Page 63 - Fundamentals of Physical Volcanology

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40 CHAPTER 3

crust to a height z and held open by a pressure
0 1000 magma
in its source region in excess of the local lithostatic
km pressure by an amount ∆P is given by
d
W = [(1 − ν)/µ]z magma {∆P d
+ [z magma g (ρ magma − ρ crust )/π]} (3.5)
North
America
where ν and µ are Poisson’s ratio and shear
modulus, respectively, two of the elastic propert-
ies of the host rocks. ∆P is the pressure at the inlet
d
to the dike at the base of the crust, and so the buoy-
ancy of all of the vertical extent H of the plume
Africa
head providing magma contributes to this pressure,
i.e., in this case

∆P =∆ρ gH (3.6)
d m
Table 3.2 shows the average widths of the dikes
that would contain the magma columns listed in
Table 3.1; the range, of order 100–200 m, seems
to be quite consistent with the field observations.
The great widths of these dikes means that magma
South America
may flow through them very easily at relatively high
speed. This is probably directly linked to the forma-
Fig. 3.6 A reconstruction of the North American, South tion of very large volume flood basalt flow fields
American and African plates as the Atlantic was opening. on the continents and large igneous provinces on
Giant dike swarms radiated out from a hot spot, the position
the ocean floors.
of which is indicated by the star. Although they are present
on now widely separated continents, these dikes have
essentially identical compositions and emplacement ages,
confirming their common origin before the Atlantic formed. 3.7 Summary

(Adapted from fig. 6 published in Earth Science Reviews,
Vol. 39, Ernst, R.E., Head, J.W., Parfitt, E.A., Grosfils, E. & • When a new convection cell forms in the mantle,
Wilson, L., Giant radiating dike swarms on Earth and Venus, its rising upper part, a mantle plume, undergoes
1–58, copyright Elsevier (1995).)
partial melting due to the pressure reduction, and
the resulting body of low-density melt mixed with
only simple explanation of the sizes and geometr- the residual solids constitutes a buoyant diapir
ies of these dikes is that they grow from very large rising through the surrounding mantle rocks.
reservoirs of magma that have been extracted from These rocks, although solid, act as a very viscous
the mantle but then trapped at or near the base of fluid on the long time scales and slow defor-
the crust. Detailed measurements of the fabric and mation rates involved.
magnetization of the dike rocks show that growth is • In a similar way, melting due to the temperature
mainly upward immediately above and near to the rise as wet sediments and ocean floor rocks are
reservoir, and mainly lateral in the more distant carried down in a subduction zone can also
parts of the swarm. create a region of lower-density partial melt that
Are the widths of these giant dikes consistent then forms a slowly rising diapir. Subduction
with what we would expect? The average width W zone diapirs are expected to be much smaller
of a dike extending from the base of the continental than mantle plume diapirs.