Papers by Martha Cary Eppes
Geological Society of America Bulletin, 2016
A dearth of direct field observations limits our understanding of individual mechanical weatherin... more A dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact. In particular, the specific contributions of solar-induced thermal stresses to mechanical weathering are poorly characterized. Here, we present an 11 mo data set of cracking, using acoustic emissions (AEs), combined with measurements of rock temperature, strain and other environmental conditions, all recorded continuously for a granite boulder resting on the ground in open sun. We also present stresses derived from a numerical model of the temperature and stress fields in the boulder, idealized as a uniform elastic sphere experiencing simple solar temperature forcing. The thermal model is validated using this study's data. Most observed cracking coincides with the timing of calculated maximum, insolationdriven, tensile thermal stresses. We also observe that most cracking occurs when storms, or other weather events, strongly perturb the rock surface temperature field at these times. We hypothesize that these weatheractuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing; these additive stresses ultimately trigger measurable cracking. Measured locations of observed cracking and surface strain support this hypothesis in that they generally match model-predicted locations of maximum solar-induced tensile stresses. Also, recorded rock surface strain scales with diurnal temperature cycling and records progressive, cumulative extension (dilation), consistent with ongoing, thermal stress-driven subcriti-cal crack growth in the boulder. Our results therefore suggest that (1) insolation-related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and (2) simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms. Our observed cracking activity does not correlate simply with environmental conditions, including temperature extremes or the often-cited 2 °C/min thermal shock threshold. We propose that this lack of correlation is due to both the ever-varying ambient stress levels in any rock at Earth's surface, as well as to the fact that ongoing subcritical crack growth itself will influence a rock's stress field and strength. Because similar thermal cycling is universally experienced by subaerially exposed rock, this study elucidates specific mechanisms by which solar-induced thermal stresses may influence virtually all weathering processes.
Geophysical Research Letters
Accurate characterizations of how climate controls weathering, the in situ breakdown of rock, are... more Accurate characterizations of how climate controls weathering, the in situ breakdown of rock, are essential because weathering drives erosion and soil formation (Murphy et al., 2016; Richter et al., 2019), impacts biomes (Lu & Hedin, 2019) including the evolution of human life (Kasting, 2019), degrades civil infrastructure (Phillipson et al., 2016), and-crucially-influences the rate of atmospheric CO 2 uptake by the lithosphere, which stabilizes climate on geological time scales (e.g., Isson et al., 2020; Macdonald et al., 2019; Walker et al., 1981). Currently, however, published quantifications of global-climate-weathering connections (e.g., Rugenstein et al., 2019; Winnick & Maher, 2018) do not take into consideration climate's influence on the mechanical component of rock weathering, the lengthening of fractures caused by stresses at Earth's surface-hereafter referred to as "cracking." Yet chemical weathering-and most other surface processes-are limited without the breakdown and porosity facilitated by cracking (e.g.
