Geology Reference
In-Depth Information
Table 3.1
Relative dating methods.
Method
Age range
Materials needed
References
Clast seismic velocity
1-100 ka
Boulders
Crook (1986), Gillespie (1982)
Obsidian hydration
1-500 ka
Obsidian-bearing lavas
Pierce
et al.
(1976)
Soils
1-500 ka
Soils
Harden (1982)
Mineral weathering
10 ka-1 Ma
Boulders
Colman and Dethier (1986)
Landform modification
10 ka-1 Ma
—
Davis (1899), Cotton (1922)
quite easy to measure the seismic velocity by
using a micro-seismic timer (Fig. 3.1A). For each
of several spacings between the accelerometer
and the hammer, many measurements of travel
time are made. The travel time is recorded and
is converted to velocity by dividing the separa-
tion distance between hammer and sensor by
the travel time. The clast seismic velocity of a
particular boulder is then calculated, and the
next boulder is selected. Many boulders on each
surface to be dated, e.g., moraine, alluvial fan,
debris-flow terrace, are measured (Fig. 3.1B).
Where the conditions are appropriate, the
resulting age progressions have made sense
(Crook and Gillespie, 1986), such that surfaces
expected to be older than others have clast seis-
mic velocities that are slower, indicating higher
microcrack densities. The applicability of the
technique is limited to sites where one may
assume that the production rate of microcracks
is uniform among the boulders to be sampled.
This restriction forces us to focus on sites with
uniform lithologies, or at least ones where many
boulders of the same rock type are available,
because the rate of production of microcracks,
from any of many possible production mecha-
nisms, is likely to be strongly dependent upon
rock type. As with many of the other relative
dating methods, however, the assignment of
absolute ages from such data is difficult, given
that we have no theoretical basis for predicting
the rate of decline of clast seismic velocity
through time. We do not know how long it will
take for the clast seismic velocity to decline by
50%. This limitation forces us to calibrate the
technique locally against surfaces of known age
or to consider the resulting differences in clast
seismic velocity in only a semi-quantitative
sense (Gillespie, 1982).
A
Clast Seismic Velocity
digital clock
accelerometer
closes
contact
starts clock
stops clock
T
L
Clast Seismic Velocity: CSV=L/DT
B
2.2
C
2.0
B
1.8
1.6
A
1.4
D
1.2
1.
0
0
2
4
6
Distance from Modern Dunes (km)
Fig. 3.1
Clast seismic velocity measurements.
A. Cartoon of clast seismic velocity (CSV) methodology.
B. Results of application of CSV method to the dating
of debris-flow benches in Panamint Valley, California.
Inferred age increases as the CSV declines, reflecting
growth of microcrack population within the boulders
sampled. Modified after Anderson and Anderson (1990).
Weathering rinds
Rocks exposed to temperature and wetness at
the surface of the Earth weather by a variety
of mechanisms. In general, this weathering
proceeds inward from the surface of the rock,
where moisture is most accessible and where
temperature changes most rapidly and with
greatest magnitude on both daily and storm
cycles. This variability can result in the pro-
duction of a weathering rind: an identifiable
