Page 113 - Global Tectonics

100 CHAPTER 5

As indicated above, a possible explanation of the however, explain why fractures in the same plate should
origin of island chains was proposed by Wilson (1963). trend in the same direction and develop at similar rates
It was suggested that the islands formed as the litho- (Condie, 1982a).
sphere passed over a hotspot. These hotspots are now Morgan (1971, 1972a) proposed that mantle plumes
thought to originate from mantle plumes rising from remain stationary with respect to each other and the
the lower mantle that thin the overlying lithosphere lower mantle, and are of long duration. If so, the hot-

(Section 12.10). The volcanic rocks are then derived spots represent a fixed frame of reference by which abso-
from pressure-release melting and differentiation within lute motions of plates can be determined (Section 5.4).
the plume. Such plumes represent material of low Between 40 and 50 present day hotspots have been
seismic velocity and can be detected by seismic tomog- suggested (Fig. 5.8) (Duncan & Richards, 1991; Cour-
raphy (Section 2.1.8; Montelli et al., 2004a). Although tillot et al., 2003). It seems unlikely, however, that all of
the mantle plume mechanism has been widely adopted, these centers of intra-plate volcanism, or enhanced
some workers (e.g. Turcotte & Oxburgh, 1978; Pilger, igneous activity at or near ridge crests, are of the same
1982) have questioned the necessity for mantle hotspots type or origin. Many are short-lived, and consequently

and suggest that magmas simply flow to the surface have no tracks reflecting the motion of the plate on

from the asthenosphere through fractures in the litho- which they occur. By contrast, others have persisted for
sphere resulting from intra-plate tensional stresses. This tens of millions of years, in some cases over 100 million
mechanism obviates the problem of maintaining a years, and can be traced back to a major episode of
mantle heat source for long periods. It does not, igneous activity giving rise to flood basalts on land or

Jan Mayen

Iceland

Bowie
Yellowstone
Cobb Azores
Bermuda
Canary
Hawaii New
Socerro
England
Cape
Afar
Verde Caroline
Marquesas Galapagos Fernando
Samoa Ascension Comores
Pitcairn
Society San Felix St Helena
Easter Reunion
‘
Austral Juan Martin Vas Lord
Howe
Fernandez East
Tristan
Marion Australia
Discovery Kerguelen Tasman
Crozet
Louisville
Shona Bouvet
Balleny
Figure 5.8 World-wide distribution of hotspots (modified from Duncan & Richards, 1991, by permission of the

American Geophysical Union. Copyright © 1991 American Geophysical Union).