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Case Hardening Vignettes from the Western USA: Convergence of Form as a Result of Divergent Hardening Processes


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The rock weathering literature contains the hypothesis that case hardening exemplifies equifinality, where the same end state can be reached by many potential processes in an open system. We present analytical data from six different sites in the western USA to assess the hypothesis of equifinality. Case hardening can be produced on: (1) sandstone in Petrified Forest National Park, Arizona, from the addition of silica glaze, rock varnish and heavy-metal skins; (2) sandstone in Whoopup Canyon, Wyoming, from silica glaze that formed originally inside subsurface joints combined with externally applied iron film, silica glaze, and rock varnish; (3) welded tuff in Death Valley, California, from the accumulation of rock varnish and heavy metal skins of Mn and Fe; (4) sandstone in Sedona, Arizona, from the protective effects of rock varnish accretion and heavy metal skins of Mn and Fe; (5) basalt on the Big Island, Hawai'i, from the accumulation of silica glaze inside vesicles; and (6) sandstone at Point Reyes, California, from a lithobiont mat of fungi and lichen. Each developed the general form of a case-hardened shell, protecting the surface from erosion. In accordance with the hypothesis of equifinality, the processes that led to similar appearance differ.
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Case Hardening Vignettes
from the Western USA:
Convergence of Form as a Result of
Divergent Hardening Processes
Ronald I. Dorn, Jacob Dorn, Emma Harrison, Eyssa Gutbrod,
Stephen Gibson, and Philip Larson
Arizona State University
Niccole Cerveny and Nicholas Lopat
Mesa Community College
Kaelin M. Groom and Casey D. Allen
University of Colorado Denver
e rock weathering literature contains the hypothesis that case hardening
exemplies equinality, where the same end state can be reached by many
potential processes in an open system. We present analytical data from six
dierent sites in the western USA to assess the hypothesis of equinality.
Case hardening can be produced on: (1) sandstone in Petried Forest
National Park, Arizona, from the addition of silica glaze, rock varnish and
heavy-metal skins; (2) sandstone in Whoopup Canyon, Wyoming, from
silica glaze that formed originally inside subsurface joints combined with
externally applied iron lm, silica glaze, and rock varnish; (3) welded tu
in Death Valley, California, from the accumulation of rock varnish and
heavy metal skins of Mn and Fe; (4) sandstone in Sedona, Arizona, from
the protective eects of rock varnish accretion and heavy metal skins of
Mn and Fe; (5) basalt on the Big Island, Hawai‘i, from the accumulation
of silica glaze inside vesicles; and (6) sandstone at Point Reyes, California,
from a lithobiont mat of fungi and lichen. Each developed the general
form of a case-hardened shell, protecting the surface from erosion. In
accordance with the hypothesis of equinality, the processes that led to
similar appearance dier.
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Differential weathering of rocks leads to varying degrees of resistivity
of dierent rock types to erosion. Such dierences in weathering provide
a fundamental control on Earths topography (Ollier 1984, Pain and Ollier
1995), as exemplied by dierent classic landforms settings. For example,
dierential weathering has led to dierences in erosion of the sedimentary
layers of the Colorado Plateau, resulting in certain resistant sedimentary
layers serving as caprocks in a staircase topography (Dutton 1882). e
talus atirons that occur in front of such cuestas develop because the talus
rock falls from the caprocks, in turn protecting the weaker rock beneath
(Gerson 1982). Of particular importance is the concept of topographic
inversion, where the most erosion-resistant materials become topographi-
cally prominent features within the landscape, despite a depositional origin
within a topographic low (Pain and Ollier, 1995). Examples include basalt
ows where lava owed down valleys, eventually becoming the high point
stream-channel deposits are now preserved as relatively high topography
(Maizels 1990); and even duricrusts like calcrete (Reeves 1993) and silcrete
(Summereld 1983), at a much larger scale.
We examine the role of dierential weathering on the scale of individual
boulders or rock outcrops. Case hardening exemplies the importance of dif-
ferential weathering inuencing the dierential erosion of rock surfaces—in
our study, at the scale of meters to millimeters. Oen darkened by coatings
such as rock varnish, case hardening creates a dierential resistance to
detachment where indurated surfaces erode more slowly than unprotected
rock. Hardening the outer shell of a rock surface can also promote the
formation of visually interesting forms such as honeycombed weathering
features and rock shelters used by prehistoric people.
Although there exists a long-held belief that weathered solutions rise
Walther 1891, Peel 1960, Longwell et al. 1950, Holmes 1965), very little
evidence exists for an internal origin of indurating agents. Instead, avail-
able evidence indicates that case hardening occurs when abiotic or biotic
materials, applied externally, increase the resistance to detachment in the
outer few millimeters of a rock surface (Dorn 2004, 1998).
Case hardening is important in physical geography and stone conserva-
tion. Geomorphologists connect case hardening to such issues as tafoni
Dorn et al: Case Hardening Vignettes from the Western USA
pedestal rocks (Crickmay 1935). Case hardening processes may play a role
contemporary buildings (McAlister et al. 2003), and rock art (Tratebas et
al. 2004, Cerveny et al. 2006, Cerveny 2005).
Figure 1.—Newspaper Rock member sandstone within Petried Forest National
Park hosts many examples of case-hardened sandstone surfaces. Each image
in this gure displays petroglyphs originally engraved into the case hardening
on a planar joint surface darkened by rock varnish. Many of the engravings
in the park range from 600 to 1100 calendar years BP (Dorn 2006). Decayed
engraved surfaces are progressively undermined by erosion of the underlying
rock. With the assumption that the engravings were manufactured when the
case hardened surface was fully intact, the rate of erosion of many panel faces
can be roughly estimated as ve to ten percent of a panel surface per century.
