Rock thermal data at the grain scale: applicability to granular disintegration in cold environments

Abstract

Consideration of the mechanisms associated with the granular disintegration of rock has been limited by available data. In most instances, both the size of the transducer and the nature of the study have negated any applicability of the resulting data to the understanding of grain-to-grain separation within rock. The application of microthermocouples (≤0·15 mm diameter) and high-frequency logging (20 s intervals) at a taffoni site on southern Alexander Island and from a rock outcrop on Adelaide Island (Antarctica) provide new data pertaining to the thermal conditions, at the grain scale, of the rock surface. The results show that thermal changes (∆T/t) can be very high, with values of 22°C min −1 being recorded. Although available data indicate that there can be differences in frequency and magnitude of fluctuations as a function of aspect, all aspects experienced some large magnitude (≥2°C min −1 ) fluctuations. Further, in many instances, large thermal changes in more than one direction could occur within 1 min or in subsequent minutes. These data suggest that the surface grains experience rapidly changing stress fields that may, with time, effect fatigue at the grain boundaries; albedo differences between grains and the resulting thermal variations are thought to exacerbate this. The available data failed to show any indication of water freezing (an exotherm) and thus it is suggested that microgelivation may not play as large a role in granular breakdown as is often postulated for cold regions, and that in this dry, Antarctic region thermal stress may play a significant role.

Automated field detection of rock fracturing, microclimate, and diurnal rock temperature and strain fields

“…McKay et al (2009) and Hall and André (2003) noted that > 1 measurement per minute temporal resolution of data may provide key insights into fracturing processes and the data acquisition system described herein was able to achieve that sampling frequency.…”

“…Instrumentation studies of diurnal variations in rock surface strain and/or temperature (while somewhat more common) have been limited to relatively short monitoring periods consisting of only one or two days (e.g., Hall and André, 2003;McKay et al, 2009); long periods between individual measurements (e.g., Wegmann and Gudmunson, 1999;Viles, 2005;McFadden et al, 2005), and/or only a single sensor per rock (e.g., Viles and Goudie, 2007). A long-term, multi-sensor study with high temporal and spatial measurements of temperature, strain, moisture, and acoustic emissions is unprecedented, but necessary to capture natural, spatial and temporal patterns of pertinent rock surface and environmental conditions that are associated with fracturing.…”

“…The importance of moisture (e.g., Hall and Hall, 1996;Nicholson, 2001), salts (e.g., Amit et al, 1993), and exposure to diurnal insolation (e.g., Blackwelder, 1933;Hall, 1999;Moores et al, 2008) in fracturing rock has been debated extensively. Although individual mechanisms of physical weathering have been addressed through field studies (e.g., McFadden et al, 2005;Eppes et al, 2010), numerical modeling (e.g., Tanigawa and Takeuti, 1983;Moores et al, 2008), and experimentation (e.g., McKay et al, 2009;Molaro and McKay, 2010), no study has collected the data Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Polygonal cracking in granite and considerations for a morphological classification (La Pedriza de Manzanares, Spain)

“…Insolation is a potential weathering agent due to the stress generated as a result of temperature differences (Rice 1976;Smith, 1977, Hall andAndré, 2003;Gomez-Heras et al, 2006, 2008Riley et al, 2012). In heterogeneous rock such as granite, the quartz and feldspar crystals have different thermal expansion coefficients and consequently react differently to weathering.…”

Salt-weathering simulations under hot desert conditions: agents of enlightenment or perpetuators of preconceptions?

“…In a series of papers that have similarly used high-frequency monitoring of surface temperatures, they established that rocks in these areas can likewise experience rapid temperature fluctuations (Hall and Andre, 2001). They have also demonstrated that these fluctuations and associated stresses are particularly manifest at the microscale, where frequent differential expansion is driven by thermal and crystallographic mismatches between crystals and grains (Hall and Andre, 2003). In this way, thermal stress fatigue failure (Hall, 1999) could occur where individual thermally induced stress events are insufficient to cause immediate failure but, collectively over time, can cause a material to fail.…”