<p>Rock fracturing can be slow and steady, comprising physiochemical processes that involve... more <p>Rock fracturing can be slow and steady, comprising physiochemical processes that involve the chemical breaking of bonds that are weakened in response to local stress loading. Whereas subaerial cracking of surface boulders is universally observed in desert environments, the rates and specific mechanisms that drive crack propagation in such conditions are yet to be completely understood.</p><p>Here, we present new field and petrographic observations from mode-1 (tensional) incipient (rocks are not yet split) fractures in alluvial boulders from the hyperarid southern Negev desert (Israel). Over 100 carbonate boulders embedded in a well-developed, 70 ka desert pavement that held visible fractures were forced apart along the incipient cracks. Doing so revealed a systematic recurring tri-zone pattern in crack morphology whose boundaries consistently paralleled the crack propagation front: Zone 1 &#8211; A weathered (as evidenced by incipient patina) zone proximal to the boulder surface; Zone 2 &#8211; A relatively fresh crack zone partly filled with aeolian particles and salts medial from the boulder up-facing surface; &#160;Zone 3 &#8211; A chemically altered (as evidenced by petrographic analyses) zone of otherwise intact rock at the crack tip. The occurrence of such micro-morphological crack zonation suggests slow sub-critical crack propagation at sufficiently long geologic timescales that support development of differential weathering within the crack. The petrographic analyses of sections perpendicular to the plane of the crack indicate chemical alteration that precedes the crack propagation in both space and time (i.e., extends in front of the crack tip), also indicates slow piecemeal propagation of the crack. This linkage between chemical weathering processes at the crack tip and slow subcritical propagation of the crack into the boulder provides additional support for first-order control of environmental and climatic conditions on boulder cracking rates, regardless of the physical stress-loading mechanism.</p>
This work constructs a fracture mechanics framework for conceptualizing mechanical rock breakdown... more This work constructs a fracture mechanics framework for conceptualizing mechanical rock breakdown and consequent regolith production and erosion on the surface of Earth and other terrestrial bodies. Here our analysis of fracture mechanics literature explicitly establishes for the first time that all mechanical weathering in most rock types likely progresses by climate-dependent subcritical cracking under virtually all Earth surface and near-surface environmental conditions. We substantiate and quantify this finding through development of physically based subcritical cracking and rock erosion models founded in well-vetted fracture mechanics and mechanical weathering, theory, and observation. The models show that subcritical cracking can culminate in significant rock fracture and erosion under commonly experienced environmental stress magnitudes that are significantly lower than rock critical strength. Our calculations also indicate that climate strongly influences subcritical cracking—and thus rock weathering rates—irrespective of the source of the stress (e.g., freezing, thermal cycling, and unloading). The climate dependence of subcritical cracking rates is due to the chemophysical processes acting to break bonds at crack tips experiencing these low stresses. We find that for any stress or combination of stresses lower than a rock's critical strength, linear increases in humidity lead to exponential acceleration of subcritical cracking and associated rock erosion. Our modeling also shows that these rates are sensitive to numerous other environment, rock, and mineral properties that are currently not well characterized. We propose that confining pressure from overlying soil or rock may serve to suppress subcritical cracking in near-surface environments. These results are applicable to all weathering processes. Plain Language Summary There is a long-standing question of how climate, particularly water, may influence the physical breakup of rock when it is exposed to the elements. It is typically assumed that the importance of water in rock cracking arises from the role that it plays in processes like freeze-thaw that require water to proceed. Here, we use classical mathematical models combined with a compendium of fracture mechanics theory and data from engineering research to show, for the first time, that water likely influences all types of rock cracking at Earth's surface whether it " requires " water or not. We also show that cracking proceeds even when only extremely low stresses are applied to the rock (akin to very small pulling forces). These stresses can be due to occurrences as simple and ubiquitous as daily heating and cooling by the sun.
A dearth of direct field observations limits our understanding of individual mechanical weatherin... more A dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact. In particular, the specific contributions of solar-induced thermal stresses to mechanical weathering are poorly characterized. Here, we present an 11 mo data set of cracking, using acoustic emissions (AEs), combined with measurements of rock temperature, strain and other environmental conditions, all recorded continuously for a granite boulder resting on the ground in open sun. We also present stresses derived from a numerical model of the temperature and stress fields in the boulder, idealized as a uniform elastic sphere experiencing simple solar temperature forcing. The thermal model is validated using this study's data. Most observed cracking coincides with the timing of calculated maximum, insolation-driven, tensile thermal stresses. We also observe that most cracking occurs when storms, or other weather events, strongly perturb the rock surface temperature field at these times. We hypothesize that these weather-actuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing; these additive stresses ultimately trigger measurable cracking. Measured locations of observed cracking and surface strain support this hypothesis in that they generally match model-predicted locations of maximum solar induced tensile stresses. Also, recorded rock surface strain scales with diurnal temperature cycling and records progressive, cumulative extension (dilation), consistent with ongoing, thermal stress-driven subcriti-cal crack growth in the boulder. Our results therefore suggest that (1) insolation-related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and (2) simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms. Our observed cracking activity does not correlate simply with environmental conditions, including temperature extremes or the often-cited 2 °C/min thermal shock threshold. We propose that this lack of correlation is due to both the ever-varying ambient stress levels in any rock at Earth's surface, as well as to the fact that ongoing subcritical crack growth itself will influence a rock's stress field and strength. Because similar thermal cycling is universally experienced by subaerially exposed rock, this study elucidates specific mechanisms by which solar-induced thermal stresses may influence virtually all weathering processes.
