Rock Weathering Rates on Subantarctic Marion Island

Subantarctic Marion island (46°S) is a shield volcano of basal “gray” lavas and younger “black” lava outflows. Rock weathering rates, as determined by annual mass loss from small clasts (<400 g), was measured over a 3-yr period at 4 sites on the island on an altitudinal transect from the coast toward the interior. Mass loss from gray lava clasts was found to be 0.02% near sea level, increasing to 0.10% yr−1 at 730 m a.s.l. Black lava clasts yielded mean losses of up to 0.72% at the sites, although no altitudinal trend in mass loss was evident. In the sea-spray zone, gray and black lava clasts monitored over 1 yr had mean losses of 0.30% and 0.41%, respectively. Weathering rates are marginally inflated by the annual weighing procedure, which has been determined to contribute approximately 0.01% to the measured gray lava clast mass losses, and 0.07% to the black lavas. Since none of the clasts showed visual signs of fracturing or flaking, mass loss appears to be on a granular scale at most, and material removal is probably assisted by rainwash. Extrapolated values suggest that black lava clasts can weather completely within 200 yr and gray lava clasts within approximately 1000 yr at high altitudes. These data have implications for the lifespan of periglacial landforms constituting small clasts, particularly those formed in the early Holocene.