Honors Theses
Geology - Environmental Studies
Fall 2011
Cracking up : lithological controls on non-tectonic
rock cracks, Mojave Desert, California
Sarah G. Evans
Penrose Library, Whitman College
This thesis has been deposited to Arminda @ Whitman College by the author(s) as part of their
degree program. All rights are retained by the author(s) and they are responsible for the content.
CRACKING UP:
LITHOLOGICAL CONTROLS ON NON-TECTONIC
ROCK CRACKS, MOJAVE DESERT, CALIFORNIA
by
Sarah G. Evans
A thesis submitted in partial fulfillment of the requirements for graduation
with Honors in Geology and Environmental Studies.
Whitman College
2011
Certificate of Approval
This is to certify that the accompanying thesis by Sarah G. Evans has been accepted in
partial fulfillment of the requirements for graduation with Honors in Geology and
Environmental Studies.
_________________________________
Professor Robert J. Carson
Whitman College
May 11, 2011
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TABLE OF CONTENTS
Title Page………………………………………………………………………...……..….i
Certificate of Approval…………………………………………………………...……….ii
List of Figures……………………………………………………………...……………. iv
Abstract……………………………………………………………….…………..……….v
Introduction………………………………………………………………………...……...1
Literature Review…………………………………………………………......…..1
Project Goals………………………………………………………….…..….…..6
Geologic Background………………………………………………………………...…...8
Geologic Setting…………………………………………………………...……...9
Environmental Application………………………………………………...……..17
Methods……………………………………………………………………...……...……19
Field Methods…………………………………………………………………...19
Lab Methods………………………………………………………….…………20
Results……………………………………………………………………………...…….23
Discussion………………………………………………………………………………..28
Conclusions……………………………………..……………………………...………...33
References Cited…………………………………………………………………...…….35
Appendix……………………………………………….………………………………..39
A: Crack Orientation Data……………………………………………………..39
B: Raw Crack Data……………………………………………………....……..40
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FIGURE LIST
Figure 1: Depiction of the diurnal pattern of solar heating
3
Figure 2: Graph of tensile stresses in heated rock cores of various diameters
4
Figure 3: Example of referred orientation data for cracks not parallel to rock shape or fabric
5
Figure 4: Field site location map
8
Figure 5: Generalized geologic history cross-section of the Mojave Desert
10
Figure 6: Photo of desert pavement
11
Figure 7: Photo of rock varnish
12
Figure 8: Photo of the Providence Mountains
13
Figure 9: Geologic map of Providence Mountains
14
Figure 10: Desert pavement at Cima Volcanic field
15
Figure 11: Geologic map of Cima Volcanic field
16
Figure 12: Photo of transect along the Cima Basalt Flow Surface-I
19
Figure 13: Photo of field methods
20
Figure 14: Crack density box and whisker plot
23
Figure 15: Table of percent of rocks with one or more cracks
24
Figure 16: Images of crack size as a function of rock type
24
Figure 17: Rose diagrams of preferred crack orientations
25
Figure 18: Quantile-quantile plot of preferred crack orientations
26
Figure 19: Image of vesicles in basalt
26
Figure 20: Graph of rock varnish versus crack density
27
Figure 21: Solar path chart for the Mojave field site
29
Figure 22: Image of differences in crack preservation
32
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ABSTRACT
Physical weathering affects erosion rates, sediment production, and atmospheric
concentrations of CO2, yet non-tectonic-related crack formation is poorly understood.
Thermal stresses related to diurnal directional insolation may play the primary role in
generating cracks initially, but it is unknown how specific rock properties affect this
process. In this study we utilized field data from the 130 ka Providence Mountain alluvial
fans and the ~140 ka Cima Volcanic Field basalt flows in the Mojave Desert to determine
if crack population characteristics vary as a function of lithology. We measured crack
density and orientation for more than 400 rocks along 19 transects.
Analysis suggests that rock type influences crack density, size, and orientation.
Basalt clasts have a median crack density of 15 c/m2 (cracks per square meter), and
metavolcanics have 35 c/m2. Generally about 70% of all rock types excluding basalt
contained one or more cracks while only 38% of basalt clasts contained at least one
crack. As the density of vesicles increases in basalts, the average number of cracks per
clast decreases, possibly due to heat dissipation and reduction of thermal stresses. All
rock types display preferred crack orientations with the majority of mean crack
orientations toward the northeast. The lone exception to this trend is limestones, which
show a southeast trend. The differences in orientations may be due to differences in
mineralogy, heat capacity, and other thermodynamic properties of different rocks and
minerals, making them susceptible to cracking at different times of the day or year.
-v-
INTRODUCTION
The rate at which rocks break down mechanically is the primary rate-limiting
process in landscape evolution and sediment production. Mechanical weathering of rocks
initiates a complicated suite of surface processes, all of which depend on the creation of
cracks and the exposure of fresh clast surfaces. Since mechanical weathering rates affect
chemical weathering rates, long-term changes in mechanical weathering influence global
climate via carbon sequestration and the weathering of silicate rocks (Riebe et al., 2004).
Furthermore, the surface breakdown of rocks permits desert pavements to form. Since
desert pavement plays an important role in stabilizing underlying silt and sand over vast
arid regions, this has far-reaching contributions to Earth’s climatic system (e.g., erosion
by wind and water is reduced).
LITERATURE REVIEW
Physical weathering is the mechanical breakdown of rocks into smaller materials.
In combination with chemical weathering it plays a key role in the rock cycle. For
example, in arid regions the in-situ weathering of existing surface rocks plays a key role
in desert pavement formation (Wells et al., 1995). Many near-surface rocks of a variety
of lithologies are commonly cracked, representing the initial stages of rock disintegration
(McFadden et al., 2005).
Thus, the start of mechanical weathering is the formation of an incipient crack.
Some cracks may be related to geological inheritance (e.g. tectonics), salt weathering, or
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forest fires. Many surface clasts exhibit discrete cracks that extend beyond individual
grain boundaries throughout a range of climatic and geomorphic settings (Eppes et al.,
2010). Cracks are even visible in images of Mars (Eppes et al., 2010). While a
substantial body of geologic and geomorphic literature (e.g., Ritter et al. 2002,
Blackwelder, 1933) and many researchers (e.g., Goudie et al., 2002) suggest several
processes that may cause crack growth and expansion in rocks, the mechanism of crack
origin continues to be disputed.