The role of solar-induced thermal stresses in the mechanical breakdown of rock in humid-temperate... more The role of solar-induced thermal stresses in the mechanical breakdown of rock in humid-temperate climates has remained relatively unexplored. In contrast, numerous studies have demonstrated that cracks in rocks found in more arid mid-latitude locations exhibit preferred northeast orientations that are interpreted to be a consequence of insolation-related cracking. Here we hypothesize that similar insolation-related mechanisms may be efficacious in humid temperate climates, possibly in conjunction with other mechanical weathering processes. To test this hypothesis, we collected rock and crack data from a total of 310 rocks at a forested field site in North Carolina (99 rocks, 266 cracks) and at forested and unforested field sites in Pennsylvania (211 rocks, 664 cracks) in the eastern United States. We find that overall, measured cracks exhibit statistically preferred strike orientations (47° ± 16), as well as dip angles (52° ± 24°), that are similar in most respects to comparable datasets from mid-latitude deserts. There is less variance in strike orientations for larger cracks suggesting that cracks with certain orientations are preferentially propagated through time. We propose that diurnally repeating geometries of solar-related stresses result in propagation of those cracks whose orientations are favorably oriented with respect to those stresses. We hypothesize that the result is an oriented rock heterogeneity that acts as a zone of weakness much like bedding or foliation that can, in turn, be exploited by other weathering processes. Observed crack orientations vary somewhat by location, consistent with this hypothesis given the different latitude and solar exposure of the field sites. Crack densities vary between field sites and are generally higher on north-facing boulder-faces and in forested sites, suggesting that moisture-availability also plays a role in dictating cracking rates. These data provide evidence that solar-induced thermal stresses facilitate mechanical weathering in environments where other processes are also likely at play.
Image Processing: Machine Vision Applications II, 2009
As of December 2008, the two Mars rover spacecraft Spirit and Opportunity have collected more tha... more As of December 2008, the two Mars rover spacecraft Spirit and Opportunity have collected more than 4 years worth of data from nine imaging instruments producing greater than 200k images which includes both raw image data from spacecraft instruments and images generated by post-processing algorithms developed by NASA's Multimission Image Processing Laboratory (MIPL). This paper describes a prototype software system that allows scientists to browse and data-mine the images produced from NASA's Mars Exploratory Rover (MER) missions with emphasis on the automatic detection of images containing rocks that are of interest for geological research. We highlight two aspects of our prototype system: (1) software design for mining remote data repositories, (2) a computationally efficient image search engine for detecting MER images that containing rocks. Datatype abstractions made at the software design level allow users to access and visualize the source data through a single simple-to-use interface when the underlying data may originate from a local or remote image repository. Data mining queries into the MER image data are specified over chronological intervals denoted (sols) as each interval is a solar day. As in other mining applications, an automatic detection and classification algorithm is used to compute a relevance score that represents how relevant a given recorded image is to the user-specified query. Query results are presented as list of records, sorted by their relevance score, which the user may then visualize and investigate to extract information of interest. Several standard image analysis tools are provided for investigation of 2D images (e.g., histogram equalization, edge detection, etc.) and, when available, stereoscopic data is integrated with the image data using multiple windows which show both the 2D image and 3D surface geometry. The combination of data mining and a high-quality visualization interface provides MER researchers unprecedented access to the recorded data.
