Geology Reference
In-Depth Information
12
Heat Flow and Thermodynamics
of the Lithosphere
Abstract
In this chapter, I present the important theme of heat conduction across
and within the continental and oceanic lithospheres. In particular, the non-
steady state heat conduction equation is solved and applied to the cooling
of the oceanic lithosphere. The chapter also describes the main forces
driving plate tectonics: ridge push and slab pull.
motion of energy carriers. In this instance, heat
is conveyed from one region to another together
with the carriers, rather than being transferred
between energy carriers. Clearly, convection and
conduction may operate at the same time within a
fluid. Finally, heat can be transported by electro-
magnetic
radiation
. This is a process that occurs
at the top of the Earth's atmosphere, thereby it
will not be investigated in this topic.
The distribution of temperatures in the con-
tinental and oceanic lithosphere is largely con-
trolled by the conductive loss of heat at the
Earth's surface, although convective heat trans-
port by water circulating through the oceanic
basalts or intrusive igneous bodies may be locally
an important mechanism of cooling for these
rocks. This heat originates both by the secular
cooling of the Earth's hot interior and by the ra-
dioactive decay of some elements that are present
in crustal and mantle rocks.
The fundamental equation describing the con-
ductive heat transport is known as
Fourier
'
slaw
.
If we define the
heat flux q
(
n
) as the quantity
of heat per unit area and per unit time that
flows by conductive transport through a small
12.1
Fourier's Law
In this chapter, we are going to study the
thermal
structure
of the lithosphere, which is strongly
related to its rheology and dynamic behavior.
By “thermal structure”, we mean the scalar field
of temperatures,
T
D
T
(
r
,
t
), and its temporal
evolution. This field depends in turn by both the
rate of heat transfer through the Earth's interior
and the heat loss at the Earth's surface. It is
known that heat can be transferred by conduction,
convection, or radiation.
Conduction
essentially
results from micro-scale interaction between en-
ergy carriers within a material. The nature of
these carriers depends from the state of the matter
and from the material structure, so that they
can be individual molecules in fluids, electrons
or phonons in solids. In any case, conduction
consists into a direct energy transfer from more
energetic carriers to low-energy carriers through
molecular or particle collisions, thereby is a dif-
fusive process that requires spatial variability of
the temperature field.
Convection
is a process that
changes the temperature field by the large-scale
