Heat - The Solid Earth - page 288

Figure 7.6. Observed heat flows for the Atlantic, Indian and Pacific Oceans. Heat
flow predicted by the plate models: solid line, GDH1; and dashed line, PSM. Heat
flow predicted by the half-space model HS is not shown - it is almost coincident with
PSM (Table 7.5). (After Stein and Stein, Constraints on hydrothermal heat flux
through the oceanic lithosphere from global heat data, J. Geophys. Res ., 99 ,
3881-95, 1994. Copyright 1994 American Geophysical Union. Modified by
permission of American Geophysical Union.)

plate moves away from the ridge, sedimentation occurs. Deep-sea sediments have
alow permeability 2 and, in sufficient thickness, are impermeable to sea water. In
well-sedimented and therefore generally older crust, measurements of conductive
heat flow yield reliable estimates of the actual heat flow. Another important factor
affecting cessation of hydrothermal circulation is that, in older crust, pores and
cracks will become plugged with mineral deposits. As a result, hydrothermal
circulation will largely cease. Loss of heat due to hydrothermal circulation is
difficult to measure, so heat-flow estimates for young crust are generally very
scattered and also significantly lower than the theoretical estimates of heat loss
(Fig. 7.6). That heat-flow measurements are generally less than the predicted
values for oceanic lithosphere younger than 65 ± 10 Ma indicates that this is the

'sealing age'.
As an oceanic plate moves away from the ridge axis and cools, it contracts and
thus increases in density. If we assume the oceanic regions to be compensated
(see Section 5.5.2), the depth of the oceans should increase with increasing age
(and thus plate density). For any model of the cooling lithosphere, the expected
ocean depth can be calculated simply (see Section 7.5.2).
Figure 7.7(a) shows the mean depth of the oceans plotted against age. For
ages less than 20 Ma a simple relation between bathymetric depth d (km) and
lithosphere age t (Ma) is observed:

d = 2 . 6 + 0 . 365 t 1 / 2 (7.57a)

Depth increases linearly with the square root of age. For ages greater than 20 Ma
this simple relation does not hold; depth increases more slowly with increasing

2

Permeability and porosity are not the same. Sediments have a higher porosity than that of crustal
rocks but lack the connected pore spaces needed for high permeability.