Martha Cary Eppes

Physical weathering in arid landscapes due to diurnal variation in the direction of solar heating

Despite the prominent role of physical weathering in arid and semi-arid landscapes, there has been little study of the specifi c processes responsible for the rapid breakdown of subaerially exposed rocks. For example, many boulders and cobbles in deserts exhibit fi ne near-vertical cracks. Although workers have hypothesized that these and other cracks are initiated by diurnal heating and cooling, no convincing specifi c mechanism for their formation has been proposed. We have characterized these cracks at eight sites on surfaces of different ages in the Mojave, Sonoran, and Chihuahuan Deserts, and the high desert of central New Mexico. Our data reveal four basic types of cracks: longitudinal, surfaceparallel, fabric-related, and meridional. The orientations of the fi rst three types are associated with clast shape and rock fabric. The azimuths of meridional cracks, however, are preferentially aligned north-south, typically with a nonrandom multimodal distribution. We propose that these cracks are caused by tensile stresses that arise in the interior of clasts due to strong radial gradients in temperature that evolve and rotate in alignment with the sun’s rays. We suggest that the multimodal nature of crack orientations may be in part attributable to the seasonally varying, latitude-dependent solar elevation angle. Over millennial time scales, we suggest that this thermal cracking is an effi cient weathering process that, together with cumulic soil epipedon development, creates the key attributes of most desert pavements. In addition to individual clasts exposed on desert surfaces, this mechanism of cracking is potentially signifi cant in other climates and on other planets, as well as for rock outcrops and for man-made structures.

Deciphering the role of solar-induced thermal stresses in rock weathering

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 subcritical 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.

Ephemeral stream response to growing folds

The response of ephemeral alluvial streams to active tectonics is not as well established as those documented for perennial alluvial streams. This study documents the response of ephemeral streams transverse to growing folds along the north fl ank of the San Bernardino Mountains in southern California. The growing folds emerge amid a broad, sloping piedmont mantled by north-dipping alluvial fans and underlain by coarse, angular gravel and sand of mixed provenance. The study area contains fans composed of two distinct lithologies that control the expression of the folds. Sediment transport processes differ among alluvial perennial and ephemeral channels and play a primary role in defi ning the range of responses that ephemeral streams have to an actively rising fold. We fi nd that ephemeral streams respond by (1) changing pattern from a single, slightly incised channel with well-defi ned banks to a braided channel upstream of the fold axis, (2) incising across the fold axis, preserving a terrace and braid bars, and (3) returning to a single-thread, less incised channel downstream of the fold axis. This channel response is documented for both a topographically obvious anticlinal fold, the Cougar Buttes anticline, as well as a suspected, but not topographically obvious anticline, the Pitzer Buttes anticline. Variations from this general model appear to be correlated with locations of slow fold growth and/or channel alluvium that is fi ne grained and of low cohesion. A growing fold, such as the Cougar Buttes anticline, provides a laboratory for the investigation of the development of transverse streams with respect to position along strike with the fold axis. Some streams crossing the Cougar Buttes anticline are antecedent, whereas others are consequent to the growth of the fold. These observations lay the foundation for a conceptual model for ephemeral stream response to active tectonics, particularly useful in identifying previously unrecognized actively rising folds. In this study, the fold axis of the Cougar Buttes anticline is revealed to extend at least 1 km beyond its current obvious topographic expression. Because ephemeral streams are sensitive to tectonic deformation, they can be used locally in paleoseismological investigations, regionally to understand the strain partitioning between the Big Bear and Mojave blocks, and conceptually to constrain geodynamic models investigating the interaction between surface processes and the geometry of and slip rates on faults responsible for fold growth.

The influence of bedrock weathering on the response of drainage basins and associated alluvial fans to Holocene climates, San Bernardino Mountains, California, USA

The primary factors that control alluvial fan evolution still remain in question particularly for the Holocene. Holocene centennial- and millennial-scale climate fluctuations are relatively subtle and more frequent than those of glacial/interglacial transitions, therefore intrinsic factors such as rock type or basin size are hypothesized to moderate significantly the influence of Holocene climate and climate change on alluvial fan processes. Here, we examine variability in styles and rates of alluvial fan aggradation along a single mountain front that is characterized by basins of varying size and rock type (carbonate versus granite). Basin rock type is more closely correlated to variability in the episodic nature and magnitude of alluvial fan aggradation than is basin area. Bedrock physical and chemical weathering properties control sediment delivery to the piedmont and thus influence alluvial fan aggradation. We suggest that the particle size of grus produced by weathering of granitic rocks fosters sediment mobilization and alluvial fan aggradation during episodes of increased precipitation in the Holocene. Sediment mobilization during wetter climates is also possibly enhanced by drought-related fires and vegetation loss that occurred during preceding drier periods. In contrast, carbonate outcrops weather to both dissolved materials and clastic sediment and relatively rapid cementation of talus precludes its transportation out onto the piedmont under almost all Holocene climatic conditions. If the scale of past Holocene climate change is the closest analogy to current global change, this study documents some mechanisms by which different rock types can exert dramatically different effects on landscape response to those changes.

