Thermal controls on the development of fractures in dolostones of the Niagara Escarpment, Hamilton, Ontario, Canada

Little research has been conducted to clarify the mechanism, extent, or factors involved in the fracturing of rocks exposed along the Niagara Escarpment in Ontario. Of particular interest are the effects of fluctuating temperatures during region’s cold winters which may be a contributor to the formation and expansion of fractures within these rocks. The results of a preliminary field-based study of temperature changes in fractured sedimentary rocks exposed at several sites along the Escarpment in Hamilton, Ontario are reported here. The objectives of the study were to examine the characteristics of operant thermal processes and to determine the effectiveness of mechanisms such as freeze-thaw and thermal stress in contributing to fracture formation and development. Fractured dolostone units were identified at three field sites along the escarpment that varied in their aspect, vegetation, and proximity to water. At each site, temperature probes were affixed to the exposed rock surface and inserted into a nearby fracture. Temperature measurements were taken at one-minute intervals throughout the winter of 2020-21.  In-situ field measurements of thermal changes within fractured dolostones on the escarpment were supplemented with recordings of rock surface and interior temperatures taken from unfractured dolostone blocks placed in a ‘controlled’ outdoor environment throughout the winter.  Initial results from the escarpment probes in the early winter show frequent, rapid shifts from warm to sub-zero temperatures and indicate that changes in temperature recorded at the rock surface closely follow diurnal atmospheric oscillations in both magnitude and timing.  However, temperature changes recorded within fractures are less extreme and show a temporal lag. Temperature fluctuations recorded at the field site with the highest degree of exposure, a southeasterly aspect, and little vegetation cover, are significantly higher and show larger thermal responses within fractures. Temperature fluctuations recorded from unfractured blocks in the ‘controlled’ outdoor environment show similar diurnal trends to those recorded on the escarpment but with reduced differential between temperatures at the block surface and interior. Together, these data indicate that temperature fluctuations sufficient to generate freeze-thaw cycles are abundant during the early winter months, temperature variability within fractures does occur, and slope aspect and exposure plays an important role in the determining the magnitude of diurnal temperature fluctuations experienced by surface rocks on the escarpment. The role of thermal stress in fracture development, created by rapid and substantial thermal changes, has yet to be determined.