. They show a distinctive northeast–southwest alignment that has been the source of some interest. Ceraunius Tholus
and Uranius Patera
follow the same trend to the northeast, and aprons of young lava flows on the flanks of all three Tharsis Montes are aligned in the same northeast–southwest orientation. This line clearly marks a major structural feature in the Martian crust, but its origin is uncertain.
and patera
e. The tholi are dome-shaped edifices with flanks that are much steeper than the larger Tharsis shields. Their central calderas are also quite large in proportion to their base diameters. The density of impact craters on many of the tholi indicate they are older than the large shields, having formed between late Noachian and early Hesperian times. Ceraunius Tholus
and Uranius Tholus
have densely channeled flanks, suggesting that the flank surfaces are made up of easily erodible material, such as ash. The age and morphology of the tholi provide strong evidence that the tholi represent the summits of old shield volcanoes that have been largely buried by great thicknesses of younger lava flows. By one estimate the Tharsis tholi may be buried by up to 4 km of lava.
Patera
(pl. paterae) is Latin for a shallow drinking bowl. The term was applied to certain ill-defined, scalloped-edged craters that appeared in early spacecraft images to be large volcanic calderas. The smaller paterae in Tharsis appear to be morphologically similar to the tholi, except for having larger calderas. Like the tholi, the Tharsis paterae probably represent the tops of larger, now buried shield volcanoes. Historically, the term patera has been used to describe the entire edifice of certain volcanoes on Mars (e.g., Alba Patera). In 2007, the International Astronomical Union
(IAU) redefined the terms Alba Patera
, Uranius Patera
, and Ulysses Patera
to refer only to the central calderas of these volcanoes.
Olympus Mons
is the youngest and tallest large volcano on Mars. It is located 1200 km northwest of of the Tharsis Montes, just off the western edge of the Tharsis bulge. Its summit is 21 km above datum (Mars "sea" level) and has a central caldera complex consisting of six nested calderas that together form a depression 72 x 91 km wide and 3.2 km deep. As a shield volcano, it has an extremely low profile with shallow slopes averaging between 4-5 degrees. The volcano was built up by many thousands of individual flows of highly fluid lava. An irregular escarpment, in places up to 8 km tall, lies at the base of the volcano, forming a kind of pedestal on which the volcano sits. At various locations around the volcano, immense lava flows can be seen extending into the adjacent plains, burying the escarpment. In medium resolution images (100 m/pixel), the surface of the volcano has a fine radial texture due to the innumerable flows and levee
d lava channels that line its flanks.
Because Alba Mons lies antipodal
to the Hellas impact basin, some researchers have conjectured that the volcano’s formation may have been related to crustal weakening from the Hellas impact, which produced strong seismic wave
s that focused on the opposite side of the planet.
A smaller volcanic center lies several thousand kilometers west of Tharsis in Elysium
. The Elysium volcanic complex is about 2,000 kilometers in diameter and consists of three main volcanoes, Elysium Mons
, Hecates Tholus
, and Albor Tholus
. The northwestern edge of the province is characterized by large channels (Granicus
and Tinjar
Valles) that emerge from several graben
s on the flanks of Elysium Mons. The grabens may have formed from subsurface dike
s. The dikes may have fractured the cryosphere
, releasing large volumes of ground water to form the channels. Associated with the channels are widespread sedimentary deposits that may have formed from mudflows or lahar
s. The Elysium group of volcanoes is thought to be somewhat different than the Tharsis Montes, in that development of the former involved both lavas and pyroclastics.
Elysium Mons is the largest volcanic edifice in the province. It is 375 km across (depending on how one defines the base) and 14 km high. It has single, simple caldera at its summit that measures 14 km wide and 100 m deep. The volcano is distinctly conical in profile, leading some to call it a stratocone; however, given the predominantly low slopes, it is probably a shield. Elysium Mons is only about one-fifth the volume of Arsia Mons.
