『Abstract
Despite the prominent role of physical weathering in arid and
semi-arid landscapes, there has been little study of the specific
processes responsible for the rapid breakdown of subaerially exposed
rocks. For example, many boulders and cobbles in deserts exhibit
fine near-vertical cracks. Although workers have hypothesized
that these and other cracks are initiated by diurnal heating and
cooling, no convincing specific 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, surface-parallel,
fabric-related, and meridional. The orientations of the first
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 i part attributable to the seasonally varying, latitude-dependent
solar elevation angle. Over millennial time scales, we suggest
that this thermal cracking is an efficient weathering process
that, together with cumulic soil epipedon development, creates
the key attributes of most desert pavements. I addition to individual
clasts exposed on desert surfaces, this mechanism of cracking
is potentially significant in other climates and on other planets,
as well as for rock outcrops and for man-made structures.
Keywords: physical weathering; desert soils; fractures; cracks;
desert pavement; thermal stresses』
Introduction
Methods and field sites
Statistical analysis of crack orientations
Collection of clast surface temperature data
Results
Crack orientations
Meridional cracks
Fabric-related cracks
Surface-parallel cracks
Longitudinal cracks
Other crack characteristics
Changes in daily clast surface temperature
Discussion
Mechanisms for crack initiation
Meridional cracks
Fabric, surface-parallel, and longitudinal cracks
Other factors that influence cracking
Clast size
Rock color
Inheritance of rock surface microcracks
Possible cause of the bimodal crack orientations
Implications of solar-generated cracking for desert pavements
and weathering in other subaerial environments
Conclusions
Acknowledgments
References cited