Salt weathering has been suggested to be the primary physical weathering process
in deserts (e.g., Amit et al., 1993). However, salt weathering appears to be more effective
in crack propagation than crack formation (McFadden et al., 2005). Only after salts fit
into the clast’s interior via small cracks can crystallization, expansion, or hydration of
salts induce thermal stresses significant enough to crack the rock (Amit et al., 1993).
Thermal stresses have also been suggested to play an important role in rock weathering
and crack formation, especially in deserts. It was recognized as early as 1832 that solar
heating caused steep temperature gradients that produced stresses in the outer few
centimeters of a clast (Bartlett, 1832). As these stresses increase, exceeding the tensile
strength of the rock, brittle failure occurs. However, classic papers of experimental
findings by Griggs (1936a, 1936b) cast doubt on the idea that thermal stresses produced
by insolation are important in rock cracking. In his experiments Griggs simulated 244
years of diurnal cycling with a uniform temperature range of 10-110 ºC. He found no
cracking in his samples.
-2-
Since Griggs’ experiments, some geomorphologists (e.g., Rice, 1976) continue to
argue that insolation is an important process in arid regions, while others continue to rule
it out (e.g., Twidale, 1968). Even more recent studies conclude that thermally induced
cracks may form, but likely only in secondary association with other processes or in the
presence of high levels of moisture or salts (Ritter et al., 2002; Bloom, 1998; Ahnert,
1996). Nearly seventy years after Griggs’ initial papers, geologists continue to dispute
the effectiveness of solar insolation in the mechanical breakdown of rocks.
Recently, McFadden and others (2005) re-evaluated Griggs’ experiment and
Figure 1. Cartoon depicting diurnal pattern of solar heating that creates tensile
stresses in rocks, view to north. Boulder exhibits cracking in a direction
perpendicular to the direction of the sun’s rays in the morning and afternoon.
On the associated sketch warm colors are hot temperatures; cool colors are cold
temperatures (McFadden et al., 2005).
-3-
found a flaw in his methods: clasts in natural environments on the Earth’s surface are not
subject to such uniform heating.
On the contrary, he discovered clasts on the Earth’s
surface experience tensile stresses caused by directional heating and cooling during the
sun’s east to west transition across the sky (Fig. 1). McFadden further suggested that
these stresses may play the key role in the initial generation of cracks (McFadden et al.,
2005).
When a clast is heated by insolation cracking, a steep temperature gradient
develops in the outer few centimeters of the clast (Warke and Smith, 1994). Clast surface
temperature gradients are rapid and not spatially uniform (Sumner, 2004). They produce
sufficient stresses (Fig. 2) within the rock surface which exceed the tensile strength of
Normal stress (vertical; tensile is positive) at core of clast
Figure 2. Temperature and normal stress at the core of rock spheres of different sizes
(diameters 0.05, 0.5, and 5.0 m). Positive stresses are tensile. Stresses produced upward
of 7-15 Megapascals (MPa = Newton/mm2) (Eppes et al., 2010).
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most rocks and cause brittle failure (Eppes et al., 2010). Fire-induced rock spalling is a
common example of this. Although the diurnal cycles of solar heating and cooling that
cause expansion and contraction are of a much smaller magnitude than temperature
changes during fires, they tend to penetrate more deeply than fires do (Bloom, 1998).
How then may one prove that insolation is the mechanism of crack formation?
The majority of cracks on geomorphic surfaces have preferred orientation independent of
rock shape and fabric (Fig. 3) (Eppes et al., 2010). These preferred orientations are
difficult to explain by mechanisms other than directional insolation heating, as they
reflect the largest recurring thermal stresses. Cracks produced by processes such saltshattering would exhibit random azimuthal crack orientations. Likewise, cracks
produced by the freezing and thawing cycle of water (freeze-thaw weathering and frost
wedging) would not necessarily form depending on the sun’s angle, and would not
produce unimodal or bimodal orientations.
Figure 3. Rose diagrams of preferred orientation data for cracks not parallel to
rock shape or fabric. A. boulders on a variety of geomorphic surfaces (McFadden
et al., 2005), B. west-facing boulder field in Virginia, C. a single large boulder on
south-facing slope in North Carolina (Eppes et al., 2010).
-5-
McFadden and others (2005) tested the possibility of insolation crack generation
by measuring the orientations of approximately 700 cracks in the southwestern United
States (California, Arizona, and New Mexico). They found that a significant number of
cracks (462 out of ~700) had a mean orientation of N 5º E ± 12 º. This, they concluded,
was fundamentally related to thermal stresses that arise daily from non-uniform heating.
Furthermore, their research supported the hypothesis that insolation plays a key role in
the physical weathering of rocks exposed to the sun, and that this mechanism is viable in
other climates, as well as outcrops, man-made structures, and rocks in non-terrestrial
environments.
Eppes and others (2010) further explored these north-south insolation-generated
cracks. Azimuth orientation of ~470 cracks was measured in three desert study sites: the
Mojave Desert of southern California, the Gobi Desert of Mongolia, and the Strzelecki
Desert of Australia. The mean orientation for all data over all three sites was N 26º E ±
30 º, again supporting a preferred crack orientation in deserts of various latitudes and
around the globe.
Based on this data they concluded that thermal conditions resulting in
maximum temperature gradients on the rock surface are the most important for inducing
rock fractures with preferred orientations. Additionally, Eppes and others (2010)
proposed that rock size, surface age, latitude, aspect, and lithology play important roles in
the formation of on rock cracks.
PROJECT GOALS
Despite these recent investigations, it is still unknown how specific rock
properties affect cracks initiated by thermal stresses related to diurnal directional
-6-
insolation. In this study I measured rock cracks in the field to determine if non-tectonic
rock crack population characteristics vary as a function of rock type. We measured over
400 rocks and 600 cracks in the Mojave Desert, California. My fellow REU researchers
examined three other aspects of insolation cracking: age, rock shape, and relations
between rock cracks and surface parallel faces. By analyzing our data and comparing our
results we were able to constrain the relationship between rock characteristics and
insolation-generated cracks, further characterizing the role of the sun in the propagation
of rock fractures.
