Geology Reference
In-Depth Information
layer that has experienced more weathering
than the material beneath it. If you crack open
most clasts from an old surface, you will see that
a thin layer near the surface of the rock is
discolored, revealing that the minerals near the
surface have been altered in some way. The
thickness of this layer is thought to be a proxy
for the time the rock has spent in near-surface
conditions. Like the clast seismic velocity
technique, this technique is fraught with pro-
blems associated with variability of the rind
thickness among the surface clasts, the lithologic
dependence of the rate of rind growth, and
inheritance of a weathering rind from previous
exposure. Nonetheless, one may control for
many of these problems at selected sites.
Moreover, the method is inexpensive, low-tech,
and readily applied to a variety of deposits.
It has been argued that the growth of the
weathering rind ought to proceed as the square
root of time (Colman, 1986). Presumably, this
inference reflects the fact that the mineralogical
changes required to create a visible rind are
mostly chemical and require diffusion of species
into and out of the rock. Because diffusive
processes always result in thicknesses,
L
, that
vary as the square root of the diffusivity,
k
,
multiplied by the time,
t
, since the process was
initiated (
A
Weathering Rinds
McCall
(basalts)
Lassen
(andesites)
3
2
1
0
Age (ka) New Zealand
0
2
4
6
8
10
6
B
New Zealand
2
Bohemia
4
1
2
Yellowstone
0
0
0
50
100
150
200
Age (ka), Bohemia and Yellowstone
Fig. 3.2
Weathering rinds.
A. Weathering rind thicknesses are used to distinguish
glacial deposits in Mt Lassen and Mt McCall, Idaho.
Absolute age control is lacking on these moraines; the
bottom axis is arbitrary. B. Growth histories of rinds in
Bohemia and Yellowstone (left and bottom axes) and
New Zealand (right and top axes). Note decline in growth
rate with time. Modified after Colman and Pierce (1992).
L
k
), one might expect a square-
root relation of rind thickness to age. Only rarely
have sufficient data been arrayed to test this
model (Fig. 3.2B), because absolute ages on a
variety of surfaces are required for the test.
In a review of chronologies derived for alpine
glaciations in the western United States, Colman
and Pierce (1992) noted that this decline in the
rate of rind growth through time (no matter
what the exact nature of the nonlinearity) allows
one to place limits on the relative ages of depos-
its based on the ratios of the rind thicknesses. In
other words, the ratio of the ages (old/young)
should always be greater than the ratio of the
rind thicknesses (thick/thin).
The primary problems with the technique are
the need for calibration of the effects of local cli-
mate, the dependence of weathering rates on the
lithology of the clasts, and the possibility of inher-
itance of rinds from prior exposure. This latter
complexity is especially important if the clasts are
~
highly resistant to abrasion within the transport
system that delivers the clasts to the site to be
dated. For instance, quartzite clasts weathering
out of Jurassic conglomerates in central Utah
appear on the surface with both weathering rinds
and percussion marks from transport during the
Jurassic (and perhaps even earlier episodes of
transport). Most studies have therefore focused
on weathering rinds developed in easily weath-
ered, more abradable clasts such as andesites and
basalts (Colman and Pierce, 1992) (Fig. 3.2).
Obsidian hydration rinds
A technique used to great effect in archeology is
based on the growth of a hydration rind as a
glassy surface weathers. Obsidian is a natural
