Plume

3.4418250950552 (1315)
Posted by bender 04/04/2009 @ 20:12

Tags : plume, plum creek timber co. inc., real estate, business, a plumm summer, movies, cinema, entertainment, judy blume, authors, books, fine arts

News headlines
Arsenic, selenium plumes continue to grow under East Helena - The Missoulian
By EVE BYRON of the Helena Independent Record HELENA - Concerns over the ongoing progression toward the Helena Valley of arsenic and selenium plumes under East Helena has prompted the installation of five new monitoring wells and a hazard rating of...
Valkyrie Profile: Covenant of the Plume Confirmed for Europe - Electronic Theatre
Square Enix have today announced that Valkyrie Profile: Covenant of the Plume will be released in Europe and across PAL territories on 3rd April 2009, exclusively on the NintendoDS handheld system. Valkyrie Profile: Covenant of the Plume is the next...
Greek students pay a visit to Plume School, Maldon - BrainTree and Witham Times
By Lauren Hockney » A group of 33 students from a school in Kephalonia spent the day at Plume School in Fambridge Road. During their visit the youngsters were paired with year ten students who showed them around and gave them an insight into what it is...
St. Paul's Boulevard, Plume Street closure put off - The Virginian-Pilot
By Debbie Messina The intersection of St. Paul's Boulevard and Plume Street will not close Monday as previously announced. The utility work for light rail has been delayed until June because of inclement weather and a problem with deliveries of...
Activity at Redoubt increasing as steam plume grows - KTUU
by Channel 2 News staff ANCHORAGE, Alaska -- Seismic activity at Mount Redoubt has increased over the last 24 hours, according to the Alaska Volcano Observatory and a steam and as plume rising above the volcano increased in size dramatically in the 9...
Here's Looking @ Earth — Shiveluch Getting All Steamed Up - SatNews Publishers
The Shiveluch Volcano on Russia's Kamchatka Peninsula released a small plume of vapor as the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite passed overhead on May 10, 2009. According to the Kamchatkan...
Salt in ice plumes hint at liquid sea on Saturn's moon - Times of India
LONDON: Scientists, studying measurements made by Cassini spacecraft, have found salt in the ice plumes that bloom above Saturn's moon Enceladus, which suggest the presence of a liquid ocean on the satellite. The Cassini spacecraft flew through a plume...
Ellsworth Restoration Advisory Board meeting Tuesday - Rapid City Journal
Agenda items to be discussed at this meeting are the status of landfill sites, petroleum release sites, and chlorinated plume sites, including biodechlorination progress, the off-base plume and plans for the summer....
Voluntary Evacuations near Chemical Fire Site - Today's TMJ4
In addition, they are attempting to test the air inside the plant and in a plume of smoke that still surrounds it, for dangerous chemicals. Dodge County Sheriff Todd Nehls says it could be weeks before they know what caused the fire....

Plume Latraverse

Plume Latraverse (born Michel Latraverse May 11, 1946) is a prolific singer, musician, songwriter and author from Quebec. His career spans over 30 years; Latraverse is probably one of the most influential names in Quebec counterculture.

To the top



Thomas Plume

The Reverend Doctor Thomas Plume, B.A., D.D. (1630 – 20 November 1704) was an English churchman and philanthropist, founder of a school which still stands today, the Plume School, Essex.

He was born in 1630 in Maldon, and educated at Chelmsford, Essex, and Christ's College, Cambridge. In 1658 he was appointed Vicar of East Greenwich, Kent, in 1662 Rector of Merston, Sussex, and in 1665 Rector of Little Easton, Essex. From 1679 until his death, unmarried, on 20 November 1704, Thomas Plume was Archdeacon of Rochester, Kent. He was buried at Longfield, Kent.

Thomas was three times married. Deborah, maiden name unknown, Elizabtih Pratte, 24 July 1624 and Elinor (Hellen) surname unknown.

At the time of his restoration Thomas Plume was admitted Vicar of Greenwich and subscribed the declaration under the Act of Uniformaity on July 28, 1662. This is noteworthy as this time 2,000 to 2,500 ministers were ejected for nonconformity, and his father at Maldon had been a prominent Presbyterian. Thomas was admitted Vicar of Greenwich at the age of twenty-eight, on September 22, 1658. He remained in this role for the next forty-six years until his death on November 20, 1704.

Even though he lived in Greenwich most of his life, Plume left his collection of 7,000 books, printed between 1470 and his death, to the town of Maldon. It was kept in St. Peter's Church, of which only the original Tower survives; the rest of the building was rebuilt by Plume to house his library. The library was to be "for the use of the minister and clergy of the neighbouring parishes who generally make this town their place of residence on account of the unwholesomeness of the air in the vicinity of their churches" Plume left specific instruction for the use of the library: "and Gentlemen or Scholar who desires, may go into it, and make use of any book there or borrow it, in case he leaves a vadimonium with the Keeper for the restoring thereof fair and uncorrupted within a short time". Plume's library continues to grow after his death with contributions from others.

