Volcanism throughout the solar system.
AA2281 Exploring the Solar System
Assessment
F: Volcanism throughout the solar system
"Volcanism is one of the major processes, whereby a planet transfers heat produced in its interior outward to the surface" (Encyclopedia of the solar system).Volcanism is part of the process of bringing material up from the deep interior of a planet and spilling it forth on the surface where it cools and becomes solid. At least three quarters of the surface rocks on earth and Venus can be attributed to volcanic activity. There are two styles of volcanic activity explosive or effusive. Explosive is when magma is disrupted into fragments which are called pyroclasts by the expansion of exsolved gases. Effusive eruption is when magma escapes through a vent, there is no significant disruption, but a lava flow is formed. To understand the volcanic processes of the earth the products of volcanic activity have been studied under a wide range of environmental conditions such as gravity and atmospheric pressure. There are quite a few other planets which have volcanoes on the surface, including Venus, Mars, and Jupiter's moon Io. Other planets exhibit the results of volcanic activity. These include Mercury, the Earth's Moon, Jupiter's moon Europa, and perhaps Neptune's moon Triton. Planetary bodies, like Jupiter's moon Europa also exhibit icy volcanism where water takes the place of lava.
Eruptions allow fresh gases to the surface from the melted material below. Volcanism is part of the process by which a planet cools off. Even though they are not volcano's, geysers and hot springs are also part of the volcanic process, involving water and hydrothermal activity. Much of the earth's surface especially the crust of the ocean floor consisted of very young rocks, geologically. The rocks came from long lines of volcanoes, which were located along ridges near centres of ocean basins, within the last 300 million years. When this was concluded the theory of plate tectonics was developed, this explained the locations and distributions of volcanoes on Earth. Volcanoes that erupted basalt tended to be located along midocean ridges, this marks constructional margins of Earth's rigid crust plates. There are several ways in which a volcano forms, just as there are several different kinds of volcanoes.
Hot magma, rising from lower reaches of the Earth, eventually, but not always, erupts onto the surface. During the eruption lava and ash form a cone. This cone is what we know as a volcano. Strong earthquakes accompany rising magma just before an Eruption. Among the different kinds of volcanoes are shield volcanoes, cinder cones and composite volcanoes. On Earth, the most general cause of volcanism is caused by lithospheric subduction which are composite volcanoes. These volcanoes form as a result of subduction of the nearby lithosphere.
The Earth's Moon has no large volcanoes like Hawaii or Mount St. Helens. However, vast plains of basaltic lava cover much of the lunar surface. Volcanism on the Moon differs in several ways from volcanism on the Earth. First, there is the matter of age. Volcanism on the Earth is an ongoing process. Many of Earth's volcanoes are quite young in geologic terms, often less than a few 100,000 years old. In contrast, most volcanism on the Moon appears to have occurred between 3 and 4 billion years ago. Typical mare samples are ~3,500,000,000 years old. Even the youngest mare flows have estimated ages of nearly 1 billion years. These "young" rocks have not been sampled or directly dated, so this age is could be incorrect. For comparison, the oldest dated rock on the Earth is ~3.9 billion years old. The oldest sea floor basalts on Earth are only about 200 million years old. Because the Moon does not show any evidence for recent volcanic or geologic activity, it is sometimes referred to as the "dead" planet.
The settings of mare volcanism reveal another major difference from volcanism on the Earth. Specifically, Earth's volcanoes mostly occur within long linear mountain chains. Mountain chains like the Andes mark the edge of a lithospheric plate. Mountain chains like the Hawaiian Islands mark past plate movements over a mantle hotspot. In contrast, the mare typically occur in the bottoms of very large, very old impact craters. Thus, most of the mare are nearly circular in shape. Further, lunar mountain chains form the edges of these impact basins and tend to surround the lunar mare. There is no evidence ...
