Environmental Engineering Reference
In-Depth Information
2.8 (a) Normal geothermal gradient (Sleep 2005).
(b) Earth's heat flow map (based on Pollack and
Chapman 1977).
(97 nW/g for
235
U), but experimental evidence suggests
that it could be a substantial radioactive heat source not
only in the crust but also in the core (Murthy, van West-
renen, and Fei 2003).
The geothermal gradient in the Earth's upper crust
is typically 25
C-30
C/km; it decreases with depth
through the middle and lower crust, and it is difficult to
extrapolate into the mantle (fig. 2.8) (Sleep 2005). Its
variability, as demonstrated by hot springs and volcanic
regions, is large. Measurements of heat flow conducted
through the rocky crust (gradients are established by tak-
ing temperatures at intervals in bore holes on land and by
using hollow probes in the soft sediments at the ocean
bottom) began only in 1939 on land and in 1952 in the
ocean. The total number of measurements rose from 47
2.5 Geoenergetics: Heat, Plate Tectonics,
Volcanoes, Earthquakes
A celestial body without any internal heat has its surface
shaped only by external forces; the Earth's grand features
are sculpted primarily by a heat flux whose power density
is 3 OM lower than the mean insolation but whose inces-
sant action is refashioning the ocean floor, breaking up
and reassembling continents, creating ocean ridges and
mountain ranges, and generating earthquakes, volcanic
eruptions, and tsunamis. The planet's early thermal his-
tory remains speculative, and there are also major uncer-
tainties regarding the relative contributions of basal
cooling of the Earth's core and of heat-producing iso-
topes of
235
U,
238
U, and
232
Th. The role of
40
K is par-
ticularly intriguing: its decay generates a mere 3.6 pW/g