Hecates Tholus is 180 km across and 4.8 km high. The slopes of the volcano are heavily dissected with channels, suggesting that the volcano is composed of easily erodible material such as volcanic ash. The origin of the channels is unknown; they have been atrributed to lava, ash flows, or even water from snow or rainfall. Albor Tholus, the southern-most of the Elysium volcanoes, is 150 km in diameter and 4.1 km high. Its slopes are smoother and less heavily cratered than the slopes of the other Elysium volcanoes.
bearing the same name. The volcano is 1200 km in diameter but only 2 km high. It has two calderas, Meroe Patera and Nili Patera. Studies involving the regional gravity field suggest a solidified magma chamber at least 5 km thick lies under the surface. Syrtis Major is of interest to geologists because dacite
and granite
have been detected there from orbiting spacecraft. Dacites and granites are silica-rich rocks that crystallize from a magma that is more chemically evolved and differentiated than basalt. They may form at the top of a magma chamber after the heavy minerals, such as olivine
and pyroxene
(those containing iron
and magnesium
), have settled to the bottom. Dacites and granites are very common on Earth but rare on Mars.
, Peneus Patera
, and Pityusa Patera
. Geomorphologic evidence suggests that the highland patera were produced through a combination of lava flows and pyroclastics from the interaction of magma with water. Some researchers speculate that the location of the highland paterae around Hellas is due to deep-seated fractures caused by the impact that provided conduits for magma to rise to the surface. Although they are not very high, some paterae cover large areas—Amphritrites Patera, for example, covers a larger area than Olympus Mons.
flows can form a lava tube
when the exposed upper layers of lava cool and solidify to form a roof while the lava
underneath continues flowing. Often, when all the remaining lava leaves the tube, the roof collapses to make a channel or line of pit craters (catena
).
An unusual type of flow feature occurs in the Cerberus plains south of Elysium and in Amazonis. These flows have a broken platey texture, consisting of dark, kilometer-scale slabs embedded in a light-toned matrix. They have been attributed to rafted slabs of solidified lava floating on a still-molten subsurface. Others have claimed the broken slabs represent pack ice that froze over a sea that pooled in the area after massive releases of groundwater from the Cerberus Fossae
area.
The second type of volcanic plains (ridged plains) are characterized by abundant wrinkle ridges. Volcanic flow features are rare or absent. The ridged plains are believed to be regions of extensive flood basalt
s, by analogy with the lunar maria. Ridged plains make up about 30% of the Martian surface and are most prominent in Lunae, Hesperia, and Malea Plana, as well as throughout much of the northern lowlands. Ridged plains are all Hesperian in age and represent a style of volcanism globally predominant during that time period. The Hesperian Period is named after the ridged plains in Hesperia Planum.
orbiter photographed lava flows that must have occurred within the past two million years, suggesting a relatively recent geologic activity.
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
, has played a significant role in the geologic evolution of 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...
. Scientists have known since the Mariner 9
Mariner 9
Mariner 9 was a NASA space orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit...
mission in 1972 that volcanic features cover large portions of the Martian surface. These features include extensive lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows, vast volcanic plains, and the largest known volcanoes in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. Martian volcanic features range in age from Noachian
Noachian
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...
(>3.7 billion years) to late Amazonian (< 500 million years), indicating that the planet has been volcanically active throughout its history and probably still is so today.
Evolution of magma on Mars
Both Earth and Mars are large, differentiatedPlanetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...
planets built from similar chondritic materials. Many of the same magmatic processes that occur on Earth also occur on Mars, and both planets are similar enough compositionally that the same names can be applied to their igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s and mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s.
Volcanism is a process in which magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
from a planet’s interior rises through the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and erupts on the surface. The erupted materials consist of molten rock (lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
), hot fragmental debris (tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
or ash), and gases
Volcanic gas
|250px|thumb|right|Image of the [[rhyolitic]] [[lava dome]] of [[Chaitén Volcano]] during its 2008-2010 eruption.Volcanic gases include a variety of substances given off by active volcanoes...
. Volcanism is a principal way that planets release their internal heat. Volcanic eruptions produce distinctive landform
Landform
A landform or physical feature in the earth sciences and geology sub-fields, comprises a geomorphological unit, and is largely defined by its surface form and location in the landscape, as part of the terrain, and as such, is typically an element of topography...
s, rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
types, and terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
s that provide a window on the chemical composition, thermal state, and history of a planet's interior.
