Schmidt-hammer exposure-age dating: a review of principles and practice

Abstract

Schmidt-hammer exposure-age dating (SHD) is a numerical, calibrated-age dating technique based on the degree of weathering of rock surfaces. The Schmidt hammer measures the hardness or strength of a rock surface by means of R-values (rebound values), which are produced as the hammer impacts the surface. Age calibration is carried out with reference to at least two surfaces of known age (control surfaces). Mean R-values then yield numerical estimates of the time elapsed since exposure of rock surfaces to subaerial weathering. Under favourable circumstances with a sufficiently large number of impacts, SHD has an age resolution of 300–500 years for rock surfaces exposed during the Holocene and Late Pleistocene, and a current age range of ~25 ka. Accurate independent dating of control surfaces and careful selection of sites to reduce the effects of lithological variability are essential. R-value distributions are an important aid in the interpretation of SHD ages from active and relict, synchronous and diachronous, single and multiple-event surfaces. Important research applications have been made within the fields of Quaternary geochronology, palaeoenvironmental reconstruction, geomorphology and subaerial processes, and landscape dynamics. SHD has been found particularly useful in relation to boulder deposits and bedrock surfaces in glacial and periglacial environments. There is much untapped potential in coastal, temperate and tropical landscapes wherever rock surfaces are exposed to undisturbed rock weathering. The advantages and limitations of SHD compare favourably with the complementary exposure-age technique of terrestrial cosmogenic nuclide dating (TCND). The capability to provide a large number of exposure ages at low cost, and non-destructively in sensitive natural and artefactual settings, are special strengths of SHD.