394 
PACIFIC SCIENCE, Vol. XIX, July 1965 
Cooling 
FIG. 1. Pyromagnetization curve established experi- 
mentally for the EM 7 basalt. 
to us two primary experiments: first, to see if 
the overlying sediment had been heated suffi- 
ciently to acquire a thermo-remanent magneti- 
zation; and, second, to determine whether this 
thermo-remanent magnetization agreed in di- 
rection with that of the basalt. If it did so, 
this would be strong evidence that the whole 
event had taken place during the period when 
the main magnetic field of the earth was re- 
versed in polarity from the sense that it has 
today. 
A second study would be to extend the 
examination for thermo-remanence upward 
from the contact to see the manner in which 
it diminished with distance. If intrusion had 
occurred under a thick overburden, one could 
expect the thermo-remanent magnetization to 
give way to a partial thermo-remanent mag- 
netization and the total effect to diminish in 
an exponential manner. On the other hand, if 
intrusion had occurred under a very thin over- 
burden of sediment, then the heat pulse from 
the basalt might well have broken through to 
the sea floor and been dissipated before any 
appreciable thickness of sediment had been 
further deposited in the area. In this case, the 
upward change of thermal magnetizations 
would show a discontinuity, diminishing sud- 
denly to the level common for sedimentary 
type magnetizations. The time scale over which 
such a cooling might take place and the extent 
to which temperature isotherms might extend 
from the basalt could be judged from studies 
made earlier by Jaeger (1957). Figure 2 is a 
diagram prepared from such considerations for 
the case where the overburden equals the thick- 
ness of an intruded sill It may be seen from 
this that the 600° isotherm would extend out- 
ward to distances of about 20 per cent of the 
total thickness of the sill. Lower isotherms which 
could still produce an appreciable partial thermo- 
remanence might extend out much farther, in 
excess of half the thickness of the sill. The 
cooling period would extend over several hun- 
dreds of years. 
Studies of the sedimentary rock core above 
the basalt have been hampered for two reasons. 
First, recovery was very incomplete, particu- 
larly in the drill hole which penetrated through 
the basalt (EM 7). Second, the material was 
rich in calcareous ooze and had abnormally low 
magnetic properties as compared with other 
abyssal sediment. The material placed at our 
disposal for study was largely confined to Run 3 
of the EM 7 drilling. Within this section we 
had 13 samples ranging from distances of 32 
cm above the basalt upwards to a distance of 
114 cm. The 6 lowest samples which ranged 
to distances of 58 cm from the contact showed 
a positive inclination. The 6 samples overlying 
this, ranging to a height of 113 cm, had a 
negative inclination paralleling quite closely 
that found in the basalt. One positive inclina- 
FlG. 2. Change of isothermal lines with time dur- 
ing the cooling of a thick intrusive body. 