Magma is a complex, high-temperature mixture of molten silicates
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
, suspended crystals, and dissolved gases. Magma on Mars likely ascends in a similar manner to that on Earth. It rises through the lower crust in diapir
Diapir
A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress...
ic bodies that are less dense than the surrounding material. As the magma rises, it eventually reaches regions of lower density. When the magma density matches that of the host rock, buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
is neutralized and the magma body stalls. At this point, it may form a magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
and spread out laterally into a network of dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
and sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
. Subsequently, the magma may cool and solidify to form intrusive igneous bodies (plutons). Geologists estimate that about 80% of the magma generated on Earth stalls in the crust and never reaches the surface.
As magma rises and cools, it undergoes many complex and dynamic compositional changes. Heavier minerals may crystallize and settle to the bottom of the magma chamber. The magma may also assimilate portions of host rock or mix with other batches of magma. These processes alter the composition of the remaining melt, so that any magma reaching the surface may be chemically quite different from its parent melt. Magmas that have been so altered are said to be "evolved" to distinquish them from "primitive" magmas that more closely resemble the composition of their mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
source. (See igneous differentiation
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
and fractional crystallization
Fractional crystallization
Fractional crystallization may refer to:* Fractional crystallization , a process to separate different solutes from a solution* Fractional crystallization , a natural process occurring in igneous rocks during which precipitation of minerals takes place...
.) More highly evolved magmas are usually felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
, that is enriched in silica, volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
, and other light elements compared to iron- and magnesium-rich (mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
) primitive magmas. The degree and extent to which magmas evolve over time is an indication of a planet's level of internal heat and tectonic activity. The Earth's continental crust is made up of evolved granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
rocks that developed through many episodes of magmatic reprocessing. Evolved igneous rocks are much less common on cold, dead bodies such as the Moon. Mars, being intermediate in size between the Earth and the Moon, is thought to be intermediate is its level of magmatic activity.
At shallower depths in the crust, the lithostatic pressure on the magma body decreases. The reduced pressure can cause gases (volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
), such as carbon dioxide and water vapor, to exsolve from the melt into a froth of gas bubbles. The nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
of bubbles causes a rapid expansion and cooling of the surrounding melt, producing glassy shards that may erupt explosively as tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
(also called pyroclastics). Fine-grained tephra is commonly referred to as volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
. Whether a volcano erupts explosively or effusively as fluid lava depends on the composition of the melt. Felsic magmas of andesitic
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
and rhyolitic
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
composition tend to erupt explosively. They are very viscous
Viscosity
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...
(thick and sticky) and rich in dissolved gases. Mafic magmas, on the other hand, are low in volatiles and commonly erupt effusively as 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...
ic lava flows. However, these are only generalizations. For example, magma that comes into sudden contact with groundwater or surface water may erupt violently in steam explosions called hydromagmatic (phreatomagmatic or phreatic) eruptions. Also, erupting magmas may behave differently on planets with different interior compositions, atmospheres, and gravity fields.
Differences in volcanic styles between Earth and Mars
The most common form of volcanism on the Earth is basaltic. BasaltBasalt
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 are extrusive igneous rocks derived from the partial melting of the upper mantle. They are rich in iron and magnesium (mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
) mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and commonly dark gray in color. The principal type of volcanism on Mars is almost certainly basaltic too. On Earth, basaltic magmas commonly erupt as highly fluid flows, which either emerge directly from vents or form by the coalescence of molten clots at the base of fire fountains
Lava fountain
A lava fountain is a volcanic phenomenon in which lava is forcefully but non-explosively ejected from a crater, vent, or fissure. Lava fountains may reach heights of up to . They may occur as a series of short pulses, or a continuous jet of lava. They are commonly seen in Hawaiian eruptions.-See...
(Hawaiian eruption
Hawaiian eruption
A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relative gentle, low level eruption, so called because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high...
