EVIDENCE island at 0.013 /a. Saunders points out

EVIDENCE OF PLUMES

The first paper was
the comprehensive review by Ian Campbell
of the theory of mantle plumes. Review shows that mantle plume theory makes
numerous testable predictions which are both quantitative and exacting. The
hypothesis correctly predicts that; rapid initial volcanism from the plume head
is followed by reduced volcanism from the plume tail, the size of the flattened
plume head in the upper mantle is 2000 to 2500 km across, flood volcanism
preceded by domal uplift. Campbell
concludes the excellent agreement between the predictions made from plume
theory and the observations made from large igneous provinces and hotspot
tracks leaves little room for doubt that the plume hypothesis is correct.

Michael Story et al. present new /  ages for the early tertiary lavas of East
Greenland and the Faeroe Islands (part of the North Atlantic Igneous Provinces)
and used these with published data to estimate melt production rates leading up
to, during and following continental break-up between Greenland and Europe. The
authors show that following commencement of volcanism at ?
61 Ma, volcanic activity was irregular until 55.7 ± 0.5 Ma when the average
melt production rate increased by more than an order of magnitude (from <100 to >3000  /km
of rift/Ma). This increase coincided with continental rifting between Europe
and Greenland and shallow decompression melting of hot mantle. It is concluded
that a start-up mantle plume head and tail model, with moderate excess
temperature (?T?100 °C) and active upwelling, best
explains the time, spatial and compositional aspects of volcanism in this
province.

Saunders et al (2003) cite Hawaii in
support of the model, In Hawaii despite the very high lithostatic pressures
found at 70-80 km the plume heat source was able to produce melting. The basalt
production rate at Hawaii currently is 0.13 /a, which is lower than an average production
during lifetime of the island at 0.013 /a. Saunders
points out that at a modern mid ocean ridge where asthenosphere is within few
kilometers of the earth surface with melting rates at 104 km long, 1 km wide
and 1cm/a spreading ridge produce magma at an average rate of 1.2 /a (Mc
Lean M.D. 1996). This average volume of melt reaching the surface of Hawaii 

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