Many geolists attribute hot spots and volcanos to plumes arising vertically straight-up directly from the outer core. This is the predominant theory: but still hotly debated. In any case, what has been this ridiculous two-page fuss about magma not being molten mantle (-or molten crust)? And all these useless mudslinging posts above: challenges, snipey little comments, condescending remarks - void of any content. This is an embarrassment to sciforum and a regression of progress in science. The content of this forum has gone far adrift from the subject of the original posting. My own posting here belongs on a new thread called "What are Plumes: the Debate" or "Are Plumes the Origin of Hot Spots and some Volcanos?"
The fact that magma near the Earth's surface does not consist of 90% iron and some nickel is proof in itself that that molten mantle material does not directly flow straight-up as a fluid from the outer core. If there are such straight-up plumes, they would be more like segregated molten chambers, or vertical heat convection currents, but by the time the heat is convected to the surface, the magma is dilluted molten mantle. I am very sceptical of all this, even though it was what I was taught many moons ago. Therefore, I am more inclined to agree with you. Nevertheless, the majority views:
"Plume: an upwelling of molten material from the Earth's mantle."
Source:
http://www.thefreedictionary.com/plume
"Plume theory: The fluid mechanics of low-Reynolds-number plumes is well understood. A mantle starting plume is predicted to consist of a large spherical head followed by a small tail. When it reaches the top of its ascent the head flattens to form a disk with a diameter twice that of the spherical head. Plume heads have been proposed to be responsible for Earth's major flood basalts and plume tails for aseismic ridges and island chains that connect the flood basalt to the current position of the hot spot. Plume theory makes the following testable predictions:
1) plume heads have a surface expression up to 2,500 km across,
2) lood volcanism is preceded by 1000 ± 500 m of uplift,
3) temperatures within the plume are 100 to 300 ?C higher than the adjacent mantle,
4) plume tails extend to the core-mantle boundary.
Source:
http://www.agu.org/meetings/cc05gcall.html
"A mantle plume is an upwelling of anomalously hot rock in the Earth's (or another planet's) mantle. Mantle plumes are thought to be the cause of volcanic centers known as hotspots and probably also have caused flood basalts. 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. Plumes of hotter-than-average material rise through the mantle till it reaches the Earth's crust where it causes a hotspot. The plumes originate at a thermal boundary layer at the core-mantle boundary. Also, a "superplume" is the term for a larger-scale plume. It is usually defined as a plume that has a diameter of at least 1500-3000 km by the time the plume head spreads at the base of the lithosphere."
http://en.wikipedia.org/wiki/Magma_plume
Melting anomalies can result from upwelling jets – plumes...The usual explanation for melting anomalies is that they result from active hot upwellings from a deep thermal boundary layer.
http://www.mantleplumes.org/PlumeDLA.html
Hypothetical View of Outer Core to Upper Lithoshere Plumes:
"Plumes are thought to originate very deep in the Earth at the core-mantle boundary for the larger ones and at a depth of about 600 km deep for the smaller ones - but they seem to be related to the breakup of continents (rifting), so there is some influence from global plate tectonic processes."
"This may represent a typical pattern of plume distribution at any period of geological time....The plumes are proposed to be richer in lighter elements and hotter than the surrounding mantle. As they rise, magma (liquid rock) is generated by partial melting of the plume material. The magma is injected into the lithosphere and erupted onto the Earth's surface to form huge basalt lava flows."
Source:
http://www.geolsoc.org.uk/template.cfm?name=fbasalts
"All upward movement of mantle material is confined to about 20 plumes, each plume a few hundred kilometers in diameter, rising from the core–mantle boundary. The plume effectively burns a hole through the overlying crustal plate .. and a volcano results." Source: “Volcanoes,” by P. Francis, 1976.
"A mantle plume is a localised, roughly axisymmetric upwelling of buoyant rock, originating from a boundary layer deep within the Earth...Rock, plus melt at higher levels. Unspecified – 670 km or core-mantle boundary are likely candidates."
Source: “Plumes and Plumage,” by Andy Saunders, Symposium of the Herdman Society, Dept. Earth Sciences, Liverpool University, March 2004.
"Plume: an upward flow of hot material from the Earth's mantle into the crust.”
Source: "Geology: An Introduction to Physical Geology," by Chernicoff, C. and R. Venkatakrishnan, 1995, p. 593.
"Mantle Plume: a buoyant mass of hot mantle material that rises to the base of the lithosphere. Mantle plumes commonly produce volcanic activity and structural deformation in the central part of lithospheric plates."
Source: "The Earth's Dynamic Systems," W.K. Hamblin, 1989, p. 555.
"Plume: a persistent pipelike body of hot material moving upward from the Earth's mantle into the crust. Its surface expression may be a hot spot.”
Source: "Dictionary of Geological Terms," by Bates R.L. and J.A. Jackson 1984, p. 391.
"Plume: A column of magma rising from the lower mantle and spreading sideways on reaching the base of the lithosphere, proposed as an explanation of the motion of lithospheric plates and of sites of volcanic activity away from plate margins"
Source: Oxford Educational Dictionary
"A solid-state, narrow upwelling current in the mantle with a diameter of the order of 100 km and originating from a hot, low-density boundary layer located either above the seismic discontinuity at 660 km depth or near the core-mantle boundary at 2,900 km depth." Source: Hoffmann, 1997.
"A cactolith is an irregular intrusive igneous body of obscurely cactuslike form, more or less confined to a horizontal zone and appearing to consist of irregularly related and possibly distorted branching and anastamosing dikes that fed a laccolith.
Source: " Glossary of Geology," Erik Lundin, 23rd December, 2003
http://www.mantleplumes.org/DefinitionOfAPlume.html