). These styles are also common on Mars, but the lower gravity and atmospheric pressure on Mars allow nucleation of gas bubbles (see above) to occur more readily and at greater depths than on Earth. As a consequence, Martian basaltic volcanoes are also capable of erupting large quantities of ash in Plinian-style
Plinian eruption
Plinian eruptions, also known as 'Vesuvian eruptions', are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in AD 79 ....
eruptions. In a Plinian eruption, hot ash is incorporated into the atmosphere, forming a huge convective column (cloud). If insufficient atmosphere is incorporated, the column may collapse to form pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...
s. Plinian eruptions are rare in basaltic volcanoes on Earth where such eruptions are most commonly associated with silica-rich andesitic
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
or rhyolitic
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
magmas (e.g., Mount St. Helens
Mount St. Helens
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It is south of Seattle, Washington and northeast of Portland, Oregon. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a...
).
Because the lower gravity of Mars generates less buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
forces on magma rising through the crust, the magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
s that feed volcanoes on Mars are thought to be deeper and much larger than those on Earth. If a magma body on Mars is to reach close enough to the surface to erupt before solidifying, it must be big. Consequently, eruptions on Mars are less frequent than on Earth, but are of enormous scale and eruptive rate when they do occur. Somewhat paradoxically, the lower gravity of Mars also allows for longer and more widespread lava flows. Lava eruptions on Mars may be unimaginably huge. A vast lava flow the size of the state of Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
has recently been described in western Elysium Planitia
Elysium Planitia
Elysium Planitia is the second largest volcanic region on Mars, after Tharsis Montes. It is centered at . It includes volcanoes, from north to south, Hecates Tholus, Elysium Mons and Albor Tholus. Another large volcano, Apollinaris Mons, lies south of the others. Besides having large volcanoes,...
. The flow is believed to have been emplaced turbulently over the span of several weeks and thought to be one of the youngest lava flows on Mars.
The tectonic
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
settings of volcanoes on Earth and Mars are very different. Most active volcanoes on Earth occur in long, linear chains along plate boundaries, either in zones where the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
is spreading apart (divergent boundaries
Divergent boundary
In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...
) or being subducted
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
back into the mantle (convergent boundaries
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
). Because Mars currently lacks plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, volcanoes there do not show the same global pattern as on Earth. Martian volcanoes are more analogous to terrestrial mid-plate volcanoes, such as those in the Hawaiian Islands
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
, which are thought to have formed over a stationary mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
. (See hot spot.)
The largest and most conspicuous volcanoes on Mars occur in 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...
and Elysium
Elysium Planitia
Elysium Planitia is the second largest volcanic region on Mars, after Tharsis Montes. It is centered at . It includes volcanoes, from north to south, Hecates Tholus, Elysium Mons and Albor Tholus. Another large volcano, Apollinaris Mons, lies south of the others. Besides having large volcanoes,...
regions. These volcanoes are strikingly similar to 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...
es on Earth. Both have shallow-sloping flanks and summit 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...
s. The main difference between Martian shield volcanoes and those on Earth is in size: Martian shield volcanoes are truly colossal. For example, the tallest volcano on Mars, Olympus Mons
Olympus Mons
Olympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
, is 550 km across and 21 km high. It is nearly 100 times greater in volume than 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...
, the largest shield volcano on Earth. Geologists think one of the reasons that volcanoes on Mars are able to grow so large is because Mars lacks plate tectonics. The Martian lithosphere does not slide over the upper mantle (asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
) as on Earth, so lava from a stationary hot spot is able to accumulate at one location on the surface for a billion years or longer.
Tharsis volcanic province
The western hemisphere of Mars is dominated by a massive volcano-tectonic complex known as the TharsisTharsis
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 or the Tharsis bulge. This immense, elevated structure is thousands of kilometers in diameter and covers up to 25% of the planet’s surface. Averaging 7–10 km above datum (Martian “sea” level), Tharsis contains the highest elevations on the planet. Three enormous volcanoes, 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...
, Pavonis Mons
Pavonis Mons
Pavonis Mons is a large shield volcano located in the Tharsis region of the planet Mars. It is the middle member of a chain of three volcanic mountains that straddle the Martian equator between longitudes 235°E and 259°E. The volcano was discovered by the Mariner 9 spacecraft in 1971 and was...