In 1704 Thomas Plume founded the chair of Plumian Professor of Astronomy and Experimental Philosophy at the University of Cambridge in order to "erect an Observatory and to maintain a studious and learned Professor of Astronomy and Experimental Philosophy, and to buy him and his successors utensils and instruments quadrants telescopes etc".

To the top



Plume tectonics

Plume tectonics is a relatively new theory in geophysics which studies the movements of Mantle plumes under Tectonic plates at the depth of 2900km in the earth. Japanese geophysicists Fukao Yoshio (ja:深尾良夫, Nagoya University) and Maruyama Shigenori(ja:丸山茂徳, visiting scholar at Stanford University, Tokyo Institute of Technology) have advanced the theory. Fukao studied the internal structure of the Earth with p-wave tomography. Maruyama interpreted Fukao's data in terms of convection in the mantle in 1994. Plate tectonics could not explain all of geological activities, and thus was considered incomplete before Fukao and Maruyama hypothesized plume tectonics.

Nevertheless, this is still a subject of intense debate. Of particular note is the current vigorous debate regarding whether mantle plumes exist or not.

To the top



La Plume Township, Pennsylvania

Location of La Plume Township in Pennsylvania

La Plume Township is a township in Lackawanna County, Pennsylvania, United States. The population was 603 at the 2000 census. It is home to Keystone College.

According to the United States Census Bureau, the township has a total area of 2.4 square miles (6.2 km²), all of it land.

As of the census of 2000, there were 603 people, 243 households, and 164 families residing in the township. The population density was 251.0 people per square mile (97.0/km²). There were 264 housing units at an average density of 109.9/sq mi (42.5/km²). The racial makeup of the township was 96.52% White, 0.50% African American, 2.16% Asian, 0.17% from other races, and 0.66% from two or more races. Hispanic or Latino of any race were 1.00% of the population.

There were 243 households out of which 30.0% had children under the age of 18 living with them, 49.0% were married couples living together, 13.2% had a female householder with no husband present, and 32.5% were non-families. 25.9% of all households were made up of individuals and 9.5% had someone living alone who was 65 years of age or older. The average household size was 2.48 and the average family size was 2.97.

In the township the population was spread out with 24.4% under the age of 18, 8.0% from 18 to 24, 29.4% from 25 to 44, 24.0% from 45 to 64, and 14.3% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 92.7 males. For every 100 females age 18 and over, there were 87.7 males.

The median income for a household in the township was $32,083, and the median income for a family was $39,107. Males had a median income of $29,773 versus $19,688 for females. The per capita income for the township was $16,491. About 8.3% of families and 13.3% of the population were below the poverty line, including 31.8% of those under age 18 and 4.3% of those age 65 or over.

To the top



Night Plume

Night Plume is a fictional character in the fantasy novel trilogy The Echorium Sequence by Katherine Roberts and features third novel only; Dark Quetzal. Night Plume is one of the main characters in the third novel featuring throughout most of the novel. Night Plume is a quetzal; half-man and half-bird corrupted by Frazhin's use of the khiz crystal. Frazhin attempts to use Night Plume to torture Rialle into giving him the information he wants about his lost daughter which The Echorium took into their own hands when they captured and destroyed the Khizalace in the Crystal Mask who at the time was in her mother's womb fail.

To the top



White Plume Mountain

S2 White Plume Mountain.jpg

White Plume Mountain is a module for the Advanced Dungeons & Dragons fantasy role-playing game, written by Lawrence Schick and published by TSR in 1979. The 16-page adventure bears the code "S2" ("S" for "special") The adventure is a dungeon crawl where the players' characters are hired to retrieve three "notorious" magical weapons: a trident, a war hammer and a sword, each possessing its own intelligence. The adventure contains art by Erol Otus, and a cover by Jeff Dee. A sequel, Return to White Plume Mountain, was published in 1999, and an updated version conforming to v3.5 rules was released online in 2005.

White Plume Mountain was well received by critics. It was ranked the 9th greatest Dungeons & Dragons adventure of all time by Dungeon magazine in 2004. One judge, commenting on the ingenuity required to complete the adventure, described it as "the puzzle dungeon to end all puzzle dungeons." A review for British magazine White Dwarf gave it an overall rating of 8/10, noting that the adventure focuses on problem solving. It is also the favorite adventure of Wired magazine's Ken Denmead, who described it as the "amusement park of dungeons". Other adventures in the S series include Tomb of Horrors, Expedition to the Barrier Peaks, and Lost Caverns of Tsojcanth.

In the World of Greyhawk, a campaign setting for Dungeons & Dragons, White Plume Mountain is a volcano. The earliest known inhabitant of White Plume Mountain is the druid Aegwareth. Aegwareth is later slain by the evil wizard Keraptis, who took over the mountain with his gnomish servitors. The premise of White Plume Mountain is that thirteen hundred years ago, Keraptis descended into the volcanic mountain with a company of gnomes, and disappeared. The adventure is a dungeon crawl, and hinges on the theft of three powerful magical weapons: a trident named Wave, a war hammer named Whelm, and a sword named Blackrazor, all of which have their own intelligence and were first introduced in this adventure. The weapons' former owners each received a copy of a taunting poem, instructing them that the weapons are in White Plume Mountain. The characters' goal is to enter into the Wizard's Mouth, a fissure in the side of the active volcano, to rescue the magical weapons from Keraptis' lair.