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Weathering forms such as case hardening support the geomorphic
concept of equinality (Goudie and Viles 1999, Turkington and Paradise
potential means in an open system. We present analytical data from dierent
sites in the western United States to assess the hypothesis of equinality for
case-hardened surfaces.
Field Sites
Sampling sites analyzed for this study were not selected randomly. We
compiled analyses collected for other projects that have been repurposed
to understand case hardening in selected western United States sites. For
example, the granodiorite of South Mountain, Arizona, was sampled to
explore the eect of paintball on stone surfaces. Welded tu in Death Valley
was collected to understand rock varnish processes, and the basalt on the
Big Island of Hawai‘i was collected as part of a petroglyph dating project.
us, the electron-microscope imagery gathered in previous studies can be
used in our attempt to assess equinality of case-hardened surfaces.
Four sampled sites in three very dierent environments involve sand-
stone. Samples from the high desert of Petried Forest National Park (PEFO)
on the Colorado Plateau were collected to understand rock art chronology
(Dorn 2006). e pine forests of the Black Hills provide the environmental
backdrop for samples collected to understand how re impacts rock art pan-
els (Tratebas, Cerveny, and Dorn 2004). Marine terrace-derived sandstone
from Point Reyes was sampled to understand the nature of rock coatings in
this maritime climatological setting, as were Sedona’s semi-arid cli faces.
In addition to samples collected for electron microscopy, Petried Forest
National Park provided the setting for a systematic study of petroglyph panel
stability. Students from multiple institutions collected data on over 2,500
panels distributed across the park.
Dierent microscopic techniques generated imagery and chemistry to un-
derstand the nature of case hardening at dierent sites. Light microscopy
provides information on the scale of microns to millimeters. Scanning
electron microscopy (SEM) generates a topographic perspective using sec-
ondary electrons (SE). SEM with a back-scattered electron detector (BSE)
generates an image of average atomic number (Z) of a at surface that, here,
is typically a cross-section from the surface of a sample down into the rock.
Dorn et al: Case Hardening Vignettes from the Western USA
Energy dispersive X-ray (EDS) and wavelength dispersive (WDS) analyses
measures the elemental composition of specic areas—typically micron-
sized spot sizes.
In addition to electron microscopic techniques, case hardening was
studied in the context of a rock art project carried out at PEFO. e Rock
Art Stability Index (RASI) is a triage technique for condition assessment
of rocks containing Native American rock art (Dorn et al. 2008, Cerveny
2005). RASI is used extensively throughout PEFO, and in this study col-
lege students and K–12 teachers gathered data relating case-hardening
with adjacent aking and scaling erosion. e methodology requires eld
researchers, typically docents or introductory science students in college,
to utilize basic training to gather observation eld-data for approximately
three dozen weathering characteristics. Scores are compiled in the laboratory
and a value of stability assigned on a panel-by-panel basis. Generalizations
or regional causation for specic weathering patterns can then be analyzed
and communicated, giving sound data for site managers to make informed
conservation and preservation decisions. In the PEFO research, over 2,500
panels were measured using RASI, resulting in data being gathered on case
hardening for an unprecedented number of sites.
Petried Forest National Park
Case hardening at Petried Forest National Park (Figure 2A) results from a
mixture of dierent added constituents. e process starts when silica glaze is
added to the walls of buried joint faces. Water moving through joints carried
dissolved silica that reprecipitated in pores (Figure 2C). en, upon exposure
of a joint at the surface, rock varnish formed. Although portions of varnish
are geochemically stable, much of the varnish can dissolve. Remobilized
iron and manganese has then moved into the pores in the sandstone and
added to the case hardening (Figure 2B).
Reports were collected on the weathering condition of over 2,500 rock
art panels in Petried Forest National Park. e RASI scoring system is an
index amounting to a score of 0 if there is no weathering and over 60 if the
panel is falling apart from rock decay. Because case hardening provides
a stabilizing inuence, researchers scored case hardening with negative
numbers in an ordinal scale of not present (0), present (-1), obvious (-2),
or dominant (-3). e overall pattern (Figure 3) reveals that case harden-
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ing is present in about forty-two percent of the scored panels, and that case
hardening dominates only ve percent of the rock art panels.
e spatial distribution reects the presence of the Newspaper Rock
member sandstone, the locations of petroglyph manufacturing, as well
as a clustering of dominant case hardening whose cause has not yet been
Figure 2.—Case hardening of sandstone at Petried Forest National Park. (A) A case-
hardened joint face forms a stable surface used for carving of a bird motif and an earlier
anthropomorphic image. Note, however, that the case hardening has been breached—
with wristwatch for scale. (B) Back-scattered electron-microscope image of a cross-section
collected at location c” in image A. The bright material between the grains is a heavy metal
skin of iron and manganese constituents, likely dissolved from rock varnish and mobilized
into the sandstone to stabilize the upper millimeter of a surface. Note also the abundant
porosity underneath the case hardened heavy metal skin. Once the case hardening is
breached, this porosity makes erosion rapid. (C) Another type of case hardening exists
at the very surface of this panel. A mixture of silica glaze and rock varnish can oer
additional stability in the upper few microns, where this secondary electron image was
from a sample also collected from the letter C above the watch in image A.
Dorn et al: Case Hardening Vignettes from the Western USA
Whoopup Canyon, Black Hills, Wyoming
Whoopup Canyon in the Black Hills, Wyoming, exhibits sandstone joint
faces that were originally case hardened by silica glaze before the joints
became exposed. Iron lm then remobilized under acidic conditions and
further impregnated the sandstone. Later rock varnish formation led to
remobilization of the varnish constituents that also moved downward into
the rock to mix with silica glaze. us, the case hardening is a complex mix-
ture of dierent materials. e porosity from enhanced weathering in the
underlying sandstone allows detachment of rock fragments as rock aking.
Figure 3. —Histogram and mapping of observations of case hardening at rock
art panels in Petried Forest National Park. “Dominant” observations of case
hardening are the smallest symbol on the map; they represent the smallest
number of observations and hence are visible because they are portrayed on top.
