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to slope and thickness (Janke and Frauenfelder
2008
). Perhaps a warmer climate is
transforming ice near the toe that was previous too cold and rigid to produce greater
ratesofflow.
Water may also be affecting rates of flow near the toe. Ikeda et al. (
2008
) found
rapid rock glacier acceleration during periods of snowmelt in the Swiss Alps. Air
voids in the frozen debris fill with water, which may improve the capability of ice
flow (Ikeda et al.
2008
). Krainer and Mostler (
2006
) noted several springs at the
base of the rock glaciers in the Austrian Alps in correlation with large displace-
ments that cannot be explained by ice deformation alone. This suggests that water
acts as a lubricant between shear planes. On the California rock glacier, water could
accumulate in the toe from drainage down a longitudinal furrow (Fig.
5.7
). After
comparing surface characteristics of a region around points 1-3 (decreasing veloc-
ities) versus points 5-7 (increasing velocities), it appears that curvature varies the
most between these areas (Table
5.4
). More specifically, profile curvature of the
surface is slightly negative in the region surrounding points 5-7 (Table
5.4
). The
concaveness in the direction of the longitudinal furrow suggests potential for water
capture. In turn, water may warm ice, fill void spaces, or act as a lubricant between
shear planes to increase flow rates.
Although far from comparable environmental conditions exist, an attempt was
made to compare climatic data from nearby stations for two periods in which veloc-
ities were calculated: the mid-1980s to 1998 and 2003 to 2008. Total precipitation
and Mean Annual Air Temperature (MAAT) were averaged for a climate station
northwest of the field site (Great Sand Dunes at 2,475 m [37.717
◦
N, 105.533
◦
W])
and a station northeast of the rock glacier (Sheep Mountain at 2,362 m [37.717
◦
N,
105.233
◦
W]). Unfortunately, only two sites are available for comparison because
observations are often incomplete or do not correspond to the same period of this
study. Average total precipitation has decreased 3.8 cm at Great Sand Dunes and
0.8 cm at Sheep Mountain; average MAAT has shown a 1.0
◦
C increase at Great
Sand Dunes and a 1.5
◦
C at Sheep Mountain between the two aforementioned peri-
ods. Therefore, increased snowfall and associated melt are less likely to cause
greater velocities; however, warmer temperatures may affect the timing and amount
of melt from permanent snowbanks located on the rockwalls surrounding the rock
glacier, or ice at lower elevations may be melting quicker and produce greater rates
of flow. Neither the change in climate nor the change in rates of flow are substantial
Table 5.4
Average topographic variables for regions surrounding GPS survey points
Slope (
◦
) r at re
a
Plan curvature
b
Profile curvature
c
ID
Elevation (m)
1-3
3,640
15.2
0.05
0.10
0.05
5-7
3,576
16.0
0.27
0.11
- 0.18
a
Positive values indicate a convex slope; negative indicate concave
b
In the direction of the slope
c
Perpendicular to the slope