The adventure's 16 pages are divided into 27 encounters. The player's characters begin in a cave at the base of White Plume Mountain. In the first numbered encounter, the characters find a spiral staircase in the cave which leads to a "mangy, bedraggled" gynosphinx.:5 Encounter seven involves a large cave with a floor of boiling mud. Circular wooden platforms are suspended from the ceiling, and the characters must jump from platform to platform while dodging geysers of hot mud. In the eighth encounter, the characters confront a vampire who is guarding the magical war hammer Whelm in a room of permanent darkness.

In encounter 17, a corridor leads the characters to a boiling lake. According to the adventure, "The corridor from the dungeon continues out into the lake under a rubbery magical forcefield that keeps out the waters by forming a sort of elastic skin of super-tension.":9 The watery tunnel opens into a watery dome, where the characters must defeat a giant crab in order to collect the magical trident Wave. Encounter 22 involves a frictionless room with spikes, and in encounter 23, the characters kayak on a stream suspended in mid air. In the 26th encounter, the characters must fight various creatures in a magical ziggurat where each level is guarded by a different monster including sea lions, giant crayfish, giant scorpions, and manticores. In the last encounter, an ogre mage must be defeated in order to win the magical sword Blackrazor. An end note recommends that the Dungeon Master add an encounter with two efreet if the characters have succeeded in taking two or three of the magic weapons.

The original White Plume Mountain adventure was written by Lawrence Schick, and was published by TSR in 1979. The interior contains art by Erol Otus,:3 Bill Willingham,:13, and David C. Sutherland III:4 among others, and the front cover is by Jeff Dee.:cover The adventure was included as part of the Realms of Horror abridged compilation produced in 1987. Wizards of the Coast also released a sequel to the adventure in 1999, Return to White Plume Mountain, as part of the TSR 25th Anniversary series of publications. The events in the sequel are assumed to take place 20 years following those in the original.

In 2005, an online version of the adventure was released as a free download, updated to conform with v3.5 rules (Wizards of the Coast periodically alters the rules of Dungeons & Dragons and releases a new version). The revised module is designed for characters of the seventh level of experience. Return to White Plume Mountain has also received a v3.5 update and is likewise available for free download on their website. In both of the revised modules, the classic weapons associated with them (Blackrazor, Whelm and Wave for White Plume Mountain, Frostrazor for Return to White Plume Mountain) have been converted into Legacy Weapons.

White Plume Mountain was well received by critics. It was ranked the 9th greatest Dungeons & Dragons adventure of all time by Dungeon magazine in 2004, on the 30th anniversary of the Dungeons & Dragons game. Judge Mike Mearls commented on the ingenuity required to complete the adventure, describing it as "the puzzle dungeon to end all puzzle dungeons". Further, when speaking to why it is one of the top adventures ever, he said that while it lacked the "sheer brutality" of Tomb of Horrors, it made up for it with "crazy, over the top, pure fun". Another Judge, Clark Peterson, said that he liked the three magical weapons: Wave, Whelm, and Blackrazor. To Peterson, just the inclusion of Blackrazor makes White Plume Mountain a "classic". The editors of Dungeon felt that the adventure was defined by the ziggurat and its monsters.

Jim Bambra reviewed White Plume Mountain for the British magazine White Dwarf, and rated it favorably at 8/10 overall. He gave playability, enjoyment, and skill ratings of 9/10, and a complexity rating of 7/10. He noted that the adventure focuses on problem solving. Comparing it to the previous S series adventure Tomb of Horrors, Bambra found White Plume Mountain "quite lenient." Where in Tomb of Horrors a wrong decision would leave the player's character dead, in White Plume Mountain it merely leaves the player frustrated. The adventure's tests are "designed to stretch a party to its limits, not deal death at every opportunity".

Ken Denmead of Wired says that White Plume Mountain is his favorite adventure, if not necessarily the best. For him, it was the "amusement park of dungeons". He describes the story arc, where the adventurers are hired to retrieve three magic items, as similar to the A-Team or The Equalizer: "You've been hired to help when no one else has been able." He felt that while the sword Blackrazor was a "blatant ripoff of Elric", it was "still way cool".

Denmead commented on several of the adventure's encounters. He felt the cavern with boiling mud, hanging disks, and geysers, was "just cruel". Concerning the permanently dark room that houses a vampire, he commented on the ease with which one player's character can easily hit that of another in the gloom. He describes the room where a giant crab guards the trident Wave as "basically a bubble inside a tank of boiling water", noting the crab knows not to pierce the walls and asks, "Are you that smart?" Also, if the players end the adventure with the magic weapons, it's up to them to "bribe" their Dungeon Master to keep them.