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Death Valley, California
Studies of rock varnish in Death Valley have yielded insight into the impor-
tance of lichens, even in hyperarid settings. Lichens colonizing the surfaces
of rock varnish secrete acids.
Figure 4.—Sandstone
surface at Whoopup
Canyon, Black Hills,
Wyoming (width of upper
image about 30 cm) has
case hardened due to
the accumulation of
three types of materials
as revealed in the lower
back-scattered electron-
microscope image. First,
silica glaze impregnated
a joint surface, while it
was still unexposed in the
subsurface. Second, iron
seeped into the sandstone
and lled in pore spaces
to cement sand grains
together. Third, rock
varnish formed on
the surface and also
remobilized to mix
with silica glaze.
Enhanced dissolution
of quartz underneath
the case hardening led
to detachment of the
millimeter-scale ake.
Dorn et al: Case Hardening Vignettes from the Western USA
e acidity reduces Mn (IV) and Fe (III), which causes the varnish to
develop nanoscale pore spaces (Figure 5C) as the Mn (II) and Fe (II) migrate
into the underlying rock.
Figure 5.—Death Valley rock varnish undergoing dissolution from the acid-
producing activity of lichens. (A) Secondary electron (SE) image that shows the
topography of a cross-section. (B) Back-scattered electron (BSE) image that shows
the atomic number (Z) of the material in this cross-section. Notice that most of
the lichen turns black in the BSE image, because of its lower atomic number.
The bright specks in the lichen are oxalate minerals that often precipitate inside
lichens. (C) Close-up BSE image of the pocket of porous material underneath
the lichens.
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e process of varnish remobilization was studied at the nanoscale
level through energy-dispersive X-ray analyses. Prior electron-microscope
elemental data on rock varnish chemistry generated only data from microm-
eter-scale spot analyses that are typically two microns in diameter. rough
the use of higher-resolution electron microscopy and higher energy levels,
spot sizes were reduced to about 1.3 nanometers. e descriptive statistics
of these data reveal tremendous variability that could be from the ongoing
dissolution of the varnish. Whereas larger spot sizes would “average” these
tiny pores, the tremendous nanoscale chemical variability suggests that
varnish undergoes uneven dissolution on a submicron scale.
Figure 6.—(A) Welded tu, Death Valley, forms a rock shelter, because of a
case-hardened outer shell. Image A is 2 meters wide. (B) In this back-scattered
electron image, the brighter material accumulating in the fractures is manganese
(Mn)-Iron (Fe)-clay rock varnish. C) In addition, some of the Mn and Fe have
reprecipitated in micron-sized pore spaces. The lichens (li) appear in images A
and B because of its lower atomic number.
Dorn et al: Case Hardening Vignettes from the Western USA
Table 1. Descriptive statistics of 128 nanoscale measurements of rock varnish
undergoing remobilization from lichens.
Element Average Median Standard
Value Range
C 0.14 0.00 1.20 0.00 11.21 11.21
F 3.32 1.45 3.71 0.00 13.18 13.18
Mg 1.46 1.31 1.42 0.00 18.34 18.34
Al 17.89 18.11 1.24 10.72 19.90 9.18
Si 11.43 10.96 2.78 1.58 21.56 19.98
P 1.17 1.22 0.24 0.37 1.69 1.32
Cl 0.00 0.00 0.03 0.00 0.20 0.20
K 0.48 0.44 0.21 0.23 1.56 1.33
Ca 0.69 0.58 1.41 0.26 17.78 17.52
Ti 0.73 0.00 2.86 0.00 15.50 15.50
Mn 14.90 15.48 4.08 3.96 24.21 20.25
Fe 6.57 6.85 2.02 0.77 12.04 11.27
Cu 0.95 0.92 0.18 0.50 1.76 1.26
Zn 0.00 0.00 0.06 0.00 0.68 0.68
Ba 0.94 1.24 0.84 0.00 2.97 2.97
Ce 0.28 0.00 0.38 0.00 1.32 1.32
Re 0.01 0.00 0.09 0.00 0.80 0.80
O 39.06 39.06 2.96 33.10 55.23 22.13
e instability of varnish very much relates to case hardening. e
indurated rock has formed a rock shelter where rock varnish and heavy
metals have been dissolved by lichens. Aer uneven dissolution of nanoscale
pockets, some of the varnish moves into fractures in the rock and reprecipi-
tates. Occasionally, just the iron and manganese ll in pore spaces as heavy
metals without the clays. us, even in a hyperarid environment, the acid-
ity produced by lithobionts can be important in freeing up case-hardening
agents to penetrate into the rock.
Sedona, Arizona
e case hardening of sandstone by the accumulation of manganese in the
pore spaces does not necessarily have to do with the dissolution of varnish
constituents, as exemplied in the previous results sections. e dark streaks
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e manganese moves into the pore spaces between the sand grains
and sometimes pushes the grains apart. e material composing these dark
streaks is oen a mix of true clay-Mn-Fe rock varnish and heavy metal skin
dominated by manganese. e case hardening can extend into the rock up
to a millimeter.
Basalt surfaces on the rainshadow portions of the island of Hawai‘i oen
accumulate silica glaze. e silica glaze that coats ow surfaces dissolves
and reprecipitates in vesicles inside the basalt. One result is a color change
from the dark black of a basalt ow to the light brown of a silica-glaze coated
surface. A second result is the case hardening of the basalt surface.
Figure 7.—Waterow streaks at Sedona can form through the accumulation of
fungi, lichens, heavy metal skins, and rock varnish. In this case, the sampled streak
is a mixture of rock varnish (at the surface) and heavy metal skins (remobilized
magniferous varnish) that move downward through pore spaces to impregnate
the sandstone of Sedona, at Schnebly Hill Road, Arizona.