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The settings of mare volcanism reveal another major difference from volcanism on the Earth. Specifically, Earth's volcanoes mostly occur within long linear mountain chains. Mountain chains like the Andes mark the edge of a lithospheric plate. Mountain chains like the Hawaiian Islands mark past plate movements over a mantle hotspot. In contrast, the mare typically occur in the bottoms of very large, very old impact craters. Thus, most of the mare are nearly circular in shape. Further, lunar mountain chains form the edges of these impact basins and tend to surround the lunar mare. There is no evidence that any system of plate tectonics ever developed on the Moon. Finally, the lunar mare are primarily found on one side of the Moon. They cover nearly one third of the lunar nearside, but less than 2% of the lunar farside. The surface is much higher on the farside and the crust is typically much thicker there as well. Therefore, the primary factors controlling volcanism on the Moon appear to be the crustal thickness and the surface elevation.
Finally, there are some major physical differences between volcanism on the Earth and on the Moon. First, lunar gravity is only one sixth that of the Earth's. This means that the forces driving lava flow are weaker on the Moon. Thus, the very flat and smooth mare surfaces imply that mare lava was very fluid. They could both flow very easily and spread out over large areas. Also, the low gravity means that explosive eruptions can throw debris further on the Moon than on the Earth. Indeed, such eruptions on the Moon should spread lava out into a broad flat layer and not into the cone-shaped features seen on the Earth. This gives one reason for why large volcanoes are not seen on the Moon. Also, the Moon has essentially no dissolved water. The lunar mare are all bone dry. In contrast, water is one of the most common gases in Earth lava. Water also plays a major role in driving violent eruptions on the Earth. Therefore, this lack of lunar water should strongly affect lunar volcanism. In particular, without water, violent explosive eruptions are much less likely on the Moon. Instead, lava should just flow smoothly and quietly out onto the surface.
Like the moon, Mercury has many craters flooded by lava and other lava plains. These indicate that there may have been volcanism on Mercury at one time.
There is much volcanism on Venus. Many are high peaks with extensive lava flows . The volcanoes are shield volcanoes, like the Hawaiian islands. This type of volcano indicates a very fluid lava. No active volcanoes have been found on Venus to date but evidence seems to suggest that some do exist. There are two pieces of evidence that point to volcanism still taking place on Venus.
Levels of sulphur dioxide in the atmosphere show variations. One spacecraft measured very high levels of sulphur dioxide but levels dropped off over time. Sulphur dioxide is a gas which is commonly released in volcanic eruptions. On Earth, when a volcano erupts, our atmospheric levels of this gas rise. Another bit of evidence is that two orbiters have measured bursts of radio energy on Venus. These bursts are similar to those produced by lightening discharges which often occur in the plumes of erupting volcanoes. Unlike the Earth, where volcanoes are generally confined to plate margins, the distribution of volcanoes on Venus is quite random, and this is perhaps the best argument that Venus loses all of its internal heat via hotspot volcanism, rather than plate tectonics. Volcanic features on Venus can be classified as shields and shield fields. Shields are small (probably basaltic) volcanoes, less than 20km in diameter, they resemble terrestrial shield volcanoes, or domes. They are typically circular or elliptical in outline, and often have summit pits or craters. Shield lava flows are too narrow to be resolved in Magellan imagery. Individual shield volcanoes are the most abundant landform on Venus, numbering well over 1000. Clusters of shield volcanoes 100-200km across are commonly found in association with linear fracture belts, or coronae. The closest analogue to the shield clusters is the volcanic regions of the Snake River Plains in southern Idaho, or the clusters of central vent edifices found on parts of the Mid-Atlantic Ridge.
Cones are relatively small volcanic structures with steeper slopes than shield volcanoes (12-23°), and heights of up to 1700 metres. Cones are usually brighter than shields in radar images, demonstrating that they have rougher surfaces, possibly composed of coarser lava flows. Cones also occur in clusters, frequently in association with shield fields.