, and Arsia Mons
Arsia Mons
Arsia Mons is the southernmost of three volcanos on the Tharsis bulge near the equator of the planet Mars. To its north is Pavonis Mons, and north of that is Ascraeus Mons. The tallest mountain in the solar system, Olympus Mons, is to its northwest...
(collectively known as the 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...
), sit aligned northeast–southwest along the crest of the bulge. The vast Alba Mons (formerly Alba Patera) occupies the northern part of the region. The huge 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...
Olympus Mons
Olympus Mons
Olympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
lies off the main bulge, at the western edge of the province.
Built up by countless generations of lava flows and ash, the Tharsis bulge contains some of the youngest lava flows on Mars, but the bulge itself is believed to be very ancient. Geologic evidence indicates that most of the mass of Tharsis was in place by the end of the Noachian Period, about 3.7 billion years ago (Gya). Tharsis is so massive that it has placed tremendous stresses on the planet’s lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
, generating immense extensional fractures (grabens and rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...
s) that extend halfway around the planet. The mass of Tharsis may have even altered the orientation of Mars' rotational axis, causing climate changes.
Tharsis Montes
The three Tharsis Montes are shield volcanoShield 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...
es centered near the equator at longitude 247°E. All are several hundred kilometers in diameter and range in height from 14 to 18 km. Arsia Mons
Arsia Mons
Arsia Mons is the southernmost of three volcanos on the Tharsis bulge near the equator of the planet Mars. To its north is Pavonis Mons, and north of that is Ascraeus Mons. The tallest mountain in the solar system, Olympus Mons, is to its northwest...
, the southernmost of the group, has a large summit caldera that is 130 km across and 1.3 km deep. Pavonis Mons
Pavonis Mons
Pavonis Mons is a large shield volcano located in the Tharsis region of the planet Mars. It is the middle member of a chain of three volcanic mountains that straddle the Martian equator between longitudes 235°E and 259°E. The volcano was discovered by the Mariner 9 spacecraft in 1971 and was...
, the middle volcano, has two nested calderas with the smaller one being almost 5 km deep. 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...
in the north, has a complex set of internested calderas and a long history of eruption that is believed to span most of Mars' history.
The three Tharsis Montes are about 700 km apart. They show a distinctive northeast–southwest alignment that has been the source of some interest. Ceraunius Tholus
Ceraunius Tholus
Ceraunius Tholus is a volcano on Mars located in the Tharsis quadrangle at 24.2 degrees north latitude and 97.4 degrees west longitude and it is a part of Uranius group of volcanoes. It is 130 km across, 5.5 km high and is named after a classical albedo feature name.It is generally...
and Uranius Patera
Uranius Patera
Uranius Patera is a volcano on Mars located in the Tharsis quadrangle. It is about 3 kilometers high and it has shallow slopes. It belongs to Uranius group of volcanoes in the Tharsis area.Side of Uranius Patera consist of radial lava flows with few low shields with the large caldera which is...
follow the same trend to the northeast, and aprons of young lava flows on the flanks of all three Tharsis Montes are aligned in the same northeast–southwest orientation. This line clearly marks a major structural feature in the Martian crust, but its origin is uncertain.
Tholi and Paterae
In addition to the large shield volcanoes, Tharsis contains a number of smaller volcanoes called tholiTholus
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...
and patera
Patera
A patera was a broad, shallow dish used for drinking, primarily in a ritual context such as a libation. These paterae were often used in Rome....
e. The tholi are dome-shaped edifices with flanks that are much steeper than the larger Tharsis shields. Their central calderas are also quite large in proportion to their base diameters. The density of impact craters on many of the tholi indicate they are older than the large shields, having formed between late Noachian and early Hesperian times. Ceraunius Tholus
Ceraunius Tholus
Ceraunius Tholus is a volcano on Mars located in the Tharsis quadrangle at 24.2 degrees north latitude and 97.4 degrees west longitude and it is a part of Uranius group of volcanoes. It is 130 km across, 5.5 km high and is named after a classical albedo feature name.It is generally...
and Uranius Tholus
Uranius Tholus
Uranius Tholus is a volcano on Mars located in the Tharsis quadrangle at 2.9° north latitude and 121.6° west longitude. It is 101.7 km across and was named after a classical albedo feature name.- Volcanoes :...
have densely channeled flanks, suggesting that the flank surfaces are made up of easily erodible material, such as ash. The age and morphology of the tholi provide strong evidence that the tholi represent the summits of old shield volcanoes that have been largely buried by great thicknesses of younger lava flows. By one estimate the Tharsis tholi may be buried by up to 4 km of lava.
