Geology Reference
In-Depth Information
2000
3000
(b)
(a)
1800
10
4
10
4
10
3
1600
T
s
= 200
°
C
1400
2000
1200
400
°
C
1000
800
800
°
C
1000
600
400
Q
b
= 10
5
N/s
200
0
0
200
400
600
800
1000
1200
10
3
10
4
10
5
diameter of head (km)
buoyancy flux (N/s)
Figure 7.12. (a) Predicted plume head diameter versus height risen in a mantle of
viscosity 10
22
Pa s (thick lines) and 10
21
Pa s (thin line). Curves are labelled with
buoyancy flow rate
Q
b
. (b) Predicted plume head diameter at the top of the mantle
for a mantle viscosity of 10
22
Pa s and a range of values of the buoyancy flow rate
in the plume tail and fluid excess temperature
T
s
. From Griffiths and Campbell
[68]. Copyright by Elsevier Science. Reprinted with permission.
flows up to several kilometres. A map of the main identified flood basalt provinces
is shown in Figure 7.13. Total volumes of extrusive eruptions range up to 10
million cubic kilometres, and evidence is accumulating that much of this volume
is erupted in less than one million years [70]. It has also been recognised that some
oceanic plateaus are oceanic equivalents of continental flood basalts. The largest
flood basalt province is the Ontong-Java Plateau, a submarine plateau east of New
Guinea.
Morgan proposed that, if flood basalts and hotspot tracks are associated, then the
head-and-tail structure of a new plume, which had been demonstrated by Whitehead
and Luther [67], would provide an explanation. Figure 7.14 illustrates the concept.
The flood basalt eruption would be due to the arrival of the plume head under the
lithosphere, and the hotspot track would be formed by the tail following the head.
If the overlying plate is moving, then the flood basalt and the underlying head
remnant would be carried away, and the hotspot track would emerge from the flood
basalt province, and connect it to the currently active volcanic centre, which would
be underlain by the active plume tail.
Not a lot of attention was given to Morgan's proposal until Richards, Duncan and
Courtillot [71] revived and advocated the idea. Subsequently Griffiths and Campbell
[68] demonstrated the thermal entrainment process and argued in more detail for