The northern piedmont of the San Bernardino Mountains contains kinematic elements characteristic ... more The northern piedmont of the San Bernardino Mountains contains kinematic elements characteristic of the Mojave block dextral plate boundary zone between the North American and Pacific plates and the complex convergent Transverse Ranges partition of that motion here represented by the North Frontal thrust system. Predominantly lateral slip in the central Mojave block is carried by the Helendale fault through
ABSTRACT NASA&#39;s two Mars rover spacecraft, Spirit and Opportunity, have collected more th... more ABSTRACT NASA&#39;s two Mars rover spacecraft, Spirit and Opportunity, have collected more than 4 years worth of data from nine imaging instruments producing greater than 200k images. To date, however, the potential `field&#39; data that these images represent has remained relatively untapped because of a lack of software with which to readily analyze the images quantitatively. We have developed prototype software that allows scientists to locate and explore 2D and 3D imagery captured by the NASA&#39;s Mars Exploratory Rover (MER) mission robots Spirit and Opportunity. For example, using our software, a person could measure the dimensions of a rock or the strike and dip of a bedding plane. The developed software has three aspects that make it distinct from existing approaches for indexing large sets of imagery: (1) a computationally efficient image search engine capable of locating MER images containing features of interest, rocks in particular, (2) an interface for making measurements (distances and orientations) from stereographic image pairs and (3) remote browsing and storage capabilities that removes the burden of storing and managing these very large image sets. Two methods of search are supported (i) a rock detection algorithm for finding images that contain rock-like structures having a specified size and (ii) a generic query-by-image search which uses exemplar image(s) of a desired object to locate other images within the MER data repository that contain similar structures (i.e. one could search for all images of sand dunes). Query by image capabilities are made possible via a bag-of-features (e.g. Labeznik et. al. 2003; Schmid et. al. 2002) representation of the image data which compresses the image into a small set of features which are robust to changes in illumination and perspective. Searches are then reduced to looking for feature sets which have similar values; a task that is computationally tractable providing quick search results for complex image-based queries. Images recorded by the MER pancam instrument often include an (x,y,z) position information for each image pixel which can be localized to the global Mars Body Fixed (MBF) coordinate system. This allows position, distance, and orientation measurements from objects imaged by the pancam as stereographic image pair. Analysis tools paired with our search capabilities allow researchers to obtain measurements directly from MER imagery, to visualize these measurement(s) and the data in 2D and in 3D, to collate numerous measurements for statistical analysis and to estimate the uncertainty of each recorded measurement. The combination of data mining functionality in conjunction with analysis and visualization tools provides MER researchers unprecedented abilities to extract quantitative information from this important data not provided by current MER software systems. Our group will be employing the software to test recent hypotheses (e.g. McFadden et al., 2005; Eppes et al., 2010) related to the efficacy of diurnal heating and cooling in cracking rocks on the surface of Mars.
Journal of Maps, 2010
During the Last Glacial Maximum (LGM), the San Juan Mountains of southern Colorado, USA were cove... more During the Last Glacial Maximum (LGM), the San Juan Mountains of southern Colorado, USA were covered by one of the largest ice caps in North America. The deposits formed subsequent to LGM retreat provide a record of the interaction between post-LGM climate change and late Quaternary landscape evolution. In order to determine the role of post-LGM climate change in alpine landscape modification, a high resolution surficial geologic map was produced for the four primary tributaries and the main stem of the upper Conejos River watershed in the southeastern San Juan Mountains.