Intermediate volcanoes are classified as those structures which fall into the size range of 20-100km in diameter. Shields and domes are the dominant forms of intermediate volcanoes, and these are generally symmetrical, and characterised by radial patterns of lava flows and fractures. Most of the domes on Venus fall into this classification. They can be up to 2000 metres high, and have steep sides with flat tops. Summit pits or craters are common, though not necessarily centrally located. The steep bounding slopes of the domes are frequently subject to wall failure, producing landslides, and a characteristic scalloped outline to many domes.
There are large volcanoes on Venus which measure 100-600km in diameter and can be several kilometres high. They occur mostly at high elevation along broad rises, or at the junctions of rift systems. Systems of radial lava flows extend for several hundred kilometres, originating either from the summit, or cones on the flanks of the volcano.
Calderas are circular to extended depressions surrounded by concentric fractures, which are identical to caldera structures on Earth, formed by the collapse of a volcano into its magma chamber. The caldera floor may be filled with late-stage flood lava. Calderas resemble meteorite craters, but can be distinguished by the presence of radial lava flows, the presence of the concentric fracturing, absence of a raised rim. Sacajawea Patera is an elliptical caldera over 260km long, and 175km wide.
Coronae are unique to Venus, and probably are an expression of the way that Venus loses heat via hotspot activity. Coronae are circular to elongate domes or plateaux, often with an interior depression, and always surrounded by a trough or moat. Also found in association with coronae are shield and cone fields, intermediate volcanoes, radial lava flows, and annuli of troughs and ridges. Coronae can be concentrated into clusters or chains, such as those found along Hecate or Parga Chasmata. Coronae are interpreted as the surface expressions of mantle upwellings, that subsequently vent magma to the surface and "deflate".
Arachnoids are very similar to coronae, and may in fact be genetically related to them, though they are typically much smaller. Arachnoids were named as such because their complex patterns of radial and concentric fractures were thought to resemble cobwebs.
Novae are prominent starburst, or stellate, patterns of fractures centred on a broad dome, usually over 100km in diameter. Very few other volcanic features, in sharp contrast to the coronae, are found in association with novae. Novae are also notably less common than coronae.
A number of sinuous channels can be found in Magellan imagery of some lava plains. They are from 0.5 to 2km across, and can be several hundred kilometres in length. These channels may be braided, or exhibit remarkable features such as meanders. There can be little doubt that these channels are the product of erosion by a flowing magma, but the composition of the magma is unknown. Modelling of the cooling of silicate magmas under Venusian conditions shows that they would solidify far to quickly to cut channels so long. Work in this department is currently investigating the stability of carbonatite magmas, to determine whether they might be responsible for carving the canali, though it should be recognised that the nature of the liquid could conceivably be something we have never encountered before. Sinuous rilles, resembling those on the Moon, are also found, and are likely to have the same origin.
Some of the domes on Venus may look a little bit like craters, but they are really flat-topped mountains. Mt Pelee in Martinique, West Indies, and Lassen Peak domes in California are examples of terrestrial lava domes. These domes are built the same way some islands are built on Earth, by a hot plume of material from the deep interior of Venus, which rises to the surface, pushing out the crust and causing volcanism.
Volcanic rises are thought to be formed by plumes, in a similar fashion to island formation on Earth. On Venus, there are three types of volcanic rises.
Rift-dominated rises which contain two or more deep rift valleys. These valleys cut through most of the rise itself, but also extend several hundred kilometres beyond the rise itself. Atla Regio and Beta Regio are rift-dominated rises. Volcano-dominated rises which lack deep rift valleys. They do contain one or more large shield volcanoes. Western Eistla Regio and Imdr Regio are volcano-dominated rises.
Corona-dominated rises which lack large rift valleys, like volcano-dominated rises. But instead of being dominated by shield volcanoes, their surfaces are dominated by coronae. Corona-dominated rises include Eastern Eistla Regio and Themis Regio.
During its earliest history, Mars was bombarded with planetismals. The impacts of these asteroid-like boulders caused the surface regions of Mars to become warm enough for continents to drift across the surface just as they do on Earth to this day. When the lithosphere became immovable, what may once have been a supercontinent froze in place, in the southern hemisphere, becoming what is now the highlands of Mars.
