Geology Reference
In-Depth Information
1
0.1
0.01
15% eclogite
pyrolite
0.001
0.1
1
10
time (Myr)
Figure 7.16. Rate of melting versus time from high-resolution computations of
melting in plume heads: comparison of melting of normal mantle composition
(pyrolite) and a composition including an additional 15% basaltic (eclogite) com-
ponent. The plumes are grown from a thermal boundary layer and rise through a
mantle viscosity structure that decreases in the upper mantle, as in Figure 7.15.
From Leitch and Davies [76]. Copyright by the American Geophysical Union.
is that the head can generate millions of cubic kilometres of melt within a timescale
of about one million years (Figure 7.16). Thus the plume-head hypothesis seems
to account quantitatively for flood basalt eruptions, as well as for their relationship
with hotspot tracks.
7.5 Irregular volcanism and thermochemical plumes
Not all intra-plate volcanism fits the simple Hawaiian pattern of an age-progressive
line. Two examples from the Pacific basin are shown in Figure 7.17. Nonlinear
ridges, splayed sets of ridges, multiple phases of volcanism and no clear age
progression are among the features from this region. Some caution is appropriate
in drawing conclusions from this, because of possible complications in hotspot
tracks. For example, the lack of a clear age progression in a volcanic ridge or chain
might reflect overprinting of separate volcanic episodes, jumps of adjacent ridge
locations (as is evident with the Ninety East Ridge), complications engendered
by continental crust, and slow motion combined with large erupted volumes, as
in the case of Iceland. Nevertheless, the complicated morphologies evident in
Figure 7.17 make a strong case for something other than a classical thermal plume
as a source.
Nor do all hotspot tracks connect with a flood basalt province, as would be
expected from the head-and-tail structure of plumes. Clouard and Bonneville [78]
argue that, of 14 identified hotspots in the Pacific, only four have plausible or possi-
ble connections with an oceanic plateau that might be the product of a plume head.