Processes of mechanical weathering related to diurnal insolation are largely unexplored. Recent s... more Processes of mechanical weathering related to diurnal insolation are largely unexplored. Recent studies (McFadden et al., 2005, Eppes et al., 2010) demonstrated that rocks in a range of environments exhibit preferentially orientated (~N-S) cracks that are hypothesized to form as rocks are heated and cooled during the sun's daily transit across the sky. In this study, we attempt to better understand the association between rock fracture and directional insolation. In Charlotte, NC we instrumented a ~30 cm diameter granite boulder sitting in full sun exposure with 8 thermocouples, 8 strain rosettes, 6 acoustic emission sensors and a moisture sensor, in order to spatially and temporally correlate rock cracking with rock surface conditions. Temperature and strain are recorded every minute along with a suite of meteorological data, and acoustic emissions are continuously monitored. As part of an NSF REU, in the Providence Mountains of the Mojave Desert of Southern California, we exa...
Geomorphology, 2010
The formation of cracks is a fundamental first step in the physical weathering of rocks in desert... more The formation of cracks is a fundamental first step in the physical weathering of rocks in desert environments. In this study we combine new field data from the Mojave (U.S.), Gobi (Mongolia) and Strzelecki (Australia) deserts that collectively support the hypothesis that meridional cracks (cracks with orientations not readily attributable to rock anisotropies or shape) in boulders or cobbles form due to tensile stresses caused by directional heating and cooling during the sun's daily transit. The new studies indicate that rock size, surface age, and latitude play important roles with respect to their influence on rock fracture. Rock size and pavement surface age exert an influence on the development of rock cracks as the average clast size of mature desert pavements may be at or below the threshold-clast size for thermal cracking of rocks. Latitude-controlled seasonal temperature variations play a key role, as demonstrated by: 1) tightly clustered mean resultant orientations that differ by latitude, as predicted in McFadden et al. (2005), and 2) very cold wintertime temperatures and strong diurnal gradients that may favor crack development in wintertime, given the likelihood for strong clast heating during early morning hours. The consistent evidence for meridional cracks in surfaces of diverse age and desert environments, climate, vegetation, and distance of clast transport indicate that directional insolation may play the key role in initially generating and propagating rock fractures, rather than a secondary role as implied in recent field and modeling studies of physical weathering in deserts.
Low-gradient alluvial piedmonts are common in desert areas throughout the world; however, long-te... more Low-gradient alluvial piedmonts are common in desert areas throughout the world; however, long-term rates of processes that modify these landscapes are poorly understood. Using cosmogenic 10Be and 26Al, we attempt to quantify the long-term (>103 y) behavior of desert piedmonts in Southern California. We measured the activity of 10Be and 26Al in three samples of drainage basin alluvium and six amalgamated samples from transects spaced at 1-km intervals down a piedmont in Fort Irwin, Mojave Desert, California. Each transect sample consists of sediment from 21 collection sites spaced at 150 m intervals. Such sampling averages the variability of nuclide activity between sub-sample locations and thus gives a long-term dosing history of sediment as it is transported from uplands to the distal piedmont. The piedmont is heavily used during military training exercises during which hundreds of wheeled and tracked vehicles traverse the surface. The piedmont surface is planar, and fan-head incision is minimal at the rangefront decreasing to zero between the first and second transects, 1.5 km from the rangefront. 10Be activity increases steadily from 5.87 X 105 atoms g-1 at the rangefront to 1.02 X 6 atoms g-1 at the piedmont bottom. Nuclide activity and distance are well correlated (r2 = 0.95) suggesting that sediment is dosed uniformly as it is transported down piedmont. We have measured similar increases in nuclide activity in transect samples collected from two other Mojave Desert piedmonts, those fringing the Iron and Granite Mountains (Nichols et al, in press, Geomorphology). These piedmonts have nuclide activities that also correlate well with distance (r2 = 0.98 and 0.96, respectively) from their rangefronts, but nuclides increase at a lower rate down piedmont. Modeled sediment transport speeds for the Iron and Granite Mountain piedmonts are decimeters per year. The regular increase in nuclide activities down three different Mojave Desert piedmonts suggests that sediment is dosed and transported on these planar, minimally incised surfaces in a systematic fashion.