Dorn et al: Case Hardening Vignettes from the Western USA
Point Reyes, California
Sandstone weathering at Point Reyes includes a variety of forms, such as
tafoni, alveoli, gnamma pits, and case hardening. A sample of case-hardened
sandstone reveals the presence of a lithobiontic crust composed of lichen
mixed with fungi. e biolm also contains calcium oxalate minerals, per-
haps secreted by the lichens. e biolm was typically less than 0.1 mm thick.
However, the laments are clearly able to bind the underlying sandstone in
a way that indurates the surface.
Figure 8.—Hawai‘ian petroglyph panels and petroglyphs are often case hardened
by silica glaze that originally accretes on surfaces, and then remobilizes into the
rock and inlls vesicles. The back-scattered electron-microscope image on the
left shows dark-colored silica glaze, where about 30% of a vesicle has been lled
by silica glaze. In addition to silica glaze, other precipitates (e.g. iron skins, rock
varnish, carbonate) formed inside the vesicle. The image on the right from the
Ki’I site on the Big island of Hawai‘i shows a panel composed of basalt, but the
lighter color of the engraved surface in the foreground derives from silica glaze.
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South Mountain, Arizona
We think it is important to mention anthropogenic agents in this discussion
of case hardening, since humans continue to inuence natural surfaces. One
way that natural rock surfaces are impacted rests in grati and activities like
paintball gaming. e study of modern paints on rock surfaces has heretofore
been limited to conservation eorts designed to remove or hide these scars.
In a pilot study to examine the inuence of paintballs, we examined
samples of natural rock surfaces hit by this recreational activity using
back-scattered electron microscopy. Preliminary results indicate that the
paint has begun the process of physically separating from the underlying
rock coating. ere are still abundant attachment points. However, in the
unknown amount of time between the paintball attachment and sampling,
substantial physical separation has occurred. us, paintballing may not
become a case-hardening agent and will eventually detach itself from rock
surfaces, eliminating paintballs as agents of rock case hardening.
Figure 9.—Point Reyes case hardening in Image A, where the ice plant in the lower
right provides scale. The indurated sandstone erodes because of undermining
as the unhardened sandstone detaches. A secondary electron image of the case
hardened shell reveals that it is composed of a ~40 µm thick layer of a mat of
lichen mixed with fungi. The angular materials underneath the lamentous mat
are minerals in the underlying sandstone.
Dorn et al: Case Hardening Vignettes from the Western USA
Figure 10.Anthropogenic paint balls applied to granodiorite rock surfaces
at South Mountain Park, Arizona. BSE imagery reveals that the anthropogenic
paint (p) separates from the underlying rock surface after application of epoxy
(ep) application of the vacuum used to carbon coat the samples. The BSE
imagery and EDS analyses also reveals two case-hardening agents are present
on the rock surface: rock varnish (rv) and iron lms (if). The iron lms tend to
incorporate silt-sized fragments as preprecipitating containing iron envelope
dust particles that are ubiquitous on rock surfaces. Note: normally, BSE imagery
shows epoxy as black. However, the contrast and brightness were adjusted in
a way to minimize contrast and reveal the epoxy and the paint.
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Our ndings indicate that equinality does appear to be valid for the wide
variety of study sites and environments in the western United States that
were studied using electron microscopy. Case hardening can occur through
lithobiontic crusts composed of fungi and lichen, by lichens dissolving heavy
metals that are then reprecipitated in the outer shell of a rock, by silica glaze
infusing into vesicles in basalt ows, and by mixtures of silica glaze, rock
varnish, and heavy metals that hold sandstone grains together.
e persistence of equinality can be understood further by examin-
ing environmental change over time for the studied sandstone surfaces of
Petried Forest National Park and Whoopup Canyon.
The first step in-
volved the formation of
silica glaze inside un-
opened joint faces that
started the process of
case hardening. Second,
erosion of soil and more
weathered rock exposed
silica-glaze impregnated
joint faces to the subaer-
ial environment. ird,
subaerial rock varnish
formed on the surface.
Fourth, leaching of the
iron and manganese out
of the rock coating and
into the underlying rock
added to the initial case
hardening started by
the silica glaze. Thus,
the equinality of case
hardening forms that
started with silica glaze
inside a subsurface joint
Figure 11.Weathering profiles in many different
rock types develop in a fashion generalized in this
diagram. Then, enhanced erosion can strip the overlying
weathering zone. At Petried Forest National Park, for
example, the soil and highly weathered rock materials
that may have been present during the late Pleistocene
have eroded away. Moderately weathered and slightly
weathered rocks are now exposed at the surface. The
silica glaze that formed inside joint faces represents the
rst step in case hardening. This diagram is modied
from (Ehlen 2005).
Dorn et al: Case Hardening Vignettes from the Western USA
was enhanced by completely dierent processes operating in a surcial
Another perspective on equinality can be understood by examin-
ing a time sequence of case hardening on a single rock face (Turkington
and Paradise 2005), where case hardening stabilizes a surface for a time
as the underlying rock continues to weather. When the hardened surface
is breached, this leads to rapid erosion of the underlying heavily decayed
weathering rind. Aer the weathered material erodes away, the surface can
restabilize and begin to case harden.
is generalized sequence can be seen in operation at the Petried For-
est site. Case hardening has stabilized the surface, but the back-scattered
electron-microscope image reveals that weathering has continued under-
neath as indicated by the abundant porosity in this weathered rind. e
breach keeps growing until the weathered rind is eroded down to fresher
rock, at which time a new round of case hardening takes place. e cycle
continues when this new case hardening is breached. With forty-two percent
of panels hosting case hardening, and a process that reforms induration aer
Figure 12.From Turkington and Paradise (2005), a sandstone surface can
undergo a series of changes. In this idealized diagram, subsurface dissolution
leads to the creation of a weathering rind (A), followed by the surcial
accumulation of minerals and lithobions (B) to case harden surfaces (C). Then, the
case-hardened surface is breached (D), leading to rapid erosion of the decayed
weathering rind underneath (E and F). Eventually, the new surface stabilizes (G).