-7-
GEOLOGIC BACKGROUND
Data was obtained from the Providence Mountain alluvial fans and the
Cima Volcanic Field basalt flows in the arid eastern Mojave Desert (Fig. 4). Both
the Providence Mountain alluvial fans and Cima Volcanic basalt flows are classic
localities of desert geomorphology, desert ecology, and desert soil
chronosequences. They have been studied extensively for their classic examples
of desert pavement (e.g., McFadden et al., 1989).
Thus they represent a unique
location to build on existing work in order to further understand the role of
Cima
Volcanic
Field
physical weathering in landscape evolution.
Mojave Desert Preserve
Cima
Providence Mtns.
Prov. Mtns.
Figure 4. Field sites were located in California, in the western portion of the Mojave
National Preserve (USGS, 2009). Google Earth images illustrate the Qf3 Providence
Mountain alluvial fans in the southern portion of the Mojave and Cima Volcanic Field basalt
Surface-I in the northern portion of the Mojave. Stars identify the fans or flows where rocks
were measured.
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In the southern Mojave Desert, elevations range from ~600 m in the distal fan
areas to more than 2000 m at mountain tops (McDonald et al., 2003). Plant communities
are characterized by sparsely scattered scrubs, Larrea tridenata (creosotebush), Ambrosia
dumosa (white bursage), Yucca schidigera (Mojave yucca), juniper, and grasses with no
large trees (McDonald et al., 2003). The climate ranges from arid to semi-arid with an
average rainfall of approximately 150 mm which is associated with winter Pacific frontal
storms (McDonald et al., 2003).
GEOLOGIC SETTING
During the Proterozoic the region that is now the Mojave Desert in southern
California experienced little structural change as erosion gradually wore down the
landscape to nearly sea level. As the supercontinent Rodinia began to rift apart, a protoPacific basin developed. The edge of the North America continent gradually sank
beneath the ocean surface, and a thick sequence of sedimentary rocks began to
accumulate on the continental margin. This miogeosynclinal, passive continental margin
(Fig. 5 ―A‖) continued to be passive for approximately 800 Ma. The great carbonate-rich
sedimentary rock section preserved in the Mojave region records that during this time
North America gradually drifted northward across the equatorial region, becoming home
to a warm, shallow continental platform (USGS, 2009). This passive margin continued
until the Triassic Period (~ 240 Ma) when the supercontinent Pangea began to rift apart,
forming a new subduction zone and the Atlantic Ocean basin. The relative westward
motion of the North American continent caused the its western margin to override the
adjacent oceanic crust (Fig. 5 ―B‖). In addition to subduction, erosion began to degrade
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the exposed rocks and sediment, and magma generated by subducting plates caused
igneous activity. Beginning in the Jurassic Period, an extensive volcanic arc developed
across the Mojave region, and granitic batholiths were emplaced (USGS, 2009).
Figure 5. Generalized geologic history cross-section of the Mojave Desert
(USGS, 2009).
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Extrusive and intrusive volcanism ended in Late Cretaceous time (~80 Ma).
Then, beginning in the Late Oligocene Epoch (~ 30 My\a), new tectonic forces acted on
the landscape. A great rift-style fault system developed across the region as the Great
Basin began to spread apart (Fig. 5 ―C‖). The Mojave Block was uplifted around 25 Ma.
Around 18 to 14 Ma two detachment fault systems affected parts of the Mojave. Today
active fault systems in the region are a result of right-lateral shearing forces associated
with the San Andreas Fault system (USGS, 2009).
Throughout the Late Tertiary and Quaternary periods, large volcanic eruptions
occurred fairly frequently in the region. Ash recurrently blanketed the landscape; many
ash beds are preserved in the alluvial deposits today. The last volcanic episode in this
area was about 8,000 ka. Climate changes during the last millions of years were mostly
Figure 6. Desert pavement is a distinctive surficial feature where at least 65% of
the soil surface is clast-covered (Wood et al., 2004).
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responsible for the majority of the landscape features evident today (USGS, 2009). There
is no evidence of tectonic activity along the mountain front during the late Quaternary
(McDonald et al., 2003).
In more recent time, inflationary desert pavements have formed (Fig. 6). These
pavements originate as a surface of coarse particles. The protruding cobbles attract fines,
which settle and sift downward through the spaces between the larger clasts, ejecting the
cobbles upwards. Infiltrating rainwater aids this process. Such desert pavements are
marked by silty layers within the soil Vesicular A Horizon (Av).
Additionally, many of the Mojave rock surfaces have rock varnish (Fig. 7). Rock
varnish is formed by wind-transported manganese- and iron-rich clays that become
plastered to wet desert rocks, becoming oxygenated with dew or carbonic acid in rain.
Figure 7. Rock varnish as pictured here is a shiny, thin, red/brown layer of
manganese oxides and iron oxides.
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N
Figure 8. The Providence Mountains of the Mojave range in elevation from 600 to
2000 m, shedding sediment to create eight different Quaternary alluvial fan
depositional units.
The carbonic acid in rain separates manganese from iron, and then oxidizing conditions
concentrate manganese oxides and hydroxides as a cement to form the varnish (Dorn,
2007). In recent times both desert pavement and rock varnish serve as signs that the
clasts have not been disturbed by humans or recent floods.
The Providence Mountains (Fig. 8) are a prominent feature along the eastern edge
of the Mojave. Eight alluvial fans have been identified in the Providence Mountains
based on stratigraphic relations, gravel bar relief, soil development, pavement particle
size, and distinct weathering characteristics (McDonald et al., 2003). These aggregated
over one and a half million years (McDonald et al., 2003). Locally fans are labeled from
Qf8, modern fans, to Qf1, from the early Pleistocene Epoch (Fig. 9). All fans are
suggested to have accumulated during wet times or transitions between climatic regimes.
Fans were deposited along the western margin of the Providence Mountains and grade to
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an extensive draa associated with the Kelso Dunes (McDonald et al., 2003).
Each fan
has bar and swale depositional topography, with a half to a whole meter of relief.
Figure 9. Geologic map of Providence Mountains showing Qf3 fans in
dark gray (McDonald et al., 2003).
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For this study, measurements were taken only from the Qf3 Middle Pleistocene,
130 ka alluvial fan surfaces in the Providence Mountains (McDonald et al., 2003). Qf3
fans were distinguished by having a strong desert pavement, clay loam between clasts,
and an argillic to petrocalic B horizon (McDonald et al., 2003). Limestone clast faces are
very etched. Qf3 was deposited during OIS 6, when climate was wetter than present
(Eppes et al., 2003). The elevation here is approximately 975 m.
