Heat - The Solid Earth - page 312

period of prograde metamorphism throughout the pile, with the removal of water
to higher levels during recrystallization. Finally, partial melting takes place at
the base of the pile, and the radioactive heat production is redistributed until
thermal stability is reached. This upward redistribution of the radioactive ele-
ments is an episodic process. When a partial melt forms, it tends to be rich in
potassium, thorium and uranium. Thus, over time this process of melting and
intrusion effectively scours the deep crust of heat-producing elements and leads
to their concentration in shallow-level intrusions (which would be recognized as
'late' or 'post-tectonic') and in pegmatites. Eventually, after erosion, they tend to
be concentrated in sediments and sea water. The net effect is a marked concen-
tration of heat production in the upper crust; whatever the initial distribution of
heat production, this leads to the stabilization of the rock pile to a non-melting
equilibrium.
7.8.3 Erosion and deposition
Erosion and deposition are two processes that are able to change a geotherm
rapidly. They are also interesting to geologists because no sedimentary rock can
exist without deposition; neither can any metamorphic rock become exposed at
today's surface without erosion. Erosion represents the solar input to the geolog-
ical machine, and the volcanism and deformation that provide the material to be
eroded are driven from the interior. Figure 7.22(a) shows the effect of eroding the
model rock column of Fig. 7.3(a) at a rate of 1 km Ma − 1 for 25 Ma. The shallow-
level geotherm is raised to 50 ◦ Ckm − 1 after the 25 Ma of erosion, after which
it slowly relaxes towards the new equilibrium. If, instead of erosion, the model
column is subject to sedimentation, then the shallow-level geotherm is depressed.
Sedimentation at 0.5 km Ma − 1 for 25 Ma depresses the shallow-level geotherm
to about 23 ◦ Ckm − 1 (Fig. 7.22(b)). After sedimentation ceases, the temperatures
slowly relax towards the new equilibrium.
Alternatively, instead of considering the effects of erosion and deposition on
the geotherm, we could trace the temperature history of a particular rock (e.g.,
the rock originally at 30 km depth). In the erosion example, the temperature of
this rock decreases, dropping some 500 ◦ C during erosion and a further 200 ◦ C
during the re-equilibration. In the depositional example, the temperature of this
rock is not affected much during the deposition, but it increases some 400 ◦ C
during the subsequent slow re-equilibration.
7.8.4 Erosional models: the development of a
metamorphic geotherm
Erosion is essential in the formation of a metamorphic belt since without it no
metamorphic rocks would be exposed at the surface. However, as illustrated
with the simple one-dimensional model, the process of erosion itself has a