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be generally present, but the lower crust is characterized by a velo-
city of Ö.3-7.2 km/sec, wbich is identified 'with two or three geologi-
cally distinct structures, viz. the original oceanic crust, a dense intru-
sive system of dikes and sills, and a stack of floored magma chambers
filled with ultramafic cmnulates. The higher velocity material ap-
pears to bulge upwards beneath the volcanoes Mauna Kea, Kohala
Mountain and Kilauea.
It appears that the upward bulge in the higher velocity material
under some of the major volcanoes in Hawaii may be caused by the
same process that appears to be working under some of the major
volcanoes in Iceland. However, no subsurface temperature data, that
would be indicative of deeper conditions, appear to be available on
the island of Hawaii, and these would probably be difficult to obtain
due to the permeability of the near-surface formations.
13. CRUSTAL SPREADING AND SEISMIC
STRUCTURE
In most present-day discussions on the origin and the nature of
the oceanic crust, the concept of ocean-floor spreading (Hess, 1962)
is considered to be an established fact or a near-fact. The Vine and
Matthews (1963) theory of the linear magnetic anomalies over the
ocean ridges, the sediment distribution at various distances from the
ridge crests (Ewing and Ewing, 1967), earthquake mechanism stu-
dies (Stkes, 1967) and high heat flow values at the ridge crests
(Lee and Uyeda, 1965) are all considered as supporting evidence
for ocean-floor spreading. Bödvarsson and Walker (1964) have pro-
posed a process of crustal drift by dyke injection, based on observa-
tions in Iceland.
An agreement on the ocean-floor spreading hypothesis is, however,
not general. Einarsson (1968) proposed, mainly on the basis of ob-
servations in Iceland, that the mid-ocean ridges are expressions of
shear faulting in the earth’s crust. The Mid-Atlantic Ridge should
be caused by an Er-W maximum compression with shear planes at
45° to this direction. Hast (1969) has made absolute stress measure-
ments in Iceland and along the eastern margin of the North Atlantic.
He finds compressive horizontal stresses everywhere, and suggests
that the Mid-Atlantic Ridge and the associated fracture zones repre-