Approximately 30 miles north of the Providence Mountains lies the Cima
Volcanic field (Fig. 10). Here K-Ar dated basalt flows range in age from early to latest
Pleistocene (McFadden et al., 1987; Wells et al., 1985). Flow surfaces have developed
coarse accretionary mantles composed of weathered bedrock with fine eolian deposits
trapped by the irregular flow-top surface. The lava flows overlie Tertiary crystalline
rocks and gravels (Wells et al., 1985). There is well-preserved desert pavement on all
N
Figure 10. Desert pavement in the Cima Volcanic Field.
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flows. Flow slopes range from ~3% to ~6% with a mean slope of ~4.5%. Many basalt
clasts have calcium carbonate rinds. For this study to be comparable with the Qf3
Providence Mountain alluvial fan, data was only taken from Surface-I (Fig.11), a 140 ka
basalt flow surface with an approximate elevation of 1000 m (Wells et al., 1985). This
flow was chosen in hopes of avoiding confounding variables because it most closely
matched the 130 ka, 975 m elevation Providence Mountain alluvial fan surface.
Figure 11. Geologic map of Cima Basalt Flows showing Surface-I in yellow
(Wells et al., 1985).
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ENVIRONMENTAL IMPLICATIONS
Insolation-generated cracking affects mechanical weathering, which increases
surface area, therefore affecting chemical weathering rates. Long-term changes in
mechanical and chemical weathering influence global climate via carbon sequestration
and the weathering of silicate rocks (Riebe et al., 2004). Furthermore, the surface
breakdown of rocks permits desert pavements to form in arid lands. Since desert
pavement plays an important role in stabilizing underlying silt and sand over vast arid
regions, this has far-reaching contributions to Earth’s climatic system (e.g., erosion by
wind and water is reduced) (Eppes et al., 2010).
Rock breakdown is closely associated with chemical weathering of silicate
minerals which influences the amount of carbon dioxide (CO2) in the atmosphere
(Chernicoff and Whitney, 2002). When the atmosphere is rich in CO2 the climate warms
via the greenhouse effect. In this situation, atmospheric CO2 allows incoming shortwave, ultraviolet radiation to pass through to the Earth’s surface. This CO2 also traps
outgoing, long-wave radiation near the surface, warming the Earth’s atmosphere.
Conversely, when the amount of heat-trapping CO2 is relatively low, the global
climate cools. CO2 is added to the Earth’s atmosphere through volcanic eruptions and
extrusion of mantle-derived magma at oceanic divergent boundaries (Chernicoff and
Whitney, 2002). This CO2 is then integrated into many surface processes, including
chemical weathering. The following equation outlines the general reaction illustrating
the role of CO2 in chemical weathering:
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silicate minerals + CO2 + H2O→ carbonate minerals + SiO2 (in solution)
Thus, as the rate of chemical weathering increases, the amount of CO2 that is removed
from the atmosphere and stored in carbonate minerals will increase, perhaps driving
global cooling (Chernicoff and Whitney, 2002).
In arid regions the in-situ weathering of existing surface rocks plays a key role in
desert pavement formation (Wells et al., 1995). The formation of desert pavements also
has plays a unique effect on global climate. Desert pavement limits infiltration and
delivers rainfall as runoff to nearby bare ground areas where scrubs cluster (Wood et al.,
2004). Additionally, desert pavements shield surfaces from eolian processes while
capturing eolian sand, silt, clay, and salts.
While quantitative models of desert pavement development explain many features
of landforms, until now, no mechanism has been proposed to account for the rapid
breakdown of both cobbles and boulders into the stable clasts that form the pavement of
these fans and flow surfaces. Insolation-generated cracks allow desert pavement
formation. A distant and perhaps unexpected connection arises between desert pavement
and global climate, since desert pavement plays an important role in stabilizing
underlying silt and sand over vast arid regions (Eppes et al., 2010). Thus, any temporal
or spatial variation in the process that changes the rate of formation of desert pavements
has the potential to alter climate by changing the dustiness of our planet.
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METHODS
FIELD METHODS
Over 400 rocks and 600 cracks were measured along 19 transects; nine along the
Qf3 (130 ka) alluvial fan surface and ten along the surface-I (140 ka) of the Cima basaltic
field. These surface ages (130 ka and 140 ka, respectively) were determined in previous
surveys by C-14 dating and K-Ar dating, respectively (e.g., McDonald et al., 2003; Wells
et al., 1985). Transects (Fig. 12) were completed by laying a 100-m tape parallel to the
fan or a’a’ flow surface within the area of highest rock density. Data was collected for
each rock that fell under every half-meter tick mark of tape, moving up each fan or flow.
If a rock under the half-meter tick mark did not meet the size specifications that follow or
there was no rock under the mark, the area
under the mark was scanned in a clockwise
direction to locate the closest rock to the tick
mark. The following data was collected for
each rock: rock type, width, length, depth,
azimuthal orientation of the rock’s longest axis,
and number of total cracks. It was also noted
whether or not the rock had granular
disintegration, fabric, spalling, vesicles, rock
Figure 12. Transect along the Cima
Basalt Flow Surface-I.
varnish, microcracks (>2 cm in length), and if
it was embedded.
- 19 -
Characteristics were measured for every crack. A crack is defined as a linear void
of which ambient light did not illuminate the bottom.
Cracks shorter than 2 cm, rocks
longer than 5cm or greater than 50 cm, and rocks with evidence of disturbance such as a
displaced CaCO3 collar or unaligned rock
varnish were excluded. Current active
channels were not studied because
cracking there could be attributed to joint
inheritance and transport. If the rock was
not disturbed, it was recorded regardless
of whether or not it had any cracks. For
Figure 13. Author collecting data in
stylish extra-large golden shorts―all the
rage in the Mojave.
every crack data was collected with a ruler and Brunton compass (Fig. 13). Crack data
collected included: observed crack type (ie. fabric, longitudinal, spall, or other),
maximum width, length, strike, and dip. Any crack with a width less than 0.5 mm was
recorded as incipient. Rock types measured include: metavolcanic rocks, basalt rocks,
limestone rocks (also fine-grained marbles), and coarse- and fine-grained plutonic rock.
We attempted to minimize sample bias by collecting data with a consistent procedure for
each transect. However, inherent bias is preserved by the non-random choice of transect
locations.