CATENA, 2015
Surficial processes acting on post-glacial alpine and sub-alpine landscapes vary at small tempora... more Surficial processes acting on post-glacial alpine and sub-alpine landscapes vary at small temporal and spatial scales and are thus often difficult to conceptualize in the context of large-scale landscape evolution models. Soils developing in this setting can thus provide valuable information about landform genesis, sedimentology and age. Relatively few post-glacial chronosequences have been examined in these settings however, particularly for the variety of landforms and parent materials that exist within alpine and sub-alpine environments. Here, we examine a chronosequence of relatively young, post-glacial landforms with varying parent materials and climate histories. We dug and described 39 soil pits in the upper Conejos River Valley of Colorado on a variety of deposits and landforms, including alluvial fans, terraces, colluvium, glacial till, and terminal moraines, and compared soil properties with radiocarbon ages from the area. Our results suggest that some typical chronosequence soil properties (e.g., pH, structure, color) do not correlate with time over short time scales. However, extractable iron ratios (Fe o /Fe d ) show a relatively strong correlation with age across late-Pleistocene and Holocene time scales and maximum profile clay content shows a weak but statistically significant relationship with age. Both of these trends are stronger when examined across a single parent material. Differences in initial parent material texture and dust inputs seem to be the most significant complicating factors over post-glacial time scales. Soil property development through time is most inconsistent in cumulic alluvial fan soils. This observation may indicate that alluvial fans are more responsive to sub-basin scale processes as opposed to fluvial terraces that are more likely respond to processes active across the entire basin. These differences would explain why stratigraphically similar alluvial fans are mantled by soils with varying development. Nonetheless, horizonation, clay content, and extractable iron ratios provide a useful tool for correlating young deposits, assigning ages, and interpreting the geomorphic history of complex post-glacial environments.
The contribution of solar-driven thermal cycling to the progressive breakdown of surface rocks on... more The contribution of solar-driven thermal cycling to the progressive breakdown of surface rocks on the Earth and other planets is controversial. We introduce a current study of the physical state in boulders that integrates modern instrumental and numerical approaches to quantify the surface temperature, stresses, strains, and microfracture activity in exposed boulders, and to shed light on the processes underlying this form of mechanical weathering. We are monitoring the surface and environmental conditions of two ~30 cm dia. granite boulders (one in North Carolina, one in New Mexico) in the field for ~1 yr each. Each rock is instrumented with 8 thermocouples, 8 strain gauges, a surface moisture sensor and 6 Acoustic Emission (AE) sensors to monitor microfracture activity continuously. These sensors and a full meteorological station, including soil-moisture probes, are combined into a single, remotely accessible system. AE events can be located to within 2.5 cm. We are able 1) to sp...
Surficial processes acting on post-glacial alpine and sub-alpine landscapes vary at small tempora... more Surficial processes acting on post-glacial alpine and sub-alpine landscapes vary at small temporal and spatial scales and are thus often difficult to conceptualize in the context of large-scale landscape evolution models. Soils developing in this setting can thus provide valuable information about landform genesis, sedimentology and age. Relatively few post-glacial chronosequences have been examined in these settings however, particularly for the variety of landforms and parent materials that exist within alpine and sub-alpine environments. Here, we examine a chronosequence of relatively young, post-glacial landforms with varying parent materials and climate histories. We dug and described 39 soil pits in the upper Conejos River Valley of Colorado on a variety of deposits and landforms, including alluvial fans, terraces, colluvium, glacial till, and terminal moraines, and compared soil properties with radiocarbon ages from the area. Our results suggest that some typical chronosequence soil properties (e.g., pH, structure, color) do not correlate with time over short time scales. However, extractable iron ratios (Fe o /Fe d ) show a relatively strong correlation with age across late-Pleistocene and Holocene time scales and maximum profile clay content shows a weak but statistically significant relationship with age. Both of these trends are stronger when examined across a single parent material. Differences in initial parent material texture and dust inputs seem to be the most significant complicating factors over post-glacial time scales. Soil property development through time is most inconsistent in cumulic alluvial fan soils. This observation may indicate that alluvial fans are more responsive to sub-basin scale processes as opposed to fluvial terraces that are more likely respond to processes active across the entire basin. These differences would explain why stratigraphically similar alluvial fans are mantled by soils with varying development. Nonetheless, horizonation, clay content, and extractable iron ratios provide a useful tool for correlating young deposits, assigning ages, and interpreting the geomorphic history of complex post-glacial environments.