Patera
Patera
A patera was a broad, shallow dish used for drinking, primarily in a ritual context such as a libation. These paterae were often used in Rome....
(pl. paterae) is Latin for a shallow drinking bowl. The term was applied to certain ill-defined, scalloped-edged craters that appeared in early spacecraft images to be large volcanic calderas. The smaller paterae in Tharsis appear to be morphologically similar to the tholi, except for having larger calderas. Like the tholi, the Tharsis paterae probably represent the tops of larger, now buried shield volcanoes. Historically, the term patera has been used to describe the entire edifice of certain volcanoes on Mars (e.g., Alba Patera). In 2007, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
(IAU) redefined the terms Alba Patera
Alba Patera
Alba Mons is an immense, low-lying volcano located in the northern Tharsis region of the planet Mars. It is the largest volcano on Mars in terms of area, with volcanic flow fields that extend for at least 1350 km from its summit...
, Uranius Patera
Uranius Patera
Uranius Patera is a volcano on Mars located in the Tharsis quadrangle. It is about 3 kilometers high and it has shallow slopes. It belongs to Uranius group of volcanoes in the Tharsis area.Side of Uranius Patera consist of radial lava flows with few low shields with the large caldera which is...
, and Ulysses Patera
Ulysses Patera
Ulysses Patera is a Martian volcano. It is located in the Tharsis quadrangle at2.89° north latitude and 121.55° west longitude. It is 58 km across and is named after a classical albedo feature name. Its size changed Sept. 19, 2007 to correspond only to the central depression.Olympusis the...
to refer only to the central calderas of these volcanoes.
Olympus Mons
Olympus Mons
Olympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
is the youngest and tallest large volcano on Mars. It is located 1200 km northwest of of the Tharsis Montes, just off the western edge of the Tharsis bulge. Its summit is 21 km above datum (Mars "sea" level) and has a central caldera complex consisting of six nested calderas that together form a depression 72 x 91 km wide and 3.2 km deep. As a shield volcano, it has an extremely low profile with shallow slopes averaging between 4-5 degrees. The volcano was built up by many thousands of individual flows of highly fluid lava. An irregular escarpment, in places up to 8 km tall, lies at the base of the volcano, forming a kind of pedestal on which the volcano sits. At various locations around the volcano, immense lava flows can be seen extending into the adjacent plains, burying the escarpment. In medium resolution images (100 m/pixel), the surface of the volcano has a fine radial texture due to the innumerable flows and levee
Levee
A levee, levée, dike , embankment, floodbank or stopbank is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels...
d lava channels that line its flanks.
Alba Mons (Alba Patera)
Alba Mons, located in the northern Tharsis region, is a unique volcanic structure, with no counterpart on Earth or elsewhere on Mars. The flanks of the volcano have extremely low slopes characterized by extensive lava flows and channels. The average flank slope on Alba Mons is only about 0.5°, over five times lower than the slopes on the other Tharsis volcanoes. The volcano has a central edifice 350 km wide and 1.5 km high with a double caldera complex at the summit. Surrounding the central edifice is an incomplete ring of fractures. Flows related to the volcano can be traced as far north as 61°N and as far south as 26°N. If one counts these widespread flow fields, the volcano stretches an immense 2000 km north–south and 3000 km east–west, making it one of the most areally extensive volcanic features in the Solar System. Most geological models suggest that Alba Mons is composed of highly fluid basaltic lava flows, but some researchers have identified possible pyroclastic deposits on the volcano's flanks.Because Alba Mons lies antipodal
Antipodal point
In mathematics, the antipodal point of a point on the surface of a sphere is the point which is diametrically opposite to it — so situated that a line drawn from the one to the other passes through the centre of the sphere and forms a true diameter....
to the Hellas impact basin, some researchers have conjectured that the volcano’s formation may have been related to crustal weakening from the Hellas impact, which produced strong seismic wave
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...
s that focused on the opposite side of the planet.
