Geology Reference
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vesicles must be determined in the lab. Several
approaches have been used to determine
modal bubble size: infusing a plastic into the
sample and then dissolving the surrounding
rock; making random cuts through a sample
and using the statistics of two-dimensional sec-
tions to calculate the three-dimensional vesicle
distribution; or using X-ray tomography to
image distributions directly (see figure B).
used to determine paleoelevation at the time
of eruption. Because basalts can be dated
radiometrically, the time at which the lava
was at that elevation can be determined, and
the uplift or subsidence of the site can be
determined by subtraction from the current
elevation. Recent applications of this method
to the Colorado Plateau (Sahagian et  al. ,
2002b) suggest that rates of Cenozoic rock
uplift increased about five-fold in the past
5 Myr (see figure C).
2.5
Uplift = Current Elevation
minus Paleoelevation
2.0
40 m/My
1.5
1.0
Rise of the
Colorado Plateau
Vesicle
Tomography
B
Plateau sites
0.5
South, Southeast
220 m/My
B. Tomographic reconstruction of vesicles. Modified
from Sahagian et al. (2002a).
Northeast
Central
0
5
10
15
20
25
C
Time (Ma)
With a vesicle-size distribution and flow
thickness in hand, the equation above can be
C. Reconstructed net Cenozoic uplift of the Colorado
Plateau. Modified from Sahagian et al. (2002b).
rainfall changes. At higher altitudes, the 18 O/ 16 O
ratio is lower than at lower altitudes (Fig. 7.28).
Surface waters, as well as plants, animals, and
minerals that are in equilibrium with the surface
waters, reflect these isotopic compositions of
the water in which they grow, and therefore can
preserve a record of the variations in isotopic
signature (Fig. 7.29). The presence of these
isotopic changes suggests some ways in which
the growth of mountain ranges could be
documented. For example, the isotopic compo-
sition of flora or fauna should differ both with
altitude and across a range with strong
orographic precipitation. Thus, if you were to
0
-5
r 2 = 0.79
-10
Oxygen Isotopes
versus Altitude
-15 0
1
2 3
Change in Elevation (km)
4
5
6
Fig. 7.28 Oxygen isotopic composition as a function
of altitude.
Compilation of global data for altitude-dependent
changes in oxygen isotopic ratios as expressed in δ
18 O
ratios. Modified from Chamberlain and Poage (2000).
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