The rifts of the 14 basaltic shield volcanoes that extend from Kauai to Hawaii are composed of thousands of dykes that were fed laterally by periodic leakages from central volcanic con­duits. Individual dykes are believed to be thin, steeply dipping blades, several kilometres from top to bottom, that extend horizontally outward for as much as 120 km. The dykes are contained largely within the volcanic edifices, and, because of such shallow emplacement, the direction of dyke propagation is interpreted to be strongly influenced by the gravitational stresses within these edifices. Simple isolated shields, such as Kauai and West Molokai, had nearly symmetrical stress fields, influenced only slightly by regional stresses, and the dykes injected into these volcanoes had little or no tendency to cluster into well-defined rifts. Other volcanoes, such as Koolau and Kilauea, pierced the thick, sloping apron of pre-existing neigh­bour volcanoes. The dykes that propagated from these centres were strongly influenced by the gravitational stress fields of the sloping aprons in which they grew. Accordingly, they clustered into well-defined rifts oriented roughly perpendicular to the downslope direction of these aprons. With only minor exceptions, 8 of the 14 volcanoes forming the southeast part of the Hawaiian chain conformed to this pattern of growth; the influence of regional Pacific structure on rift orientation is suspected in only 6 volcanoes that grew as simple, iso­lated shields, away from the influence of gravitational stresses of any neighbour volcano.

Automorphic-poikilitic texture,.. _______________-47 Hypautomorphic texture ____________________ _ 4 7 Xenomorphic texture_~ ___ ~ _________ ~ _______ _ 50 Development of the Ultramafic zone __ ~ ______________-50 Conditions of crystallization and deposition ________-52 Predepositional conditions ___________________-52 The magma ____________- .-_____________-54 Primary crystallization products _________-54 Depositional conditions _______ ~ ___ .-_________-54 Agents of transportation and deposition __-Origin of the layering_ ~

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[1973] proposed that there are as many as 150 terrestrial plumes, and Wilson and Blurke [1973] classified plumes into several categories. We wish to take a conservative stance regarding the plume hypothesis, and we introduce the term 'melting anomaly' in its place to emphasize that unusual volcanic activity may ensue from a variety of processes. We also wish to encourage the idea that individual melting anomalies may arise from different dynamic situations. Indeed, considerable attention has been given to other We plan to extend the mechanism proposed by Shaw [1973] for the origin of the Hawaiian melting anomaly. Shaw proposed that the zone in which plate motion is decoupled from the deeper mantle contains variable gradients of horizontal flow velocities that depend intrinsically on the distribution of a variable melt fraction. The existence of episodic variations in 8634