Elysium volcanic province
Elysium Planitia
Elysium Planitia is the second largest volcanic region on Mars, after Tharsis Montes. It is centered at . It includes volcanoes, from north to south, Hecates Tholus, Elysium Mons and Albor Tholus. Another large volcano, Apollinaris Mons, lies south of the others. Besides having large volcanoes,...
. The Elysium volcanic complex is about 2,000 kilometers in diameter and consists of three main volcanoes, Elysium Mons
Elysium Mons
Elysium Mons is a volcano on Mars located in the Elysium Planitia, at , in the Martian eastern hemisphere. It stands about 13.9 km above the surrounding lava plains, and about 16 km above the Martian datum. Its diameter is about 240 km, with a summit caldera about 14 km across...
, Hecates Tholus
Hecates Tholus
Hecates Tholus is a Martian volcano, notable for results from the European Space Agency's Mars Express mission which indicate a major eruption took place 350 million years ago. The eruption created a caldera 10 km in diameter. It has been suggested that glacial deposits later partly filled the...
, and Albor Tholus
Albor Tholus
Albor Tholus is an extinct volcano in the Elysium Planitia area on Mars. It lies south of the neighbouring volcanoes Elysium Mons and Hecates Tholus. Albor Tholus is 4.5 kilometres high and has a diameter of 160 km at its base. Its caldera has a diameter of 30 km and is 3 km deep, it...
. The northwestern edge of the province is characterized by large channels (Granicus
Granicus Vallis
Granicus Vallis is a valley in the Amenthes quadrangle of Mars, located at 30° north latitude and 229° west longitude. It is 750 km long and is named after the ancient name for river in Turkey. It has been identified as an outflow channel....
and Tinjar
Tinjar Vallis
Tinjar Vallis is an ancient outflow channel in the Amenthes quadrangle of Mars, located at 38° north latitude and 235.8° west longitude. It is 425 km long and was named after a modern river in Sarawak, Malaysia....
Valles) that emerge from several 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 on the flanks of Elysium Mons. The grabens may have formed from subsurface dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
s. The dikes may have fractured the cryosphere
Cryosphere
The cryosphere is the term which collectively describes the portions of the Earth’s surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps and ice sheets, and frozen ground . Thus there is a wide overlap with the hydrosphere...
, releasing large volumes of ground water to form the channels. Associated with the channels are widespread sedimentary deposits that may have formed from mudflows or lahar
Lahar
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...
s. The Elysium group of volcanoes is thought to be somewhat different than the Tharsis Montes, in that development of the former involved both lavas and pyroclastics.
Elysium Mons is the largest volcanic edifice in the province. It is 375 km across (depending on how one defines the base) and 14 km high. It has single, simple caldera at its summit that measures 14 km wide and 100 m deep. The volcano is distinctly conical in profile, leading some to call it a stratocone; however, given the predominantly low slopes, it is probably a shield. Elysium Mons is only about one-fifth the volume of Arsia Mons.
Hecates Tholus is 180 km across and 4.8 km high. The slopes of the volcano are heavily dissected with channels, suggesting that the volcano is composed of easily erodible material such as volcanic ash. The origin of the channels is unknown; they have been atrributed to lava, ash flows, or even water from snow or rainfall. Albor Tholus, the southern-most of the Elysium volcanoes, is 150 km in diameter and 4.1 km high. Its slopes are smoother and less heavily cratered than the slopes of the other Elysium volcanoes.
Syrtis Major
Syrtis Major Planum is a vast Hesperian-aged shield volcano located within the albedo featureAlbedo feature
An albedo feature is a large area on the surface of a planet which shows a contrast in brightness or darkness with adjacent areas....
bearing the same name. The volcano is 1200 km in diameter but only 2 km high. It has two calderas, Meroe Patera and Nili Patera. Studies involving the regional gravity field suggest a solidified magma chamber at least 5 km thick lies under the surface. Syrtis Major is of interest to geologists because dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
and granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
have been detected there from orbiting spacecraft. Dacites and granites are silica-rich rocks that crystallize from a magma that is more chemically evolved and differentiated than basalt. They may form at the top of a magma chamber after the heavy minerals, such as 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....
and 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...
