This article is edited and drawn from:
Davies, B.J., 2022. Dating Glacial Landforms II: Radiometric Techniques, in: Haritashya, U. (Ed.), Treatise in Geomorphology (Second edition). Cryospheric Geomorphology. Elsevier, pp. 249-280. (link)
Weathering and denudation
Exposed surfaces will denude over time due to weathering and in situ breakdown of rock. The concentration of surface cosmogenic nuclides is sensitive to surface erosion (Gosse and Phillips, 2001).
Assuming that rock erosion rates are constant over time, cosmogenic nuclide data can be used to determine an erosion rate, because the concentration of 10Be at the surface of a steadily eroding outcrop surface is inversely proportional to the outcrop’s erosion rate (Balco et al., 2008; Gillespie and Bierman, 1995; Portenga et al., 2013).
Erosion rates calculated in this way are limited to rock outcrops and boulders. A detailed discussion of erosion rates is beyond the scope of this chapter. Here, this section focuses on the importance and significance of erosion rates for the production of surface exposure ages.
Erosion rates
Calculators require an erosion rate (cm yr-1 or mm kyr-1) to account for the impact of this on the surface exposure age (Granger et al., 2013). Typically low erosion rates of 0.5 to 2.5 mm kyr-1 for resistant lithologies in temperate regions will have little impact on samples younger than ~30 kyr (Balco, 2011; Davies et al., 2020; Gosse and Phillips, 2001).
Erosion rates are increasingly important for older deposits, even with low erosion rates of 1 mm kyr-1 (Hein et al., 2009). However, measured erosion rates are sparse in many regions, and may be spatially and temporally variable, as well as variable according to lithology sampled.
Practitioners presenting surface exposure ages should therefore present data with a zero erosion rate, providing a minimum age for the deposit, and one under a clearly reported constant, estimated rate of erosion that is appropriate for the regional climate and lithology (Darvill, 2013).
Typical rock erosion rates
Typical erosion rates were reviewed by Portenga and Bierman (2011), who compiled and normalized published 10Be erosion rate data (n = 1599).
They found that outcrops had a mean erosion rate of 12 ± 1.3 mm kyr-1, median 5.4 mm kyr-1, lower than the mean drainage basin erosion rate of 218 mm kyr-1. These rates varied according to climate zone, rock type and tectonic setting.
Erosion rates on sedimentary outcrops are typically 20 ± 2.0 mm kyr-1, metamorphic outcrops have a mean erosion rate of 11 ± 1.4 mm kyr-1, and igneous outcrops have a mean erosion rate of 8.7 ± 1.0 mm kyr-1.
Erosion rates of outcrops in polar climates have a mean of 3.9 ± 0.39 mm kyr-1, whilst temperate climates have a mean of 25 ± 2.5 mm kyr-1 (Portenga and Bierman, 2011).
Measurements on homogenous crystalline rocks have typically yielded low erosion rates of 0.2 mm kyr-1, with 1 mm kyr-1 for biotite-rich crystalline rocks, and 5 mm kyr-1 for carbonate sedimentary rocks (André, 2002).
Estimates of erosion rates on granite in temperate Patagonia include 1.4 mm kyr-1 (Kaplan et al., 2005), 0.049 ± 0.02 to 1.31 mm kyr-1 (Bourgois et al., 2016) and 0.2 mm kyr-1 (Douglass, 2007). Bedrock outcrop erosion rates from the central Appalachian Mountains produced mean ridgeline erosion rates of 9 mm kyr-1 (Portenga et al., 2013).
Sandstone erosion rates in Antarctica are mostly < 1 mm kyr-1 (Hein et al., 2016).