LAB METHODS
Data was analyzed with open-source GEOrient © v9.4.4 software (Holcombe, 2009)
and Oriana © v3.0 (Kovach Computing Services, 2009) to plot crack orientation data as
- 20 -
rose diagrams (Fig. 17). Data was also plotted as a quantile-quantile plot (Fig. 18).
Fisher (1993) suggested that a quantile-quantile plot of i(n-1) vs. Xi/180 is a more
objective, visual indication of preferred orientations for circular data, where n=total
number of data points, Xi is the orientation value of the data converted to 0-180 for axial
data, and i is the ranking of those data from 1 to n. If data is perfectly uniform and
random, then it will fall on a line that intersects the orgin at (0,0). Systematic, periodic
deviations from this straight line indicate preferred orientations in that azimuthal
direction (e.g., Fig. 18).
Statistics were analyzed with R © v2.10.1 (R Foundation for Statistical
Computing, 2009) software, GEOrient © v9.4.4, and Oriana © v3.0 to report circular
variance as well as the Rayleigh Uniformity Test and Rao's Spacing Test (all after Fisher,
1993; Mardia and Jupp, 2000). Circular variance is a simple measure (0 to 1) of the
scatter of recorded crack azimuths; values close to 0 indicate low variance, and values
close to 1 indicate high variance (McFadden et al., 2005). This statistic is based on a von
Mises distribution (in circular statistics, a normal distribution is referred to as a von
Mises distribution) of data about a single mean (i.e., unimodal data). Therefore, for multimodal data, the variance might be high, but the data might nevertheless be non-uniform.
―The Rayleigh Uniformity Test calculates the probability of the null hypothesis that the
data are distributed in a uniform manner. Again, this test is based on statistical parameters
that assume that the data are clustered about a single mean. If p-values for this test are
<0.05 then data are considered nonuniform or non-random. Rao's Spacing Test is also a
test for the null hypothesis that the data are uniformly distributed; however, the Rao
statistic examines the spacing between adjacent points to see if they are roughly equal
- 21 -
(random with a spacing of 360/n) around the circle. Thus, Rao's Spacing Test is
appropriate for multi-modal data and may find statistical significance where other tests do
not. If the p-value for this test is <0.05, then the data are considered to be non-random‖
(Eppes et al., 2010). These statistics are expressed in Appendix A.
- 22 -
RESULTS
Data recorded along the 19 transects in the Providence Mountains and Cima flows
is presented here. This data represents a total of 604 cracks measured on 407 rocks.
Results will be presented in terms of crack density, crack size, crack orientation, and
influential factors.
CRACK DENSITY
Most rocks on all examined surfaces exhibited cracks. Crack density, defined as
the total number of observed cracks per rock area (m2), varies as a function of rock type.
Basalt rocks have a median crack density of 15, coarse plutonic have 26, fine plutonics
have 22, limestones have 24, and metavolcanics have 35 (Fig. 14). However, it is
important to notice the large size of the error bars (Fig. 14).
Crack Density (# total
cracks/rock area [m2])
1000
100
10
1
Basalt
Coarse Plutonic Fine Plutonic
Limestone
Volcanic
Metavolcanic
Figure 14. There is a nearly significant effect of rock type on crack density (F=2.33,
p=0.059).
- 23 -
Additionally, around 70% of all rock types excluding basalts contained one or more
cracks (Fig. 15). Only 38% of basalt rocks from the Cima flow contained at least one
crack.
Figure 15. Percent of rocks with one or more rock crack (p=0.018).
CRACK SIZE
There appears to be a pattern of crack widths and lengths as a function of rock type
(Fig. 16). Basalts have the widest cracks (1.52 +/- 1.16 mm) and coarse plutonics have
the narrowest cracks (0.87 +/- 0.66 mm). Average crack lengths also vary by rock type.
Fine plutonics have the longest average length (49 +/- 30 mm) and basalts have the
shortest average length (38 +/- 20 mm).
Figure 16. Differences in crack sizes between limestone (left) and fine plutonic rocks (right).
- 24 -
CRACK ORIENTATION
A large number of all cracks have a nonrandom, moderately strong north-south
orientation. All rock types display preferred crack orientations, but crack vector mean
orientation varies by rock type (Appendix A).
N 47° E
N 79° E
N 92° E
Basalt 0.90 > p > 0.50; n=83
Coarse Plutonic 0.50>p>0.10; n=89 Metavolcanic p < 0.01; n=191
S 54° E
N 67° E
Limestone 0.50 > p > 0.10; n=89 Fine Plutonic 0.50 > p > 0.10 ; n=152
Figure 17. Rose diagrams of preferred crack orientations, rock type labeled. Rao’s
Spacing Test results listed on diagram, p<0.05 indicates non-random orientation of
data. Mean resultant direction as calculated by Oriana © v 3.0 is in red.
The majority of mean resultant directions of crack orientation is toward the northeast
with N 75° E for all plutonic rocks, N 92° E for basalts, and N 47° E for metavolcanics.
The lone exception to this northeast trend is that found in limestones with a southeast
trend, S 54° E (Fig. 17). Orientation data is also expressed in a quantile-quantile plot as
discussed in the lab methods (Fig. 18). Limestone (green) deviates most strongly from
- 25 -
the crack orientations of the other rock types on both the rose diagram and quantilequantile plot.
All Rocks
Basalt
Limestone
Coarse Plutonic
Fine Plutonic
Metavolcanic
n=total number of data
Xi=orientation values
of data (0-180)
i= ranking of data from 1
to n
Figure 18. Quantile-quantile plot of preferred crack orientations. If data are
perfectly uniform and random, they will fall on a line that intersects at the origin
(0,0). Deviations from this straight line indicate non-random, preferred orientations
(Fischer 1993).
INFLUENTIAL FACTORS
The most notable confounding
variables in this study were the presence of
vesicles and rock varnish. As the density of
vesicles increased in basalts, the average
number of cracks per rock decreased, possibly
due to heat dissipation and reduction of
thermal stresses (Fig. 19). Most rock types
with 76-100% rock varnish coverage rock had
- 26 -
Figure 19. Vesicles in basalt may
dissipate heat.
a higher average number of cracks (Fig. 20). However, this may be a function of rock
age. That is, an older rock will have accumulated more rock varnish, as well as more
cracks, as both appear to be a function of age (Eppes et al., 2010).
