Environmental Engineering Reference
In-Depth Information
in 1954 to nearly 25,000 by the year 2000, and there is
now a readily accessible global heat flow database (IHFL
2005).
The measurement sites have been concentrated in
Europe, the United States, and Japan, and are relatively
rare in southern oceans. Early results led to a mistaken
conclusion about the near-equality of continental and
oceanic values (Pollack and Chapman 1977). Subsequent
revisions raised the oceanic contributions, and reviews by
Davies (1980); Sclater, Jaupart, and Galson (1980); and
Pollack, Hurter, and Johnson (1993) make it also possi-
ble to offer some detailed subdivisions by region and
crustal age. Terrestrial heat flow generally declines with
crustal age. Where thicker sediments prevent major losses
owing to hydrothermal circulation, the heat flow decays
uniformly from rates in excess of 250 mW/m
2
for ocean
floors younger than 4 Ma to 46 mW/m
2
for those older
than 120 Ma, reaching an equilibrium value of 38 mW/
m
2
after about 200 Ma.
On the continents the time of the last orogenic event,
distribution of heat-producing elements, and the speed
of erosion are the key determinants of heat flow rates.
The youngest regions average 77 mW/m
2
, the oldest
shields (
>
800 Ma) just 44 mW/m
2
, with the non-
radiogenic share declining to a constant rate of 21-25
mW/m
2
after 200-400 Ma. Sclater, Jaupart, and Galson
(1980) calculated the total heat loss through the oceans
at 30.4 TW, through the continental shelves at 2.8 TW,
and through the continents at 8.8 TW, for a grand total
of 42 TW. With 24-38 TW, radioactivity alone could
supply at least 55% and up to 90% of this flux. Pollack,
Hurter, and Johnson (1993) assigned averages of 65
mW/m
2
to the continents (including marine continental
shelves) and 101 mW/m
2
to the ocean, for the planetary
means of 87 mW/m
2
and a grand total of about 44 TW,
of which some 70% is lost through the oceans and 30%
through the continents. Heat flow minima (
<
50 mW/
m
2
) are characteristic of old, thick Precambrian shields;
maxima are associated with the formation of new ocean
floor along the oceanic ridges, particularly in the western
Pacific (Nazca ridge with
>
240 mW/m
2
) and southern
Indian Ocean (
>
180 mW/m
2
).
Aggregate heat release through the ocean floor and
ridges adds up to about 60% of the global heat loss, with
nearly half of the total taking place in the Pacific (fig.
2.8). About 9 TW, one-fifth of the global flow, are trans-
ported by hydrothermal circulation in the oceanic crust,
with 2-4 TW coming from axial flows (Elderfield and
Schultz 1996). Exploratory dives found that relatively
small openings of hydrothermal vents associated with
ocean ridges eject water with temperatures up to 350
C
at rates between 25 MW and 330 MW (10
6
-10
7
W/
m
2
). In nature these levels of heat generation are equaled
or surpassed only by ephemeral volcanic explosions.
Hydrothermal megaplumes are produced on much larger
scales (diameters
>
10 km, rising up to 1 km above the
sea floor) by the cooling of hot (1200
C) pillow basalts
(Palmer and Ernst 1998). Sixty megaplumes would pro-
duce heat loss of 230 GW, equal to less than 10% of the
total hydrothermal heat loss from young (
<
1 Ma) oce-
anic crust.
Global mean flux of less than 90 mW/m
2
is minuscule
in comparison with the mean insolation of about 170
W/m
2
, but when acting over huge areas and across
long time spans it provides much more power than is
needed to prime the processes of global geotectonics
whose understanding was revolutionized by the formula-
tion and maturation of the plate tectonic theory that uni-
fied the earlier ideas of continental drift (Wegener 1924),
sea-floor spreading (Hess 1962), and slab subduction
