PLUME the size of plume head and the nature

PLUME MODEL

Plume origin is
unknown but it probably originates at the core of the mantle boundary where
there is a variation in topography of a few hundred meters and where the
convecting liquid core is losing heat by conduction. This heat source is
believed to be the cause of partial melting and could be the source of the
plume.

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Plumes are thought
to have long surviving period for 10’s of millions of years. During their ascent,
they are able to cross the endothermic 670 km discontinuity. Courtillot et al. (2003) suggested plume
arousal by three different sources in the mantle. One type originates in the
lower mantle based on a chemical heterogeneity in the D” layer. Another type
originates at the base of the transition zones which correspond to super
swells. A third type originates in the upper mantle. Courtillot’s model played its role in explaining the diverse
morphologies exhibited by plume related volcanism. 

Many of the
predictions, such as the size of plume head and the nature of the associated
uplift, are unambiguous. Mantle plume theory basically depends on the accuracy
of its predictions. These predictions are as follows;

·         
New
plumes consist of a large head followed by a narrower tail;

·         
Plume
heads should flatten to form a disk 2000 to 2500 km in diameter at the top of
their ascent;

·         
Plume
tail should have a diameter of 100 to 200 km in the upper mantle;

·         
Plume
must originate from a hot boundary layer, probably the core-mantle boundary;

·         
Both
heads and tails should produce high-temperature magmas;

·         
Temperature
excess of the plume head is predicted to be appreciably hotter at the center of
head than at the margin.

Out of these
predictions the most obvious one is, plumes must be hotter than the adjacent
mantle. The temperature of plumes can be obtained from the thickness of the
oceanic crust produced when spreading centers overlies a plume, using the
method of McKenzie & Bickle (1988)
to calculate the temperature of the underlying mantle. Normally the oceanic
curst thickness is 7 ±1 km. The thickness of oceanic crust beneath Iceland is
40km and 25 km below the Walvis Rdge, requiring the temperature of the mantle
under these ridges to be 250° C and 200° C respectively hotter than MORB mantle.

The most exacting
prediction of the plume theory are that plume heads flatten to form disks
2000-2500 km in diameter when they reach at the top surface. The size of the
flattened plume head can be obtained from the thickness of the oceanic crust.

Plumes are thought to
be the initiating mechanism    for the
early stages of continental breakup

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