Evidence for this theory can be found with an examination of the global geography of Mars, together with the cratering pattern. The cratering evidence suggests that Mars may have warmed from the inside late in its forming history, causing volcanism after the lithosphere became immovable. This period of volcanism is what created the Tharsis Bulge, Olympus Mons, and the other volcanoes. The volcanoes poured out a new surface over the lowlands of Mars, which received a lighter load of bombardment than did the older highlands. The lowlands, where the volcanoes are found, are cratered at a rate which suggests an age of 3.7-3.8 Billion Years. The highlands are much older. The cratering record suggests that after this period however, all volcanic activity on Mars ceased. There seem to be no features younger than 3.5 Billion Years. Martian volcanism is broadly confined to the Tharsis and Elysium regions, though there are volcanoes in the southern hemisphere in the Memnonia and Aeolis regions and around the Argyre basin. Other volcanic activity occurs in the Syrtis Major Planitia, where we see extensive flood lava and low shields.
Both Tharsis and Elysium are broad uplifts (Tharsis is 10km above the mean martian elevation) thought to be related to mantle convection patterns. The mass of the Tharsis bulge has sizeable influence on the rotation of Mars, and its formation generated widespread fracturing most important of which is the Valles Marineris and Noctis Labyrinthus. The volcanoes of Tharsis are all shield volcanoes (truly massive ones; Olympus Mons towers 27km above datum and is 600km across) with low slopes and enormous summit calderas. These represent several billion years of basalt/komatiite extrusion. The youngest surfaces on Olympus Mons may be just 200 million years old. Recent Mars Global Surveyor laser altimetry of the Tharsis bulge shows that the volcano Arsia Mons is actually tipped up. This means that Tharsis really is an uplifted piece of crust rather than a lava accumulation.
The volcanoes of Elysium resemble composite volcanoes. Elysium Mons itself is relatively steep-sided, and its flanks are incised, perhaps hinting at the presence of pyroclastics. The older Hesperian volcanics of the southern hemisphere include edifices such as Apollinaris Patera, and Tyrrhena and Hadriaca Paterae. All show evidence of the action of volcanic density currents surging down their flanks and pyroclastic activity. Deposits in Aeolis and Memnonia which are understood to be ignimbrites and have been carved by the action of the wind.
Io is the most volcanically active body known in the Solar System. Io, the innermost large moon of Jupiter, is about the same size and density as Earth's Moon. The surface is formed almost entirely of volcanic flows or pools of volcanic material. It was during the Voyager mission to Jupiter that the active volcanism of Io was first discovered. The activity of Io was a major surprise. Io is the only body in the solar system other than the Earth known to be actively volcanic. The volcanism of Io provides another example of a prominent kind of volcanism found on Earth, namely the formation of geysers. The two kinds of volcanism exhibited by Io are volcanoes, which form from silicate magmas similar to those of Earth and geysers that spout liquid SO2. Eruptions are so common and so large that the entire surface can be buried under 100 meters of material every 1 million years (it takes submarine volcanoes about 80 million years to resurface about two-thirds of the Earth). Impact craters, which are common on many planets and moons, are absent on Io because of the frequent volcanic eruptions bury them.
Hotspots of Io reveal where the volcanoes are. The most well known and active volcanic features of Io are Prometheus which is a geyser, Pelee which is a geyser, Ra which is a volcano and Loki which is a volcano.
Bibliography
J.K.Beatty, C.Petersen, A.Chaiken: The New Solar System (fourth edition) Cambridge University Press
D.Morrison, S.Wolff, A.Franknoi: Abells Exploration of the Solar System (seventh edition) Saunders College Publishing
P.Weissmen, L.Mcfadden, T.Johnson: Encyclopedia of the Solar System (First Edition) Academic Press
R.Greeley: Planetary Landscapes (First Edition) Allen and Unwin
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