Weathering rind

Rock fragments with weathering rinds normally are discrete clasts, ranging in size from pebbles to cobbles or boulders.

A weathering rind represents the alteration of the outer portion of a rock by exposure to air or near surface groundwater over a period of time.

Typically, a weathering rind may be enriched with either iron or manganese (or both), and silica, and oxidized to a yellowish red to reddish color.

The use of weathering rinds in relative dating is widely used in Arctic, Antarctic, and alpine regions and in the correlation of glacial moraines and tills and fluvial sediments and terraces.

The thickness of this band can be seen, and measured, using various techniques such as a high-power microscope with 40-80 power magnification, depth profiling with SIMS (secondary ion mass spectrometry), and IR-PAS (infra red photoacoustic spectroscopy).

This distinction can be made because it takes about 70 years for a band to enlarge sufficiently so that it is readily detectable on a freshly flaked surface of a piece of obsidian.

A broken basalt cobble (15 x 10 cm size) showing well-developed weathering rind from Brazil, where chemical weathering is quite active.
Weathering rind of a large granite glacial erratic eroding out of unconsolidated Permian till , Selwyn Rock, Inman Valley, South Australia