Problems of the Upper Mantle and 
Hawaii as a Site for the Moho Hole 3 
George P. Woollarb 
GEOPHYSICAL DATA PERTAINING 
TO THE MANTLE 
The term "mantle” is a geophysically-derived 
term believed to have geological significance. 
As used, it refers to that material lying between 
the earth’s core at a depth of 2900 km, which 
is also defined geophysically, and that depth at 
which the velocities of seismic compressional 
waves increase from values of about 6. 5-7.0 
km/sec to 8.15 ± 0.5 km/sec. The variations 
in velocity associated with the mantle are of 
both a local and regional nature, and in many 
areas are azimuth dependent. Although the 
identifying velocity of 8.15 ± 0.5 km/sec may 
incorporate some bias because of poor measure- 
ments, it is significant that out of 316 measure- 
ments 223 (71%) lie between 8.0 km/sec and 
8.3 km/sec, and that carefully conducted ex- 
periments show the same range in values as 
does the gross sample of data. Anisotropic seis- 
mic transmission in the mantle is not uncom- 
mon, and can be attributed to selective orienta- 
tion of the mineral grains. It is not to be con- 
fused with boundary slope effects or the effect 
of progressive changes in heat flow or mantle 
composition. Regional variations in mantle 
velocity based on reversed profile recordings 
are related either to variations in mantle compo- 
sition or to heat flow. Local variations in velocity 
may or may not be real and, if real,, are prob- 
ably related to anisotropic transmission. Re- 
gional transmission studies such as those based 
on the GNOME and SALMON underground nu- 
clear explosions, although possibly biased by 
slope effects due to regional changes in crustal 
thickness, clearly indicate that the velocity 
character of the mantle is not uniform beneath 
the United States, and that the western area has 
a subnormal mantle velocity probably because 
of higher heat flow. Similarly, it is found that 
1 Hawaii Institute of Geophysics Contribution No. 
85. 
certain oceanic areas as the Mid-Atlantic Ridge 
and the East Pacific Rise have a subnormal man- 
tle velocity which can be correlated with heat 
flow measurements. That there are true regional 
differences in mantle composition is suggested 
by oceanic data. The average mantle velocity 
found In the Pacific Ocean is significantly 
greater than that found in the Atlantic Ocean, 
and also greater than that found on the conti- 
nents. The respective values are shown in Ta- 
ble 1. 
TABLE 1. 
Average Mantle Velocity 
in Different Regions 
MEDIAN 
RANGE 
REGION 
(km/ sec) 
(km/sec) 
Pacific Ocean 
8.25 
7.65-8.7 
Atlantic Ocean 
8.10 
7.65-8.55 
Continents 
8.12 
7.7 -8.5 
It is to be noted that the lower limit of veloc- 
ity values is much the same in all three areas 
and is associated in most cases with areas 
known or suspected of having high heat flow. 
It is not inconceivable that in these areas there 
may also be mixing or transformations of crust 
and mantle material that would also lower the 
mantle velocity. The high upper limit of 8.7 
km/sec observed in the Pacific Ocean is not 
regarded as significant because it is confined to 
a single measurement. 
That there are variations in mantle material 
with depth is suggested by seismic refraction 
measurements, which show in some areas evi- 
dence for deep layers below the "M” discon- 
tinuity having velocity values in the range 8.5— 
10.0 km/sec. As this deep structure has been 
reported in various parts of the world, it is 
probably real, and in view of the heterogene- 
ous nature of the crust which is well established, 
it is probable that the mantle is also hetero- 
geneous. 
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