The Lithologic Constitution of the Crust and Mantle 
in the Hawaiian Area 1 
Gordon A. Macdonald 
Nearly nothing is known by direct ob- 
servation about the composition of either the 
mantle or the crust below sea level in the 
Hawaiian region. Almost the only information 
about either comes from the interpretation of 
geophysical studies. 
We generally refer to the crust beneath the 
Pacific basin as basaltic but, with the exception 
of a few samples dredged from widely sepa- 
rated localities (Engel and Engel, 1964), which 
are indeed basaltic, all we really know about 
the sub-Pacific crust is the velocity of seismic 
waves in it. 
Above sea level the Hawaiian mountains 
consist largely of tholeiitic basalt, with a minor 
cap of alkalic basalt and related rocks ranging 
from ankaramite to trachyte, and an even 
smaller volume of nephelinites and related 
rocks. Below sea level the rock constitution is 
known only by implication and from a few 
samples dredged from very limited areas. Evi- 
dence suggests considerable isostatic sinking of 
the older of the Hawaiian Islands, and it may 
be presumed that the rocks down to as much 
as 3,000 ft below sea level in the older islands 
are the same as those we see above sea level in 
the younger volcanoes. Other than that, tholei- 
itic basalts have been found by dredging along 
the east rift zones of Kilauea and Mauna Kea 
volcanoes and the south rift zone of Mauna 
Loa, down to a depth of 12,000 ft (J. G. 
Moore, personal communication). 
It has been suggested that the building of 
the volcanoes through the zone from a depth 
of a few thousand feet to sea level involved 
much steam explosion and granulation of the 
lavas in contact with water, producing large 
volumes of basaltic ash and hyaloclastite. Evi- 
dence from submarine photographs and dredg- 
ing on the aforementioned rift zones does not 
1 Hawaii Institute of Geophysics Contribution No. 
86 . 
support this suggestion; but the evidence is 
scanty, and it has been argued that the frag- 
mental material has been removed into deep 
water by currents. Certainly, however, in very 
deep water, explosion is prevented by the re- 
straining pressure of the overlying ocean, 
though hyaloclastite formation might take place. 
The flows should be dense, because the re- 
straining pressure prevents vesicle formation 
unless the gas content is very much greater 
than it is in magmas erupting above sea level. 
This theoretical conclusion is supported by the 
actual decrease in size and abundance of vesi- 
cles in samples of lava dredged from increas- 
ing depths of water. 
All of the actual evidence suggests that the 
lower slopes of the volcanic mountains and the 
adjacent archipelagic aprons, as well as the sea 
floor at greater distances from the islands, con- 
sists of layers of dense tholeiitic basalt. A hole 
through the Moho in the Hawaiian region 
would give us our first real direct knowledge 
of the suboceanic "B layer.” 
Even less is known about the constitution 
of the mantle than about that of the suboceanic 
crust. Hawaiian alkalic basalts and nephelinites 
contain numerous inclusions of ultrabasic rocks, 
predominantly dunite, Iherzolite, and wehrlite. 
Some workers have presumed these to be frag- 
ments derived from the mantle and carried up 
by the rising magma. However, at some locali- 
ties they grade compositionally into gabbro, 
and even into anorthosite; and some show pro- 
nounced compositional* banding closely resem- 
bling, if not identical with, that found in many 
big differentiated basaltic sheets such as the 
Stillwater intrusion in Montana. These are more 
probably derived from consolidated intrusive 
bodies at comparatively shallow depth beneath 
the volcano, well above the mantle boundary. 
Others may be derived from the mantle, though 
the evidence for this is slight and far from con- 
clusive. Garnet pyroxenite ("eclogite”) inclu- 
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