Geology Reference
In-Depth Information
5.2.3. Thermal Properties
The thermal properties of frozen ground include thermal conductivity (
K
), mass and volu-
metric heat capacities (
Cm
and
Cv
), thermal diffusivity (
α
), and latent heat of fusion (
Qi
).
Each of these is defi ned below.
Thermal conductivity (
K
) is a measure of the rate at which heat is driven through
a unit area of a medium (soil or rock) per unit time, under a temperature gradient.
It is usually measured in W m
1
K
1
. The mass heat capacity (
Cm
), usually measured in
J kg
−1
K
−1
, is a measure of the amount of heat that must accumulate in a unit weight
(1 kg) to produce a change in temperature (1 K). The volumetric heat capacity is obtained
by multiplying the mass heat capacity,
Cm
, by the bulk density,
ρ
, i.e.:
Cv
=×
ρ
Cm
(5.7)
The thermal diffusivity (
), measured in units of cm
−2
s
−1
, is the thermal conductivity
divided by the volumetric heat capacity (
Cv
):
α
α
=
KCv KCm
=
×
ρ
(5.8)
The latent heat of fusion (
Qi
), measured in J cm
−3
or J g
−1
, is the quantity of heat released
per unit of volume or weight of a medium (soil or rock) when frozen:
(
)
Qi
=×× −
L
ρ
d
W
Wu
(5.9)
d
is dry density,
L
is latent heat of fusion of water (80 cal g
−1
or 80 kcal kg
−1
),
W
is
total water content, and
Wu
is the unfrozen water content.
All these properties and relationships are important in understanding permafrost
conditions and permafrost-related landforms.
where
ρ
5.3. HOW DOES PERMAFROST AGGRADE?
It is sometimes useful to think in terms of permafrost being either epigenetic or syngenetic
in nature. Epigenetic permafrost occurs when soil or rock, already in existence, is subject
to perennial freezing and permafrost forms by downward penetration from the ground
surface. Syngenetic permafrost refers to permafrost that forms contemporaneous with the
deposition of the sediment that is being frozen. For example, permafrost that is forming
today in Paleozoic-age rock is obviously epigenetic in nature since it is younger than the
host material. By contrast, permafrost that is forming today in deltaic or alluvial sediments
that are currently being deposited must be regarded as largely syngenetic. This distinction
is somewhat artifi cial since it can be argued that all permafrost is syngenetic in nature.
However, the distinction is especially useful when dealing with relict permafrost (see
pp. 103-4) and when undertaking Pleistocene periglacial investigations (Part III).
Permafrost-related landforms can be divided into two broad categories: those that form
when permafrost aggrades and those that form as permafrost degrades. The latter are
often associated with melt of ground ice and subsidence and erosion of thawed sediment.
These phenomena are treated in Chapters 6, 7, and 8. Here, we examine the manner in
which epigenetic permafrost forms.