To the top



Iceland plume

Topography/bathymetry of the north Atlantic around Iceland

The Iceland Plume is an upwelling of anomalously hot rock in the Earth's mantle beneath Iceland whose origin probably lies at the boundary between the core and the mantle at ca. 2880 km depth. It is generally thought to be the cause of the formation of Iceland and its volcanism, which characterizes the island to the present day, according to the plume theory of W. Jason Morgan.

The plume, which bears the name of Iceland and lies roughly beneath the center of the island, is considerably older than Iceland. Volcanic rocks related to it are found to both sides of the coast of southern Greenland and had their ages determined to lie between 58 and 64 million years; this coincides with the opening of the north Atlantic in the late Paleocene and early Eocene. It is generally thought that the volcanism was caused by the flow of hot material from the plume head into regions beneath the lithosphere which had previously been thinned by rifting and produced large amounts of melt there. The exact position of the plume at that time is controversial, but was probably beneath central Greenland; it is also not entirely clear whether the plume had ascended from the deep mantle only at that time or whether it is much older and also responsible for the old volcanism in northern Greenland, the Ellesmere Island Volcanics on Ellesmere Island, and in the Arctic Ocean (Alpha Ridge). All of these volcanics are part of the High Arctic Large Igneous Province.

As the northern Atlantic opened to the east of Greenland during the Eocene, North America and Eurasia drifted apart; the Mid-Atlantic Ridge formed as an oceanic spreading center and a part of the submarine volcanic system of mid-oceanic ridges. In the course of these plate motions Greenland moved above the Iceland plume; different investigations located the plume beneath the coast of southeastern Greenland (Scoresby Sound) or slightly eastward from it 40 million years ago and related it to the North Atlantic Igneous Province. Upon further opening of the ocean and plate drift, the plume and the mid-Atlantic Ridge approached each other, and finally a part of the plume head reached the region of thinned lithosphere at the ridge, leading to increased generation of melt and crust; these processes grew stronger the more both structures converged. The Greenland-Iceland Ridge and the Faroe-Iceland Ridge, both regions of greatly thickened oceanic crust, are traces of this stage of the convergence preceding the formation of Iceland.

The oldest crust of Iceland itself is more than 20 million years old and was formed at an old, now extinct oceanic spreading center in the western fjord (Vestfirðir) region. The westward movement of the plates and the ridge above the plume and the strong thermal anomaly of the latter caused this old spreading center to starve 15 million years ago and lead to the formation of a new one in the area of today's peninsulas Skagi and Snæfellsnes; in the latter there is still some activity in the form of the Snæfellsjökull volcano. The spreading center, and hence the main activity, have shifted eastward again 7–9 million years ago, however, and formed the current volcanic zones in the southwest (WVZ; Reykjanes–Hofsjökull–Vatnajökull) and northeast (NVZ; Vatnajökull–Tjörnes). Presently, a slow decrease of the activity in the WVZ takes place, while the volcanic zone in the southeast (Katla–Vatnajökull), which was initiated 3 million years ago, develops.

In addition to the formation of Iceland the plume has also influenced the generation of crust at the adjacent segments of the mid-oceanic ridge, especially at the Reykjanes Ridge southwest of the island. In this region, a significant thickening of the crust and an anomalous uplift of the seafloor are observed, which are explained by a hot mantle current emerging from the plume and flowing along the bottom of the thin lithosphere at the ridge; the variations in crustal thickness, which form a chevron-like pattern, show that this current has not been constant over time. Among the different possibilities to explain this pattern are interactions of the shift of the spreading center with the plume head or pulses of the plume itself.

Information about the structure of Earth's deep interior can be acquired only indirectly by geophysical and geochemical methods. For the investigation of the Iceland Plume as well as of other plumes, gravimetric, geoid and in particular seismological methods along with geochemical analyses of erupted lavas have proven especially useful. Numerical models of the geodynamical processes attempt to merge these observations into a consistent general picture.

An important method for imaging large-scale structures in Earth's interior is seismic tomography, by which the area under consideration is “illuminated” from all sides with seismic waves from earthquakes from as many different directions as possible; these waves are recorded with a network of seismometers. The size of the network is crucial for the extent of the region which can be imaged reliably. For the investigation of the Iceland Plume both global and regional tomography have been used; in the former, the whole mantle is imaged at relatively low resolution using data from stations all over the world, whereas in the latter, a denser network only on Iceland images the mantle down to 400–450 km depth with higher resolution.

Regional studies from the 1990s (ICEMELT, HOTSPOT) show unambiguously that there is a roughly cylindrical structure with a radius of 100–150 km beneath Iceland down to at least 400 km depth, in which the velocities of seismic waves are reduced by up to 3% (P waves) and more than 4% (S waves), respectively, compared to the reference model; the most recent analyses point towards an even stronger reduction. If these values are converted into a temperature anomaly using rock-physical models, it is found that the mantle there is 150–250°C hotter than normal. Some uncertainty is introduced into the models by the limitations in spatial resolution of seismic tomography: it is difficult to distinguish a hot, thin plume from a less hot, broader one with this method.

