Mantle plumes are a geological phenomenon originally proposed by W. Jason Morgan in 1971. He described them as hot upwellings of relatively primordial material which rise from the deep mantle of the Earth and feed geological hotspots on the surface such as Iceland and Hawaii. Such plumes rise because of thermal buoyancy and must originate at a thermal boundary layer. The only such layer known to exist in the deep mantle is the core-mantle boundary (D"), and thus Morgan-type plumes are generally assumed to rise from this layer.
This theory has been popular for over 30 years. However, irrefutable evidence for such plumes has still not been confirmed, and contrary, or unexpected observations are often reported. On the other hand, low-wave-speed seismic anomalies with different shapes, e.g., shallow, or very wide bodies have been found. Such observations led to diversification of the range of features that scientists call plumes. A clear, widely accepted definition of a plume thus does not currently exist, and the word plume is often used by different scientists to mean different things. Furthermore, it has become essentially impossible to disprove the plume hypothesis because the term plume has become vague and all-embracing. A vigorous global debate has developed concerning whether plumes exist at all.
In a 2004 paper, Don L. Anderson and James H. Natland write:
"Unfortunately, the terms hotspot and plume have become confused. In recent literature the terms are used interchangeably. A plume is a hypothetical mantle feature. A hotspot is a region of magmatism or elevation that has been deemed to be anomalous in some respect because of its volume or location. In the plume hypothesis, a hotspot is the surface manifestation of a plume, but the concepts are different; one is the presumed effect, and the other is the cause."
Magma is a complex high-temperature (between 650 and 1200 °C) silicate solution that is ancestral to all igneous rocks.
Magma rises primarily because a melt is less dense than its source rock, it is propelled upward through the lithosphere by the buoyancy that its lower density creates (the way less dense wood is pushed up and floats in denser water).
Magma is composed mainly of silica; alkalis (sodium, potassium, calcium, magnesium) and iron.
Oxide-rich and sulfide-rich magmas are inferred from textural and structural evidence of fluidity as well as mineralogical evidence of high temperature, together with the results of experiments on the equilibrium relations of melts and crystals.
Magma is presumed to underlie regions of active volcanism and to occupy volumes comparable in size and shape to plutons of eroded igneous rocks.
Magma migrates either at depth or to the Earth's surface, where it is ejected as lava.
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