SOLAR INSOLATION AND PHYSICAL WEATHERING IN DESERTS: REVISITING AN OLD IDEA

Many surface rocks on late Quaternary geomorphic surfaces in deserts exhibit fine, commonly vertically dipping cracks. Studies of these cracks on rocks associated with surfaces of varying ages in the Southwest indicate that their formation is a critical physical weathering process. Based on preliminary observations of cracks, we hypothesized that (1) if these are tension cracks, their origin might be attributable to a rock-surface temperature gradient, rather than a vertical, rock-interior temperature gradient usually supposed to trigger brittle failure by surface heating, and (2) the cause of such a surface temperature gradient and associated extension-contraction cycles is the daily change in the position of the sun relative to the rock surface. Analyses of the orientations of the cracks identify four basic categories of cracks correlated with rock shape and fabric. The orientations of cracks associated with the category defined by generally rounded, spherical rocks with isotropic fabrics are markedly north-south. Hypothesis (2) is the only plausible explanation for the observed nonrandom distribution. Crack orientations are also typically bimodal, a pattern attributable to the seasonally varying, latitude-dependent solar azimuth. This controls the daily timing and spatial pattern of maximum rock-surface temperature gradients, which equal or exceed maximum acquired vertical temperature gradients. The formation of incipient cracks by dry thermal stress facilitates incorporation of foreign substances (e.g. salts, dust) and, through positive feedback mechanisms, greatly increases the efficacy of other physical weathering processes. Solar insolation is not only the unambiguous cause of at least one category of cracks, but also the probable initial cause of cracks in other categories. Over millennial time scales, this highly efficient physical weathering process, with concomitant dust incorporation and cumulic,dilatant soil development, creates most desert pavements. The failure of past studies to recognize solar insolation as the primary cause of surface cracks is likely due to the largely incorrect premise of uniform surface heating (fire-induced heating as presumed analogue) and an over-reliance on results of experimental studies based on this premise that unrealistically simulated field conditions.