Global tomography confirms that there is a strong anomaly with clearly reduced seismic velocities in the upper mantle beneath Iceland. For the lower mantle (below 660 km depth), the picture is more contradictory. In all investigations the anomaly weakens considerably there and has a more irregular shape; in some depth intervals it even seems to disappear, although those depths are not the same in the different studies. At the core-mantle boundary below Iceland, an anomalously hot region has also been found with other seismological methods , and the structure of seismic discontinuities at 410 and 660 km depth below Iceland also indicates that the temperatures are elevated there. Therefore the majority of scientists thinks that the weaker signature of the plume in the lower mantle can be explained with possible temporal variability of the plume and/or change of physical properties of the mantle with depth as well as with the limitations of the method and the available data, but that the plume does indeed reach down to the base of the mantle.

Numerous studies have addressed the geochemical signature of the lavas present on Iceland and in the north Atlantic. The resulting picture is extraordinarily complex and partly self-contradicting, but nonetheless consistent in several important respects. For instance, it is not debated that the source of the volcanism in the mantle is chemically and petrologically heterogeneous: not only the normal peridotite, but also eclogite contribute to the melts. The origin of the latter is assumed to be metamorphosed, very old oceanic crust which had sunk into the mantle several hundreds of millions of years ago during the subduction of an ocean. Moreover, isotopic ratios of noble gases yield evidence that there is also a contribution from rock from the lower mantle .

The variations in the concentrations of trace elements like helium, lead, strontium, neodymium, and others show clearly that Iceland is an anomaly also with regard to geochemistry in comparison to the rest of the north Atlantic. For instance, the ratio of He-3 and He-4 has a pronounced maximum on Iceland, which correlates well with geophysical anomalies, and the decrease of this and other geochemical signatures with increasing distance from the plume allows to estimate that the influence of the plume reaches about 1500 km along the Reykjanes Ridge and at least 300 km along the Kolbeinsey Ridge. Depending on which elements are considered and how large the area covered is, one can identify up to six different mantle sources, which, however, are not all present in any single location.

Furthermore, some studies show that the amount of water dissolved in mantle minerals is two to six times higher in the Iceland region as compared to undisturbed parts of the mid-oceanic ridges, where it is regarded to lie at about 150 ppm.

The north Atlantic is characterized by strong, large-scale anomalies of the gravity field and the geoid, with Iceland lying in their center. The geoid rises up to 70 m above the geodetic reference ellipsoid in an approximately circular area with a diameter of several hundred kilometers; this is explained by the dynamic effect of the upwelling plume which bulges up the surface of the Earth. Furthermore, the plume and the thickened crust cause a positive gravity anomaly of about 60 mGal (=0.0006 m/s²) (free-air).

Since the mid-1990s several attempts have been made to explain the observations with numerical geodynamical models of mantle convection. The purpose of these calculations was, among other things, to resolve the paradox that a broad plume with a relatively low temperature anomaly is in better agreement with observed crustal thickness, topography, and gravity, whereas a thin hot plume matches seismological and geochemical observations better. The most recent models indicate that the plume is probably 180–200°C hotter than the surrounding mantle and that its stem has a radius of ca. 100 km, i.e. the seismological findings are confirmed; crustal thickness, topography, and gravity can be explained with such a model if one takes into account that the loss upon melting of water which was dissolved in mantle rock massively alters the fluid dynamical behaviour of the plume, so that the corresponding anomalies become broader and melt production decreases. Thus far, the models do not or not fully take into account petrological heterogeneity, however.

The chevron structures of the Reykjanes Ridge mentioned in the section on the geological history are explained by geodynamical models as pulsations of the plume, i.e. as variations in the mass flux through the plume stem.

As mentioned in the beginning, the plume model is the commonly accepted concept for explaining the formation of Iceland and its volcanism. However, in particular the weak visibility of the plume in tomographic images of the lower mantle and the geochemical hints on eclogite in the mantle source have raised doubts among some scientists such as Don L. Anderson and Gillian Foulger that the plume model is really valid. As an alternative, they propose processes which are restricted to the upper mantle.

According to one of those models, a large chunk of the subducted plate of a former ocean has survived in the uppermost mantle for several hundred million years, and its oceanic crust now causes excessive melt generation and the observed volcanism. This model, however, is not backed by dynamical calculations, nor is it exclusively required by the data, and it also leaves unanswered questions concerning the dynamical and chemical stability of such a body over that long period or the thermal effect of such massive melting.

Another model proposes that the upwelling in the Iceland region is driven by lateral temperature gradients between the suboceanic mantle and the neighbouring Greenland craton and therefore also restricted to the upper 200-300 km of the mantle. However, this convection mechanism is probably not strong enough under the conditions prevailing in the north Atlantic, e.g. with respect to the spreading rate, and it does not offer a simple explanation for the observed geoid anomaly.

To the top



Mantle plume

A lava lamp illustrates the basic concept of a mantle plume.