Metavolcanic
Figure 20. Clasts with more rock varnish covering the surface have more average
cracks than those with little varnish.
- 27 -
DISCUSSION
Previous studies (e.g., McFadden et al., 2005 and Eppes et al., 2010) predicted
that the stresses related to diurnal directional insolation play the primary role in initially
generated cracks. If this solar mechanical process, related to directional insolation, is
indeed the primary cause of cracking in desert clasts, and different minerals exhibit
different thermal properties, then there should be a difference in initial crack generation
and thus crack density and orientation among rock types. To test this hypothesis, I
developed a field-based strategy to attempt to document variations in crack initiation in
different rock types. Results support initial crack generation from solar insolation in
clasts as a function of rock type, thereby providing additional evidence of the importance
of directional insolation in cracking rocks. Specifics in crack orientation, density, and
size differences among rock types may be due to differences in mineralogy, coefficients
of thermal expansion, clast energy threshold, and other thermodynamic properties of
different rock types and minerals.
COEFFICIENT OF THERMAL EXPANSION
The majority of mean resultant directions of crack orientation is toward the
northeast from N 75° E for all plutonic rocks, to N 92° E for basalts, and N 47° E for
metavolcanics. The lone exception to this northeast trend is that found in limestones,
whose cracks have a southeast trend, S 54° E. These specific variations in crack
orientations of different rock types may suggest that the coefficient of thermal expansion
and thus the efficiency of heat absorption is different for different temperature ranges of
unique minerals (Moores et al., 2008). If so, our data suggests that afternoon cracks (Fig.
- 28 -
21) play a predominant role in limestone cracks (mean orientation of S 54° E), while
other rock types might be more susceptible to morning cracking. Eppes et al., 2010
suggested that observed differences in mean resultant directions and modality reflected
the times of day that are most influential in producing insolation cracks. They found the
most influential time of the day to be the morning, proposing that the early-morning
10
20
30
40
50
60
70
80
90
Jun 21
May21
Apr 20
19h
18h
17h
16h
Mar 20
15h
14h
Jun 21
5h
6h
May21
Apr 20
7h
8h
Mar 20
9h
13h
12h
11h
10h
Feb 20
Feb 20
Jan 21
Jan 21
Dec 21
Dec 21
Limestone
Basalt Plutonic
Metavolcanics
Figure 21. Solar path chart for the Mojave field site, modified from a chart created through the
University of Oregon Solar Radiation Monitoring Laboratory website. Shaded areas represent
times and days when the sun would be at 90° to the mean resultant observed crack angles.
- 29 -
temperature gradient is most important for cracking rocks (Eppes et al., 2010). During
this morning warming, ambient air temperatures are low and the insolation of the sunrise
rapidly heats one side of a rock surface, causing the greatest stress to build up over the
clast surface (McFadden et al., 2005). This would therefore suggest that limestone rocks
behave contrary to the morning norm with preferred afternoon cracking. However, this
may also indicate that limestone is more susceptible to winter water freeze-thaw cracking
as compared to summer insolation cracking. Greater winter cracking may be explained
by limestone’s increasing likeliness to crack and weather when wet via the chemical
weathering process of etching (CO2 + H2O→ carbonic acid), which brings minerals into
solution and cracks the limestone. Etching could occur during the winter monsoons in
the Mojave.
It must be kept in mind that monsoonal or other temporally-controlled diurnal and
seasonal conditions have likely been variable over the past 130 ka. If indeed cracking
has been changing partially as a function of climate over the last 130 ka, crack
populations with varying preferred orientations may develop for a particular site. This
adds additional explanation to the observed multimodality in the crack orientation rose
diagrams.
DIFFERENCES IN MINERALOGY
If cracking varies by clast mineralogy, then cracks should occur along different
mineral boundaries depending on the chemical composition of the mineral. Ebherhardt
and others (1999) showed that the initiation and development of stress-induced
microcracks in granite varies by mineral. Samples of Lac du Bonnet granite were
- 30 -
stressed with weight in a laboratory to observe how cracks formed in relation to different
minerals. Results suggested that initial cracking occurred along grain boundaries
between quartz and feldspar grains, and intragranularly within feldspar grains. These
cracks occurred when a significant number of critically oriented cracks initiated and
propagated in the sigma one (the greatest compressive stress) direction, opening cracks
perpendicular to the axial load (Ebherhardt et al., 1999). This evidence that cracks occur
through or next to minerals depending on their composition supports the theory that crack
orientations may vary as a function of mineralogy.
DIFFERENCES IN OTHER THERMODYNAMIC PROPERTIES
Our data suggests that characteristics such as the presence of vesicles likely have
an effect on rock cracking.
Basalts may have a comparatively low average crack
density (around 70% of all rock types excluding basalts contained one or more cracks
while only 38% of basalt rocks from the Cima flow contained at least one crack) likely
due to heat dissipation and reduction of thermal stresses through vesicles. The presence
of vesicles does not allow sufficient thermal stresses to build, keeping the pressure below
7 MPa, and preventing tensile failure and the formation of a crack. This is supported not
only by correlation between decreases in the average number of cracks with increases in
the density of vesicles, but also by crack size data: cracks on basalt rocks are
comparatively short, due to a relatively low amount of tensile stress buildup on the clast
surface that would be needed to cause long cracks.
- 31 -
CRACK PRESERVATION
Basalt rocks have a median crack density of 15, coarse plutonics have 26, fine
plutonics have 22, limestones have 24, and metavolcanics have 35. However, differences
in crack density may be partly a function of crack preservation instead of susceptibility to
cracking (Fig. 22). For example, cracks may not be preserved in plutonic rocks because
cracks tend to form at grain boundaries, resulting in rapid granular disintegration.
Similarly, in limestone, edges of cracks that form at the clast surface dissolve rapidly and
challenging to identify on etched surfaces. This hypothesis is supported by crack size data
presented previously where cracks on plutonic rocks never achieve large sizes.
Figure 22. An
example of
differences in
crack
preservation
between a coarse
plutonic rock
(left) and
limestone (right).
Overall this data supports the hypothesis that there is a correlation between rock
type and crack orientation, density, and size. Also supported is the hypothesis that
surface rock cracks may exhibit preferential orientations as a function of rock type. Since
the majority of cracking is in the NE quadrant, this data also provides additional evidence
of the importance of directional insolation in cracking rocks.
