Permafrost Formation Time
VIRGIL J. LUNARDINI
INTRODUCTION
The age of permafrost is of interest to biologists, geophysicists and engineers. Clearly, permafrost must
be at least as old as the time it took for it to form; thus, the formation time of permafrost can be considered
its minimum age. A volume of permafrost can be much older than this since it may exist for many years af-
ter formation. This report will examine the formation time of permafrost using a pure heat conduction mod-
el. As we shall see, the surface temperature history of the soil mass is critical for any prediction of the per-
mafrost formation time. Since the formation time of permafrost is expected to be on the order of millennia,
it is necessary to examine the geophysical record to obtain some bounds on realistic surface temperatures
that the Earth has experienced during the time when permafrost was growing. First, we will discuss perma-
frost and paleotemperature scenarios, then we will formulate a mathematical model of permafrost growth,
and, finally, we will examine some predictions by the model of permafrost formation times.
Permafrost is a widespread phenomenon that has been and still is greatly misunderstood. The term
"permafrost" is generally attributed to S.W. Muller (1945), who apparently coined the name in place of the
more awkward terms: permanently frozen ground or permanent frost. Bryan (1946) suggested the term
"pergelisol," but this has not been adopted except in the French literature. In order to understand the con-
cept, let us look at a general definition given in Lunardini (1981a):
Permafrost describes the thermal condition of earth materials (sand, glacial till, organic matter, etc.)
when their temperature remains at or below 32F (0C) continuously for a significantly long time,
but not necessarily for an entire geological period. It does not include earth materials that drop be-
low 32F during one winter and remain below 32F through the following summer and into the next
winter, although for practical engineering purposes such materials may be included.
Clearly, permafrost is not so much a material as it is the thermal state of ordinary soil systems. It does
not include systems that are at or below 0C, but contain no earth materials, e.g., ice caps, glaciers and ice-
bergs. There is no agreement on the minimum time during which the material must remain below 0C to
qualify as permafrost. Soils that freeze during an exceptionally severe winter and survive for 1 or 2 more
years are called "pereletoks," and often are not classified as permafrost (Swinzow 1969).
The existence of permafrost is a result of the history and the present state of the energy balance at the
Earth's surface--measured by the surface temperature--and the deep Earth heat flow. If permafrost exists
and the net yearly gain of energy by the entire permafrost volume is equal to the net loss of energy, then the
permafrost will remain stationary, while an excess heat gain over heat loss will result in a net loss of frozen
material. Given the same energy balances, however, i.e., net gain of energy over the year, one region may
have permafrost (albeit degrading) while another will not. This is ascribable to the thermal history of the
frozen ground in the two areas. Though both are losing or have lost permafrost, one region may have start-
ed with a larger volume of permafrost than the other. Thus, the present energy flow conditions may be such
that permafrost cannot exist in one region, whereas it will subsist in another area, although in a receding
form often referred to as "relic permafrost."
Previous Page