A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcanic centers known as hotspots and probably also to have caused flood basalts. It is a secondary way that Earth loses heat, much less important in this regard than is heat loss at plate margins (see Plate tectonics). Some scientists think that plate tectonics cools the mantle, and mantle plumes cool the core.

The geometry of the Hawaiian-Emperor seamount chain and the regular progression of ages of volcanism along it were taken as important evidence in support of the mantle plume theory (Morgan, 1972 and Willson, 1963).

In 1971, geophysicist W. Jason Morgan proposed the theory of mantle plumes. In this theory, convection in the mantle slowly transports heat from the core to the Earth's surface. It is now understood that two convective processes drive heat exchange within the earth: plate tectonics, which is driven primarily by the sinking of cold plates of lithosphere back into the mantle asthenosphere, and mantle plumes, which carry heat upward in rising columns of hot material, driven by heat exchange across the core-mantle boundary. The sinking of vast sheets of oceanic lithosphere back into the mantle is the primary driving force of plate tectonics, where the sinking of these slabs is balanced by the passive upwelling of asthenosphere along mid-oceanic ridges. In contrast, mantle plumes are narrow columns of material that rise more-or-less independently of plate motions.

Fluid dynamics experiments in the early 1970's (Whitehead and Luther, 1974) produced models of mantle plumes that consist of two parts: a long thin conduit that connects the top of the plume to its base, and a bulbous head that expands in size as the plume rises upward in the mantle; the result looks like a mushroom with a thin stalk and large top. The bulbous head forms because hot material moves upward through the plume conduit faster than the plume itself rises through the surrounding asthenosphere. In the late 1980's and early 1990's, experiments with thermal models shows that as the bulbous head expands it may entrain some of the adjacent asthenosphere into the rising head.

When the plume head encounters the base of the lithosphere, it flattens out against this surface barrier and undergoes widespread decompression melting to form enormous volumes of basalt magma. This basalt may erupt onto the surface over very short time scales (less than 1 million years) to form a continental flood basalt (if it erupts through continental crust) or an oceanic plateau (if it erupts through oceanic crust). Prominent continental flood basalt provinces include the Deccan traps and the Rajmahal traps in India, the Siberian traps of Asia, the Karmutsen Formation in British Columbia, Canada, the Karoo basalts in South Africa, the Ferrar dolerite of Antarctica (conjugate with the Karoo), the Parana basalts in South America and the Etendeka basalts in Africa (formerly a single province separated by opening of the South Atlantic ocean), and the Columbia River basalts of North America. Plume-related oceanic plateaux include the Ontong Java plateau of the southwest Pacific ocean and the Maniheken plateau of the Indian ocean.

The plume tail may continue to move material from the Earth's interior to the surface, providing a continuous supply of magma in a fixed location, often referred to as a hotspot. As the lithosphere moves over this fixed hotspot due to plate tectonics, the eruption of magma from the fixed hotspot onto the surface forms a chain of volcanoes that parallels plate motion (Skilbeck and Whitehead 1978. The classic example of this is the Hawaiian island chain in the Pacific ocean.

The eruption of continental flood basalts is often associated with continental rifting and breakup, leading to the hypothesis that mantle plumes play an important role in continental rifting and the formation of ocean basins. Where this association of flood basalts with continental rifting is observed, it is not uncommon to find linear chains of volcanic islands that parallel the motion of plates on either side of the spreading center (South Atlantic ocean).

The chemical and isotopic composition of basalts found in hotspots and inferred to form by partial melting of mantle plumes suggest that several components are involved, including primordial mantle with unfractionated noble gases, subducted oceanic crust and mantle lithosphere, and subducted sediments. The processing of oceanic crust, lithosphere, and sediment through a subduction zone decouples the water soluble trace elements (e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta), concentrating the immobile elements in the oceanic slab (the water soluble elements are added to the crust in island arc volcanoes). Seismic tomography shows that subducted oceanic slabs may sink directly to the core-mantle boundary, or pause for long periods at the mantle transition zone (400-660 km depth) before sinking to the core-mantle boundary. The subducted slabs accumulate at the core-mantle boundary and form a seismically distinct layer called the D" (Dee-double prime). This appears to be the source of most deep mantle plumes, as shown by seismic tomography (Montelli et al, 2005).

Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with well-stirred adiabatic gradients above and below this boundary. As a result, the core-mantle boundary represents a significant thermal (temperature) discontinuity, with the core at temperatures several hundred degrees Celsius hotter than the overlying mantle. As heat is transferred across this boundary by conduction, material in the D" layer becomes hotter and thus more buoyant. When it becomes sufficiently buoyant, material begins to rise from the D" layer to form a mantle plume.

In concert with hypothesised slow-down in plate tectonic motion, which may be associated with prolonged periods of supercontinent formation, it is theorised that without an actively convecting asthenosphere, the lower mantle will begin to locally overheat. These overheated portions of the mantle near the core-mantle boundary become buoyant relative to their surroundings, and begin to rise via diapirism.