- 32 -
CONCLUSIONS
The agreement of this Mojave Desert data with previous studies (e.g., McFadden
et al., 2005, Eppes et al., 2010) supports the hypothesis that surface rock cracks may
exhibit preferential orientations as a function of rock type, thereby providing additional
evidence of the importance of directional insolation in cracking rocks. Evaluation of this
data indicates that rock type plays an important role with respect to rock crack density
and size. With the exception of basalt, differences in crack density and size are likely
attributed to differences in crack preservation for different rock types, not necessarily to
differences in the rate of cracking. For basalts, data correlating crack density with vesicle
volume suggest that cracks do not form as rapidly in vesicular basalt, possibly due to heat
dissipation in vesicles. Rock type should be further evaluated to explore the mechanisms
such as differences in mineralogy, clast energy threshold (ala Moores et al., 2008), and
other thermodynamic properties that might control cracking.
- 33 -
ACKNOWLEDGMENTS
This study was made possible through the National Science Foundation Research
Experience for Undergraduates, NSF Award #EAR-0844335. Thank you to the Sweeny
Granite Mountains Desert Research Center for the wonderful housing. Special thanks to
Professors Bob Carson and Nick Bader at Whitman College and Professor Missy Eppes
at University of North Carolina at Charlotte for their excellent advice, editing, and
immeasurable help along the way. A huge thank you to my fellow REU-ers: Ivy, Josh,
and Kiernan for sweating, measuring, and experiencing the Mojave with me. Lastly, a
big thank you fellow geology majors at Whitman and my parents− thank you for
laughing with and supporting me along the way.
- 34 -
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- 38 -
APPENDIX A. CRACK ORIENTATION DATA
Rock Type
Number of
Observations
Mean
resultant
direction 1
Circular
1*
Variance
Rayleigh
2*
Test (p)
Basalt
83
N 92° E
0.59
0.923
Limestone
89
S 54° E
0.52
0.17
Coarse
Plutonic
89
N 79° E
0.46
0.01
Fine Plutonic
152
N 67° E
0.51
0.083
Metavolcanic
191
N 47° E
0.44
0.638
1
Calculated with GEOrient © v9.4.4
2
Calculated with Oriana © v 3.0
*See page 21 for explanation of variables
- 39 -
Rao’s
Spacing
Test (p) 2*
0.90 > p >
0.50
0.50 > p >
0.10
0.50 > p >
0.10
0.50 > p >
0.10
<0.01
APPENDIX B. RAW CRACK DATA FROM PROVIDENCE MOUNTAINS
AND CIMA VOLCANIC FIELD, MOJAVE DESERT, CALIFORNIA
Rocks with zero cracks are not included, but were used in data analysis.
Strike Dip
Rock
Type
2
63
B
3
38
B
10
18
B
15
74
B
19
57
B
26
53
B
36
85
B
37
38
B
37
60
B
42
73
B
44
81
B
48
70
B
56
65
B
59
73
B
65
67
B
70
79
B
73
81
B
85
68
B
87
76
B
95
67
B
96
89
B
98
50
B
99
63
B
103
11
B
104
89
B
106
66
B
109
76
B
110
79
B
112
17
B
116
82
B
118
72
B
122
41
B
124
35
B
126
63
B
133
75
B
296
61
B
56
87
CP
135
73
B
298
72
B
57
82
CP
138
78
B
305
48
B
57
48
CP
139
35
B
305
88
B
58
81
CP
144
43
B
310
72
B
59
84
CP
145
89
B
311
70
B
59
77
CP
159
64
B
323
72
B
61
90
CP
170
74
B
326
61
B
62
45
CP
174
75
B
334
39
B
64
42
CP
174
73
B
336
73
B
68
54
CP
187
63
B
338
37
B
70
38
CP
196
86
B
340
64
B
85
38
CP
204
55
B
344
50
B
87
73
CP
212
50
B
351
68
B
93
80
CP
220
75
B
0
73
CP
97
59
CP
221
66
B
5
49
CP
100
72
CP
221
76
B
5
65
CP
105
4
CP
221
70
B
12
89
CP
107
62
CP
228
16
B
16
46
CP
109
54
CP
239
69
B
23
81
CP
109
68
CP
240
67
B
33
53
CP
109
67
CP
240
67
B
35
38
CP
109
54
CP
246
68
B
36
65
CP
112
80
CP
250
74
B
37
32
CP
117
73
CP
251
33
B
39
60
CP
119
74
CP
252
70
B
39
71
CP
131
76
CP
253
48
B
43
38
CP
133
77
CP
254
65
B
44
38
CP
147
79
CP
261
77
B
48
52
CP
147
90
CP
263
65
B
51
46
CP
150
20
CP
263
66
B
51
85
CP
158
10
CP
265
69
B
52
72
CP
163
59
CP
268
83
B
52
33
CP
170
19
CP
273
80
B
52
37
CP
172
65
CP
294
80
B
53
69
CP
176
45
CP
- 40 -
197
78
CP
9
56
FP
95
53
FP
176
88
FP
210
63
CP
10
77
FP
96
75
FP
177
41
FP
218
60
CP
11
7
FP
98
71
FP
184
70
FP
233
88
CP
22
76
FP
99
66
FP
185
73
FP
234
55
CP
24
74
FP
102
36
FP
187
75
FP
243
12
CP
25
81
FP
103
73
FP
188
40
FP
245
46
CP
26
76
FP
103
52
FP
189
84
FP