A multiproxy record of postglacial climate variability from a shallowing, 12-m deep sub-alpine bog in the southeastern San Juan Mountains of Colorado, USA

Pollen assemblages, diatom assemblages, and sedimentology, from Cumbres Bog in the southeastern San Juan Mountains of Colorado, provide a record of climate and environmental change since the end of the last glacial maximum (LGM). Cumbres Bog is unusually deep (basal sediments extend 12 m below the surface) for its altitude (~3050 m a.s.l.) and we extracted 7 m core of continuous sediment below ~5 m of water and peat. The resulting record provides strong evidence of: a period of warming immediately after the LGM (~18–13 cal. kyr BP), a cool interval coinciding with the Younger Dryas (~12.8–11.5 cal. kyr BP), a warm stable period from 10 to 6 cal. kyr BP, and a cooler and highly variable climate interval after 6 cal. kyr BP. More specifically, pollen ratios and fossil diatoms indicate that cold periods generally match with previously identified periods of rapid climate change that occurred at 10.6, 8.7–7.9, 7.0–6.9, 5.4–5.2, 3.3–3.0, 2.3, 2.0 and 1.5 cal. kyr BP. This record also adds resolution to previous regional records and indicates that the periodicity of climate variability changed from 2000–3000 years to 700–1100 years around 6 cal. kyr BP and to <500 years after 3.5 cal. kyr BP. Overall, our record provides important, relatively high-resolution paleoclimatic information for this remote region of the southern Rockies.

Surficial Geologic Map of the Upper Conejos River Drainage, Southeastern San Juan Mountains, Southern Colorado, USA

Abstract Please click here to download the map associated with this article. 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. Soil development, 14C ages, and stratigraphic relationships provide evidence for three distinct periods of hillslope, alluvial fan and/or stream terrace deposition (∼9.5–13 k.y.a., ∼1.2–2.1 ybp, and modern) in the upper Conejos River watershed. We interpret the first of these periods to be the result of paraglacial hillslope adjustment. The lack of subsequent early to mid-Holocene deposits suggests that an interval of relatively warm climate coincided with landscape stability. We attribute late-Holocene deposition to an interval of cooling that has been documented in nearby proxy records. Modern deposits in the field are limited both in extent and occurrence, and are interpreted to be the result of erosion localized outcrops of soft, volcaniclastic bedrock. In addition to post-LGM depositional units, relatively flat erosional bedrock surfaces that sit 1–100 m above the modern stream, and that are capped with glacial till, are present in all four tributaries of the field area. These are interpreted as former glacial valley bottoms. The modern streams are incised into the floors of these hanging valleys. Headward erosion presumably initiated following deglaciation and is still active today. Modern channel long profiles differ between the four tributaries. We attribute the variability of incision into these hanging valleys to differences in basin size and bedrock erodibility, with more extensive incision occurring in basins characterized by more erodible rocks (poorly welded volcaniclastics) and larger basin size. Overall we see that the post-LGM landscape evolution in this alpine environment is the result of the complex interaction of Holocene climate change acting on basins of varying size and bedrock type.

Introducing Field-Based Geologic Research Using Soil Geomorphology.

A field-based study of soils and the factors that influence their development is a strong, broad introduction to geologic concepts and research. A course blueprint is detailed where students design and complete a semester-long field-based soil geomorphology project. Students are first taught basic soil concepts and to describe soil, sediment and rock properties using standard description procedures. Then, with minimal geological or field experience, they are led to design and execute a project that examines how soil properties differ as a function of processes, parent material and time. By designing and executing the semester-long project, students gain familiarity with the entire geologic research process including basic field observation, hypothesis development and testing, interpretation and presentation skills. During the course, students learn 1) the basic knowledge necessary to describe geologic materials (soil, rock, sediment) in the field, 2) to make observations and interpret them in the context of geologic hypotheses which they have developed, 3) to develop, and execute a field-based research project, 4) to integrate and draw conclusions about complicated semi-quantitative data sets, 5) to map and survey in the field and 6) to present their research in a public forum. By the end of the semester students are able to make and test hypotheses relating soil properties to the depositional environment, age and/or type of geologic deposits in which they form. The knowledge, skills and research experience gained in this simple, semester-long project serve students well in upper-level courses and beyond. An example project is presented from a 2nd order stream and its adjacent tributary alluvial fans in the Piedmont of North Carolina.

Mining Remote Image Repositories with Application to Mars Rover Stereoscopic Image Datasets

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 NASAs Multimission Image Processing Laboratory (MIPL). This paper describes a prototype software system that allows scientists to browse and data-mine the images produced from NASAs 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.