Geology Reference
In-Depth Information
Soil development
Weathering
Rinds
16
lava
It is common to be faced with the need to date
a depositional surface in which a soil has devel-
oped. Several techniques have evolved, most of
them quite qualitative, to attempt to place ages
or relative ages on these surfaces based on the
degree of soil development. Soil color is often
the easiest indicator of soil age, although, as in
the identification of minerals, there are many
ways to go wrong using such a simple criterion.
More robust indicators include the accumulation
of carbonate, clays, and iron within the subsur-
face. In formal pedogenic studies that attempt to
quantify the soil development through time on a
series of geomorphic surfaces of similar parent
material (a soil chronosequence), many indica-
tors are documented. One integrated measure
that synthesizes diverse indicators of pedogenic
maturity, including soil clay content, soil color,
and soil structure, is called the Harden index
(Harden, 1982). Although it is not our purpose
here to review all such techniques (see, e.g.,
Birkeland (1990) for such a review), we will
illustrate a couple of methods that have been
used to estimate ages.
lava
12
Pinedale
bedrock
Bull Lake
moraines
8
4
Pinedale moraines
0
0
10
100
200
Age (ka)
Fig. 3.3
Hydration rind thickness as a function of age.
Semi-log plot shows logarithmic fall-off in growth rate
with time. Calibration line from rinds in cracks on
surfaces of independently dated rhyolite lava flows and
from subglacially produced cracks in Last Glacial
Maximum (Pinedale) bedrock. Open circles indicate data
from cracks on surfaces of boulders in undated glacial
moraines. Modified after Pierce
et al
. (1976).
glass, generally having a high silica content. The
glass hydrates once a surface is exposed to
the air. The thickness of the weathered rind is
measured in thin section normal to the exposed
surface, and is identified by an abrupt roll-off in
the refractive index of the glass. The rind is on
the order of several microns thick. Because the
rate of hydration-rind growth is dependent at
least slightly on the composition of the obsidian,
and surely on the temperature to which the sur-
face has been subjected, the technique requires
calibration against surfaces of known age to
yield quantitative dates.
Working in a field area containing dated lava
flows, Pierce
et al.
(1976) demonstrated the
usefulness of the technique in dating both glaci-
ated volcanic bedrock and glacial moraines that
incorporated clasts that had been glacially
abraded. The glaciation is presumed to have
generated new cracks in the rocks, which then
hydrated (Fig. 3.3). Again, this technique suffers
from the likelihood that the rind growth rate is
dependent on both rock type and climate. Given
that climate varied significantly over the Late
Pleistocene, during which most of the surfaces
with which we are concerned have evolved, this
variability can be a significant drawback.
Carbonate coatings and other
pedogenic indicators
In arid regions, soils accumulate calcium
carbonate in the near-surface. Rainwater delivers
some calcium directly and dissolves calcium-
bearing minerals (mostly carbonates) both from
the parent material and from airborne dust
that has accumulated on the soil surface. This
dissolved load can be re-precipitated at depth as
the water is wicked back up to the surface
during evaporation or uptake by plants such
that the remaining water becomes supersaturated
with respect to calcite. Precipitation is also
favored if the CO
2
content decreases or if the
temperature increases in the soil, thereby
lowering the solubility of calcite. The total mass
of carbonate in the soil, and that part of it that
occurs as coatings on the bases of soil clasts,
have both been used in documenting relative
ages of surfaces. Studies in Idaho (Vincent
et al.
,
1994) have shown that the rate of growth of
carbonate coatings has varied by at least a factor