The origins of fractures in Martian boulders are unknown. Here, using Mars Exploration Rover
3D d... more The origins of fractures in Martian boulders are unknown. Here, using Mars Exploration Rover
3D data products, we obtain orientation measurements for 1,857 cracks visible in 1,573 rocks
along the Spirit traverse and find that Mars rock cracks are oriented in statistically preferred
directions similar to those compiled herein for Earth rock cracks found in mid-latitude deserts.
We suggest that Martian directional cracking occurs due to the preferential propagation of
microfractures favourably oriented with respect to repeating geometries of diurnal peaks in
sun-induced thermal stresses. A numerical model modified here with Mars parameters
supports this hypothesis both with respect to the overall magnitude of stresses as well as to
the times of day at which the stresses peak. These data provide the first direct field and
numerical evidence that insolation-related thermal stress potentially plays a principle role in
cracking rocks on portions of the Martian surface.
Geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains has shown that thr... more Geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains has shown that three distinct periods of aggradation have occurred since the end of the last glacial maximum (LGM). The first occurred during the Pleistocene-Holocene transition (~12.5-9.5 ka) and is interpreted as paraglacial landscape response to deglaciation after the LGM. Evidence of the second period of aggradation is limited but indicates a small pulse of sedimentation at~5.5 ka. A third, more broadly identifiable period of sedimentation occurred in the late Holocene (~2.2-1 ka). The latest two periods of aggradation are concurrent with increases in the frequency of climate change in the region suggesting that Holocene alpine and sub-alpine landscapes respond more to rapid changes in climate than to large singular climatic swings. Soil development and radiocarbon dating indicate that hillslopes were stable during the Holocene even while aggradation was occurring in valley bottoms. Thus, we can conclude that erosion does not occur equally throughout the landscape but is focused upslope of headwater streams, along tributary channels, or on ridge tops. This is in contrast to some models which assume equal erosion in headwater basins.
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Papers by Martha Cary Eppes
3D data products, we obtain orientation measurements for 1,857 cracks visible in 1,573 rocks
along the Spirit traverse and find that Mars rock cracks are oriented in statistically preferred
directions similar to those compiled herein for Earth rock cracks found in mid-latitude deserts.
We suggest that Martian directional cracking occurs due to the preferential propagation of
microfractures favourably oriented with respect to repeating geometries of diurnal peaks in
sun-induced thermal stresses. A numerical model modified here with Mars parameters
supports this hypothesis both with respect to the overall magnitude of stresses as well as to
the times of day at which the stresses peak. These data provide the first direct field and
numerical evidence that insolation-related thermal stress potentially plays a principle role in
cracking rocks on portions of the Martian surface.
3D data products, we obtain orientation measurements for 1,857 cracks visible in 1,573 rocks
along the Spirit traverse and find that Mars rock cracks are oriented in statistically preferred
directions similar to those compiled herein for Earth rock cracks found in mid-latitude deserts.
We suggest that Martian directional cracking occurs due to the preferential propagation of
microfractures favourably oriented with respect to repeating geometries of diurnal peaks in
sun-induced thermal stresses. A numerical model modified here with Mars parameters
supports this hypothesis both with respect to the overall magnitude of stresses as well as to
the times of day at which the stresses peak. These data provide the first direct field and
numerical evidence that insolation-related thermal stress potentially plays a principle role in
cracking rocks on portions of the Martian surface.