Geoscience Reference
In-Depth Information
Chapter 26
Terrestrial heat flow
During the thirty-five years which
have passed since I gave this
wide-ranged estimate [of 20--400
million years] experimental
investigation has supplied much of the
knowledge then wanting regarding
the thermal properties of rocks to
form a closer estimate of the time
which has passed since the
consolidation of the earth, we have
now good reason for judging that it
was more than 20,000,000 and less
than 40,000,000 years ago, and
probably much nearer 20 than 40.
Lord Kelvin
pre-existing TBL. The cooling of the mantle is
mainly accomplished by the cooling of the sur-
face plates. In early Earth history a transient
magma ocean allowed magmas to transfer their
heat directly to the atmosphere. As buoyant mate-
rial collected at the top, the partially molten inte-
rior became isolated from the surface. Magma,
however, could break through a possibly thick
buoyant layer and create 'heat pipes' to carry
magma and heat to the surface. Io, Venus and
early Mars are objects that may utilize this mech-
anism of heat transfer. It is also an alternative to
plate tectonics on early Earth. The surface bound-
ary condition in these cases can be viewed as
a permeable plate. Intrusion affects the topog-
raphy and heat flow, making these parameters
non-unique functions of age.
The Earth's interior is cooling off by a combi-
nation of thermal conduction -- and intrusion --
through the surface boundary layer and the deliv-
ery of cold material to the interior by slabs, a
form of advection. An unknown amount of heat
is transferred to the surface by hydrothermal
circulation. The delamination of the bottom of
over-thickened crust also cools off the underly-
ing mantle. The heat generated in the interior
of the Earth, integrated over some delay time, is
transferred to the surface conduction boundary
layer by a combination of solid-state convection,
fluid flow, radiation and conduction. The con-
ducted heat through the surface TBL (there may
be deeper ones as well) can be decomposed into
a steady-state (or declining) background term, a
transient term, and a crustal contribution. In
Heat losses
The nature of the surface boundary condition
of the mantle changes with time. Currently, the
mantle has a
conduction boundary layer
with a thickness that averages 100--200 km. The
boundary layer is assumed to start out at zero
thickness at volcanic ridges; it is pierced in places
by volcanoes that deliver a small fraction of the
Earth's heat to the surface via magma, and it
may be invaded at greater depths by sills and
dikes that affect the bathymetry and heat flow.
Ridges also jump around, migrate or start on a