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Figure 13.A petroglyph panel of sandstone at Petried Forest National Park
supports the general model of sandstone face erosion (Turkington and Paradise
2005), where the outermost case-hardened panel was breached, leading to
the enlargement of the cavity. Then, after the weathered rind eroded away,
the surface restabilized with the formation of rock varnish and some case
hardening. Then this restabilized surface was breached in turn. The lower,
back-scattered electron-microscope image (collected from SEM sample arrow)
shows how the rock underneath the varnish-stabilized surface continues to
decay. Much of the quartz has dissolved, and the porosity of the weathering
rind has increased over time.
Dorn et al: Case Hardening Vignettes from the Western USA
breaching of the initial surface, case hardening appears to be a persistently
developing land surface within the Park.
e general eld of rock decay concerns itself with processes and forms
produced by biogeochemical and biophysical mechanisms. e end result
of rock decay is typically thought of as unidirectional, where processes of
rock decay promote further rock decay. e output” of decay is typically
thought of as products transported by erosional processes, where sucient
particle-size dimunition eventually generates transportable material.
rough investigation of rock data from six dierent sites around the
western United States, we explored whether the concept of equinality ap-
plies to case hardening. In each of the sites from which data were collected,
case hardening had occurred in a way that was both unique and consistent
with the environment that housed the rock. Stone surfaces collected from all
locales demonstrate evidence of case hardening due to a variety of external
factors. Although silica glaze is present in half the sites we considered, the
conditions under which that glaze formed were dierent at each location.
Furthermore, case hardening caused by rock varnish and heavy metal skins
of Mn and Fe was observed at two sites in Arizona, and we determined
that case hardening was caused by an accumulation of fungi and lichen
in California. us, we concluded that despite dierent processes coming
into eect at each of the sites, a protective covering or shell that performs
the same ultimate function—case hardening—is present in all cases. ese
ndings support our hypothesis of equinality.
Case hardening stands as a testament to the existence of negative feed-
backs in this progression of rock decay, where the products of rock decay
act to inhibit erosion. In doing so, case-hardened rock surfaces stand out as
similar forms in a host of dierent rock types and dierent environmental
settings. ere is an outer shell, typically only millimeters thick, that stands
out in relief through the more rapid erosion of unprotected rock.
Negative feedbacks, where the products of rock decay promote surface
stability, appear to be a universal phenomenon where many dierent types
of processes can produce the form of case-hardened surfaces (Turkington
here is consistent with and fully supportive of the hypothesis of equinality.
In dierent open systems involving the decay and reprecipitation of rock
materials, many dierent processes have produced case hardening.
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Our conclusion is that with equinality being such a persistent negative
feedback to surface denudation in a host of dierent contemporary bare-
rock geomorphic settings, case hardening may have been a very important
years ago and land fungi 1.3 billion years ago (Heckman et al. 2001), Earths
surface could have been protected from erosion by the action of case hard-
ening. To the best of our knowledge, the possible role of case hardening in
protecting the early Earth is a hypothesis that has not yet been considered
in the scholarly literature.
Research was supported by National Science Foundation awardsDUE-
Park. We thank the many Mesa Community College, Universityof Colorado
Denver, and Arizona State University students who compileddata on case
hardening at Petried Forest. We also thank Death ValleyNational Park
and Point Reyes National Seashore for permission to takesmall samples
for analyses.
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... This is not surprising given the widespread evidence of biogeomorphological (e.g., secondary mineral formation, EPM swelling, and contraction) and biogeochemical processes (e.g., production of organic and inorganic acids, metalcomplexing EPM), which are vital to pedogenesis [57,58]. While SABs on stone heritage imply current or past interactions with the lithic substrates, their presence does not necessarily have a biodeteriorative role as is frequently thought [59][60][61]. A growing body of literature has reported SABs' neutral or even bioprotective effects on stones under certain conditions [62][63][64][65][66]. ...
... When rock varnish is a relict landscape feature or the rate of varnish formation is too slow to re-cover engravings, it represents a non-renewable canvas for ancient artists and the rate of varnish [76,78]. However, the partial regrowth of rock varnish is a valid relative dating method for engravings superimposition [76,79], as well as offering the opportunity to estimate petroglyphs' age via chemical measurement of elements and areal density of Mn and Fe [80][81][82][83][84]. From a different point of view, the formation of continuous and some tens of microns-thick Mn-rich coating represent a case-hardened shell [61] protecting rock surfaces against wind abrasion. Where deterioration processes are particularly severe, the development of a biomineralized Mn-and Fe-rich rock varnish inside the grooves of the engravings hampers the effect of rock dismantling, sheltering petroglyphs and promoting their preservation [33]. ...
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Rock art is a widespread cultural heritage, representing an immovable element of the material culture created on natural rocky supports. Paintings and petroglyphs can be found within caves and rock shelters or in open-air contexts and for that reason they are not isolated from the processes acting at the Earth surface. Consequently, rock art represents a sort of ecosystem because it is part of the complex and multidirectional interplay between the host rock, pigments, environmental parameters, and microbial communities. Such complexity results in several processes affecting rock art; some of them contribute to its destruction, others to its preservation. To understand the effects of such processes an interdisciplinary scientific approach is needed. In this contribution, we discuss the many processes acting at the rock interface—where rock art is present—and the multifaceted possibilities of scientific investigations—non-invasive or invasive—offered by the STEM disciplines. Finally, we suggest a sustainable approach to investigating rock art allowing to understand its production as well as its preservation and eventually suggest strategies to mitigate the risks threatening its stability.