250
62
CP
30
14
FP
103
88
FP
189
78
FP
252
39
CP
34
74
FP
105
60
FP
195
87
FP
259
18
CP
38
77
FP
106
81
FP
197
87
FP
264
47
CP
39
36
FP
110
65
FP
201
50
FP
269
64
CP
39
79
FP
110
56
FP
204
45
FP
270
56
CP
39
62
FP
112
51
FP
205
74
FP
274
80
CP
39
76
FP
114
3
FP
209
31
FP
283
64
CP
40
74
FP
115
70
FP
210
45
FP
285
38
CP
41
54
FP
115
67
FP
211
24
FP
285
34
CP
41
78
FP
118
51
FP
212
75
FP
287
6
CP
43
61
FP
119
30
FP
213
54
FP
290
74
CP
50
50
FP
121
57
FP
214
2
FP
291
32
CP
53
52
FP
127
77
FP
220
78
FP
294
82
CP
54
61
FP
129
71
FP
224
74
FP
294
76
CP
58
5
FP
130
84
FP
230
80
FP
308
58
CP
60
75
FP
134
80
FP
230
80
FP
309
89
CP
61
65
FP
136
35
FP
236
79
FP
309
21
CP
68
0
FP
140
26
FP
242
77
FP
317
76
CP
69
68
FP
140
74
FP
245
73
FP
321
42
CP
75
81
FP
142
84
FP
246
54
FP
323
39
CP
75
58
FP
144
65
FP
247
52
FP
325
30
CP
81
65
FP
145
77
FP
248
81
FP
330
63
CP
82
55
FP
148
22
FP
249
45
FP
334
30
CP
83
69
FP
156
26
FP
258
31
FP
335
82
CP
85
74
FP
156
73
FP
261
63
FP
356
71
CP
85
54
FP
157
58
FP
262
40
FP
0
64
FP
86
68
FP
159
33
FP
264
17
FP
1
71
FP
86
70
FP
164
1
FP
265
61
FP
1
29
FP
87
61
FP
165
78
FP
267
56
FP
2
30
FP
88
45
FP
165
76
FP
269
62
FP
2
55
FP
89
26
FP
168
72
FP
269
70
FP
5
63
FP
89
73
FP
171
45
FP
270
66
FP
6
39
FP
95
12
FP
175
65
FP
271
49
FP
- 40 -
274
81
FP
69
83
L
190
81
L
7
42
V
276
5
FP
71
44
L
203
86
L
8
49
V
276
48
FP
73
21
L
213
73
L
16
84
V
278
78
FP
78
71
L
213
34
L
18
66
V
279
87
FP
81
54
L
215
49
L
20
16
V
280
44
FP
85
47
L
215
90
L
20
30
V
288
76
FP
86
54
L
218
64
L
20
50
V
290
76
FP
91
80
L
225
90
L
25
61
V
291
55
FP
91
68
L
250
68
L
26
21
V
298
66
FP
94
48
L
268
72
L
27
72
V
314
26
FP
97
78
L
272
73
L
29
23
V
322
43
FP
104
55
L
280
41
L
34
56
V
323
51
FP
104
77
L
285
78
L
34
76
V
329
61
FP
107
12
L
289
74
L
35
85
V
330
39
FP
107
49
L
290
44
L
36
54
V
339
78
FP
109
24
L
292
68
L
36
25
V
342
66
FP
114
64
L
293
8
L
37
87
V
344
78
FP
114
22
L
295
79
L
38
74
V
347
68
FP
114
33
L
298
56
L
43
57
V
348
51
FP
115
66
L
298
73
L
44
44
V
352
44
FP
116
59
L
300
75
L
46
75
V
353
36
FP
116
62
L
307
45
L
49
78
V
353
71
FP
116
65
L
308
49
L
50
29
V
356
59
FP
126
71
L
309
68
L
50
13
V
359
69
FP
135
18
L
311
19
L
51
41
V
0
80
L
141
63
L
312
75
L
51
54
V
5
54
L
142
8
L
314
63
L
53
242
V
9
71
L
143
75
L
315
66
L
54
75
V
12
47
L
143
60
L
323
69
L
54
63
V
16
38
L
144
12
L
331
74
L
55
50
V
19
49
L
146
67
L
335
90
L
55
62
V
21
79
L
148
27
L
344
64
L
58
60
V
21
85
L
149
66
L
348
18
L
60
22
V
27
45
L
149
74
L
352
60
L
60
29
V
27
83
L
158
72
L
359
46
L
60
45
V
28
86
L
160
80
L
3
60
V
62
34
V
36
41
L
161
70
L
4
54
V
63
78
V
38
62
L
165
94
L
5
16
V
64
35
V
50
84
L
175
75
L
5
21
V
65
56
V
64
78
L
186
31
L
5
66
V
65
71
V
- 41 -
67
35
V
168
75
V
229
80
V
292
19
V
70
44
V
170
64
V
230
68
V
292
68
V
73
24
V
170
29
V
230
74
V
293
71
V
76
37
V
172
49
V
231
10
V
293
30
V
79
64
V
174
40
V
234
79
V
295
22
V
79
40
V
174
14
V
239
48
V
295
63
V
81
78
V
175
67
V
239
70
V
297
66
V
81
55
V
175
69
V
244
12
V
299
41
V
83
36
V
176
62
V
244
37
V
300
70
V
90
19
V
176
55
V
254
53
V
301
44
V
91
44
V
179
55
V
254
20
V
302
24
V
93
88
V
180
36
V
255
75
V
302
31
V
94
63
V
180
49
V
257
70
V
303
35
V
97
67
V
180
20
V
259
79
V
304
56
V
99
65
V
181
70
V
259
72
V
305
335
V
99
30
V
181
10
V
260
66
V
307
16
V
101
75
V
183
61
V
265
76
V
308
35
V
104
14
V
184
114
V
265
80
V
311
68
V
113
84
V
184
44
V
268
52
V
314
52
V
122
86
V
186
63
V
269
60
V
314
44
V
123
48
V
186
61
V
270
75
V
315
26
V
125
34
V
190
76
V
270
35
V
319
43
V
125
28
V
190
90
V
271
75
V
320
55
V
125
9
V
191
90
V
271
79
V
321
44
V
131
41
V
193
78
V
271
60
V
331
53
V
135
84
V
194
46
V
274
75
V
332
48
V
138
31
V
195
65
V
274
55
V
332
68
V
142
86
V
198
43
V
276
52
V
340
116
V
148
40
V
204
76
V
278
71
V
340
65
V
150
29
V
204
62
V
280
68
V
342
71
V
152
43
V
204
64
V
280
50
V
351
42
V
160
72
V
205
64
V
284
41
V
351
44
V
161
66
V
205
78
V
285
49
V
353
90
V
162
68
V
206
44
V
288
51
V
355
44
V
163
44
V
209
86
V
288
48
V
357
52
V
164
67
V
211
74
V
288
39
V
358
53
V
165
58
V
212
64
V
289
67
V
165
20
V
214
79
V
289
78
V
Key: B=Basalt, CP= Coarse Plutonic, FP=Fine Plutonic, L=Limestone, V=Metavolcanic
- 42 -