This plume of material rises through the mantle. Upon reaching shallower depths within the asthenosphere, decompression melting occurs in the plume head, creating large volumes of magma. The magma rises through the asthenosphere until it reaches the Earth's crust where it causes a hotspot.

The term mini-plumes refers to smaller plumes that may originate in the upper mantle rather than the more common deep mantle plumes. No conclusive examples have yet been identified. One possible example, however, is the Anahim plume at the Anahim hotspot in central British Columbia, Canada.

The most prominent compositional contrast known to exist in the deep (> ~400km) mantle is at the core-mantle boundary. Morgan-type plumes are generally assumed to rise from this layer for two reasons. First, this boundary represents a major thermal discontinuity because the top of the core is much hotter than the base of the mantle. Secondly, the base of the mantle is characterized by the D" layer that is seismically distinct from the overlying mantle. The D" layer appears to be compositionally distinct from the overlying mantle, and seismic tomography of subducted lithosphere suggests that the D" layer may represent the accumulation of these subducted slabs at the base of the mantle.

Very large, broad plumes that spawn a series of smaller plumes in the upper mantle are sometimes referred to as "superplumes". These are usually defined as a plume that has a diameter of at least 1500-3000 km by the time the plume head reaches the upper mantle. A "superplume event" is a short-lived mantle event (100 million years) during which a superplume and the smaller plumes that form from it bombard the base of the lithosphere (Condie et al. (2001)). It is believed that such an event may have occurred in the mid-Cretaceous.

Mantle plumes provide an explanation for intra-plate tectonic volcanism called 'hotspots'. There are several lines of evidence used to support the theory: linear volcanic centers, hotspot fixity, geochemical, noble gas isotopes, and geophysical anomalies.

The apparent linear, age-progressive distribution of the Hawaiian-Emperor seamount chain is explained in this context as a result of a fixed, deep-mantle plume impinging into the upper mantle, partly melting, and causing a "track" as the plate moves with respect to the plume source (Morgan, 1972).

Smaller plumes, arguably called petitspots, are also common within intraplate areas. For instance, tracks of ocean island basalts are found within the Indian Plate, namely the Marshall Islands hotspot.

Continental flood basalt in Oregon and Washington and the Yellowstone caldera-forming event are also used as evidence for mantle plumes, with the voluminous flood basalt envisaged as a product of the vigorous mantle plume head, and the hot 'tail' to the plume driving a progressively younger series of caldera events as the North American continental mass tracks above it.

Smaller series of intracontinental volcanic rocks are also ascribed to small plumes or petitspots. These are notably the Glasshouse Mountains in Queensland (Cohen et al. 2004), which are the oldest Tertiary (25 Ma) members of a progressively younger trend of basaltic and intraplate volcanic cones and plugs culminating in the maars and small peridotitic basalts of the Newer Volcanics in Victoria of 40,000 years ago, far to the southeast.

It is notable that these volcanic features become younger in the same vector as the motion of the Indo-Australian Plate, and matching the trend of the intraplate ocean island basalts in the Indian Ocean.

Relative abundances of osmium isotopes in Hawaiian basalts have also been taken as signatures of plume formation at the core-mantle boundary, with incorporation of some core-derived material. That explanation for the osmium isotope abundances remains controversial (Lassiter, 2006).

Geophysical anomalies associated with hotspots and plumes include thermal, seismic, and geodetic. Thermal anomalies are inherent in the term "hotspot." Thermal anomalies are reflected in high heat flow values at the Earth's surface and excess volcanism. Thermal anomalies also produce anomalies in the travel times of seismic waves.

Seismic anomalies are identified by measuring spatial variations in the time it takes seismic waves to travel through the earth. A fluid body with a lower density (e.g., a hot mantle plume or wetter mantle) exhibits lower seismic velocity compared to surrounding mantle. Observations of regions where seismic waves take longer to arrive are used as evidence for regions of anomalously hot mantle, as is observed underneath Hawaii (Ritsema et al., 1999). Other indicators of plumes would be from the dynamic uplift of the surface (Burov, 2005) and an elevated heat flow.

By deploying a dense network of seismometers and a technique known as seismic tomography, scientists can construct 3-d images of seismic velocities to try and identify vertical plume like structures (Yuan and Dueker, 2005). This is referred to as seismic tomography because it uses techniques similar to medical tomography.

Seismic waves generated by large earthquakes are used to determine structure below the Earth’s surface because they can be detected far from the earthquake epicenter. Far-travelled seismic waves (also called teleseismic waves) are especially useful for seismic tomography because they have steep travel paths that sample smaller longitudinal domains. Density differences between a mantle plume and cooler material that surrounds it enable researchers to distinguish between the two. Seismic waves slow down when they travel through low-density (hotter) material, and speed up when traveling through denser (cooler) material. Density differences may also arise from compositional differences between the plume material and the surrounding mantle.

By analyzing pressure pulses, or P-waves, a group of scientists at Princeton have identified 32 regions throughout the world where P-waves travel slower than average. They conclude that these areas are mantle plumes. The team used analysis of S-waves, another type of seismic wave generated by earthquakes, to determine that those plumes extend to the core-mantle boundary (Montelli et al. 2004).