... This is consistent with the results obtained through SEM-EDS, where a possible dissolution of the varnish in the surface area and its subsequent deeper re-precipitation may be occurring. This has been previously studied in rock varnish and case hardening (Dorn et al., 2012 and will be discussed below. ...
... The LIBS analysis also shows the same behavior at different depths, with respect to the Fe-Mn-Fe alternation (Fig. 6). This phenomenon has been observed and analyzed with back-scattered electron microscopy in the so-called geochemical case hardening, which is defined as a process by which the outer shell of an exposed rock surface hardens due to near-surface diagenesis (Dorn et al., 2012. Future studies should consider the application of this technique to differentiate between laminated microlayers of varnish (Liu, 1994(Liu, , 2017Liu and Broecker, 2000 and the reprecipitation of varnish components mixed with rock minerals. ...
Desert varnish is a dark microlayer that forms on rocky surfaces that is usually associated with arid and desert environments. It consists mainly of clay minerals (60%), while the rest are Fe and Mn oxides. Growth rates are very slow and vary from <1 to 40 μm/ky. Although different proposals exist to explain their formation mechanisms, these processes are still unknown. Around the world, various groups and human communities have created petroglyphs with different meanings in desert varnish. In the Sonoran Desert, the archaeological site La Proveedora is known for having many petroglyphs made in granite rock varnishes. It is believed that people groups occupied the area during the mid-Holocene (ca 5000-3000 BC). The research was carried out by analyzing and looking for possible signals and contributions from past environments and current processes that are preserved in the varnish layers. The X-ray diffraction, laser breakdown spectra and scanning electron microscopy with energy dispersive spectroscopy analyses in varnish samples, showed that the distribution and concentrations of Fe and Mn exhibit an alternating behavior on a depth scale from the surface until contact with granite. The DNA analysis showed a major presence of the Proteobacteria and Actinobacteria groups. Notably, some of these microorganisms can incorporate Fe and Mn into their metabolic processes and mobilize them through the varnish for its formation. The 14C radiocarbon dating (370 ± 54 cal BP) indicates a very young age associated primarily with recent microorganisms. However, a first approximation was obtained for the minimum age (5000- 1000 yr BP). The comparison between the areas of varnish and the surfaces within the petroglyphs suggests that the varnish formation occurred under conditions in the past when the humidity was higher, and that it is probably a very slow, intermittent or uncoated formation mechanism today.
... That said, comparing these values can still provide useful insight into which decay patterns and behaviours are present in the WRPA. For instance, the negative average score for the "Rock Coatings" category indicates, in general, the stabilizing influences of many rock coatings found in the region-such as mature desert varnishes and case hardening (Dorn et al. 2012;Dorn et al. 2013b), which outweigh the negative impacts of salts and anthropogenic activities in the study area. Beyond average category scores, specific averages for each assessed rock decay feature can help quickly identify which processes are most prominent within the study area. ...
... A reliable and replicable system with minimal training time, completing a RASI assessment for a rock art site (or individual rock art panel) results in a score of instability severity, providing managers with a snapshot of the current state, strength, and potential longevity of rock art panels, or rock faces, with as fine a detailed assessment as necessary for any given project. Using these data, site managers can create specific priorities by integrating RASI results for individual sites in a geographic information system (GIS) database [cf., [14], [15]], and those sites in greatest danger can then be mapped across scales-whether it be across a state, province, country, within a specific site, or the rock art panel itself. ...
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Located in northeastern Arizona (USA), Petrified Forest National Park (PEFO) presents a unique story of both geologic and human history. Though perhaps most well-known for its abundant petrified wood and being part of the Painted Desert, visitors are often surprised when they discover PEFO hosts many ancient petroglyph sites. Over the years, many attempts have been made to record the petroglyph sites, but nothing has been done to assess their geomorphic stability. To address this shortcoming, we employed the Rock Art Stability Index (RASI) to assess geologic stability and (potential) deterioration of rock art sites in PEFO. Used for more than a decade as a triage for researchers assessing which rock art panels/sites are in the most danger of eroding, RASI uses a rank-based system to assess over three-dozen rock decay parameters, resulting in an overall condition analysis of a rock art panel. The findings can then be grouped together by site location to gain a clearer understanding of overall decay processes responsible for (potential) erosion. This study highlights RASI, its use as a low-cost, non-invasive, rapid field assessment technique, and assesses the geomorphic stability of five major petroglyph sites in the Petrified Forest National Park.
Fairy chimneys of ignimbrite are well studied in Cappadocia in central Anatolia, Turkey. The ignimbrite landforms of Arizona's Basin and Range, USA, in contrast, remain relatively unexplored in geomorphic scholarship, despite decades of geological research on the characteristics of the ignimbrites themselves. This research focuses on the rock‐decay processes that modify Arizona's Basin and Range fair chimneys. Dissolution of glass and groundmass, measured along joints using digital image processing of back‐scattered electron microscope imagery, reveal a 3× higher porosity than inside the rock interior. This method also measured a 3‐4× increase in dissolution near the base of the chimney compared with the rock interior, leading to notch development that promotes chimney mass wasting and subsequent ballistic impacts. Epilithic organisms studied with electron microscopy both enhance rock decay and promote case hardening. These observations are consistent with prior understanding of how fairy chimneys evolve over time: columnar joints exposed on cliff faces experience enough decay to allow erosion of joint material, this erosion separates the chimney from the rock face and notch development near the chimney base promotes mass wasting. The rock‐decay processes studied here likely operate on very different timescales. Mass wasting events leading to ballistic impacts occur at observable timescales. Varnish microlamination dating, using ultrathin sections of rock varnish formed on fairy chimneys, indicates that the case hardening of the top of fairy chimneys are much more stable than the sides; the tops last experienced surface detachment during the early Holocene, as opposed to the sides that experienced flaking throughout the late Holocene. The timescale of basal notch development is not known, with the exception that the very surface of notches erode at rates too fast to accumulate rock varnish. The timescales of epilithic organisms enhancing rock decay and dissolution of columnar joints are not known.