(those containing iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
), have settled to the bottom. Dacites and granites are very common on Earth but rare on Mars.
Highland Paterae
In the southern hemisphere, particularly around the Hellas impact basin, are several flat-lying volcanic stuctures called highland paterae These volcanoes are some of the oldest identifiable volcanic edifices on Mars. They are characterized by having extremely low profiles with highly eroded ridges and channels that radiate outward from a degraded, central caldera complex. They include Hadriaca Patera, Amphritrites Patera, Tyrrhena PateraTyrrhena Patera
Tyrrhena Patera is the central depression of a large volcano in the Mare Tyrrhenum quadrangle of Mars, located at 21.36° south latitude and 253.47° west longitude. It was named after a classical albedo feature name. Pit chains are found at the summit of Tyrrhena Patera. They are formed by...
, Peneus Patera
Peneus Patera
Peneus Patera is a feature in the Noachis quadrangle of Mars. It is located at58.1 S and 307.5 W. It was named after an albedo feature at 48S, 290W.- Scalloped Topography :...
, and Pityusa Patera
Pityusa Patera
Pityusa Patera is a feature in the Mare Australe quadrangle of Mars, located at 67.0° S and 323.1° W. It is 230.0 km across and was named after a classical albedo feature name....
. Geomorphologic evidence suggests that the highland patera were produced through a combination of lava flows and pyroclastics from the interaction of magma with water. Some researchers speculate that the location of the highland paterae around Hellas is due to deep-seated fractures caused by the impact that provided conduits for magma to rise to the surface. Although they are not very high, some paterae cover large areas—Amphritrites Patera, for example, covers a larger area than Olympus Mons.
Volcanic plains
Volcanic plains are widespead on Mars. Two types of plains are commonly recognized: those where lava flow features are common, and those where flow features are generally absent but a volcanic origin is inferred by other characteristics. Plains with abundant lava flow features occur in and around the large volcanic provinces of Tharsis and Elysium. Flow features include both sheet flow and tube- and channel-fed flow morphologies. Sheet flows show complex, overlapping flow lobes and may extend for many hundreds of kilometers from their source areas. LavaLava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows can form a lava tube
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
when the exposed upper layers of lava cool and solidify to form a roof while the lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
underneath continues flowing. Often, when all the remaining lava leaves the tube, the roof collapses to make a channel or line of pit craters (catena
Catena
The word catena has various meanings:*The Latin word for "chain"*Catena *Catenary, a type of curve in mathematics*A chain of similarly sized impact craters, in planetary geology...
).
An unusual type of flow feature occurs in the Cerberus plains south of Elysium and in Amazonis. These flows have a broken platey texture, consisting of dark, kilometer-scale slabs embedded in a light-toned matrix. They have been attributed to rafted slabs of solidified lava floating on a still-molten subsurface. Others have claimed the broken slabs represent pack ice that froze over a sea that pooled in the area after massive releases of groundwater from the Cerberus Fossae
Cerberus Fossae
The Cerberus Fossae are a series of semi-parallel fissures on Mars formed by faults which pulled the crust apart in the Cerberus region . Ripples seen at the bottom of the fault are sand blown by the wind ....
area.
The second type of volcanic plains (ridged plains) are characterized by abundant wrinkle ridges. Volcanic flow features are rare or absent. The ridged plains are believed to be regions of extensive flood basalt
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
s, by analogy with the lunar maria. Ridged plains make up about 30% of the Martian surface and are most prominent in Lunae, Hesperia, and Malea Plana, as well as throughout much of the northern lowlands. Ridged plains are all Hesperian in age and represent a style of volcanism globally predominant during that time period. The Hesperian Period is named after the ridged plains in Hesperia Planum.
Current volcanism
Scientists have never recorded an active volcano eruption on the surface of Mars; however, the European Space Agency's Mars ExpressMars 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...
orbiter photographed lava flows that must have occurred within the past two million years, suggesting a relatively recent geologic activity.