Geodetic anomalies are reflected in topographic bulges above the plume location, and in positive geoid anomalies. The geoid is a potential surface that reflects the theoretical height to sealevel if mass was distributed uniformly within the Earth. Positive geoid anomalies reflect excess mass associated with uplift and doming over a thermal plume. The Yellowstone plume has a positive geoid anomaly of around +15 meters at its center, and over 1000 km in diameter (Smith & Braile, 1994).

Computer modeling of the mantle plume theory shows that changes of temperature and chemical composition of rising plumes can lead to plumes of varying contours as opposed to the early conceptualization that plumes developed as a homogeneous mushroom shape (Farnetani & Samuel, 2005).

Basalts associated with hotspots or mantle plumes are geochemically distinct from mid-ocean ridge basalts and from lavas associated with island arc volcanoes. In major elements, hotspot basalts are typically higher in iron (Fe) and titanium (Ti) than mid-ocean ridge basalts at similar magnesium (Mg) contents, reflecting their higher temperatures of formation. In trace elements, hotspot basalts are typically more enriched in the light rare earth elements than mid-ocean ridge basalts. Compared to island arc basalts, hotspot basalts are lower in alumina (Al2O3) and much higher in the immobile trace elements (e.g., Ti, Nb, Ta).

The significance of these differences among ocean island basalts (hotspots), mid-ocean ridge basalts, and island arc basalts rests on processes that occur during subduction of oceanic crust and mantle lithosphere. Oceanic crust (and to a lesser extent, the underlying mantle) typically becomes hydrated to varying degrees on the seafloor, partly as the result of seafloor weathering, and partly in response to hydrothermal circulation near the ridge crest. As oceanic crust-lithosphere subduct, water is released by dehydration reactions, along with water-soluble chemical elements and trace elements. This enriched fluid rises to metasomatize the overlying mantle wedge and leads to the formation of island arc basalts. The subducting slab is depleted in these water-mobile elements (e.g., K, Rb, Th, Pb) and thus relatively enriched in elements that are not water-mobile (e.g., Ti, Nb, Ta) compared to both mid-ocean ridge and island arc basalts.

Ocean island basalts, which represent the volcanic product of mantle plumes, are also relatively enriched in the immobile elements relative to the water-mobile elements, leading to the conclusion that subducted oceanic crust plays a major role in their origin.

Two of the most well known locations that fit the mantle plume theory are Hawaii and Iceland as both have volcanic activity. Other island chains that parallel plate motion include the Society Islands (e.g., Tahiti), St Helena-Ascension-Gough, and the Ninety-east ridge (Indian ocean). One of the dormant in Asia that fits the mantle plume theory is Mount Halla(Hallasan) in Jeju island(Jeju-do).

The P-wave and S-wave images show other locations that fit the mantle plume model. Ascension Island and St. Helena appear to originate from the same plume. Similarly, volcanic activity in the Azores and Canary Islands branch from a single trunk.

South of Java and in the Coral Sea, the images show possible formation of future plumes that currently extend only halfway to the surface.

The current debate has stimulated an increased interest in research to distinguish between these models. Recent advances in seismic tomography have enhanced its spatial resolution in both the upper and lower mantle. New seismic tomographs resolve anomalous features consistently within the upper mantle, and in places to the lower mantle (e.g., Montelli et al 2004). It is becoming difficult to explain these data by processes in the uppermost mantle.

To the top



Plume (hydrodynamics)

Plume of the Space Shuttle Atlantis after launch.

In hydrodynamics, a plume is a column of one fluid moving through another. Several effects control the motion of the fluid, including momentum, buoyancy and density difference. When momentum effects are more important than density differences and buoyancy effects, the plume is usually described as a jet.

Usually, as a plume moves away from its source, it widens because of entrainment of the surrounding fluid at its edges. This usually causes a plume which has initially been 'momentum-dominated' to become 'buoyancy-dominated' (this transition is usually predicted by a dimensionless number called the Richardson number).

A further phenomenon of importance is whether a plume is in laminar flow or turbulent flow. Usually there is a transition from laminar to turbulent as the plume moves away from its source. This phenomenon can be clearly seen in the rising column of smoke from a cigarette.

Another phenomenon which can also be seen clearly in the flow of smoke from a cigarette is that the leading-edge of the flow, or the starting-plume, is quite often approximately in the shape of a ring-vortex (smoke ring).

Plumes are of considerable importance in the dispersion of air pollution. A classic work on the subject of air pollution plumes is that by Gary Briggs.

A thermal plume is one which is generated by gas rising from above heat source. The gas rises because thermal expansion makes warm gas less dense than the surrounding cooler gas.

Quite simple modelling will enable many properties of fully-developed, turbulent plumes to be investigated (see eg ).

For a simple rising plume these equations predict that the plume will widen at a constant half-angle of about 6 to 15 degrees.

To the top



Source : Wikipedia