A Sonoran Desert petroglyph panel experienced an intense wildfire event in July 2021 that eroded the entire surface, removing the Hohokam-style rock art. Field observations during sampling in 1995 indicated that the panel: (1) was coated with a heavy rock varnish, (2) had a ‘fresh’ visual appearance, and (3) had some granite-derived sand (angular grus) at the panel’s base. Micron-scale back-scatter and nanoscale transmission electron microscopy of pre-fire samples revealed a minimal amount of decay (granite grussification): mainly minor grain-to-grain separation; minor internal dissolution; and a little feldspar grain cracking. Our basic finding is that even this minimal amount of grussification was enough to set the stage for the wildfire to erode the entire panel. Pre-fire micron-scale cracking may have enabled the fire’s steep thermal gradient to spall the surface. Panel erosion was likely enhanced by pre-existing grain-to-grain porosity to facilitate further fire-induced granular disintegration. Pre-fire nanoscale dissolution within mineral grains, formed along crystal defects, provided a weakness that then led to grain cracking of quartz and other granitic minerals. The implication for the conservation of rock art on granitic panels is worrisome, but clear and simple: condition assessments need to indicate whether any granitic sand occurs at a panel’s base. Given that many places experiencing climate change are also experiencing drought and enhanced risk from wildfire, the appropriate management recommendation would then be to remove all vegetation near the panel on a regular basis.
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Global Perspectives for the Conservation and Management of Open-Air Rock Art Sites responds to the growth in known rock art sites across the globe and addresses the need to investigate natural and human-originated threats to them as well as propose solutions to mitigate resulting deterioration. Bringing together perspectives of international research teams from across five continents, the chapters in this book are divided into four discrete parts that best reflect the worldwide scenarios where conservation and management of open-air rock art sites unfolds: 1) ethics, community and collaborative approaches; 2) methodological tools to support assessment and monitoring; 3) scientific examination and interventions; and 4) global community and collaborative case studies innovating methodologies for ongoing monitoring and management. The diverse origin of contributions results in a holistic and interdisciplinary approach that conciliates perceived intervention necessity, community and stakeholders’ interests, and rigorous scientific analysis regarding open-air rock art conservation and management. The book unites the voices of the global community in tackling a significant challenge: to ensure a better future for open-air rock art. Moving conservation and management of open-air rock art sites in from the periphery of conservation science, this volume is an indispensable guide for archaeologists, conservators and heritage professionals involved in rock art and its preservation.
From the 9th of March 2015 a wildfire burned an area of 25.7 km2, or approximately half of the Jonkershoek catchment (Western Cape, South Africa), over the course of three days. During this period large areas of fynbos and commercial forest plantations were razed, and rocks, including boulders and smaller rocks, were exposed to high temperatures. While a substantial body of work has been carried out to investigate the effects of wildfire on landscape development, less is known about the effect of wildfire on rock weathering within a landscape. Previous studies have reported the overall effect of wildfire on rock deterioration but the effect of intra‐fire temperature differences associated with heat behaviour on a slope has not been sufficiently addressed. In this study we investigate the effects of topography and proximity to moisture on rock deterioration processes. In particular, we focus on the use of in‐field rock deterioration measurements and GIS to investigate the relative influences of distance from burn source, distance from moisture source and topographical positioning of the sample site. The results indicate that boulder size and lithology are of secondary importance to the placement of fire‐affected rocks within the wider topography of the landscape, and that rock exposure to moisture, also a function of landscape position, is likely to exacerbate the response of the rocks to heat. No direct correlation was observed between the type and severity of the outwardly visible damage sustained on the sampled rocks, and the size, lithology or proximity to burn source. We argue that these findings call for a re‐evaluation of fire‐related damage in the wider context of rock response as a function of topographical variations.
This article overviews connections between nanoscale weathering and geomorphology. Nanoscale processes are on one side of a fundamental threshold between the coarser microscale (micrometers and up) and the finer nanoscale with different molecular dynamics. Nanoscale processes impact a variety of geomorphic research including Arctic and alpine mineral decay, biotic weathering as an explanation for deviations from Goldich's weathering series, carbon sequestration related to silicate dissolution, case hardening, detachment limited erosion, dirt cracking, geochemical pollution, the meteoric ¹⁰Be cosmogenic nuclide, rock coating behavior, silt production, and tafoni.
Fourteen different types of coatings cover rock surfaces in every terrestrial weathering environment, altering the appearance of the underlying landform. Some accretions interdigitate, while others blend together, creating a great number of variations. Rock coatings are important in geomorphology because coatings: alter weathering rates; play a role in case hardening surfaces; offer clues to understanding environmental change; and can provide chronometric insight into the exposure of the underlying rock surface. Following a landscape geochemistry paradigm, five general hierarchies of control explain the occurrence of different types of rock coatings: 1st order—geomorphic processes control the stability of bedrock surfaces on which coatings form; 2nd order—coatings originating in rock fissures are found on subaerial surfaces when erosion of the overlying rock occurs; 3rd order—the habitability of surfaces for fast-growing lithobionts such as lichens determines whether slowly accreting coatings occur; 4th order—the raw ingredients must have a transport pathway to the rock surface, and of course, they must be present; 5th order—physical, geochemical or biological barriers to transport then result in the accretion of the coating.
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In order to identify those petroglyph and pictograph panels most susceptible to damage, we propose a field-friendly index that incorporates elements of existing strategies to characterize the stability of stone. The Rock Art Stability Index (RASI) has six general categories: Site Setting (geological factors); Weakness of the Rock Art Panel; Evidence of Large Erosion Events On and Below the Panel; Evidence of Small Erosion Events on the Panel; Rock Coatings on the Panel; and Highlighting Vandalism. Initial testing reveals that training of individuals with no prior background in rock decay can be conducted within a two-day period and yield reproducible results. RASI’s use as a tool to promote cultural resource sustainability includes the use of a Geographic Information System to store, display and analyze rock art.
