Geology Reference
In-Depth Information
10
−
5
◦
C
−
1
. Because of thermal entrainment, the plume head temperature
may be only 200
◦
C higher than ambient mantle. Then
ρ
will be 16 kg/m
3
.A
plume head with a radius of 500 km will then rise at 4 cm/yr.
This rise velocity is quite comparable to plate velocities. In the upper mantle it
will increase, as the viscosity is lower, but for much of its rise through the mantle
this is a plausible velocity. At this rate it would take 50 Myr to rise 2000 km, which
would place its highest point near the top of the mantle.
The velocity of flow up the plume conduit that we estimated in Section 7.2.3 is
considerably higher than this, about 1 m/yr. This justifies the statements that the
plume head will grow as more material flows up the plume tail, as well as from
thermal entrainment.
about 2
×
7.3.5 Thermal entrainment into a plume head
The rate of thermal entrainment into a plume head was calculated by Griffiths and
Campbell [68], using a thermal boundary layer approach. A summary of that theory
is given in
Dynamic Earth
[1] and will not be repeated here, though it is similar
in style to the other analyses in this topic. The growth of a plume head is due to
two things, thermal entrainment of ambient mantle and continued flow of plume
material up the plume tail. Taking both of these into account requires a numerical
calculation. Results from such calculations by Griffiths and Campbell are given in
Figure 7.12.
The thick curves in Figure 7.12(a) are for a viscosity of 10
22
Pa s, appropriate
to the present mid-mantle. They show that the diameter at the top of the mantle is
surprisingly insensitive to the plume buoyancy flow. Recalling that the estimated
Hawaiian flow is 7
10
4
N/s and that other plumes are weaker, we can see that
diameters of about 1000 km at the top of the mantle are predicted. The thin curve
in Figure 7.12(a) is for a mantle viscosity of 10
21
Pa s, which might have applied
during the Archaean, and a smaller head is predicted, with a diameter of about
600 km. Figure 7.12(b) shows that the final head diameter is also insensitive to the
excess temperature.
×
7.4 Plume heads and flood basalt eruptions
In 1981, Morgan [69] pointed out that several hotspot tracks emerged from flood
basalt provinces. A notable example is the Chagos-Laccadive Ridge running south
from the Deccan Traps flood basalt province in western India to Reunion Island in
the Indian Ocean (Figures 2.4 and 7.13).
Flood basalts are the largest volcanic eruptions identified in the geological
record. They range up to 2000 km across, with accumulated thicknesses of basalt
