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.

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.