Geomorphology 123 2010 97 108 Contents lists available at ScienceDirect Geomorphology j o u r n a l h o m e p a g e w w w e l s ev i e r c o m l o c a t e g e o m o r p h Cracks in desert pavement rocks Further insights into mechanical weathering by directional insolation Martha Cary Eppes a Leslie D McFadden b Karl W Wegmann c Louis A Scuderi b a b c Department of Geography Earth Sciences University of North Carolina at Charlotte 9201 University City Blvd Charlotte NC 28223 United States Department of Earth and Planetary Sciences University of New Mexico Albuquerque NM 87131 United States Department of Marine Earth and Atmospheric Sciences North Carolina State University Raleigh NC 27695 United States a r t i c l e i n f o Article history Received 30 December 2009 Received in revised form 29 June 2010 Accepted 2 July 2010 Available online 3 August 2010 Keywords Desert pavements Physical weathering Desert geomorphology Insolation weathering Fractures a b s t r a c t The formation of cracks is a fundamental rst step in the physical weathering of rocks in desert environments In this study we combine new eld 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 in uence on rock fracture Rock size and pavement surface age exert an in uence 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 eld and modeling studies of physical weathering in deserts 2010 Elsevier B V All rights reserved 1 Introduction The rst stage of physical weathering unequivocally is the formation of an incipient crack These cracks are essential in enabling or accelerating virtually all other types of subsequent physical and chemical weathering processes that are in turn essential in many aspects of landscape evolution e g Goudie et al 1997 While some cracks can be linked to geological inheritance or to forest res surface boulders and cobbles regardless of rock type universally exhibit discrete vertical to subvertical cracks that extend beyond individual grain boundaries hereafter referred to as cracks throughout a range of climatic and geomorphic settings e g Yaalon 1970 Ollier 1984 Hall 1999 McFadden et al 2005 Cracks are even visible in rocks imaged in recent photographs of Mars e g Arvidson et al 2006 A large body of literature describes mechanisms for the growth and expansion of such cracks but the mechanism for their initiation has been disputed for over a century e g Blackwelder 1933 Griggs 1936 Textbooks addressing geomorphology and weathering Summer eld 1992 Watson 1992 Cooke et al 1993 Ahnert 1996 Bloom 1998 Easterbrook 1999 Corresponding author Tel 1 704 687 5993 fax 1 704 687 5966 E mail address meppes uncc edu M C Eppes 0169 555X see front matter 2010 Elsevier B V All rights reserved doi 10 1016 j geomorph 2010 07 003 Ritter et al 2002 and many researchers e g Yaalon 1970 Smith and Warke 1997 Goudie et al 2002 identify several processes as possible causes of cracks in surface rocks yet offer no clear solution to the problem of how they form New data have begun to shed light on this fundamental problem McFadden et al 2005 hypothesized that boulders or cobbles crack due to tensile stresses caused by directional heating and cooling during the sun s east to west transit across the sky Cracks produced by this process should have orientations that re ect the orientation of the largest of these recurrent thermal stresses This proposition was tested by measuring the orientations of approximately 700 cracks observed in over 300 rocks in the desert southwest of the United States A signi cant number of cracks 462 out of 700 had orientations that were not readily attributable to rock anisotropies such as fabric bedding or planar surfaces The mean azimuthal orientation of these remaining cracks was 5 12 These north south cracks were termed meridional cracks Additional documentation of the presence of these cracks in desert environments has been provided by a recent study of desert pavements in northern Africa that documents the predominance of meridional cracks in pavement clasts located there Adelsberger and Smith 2009 Meridional cracks provide strong evidence for the importance of directional solar heating and cooling as a fundamental cause of such 98 M C Eppes et al Geomorphology 123 2010 97 108 cracking because their preferential orientations are very dif cult to explain by other mechanisms For example cracks caused by freeze thaw salt shattering or other wedging processes alone presumably should exhibit largely random orientations because there is no inherent mechanism involved in these processes that would result in oriented cracks Modeling suggests that preferential shading of cracks that are already open could lead to a disproportionate propagation by these mechanisms of certain favorably oriented cracks Moores et al 2008 The speci c north south array of observed meridional crack orientations however would be predicted to preferentially propagate through this shading process only for a limited range of crack depths Moores et al 2008 Some relatively recent studies do consider a possible role for insolation but do not speci cally address directional insolation e g Hall 1999 Goudie and Viles 2000 Viles 2005 Warke 2007 These studies focus on a variety of thermal mechanical processes that may lead to cracking including thermal shock