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

Abstract

Abstract. The rates and processes that lead to non-tectonic rock fracture on Earth's surface are widely debated but poorly understood. Few, if any, studies have made the direct observations of rock fracturing under natural conditions that are necessary to directly address this problem. An instrumentation design that enables concurrent high spatial and temporal monitoring resolution of (1) diurnal environmental conditions of a natural boulder and its surroundings in addition to (2) the fracturing of that boulder under natural full-sun exposure is described herein. The surface of a fluvially transported granite boulder was instrumented with (1) six acoustic emission (AE) sensors that record micro-crack associated, elastic wave-generated activity within the three-dimensional space of the boulder, (2) eight rectangular rosette foil strain gages to measure surface strain, (3) eight thermocouples to measure surface temperature, and (4) one surface moisture sensor. Additionally, a soil moisture probe and a full weather station that measures ambient temperature, relative humidity, wind speed, wind direction, barometric pressure, insolation, and precipitation were installed adjacent to the test boulder. AE activity was continuously monitored by one logger while all other variables were acquired by a separate logger every 60 s. The protocols associated with the instrumentation, data acquisition, and analysis are discussed in detail. During the first four months, the deployed boulder experienced almost 12 000 AE events, the majority of which occur in the afternoon when temperatures are decreasing. This paper presents preliminary data that illustrates data validity and typical patterns and behaviors observed. This system offers the potential to (1) obtain an unprecedented record of the natural conditions under which rocks fracture and (2) decipher the mechanical processes that lead to rock fracture at a variety of temporal scales under a range of natural conditions.

The influence of solar‐induced thermal stresses on the mechanical weathering of rocks in humid mid‐latitudes

“…Warren et al . () also observed that cracking was typically coincident with short‐term temperature cycling driven by weather such as wind or cloud‐cover. Such short‐term temperature cycling has been observed in a range of climates (e.g.…”

“…Furthermore, calculated times of peak solar‐induced rock scale (Shi, ) and grain‐to‐grain (Eppes et al ., ) stresses coincide with peaks in the magnitude of observed cracking, as measured by acoustic emissions in boulders placed in humid temperate sites within 20 km of the NC Forested site (Eppes et al ., ; Warren et al ., ). Warren et al .…”

“…The recurrence of a majority of cracking at specific times of day as predicted by numerical modeling using temperate‐climate weather data as inputs (Shi, ) and as observed in instrumentation data (Warren et al ., ) leads to an explanation of how insolation contributes to directionality of visible cracks in subaerially exposed boulders. Instrumentation studies show that for boulders exposed to diurnal insolation, unique temperature fields are imparted across the rock surface throughout the day (e.g.…”

Thermal weathering of granite spheroidal boulders in a dry‐temperate climate, Northern Dobrogea, Romania

“…The active influence of the diurnal directional solar weathering has also been proved by acoustic emission records in natural field conditions. Measurements performed by Warren et al () on a small (25 cm) granite boulder showed that acoustic emission events associated with rock cracking occur indeed in relation to the diurnal cooling of the rock surface (at around 6:00 p.m. in the study settings) and also in relation to STCs associated with wind intensification, cloudiness and rain, as previously assumed based on thermal records (Hall and André, ; Gómez‐Heras et al , ). Compared to the continuous daily insolation, STCs related to summer rain occurrence (which we consider highly active in the study area) are thought to impact the rock dissimilarly, i.e.…”

“…Both short‐term temperature changes (STCs) and diurnal thermal stress induced by solar action have been emphasized as inducing permanent mechanical damage in rocks and triggering thermal weathering in various humidity conditions, their occurrence and efficiency being discussed in different climatic environments (Hall, ; Guglielmin et al , ; Hall et al , ; Viles, ; McKay et al , ; Molaro and McKay, ). Grain and crystal‐size temperature studies (Hall and André, , ; Gómez‐Heras et al , ) are available to prove the existence of significant mineral expansion and damages produced in polymineral rocks consequent to natural thermal oscillations even at mid‐latitudes (Gómez‐Heras et al , ), and unequivocal proofs of rock damage occurrence have been recently provided to sustain the effective action of diurnal directional isolation stress (Warren et al , ) or freeze–thaw action upon rock surfaces (Sass, ; Amitrano et al , ; Girard et al , ; Duca et al , ).…”