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This paper presents results of the first study of pre-fire and post-fire samples collected from rock engravings and adjacent sandstone joint faces. A 2001 wildfire at Whoopup Canyon, Wyoming, stimulated a comparison of 1991 and 2003 samples. Opti- cal microscopy of ultra-thin sections, backscattered electron microscopy, x-ray (energy dispersive and wavelength dispersive) analysis of cross sections, and high-resolution trans- mission electron microscopy reveal that fires create some thermal fractures that enhance panel erosion, but most of the fire-induced erosion occurs along weathering rinds that form long before petroglyph manufacturing. In addition, rock varnish on top of petro- glyphs experiences spalling, and fire ash with a clear potassium spike strongly adheres to rock varnish on petroglyphs and spalled sandstone. In the past, site managers assumed minimal damage away from massive spalls and other macrodamage on fire affected petro- glyphs, an assumption no longer tenable. Since it is difficult to protect rock art after a fire starts, mitigation efforts can include identification of areas of intense weathering-rind development as locales most susceptible to erosion, and clearing trees and shrubs near rock art by hand. (Key words: geomorphology, fire, rock art, sandstone, weathering.)
Sandstones of the Paleozoic Beacon Supergroup in the Dry Valley region of Antarctica (also known as the Ross Desert), undergo at least three chemical weathering processes. (1) Exudation of oxalate ± other chelators by endolithic microorganisms causes translocation of elements, producing distinctive Fe pigment patterns and accelerating mechanical weathering (exfoliation) in rocks colonized by endolithic organisms. (2) Formation of thin siliceous crusts (<0.1 mm) stabilizes rock surfaces. The siliceous crusts form by accumulation and in situ alteration of airborne dust composed of quartz, clays, and Fe oxy hydroxide s. Crust textures include petrographically amorphous lenses of silica, stained and laminated birefringent coatings, and chalcedonic protrusions in surface pits. (3) Silicification of porous quartz sandstones by growth of quartz in optical continuity with host grains produces impermeable rinds up to several cm thick. The rinds form by transport of silica toward subaerially-exposed surfaces due to wet/dry and warm/cold cycling of microenvironments. The resulting interstitial precipitates commonly trap pre-existing grain coatings in pores, thereby preserving biogenic pigment patterns in colonized rocks as trace fossils of endolithic microorganisms.
Pedestal rocks of granite, commonly associated with residual boulders, occur at many places in the southern Appalachian Piedmont. The rocks are essentially homogeneous in composition but exhibit marked contrasts in weathering of the cap and the shaft. They have previously been thought to have originated from joint blocks through frost action, rain wash, and decomposition. It is here shown that granular disintegration initiated by expansion through hydration is the dominant process involved. Hydra-tion is at a maximum on the lower slopes where there is some protection from the direct rays of the sun and where evaporation is at a minimum. The pedestal rocks are restricted to peneplains of Tertiary? age.
The recent discovery of two calcium carbonate-cemented drainage channels in the Pliocene Ogallala section of the Southern High Plains, Texas and New Mexico, indicates that Ogallala drainage channels were commonly cemented with calcium carbonate, due to leaching of calcrete from adjacent Ogallala sands. Parallel drainage lines near Lovington, New Mexico, originally thought to represent calcrete incisement along ancient dune swales, are now interpreted as an example of recent drainage between a suspenparallel drainage pattern in bas-relief.
Silcrete has been widely cited as an indicator of arid or semi-arid environments, such a palaeoclimatic inference deriving from a number of specific studies of Cenozoic silcretes in southern Africa, Australia and elsewhere. Recent investigations of Cenozoic silcretes in southern Africa reveal, however, the existence of two petrographically and geochemically distinct silcrete types, one of which has apparently formed under humid conditions.Silcretes in the Kalahari Basin (Botswana) have developed through silicification of a variety of host materials, especially playa sediments and calcrete. Petrographic characteristics (including length-slow chalcedony vugh-fills), lack of associated deep westhering profiles and limited independent palaeoclimatic evidence, suggest silcrete genesis in an alkaline, arid/semi-arid environment. In contrast, silcretes in the Cape coastal zone (South Africa) are almost invariably associated with deeply weathered bedrock, and are TiO2-rich in comparison with Kalahari silcretes. Local mobilisation and co-precipitation of silica and titanium is indicated by the presence of authigenic anatase in colloform features and glaebules in silcrete. This implies a low pH (< 4) during silcrete formation by silica replacement of weathering profile clays. Such a highly acidic weathering environment suggests abundant vegetation and a humid tropical or subtropical climate.Other recent reports of occurrences in Australia and France support the view that TiO2-rich weathering profile silcretes form under a humid climate. A semi-arid/arid palaeoclimatic interpretation remains valid for non-weathering profile silcretes, and for silcretes occurring within, but not genetically related to, deep weathering profiles.
Rock varnish, erosional grooves, and well-developed cavernous weathering phenomena occur in close association on a small biotite-monzogranite nunatak in the Northern Foothills region, Northern Victoria Land, Antarctica. The grooves, similar in appearance to the 'rinnenkarren' described in the karst literature, are developed on steeply inclined (>35degrees) bedrock surfaces while the rock varnish occurs on adjacent, more gently sloping (<15degrees) bedrock surfaces. The varnish forms a resistant carapace through which small weathering pits have developed and below which are large cavernously weathered hollows (taffoni). We argue that the intimate association between the grooves and the rock varnish indicate the nunatak has been exposed to a long period of subaerial weathering. The preservation of both phenomena supports (a) the idea that landscape modification in this exceptionally cold and and region of Antarctica is very slow and (b) the long-term stability of the Antarctic ice sheet. Copyright (C) 2002 John Wiley Sons, Ltd.