HOW IGNEOUS AND METAMORPHIC ROCK TYPE CONTROL THE RATE AT WHICH COBBLES ARE ABRADED

Abrasion rates for a set of 17 mostly igneous and metamorphic rocks in a stream were analyzed experimentally. The blocks were placed into a cement mixer with a 248 liter (9 ft³⁾ drum with 20 liters of water and 2 liters of pure quartz sand. The samples include different types of Gabbro, Gneiss, Alkali Feldspar Granite, Tonalite, Granite, Limestone, and Sandstone. The cement mixer was used to simulate transport by rolling and saltation in a stream. The drum rotates counter-clockwise at 27 rotations per minute, with each rock being 'transported' 80 cm to 120 cm per rotation. A total of 1620 rotations per hour resulted in 'transport' distances (via 'saltation') of between 1.296 km/hr and 1.944 km/hr. The data obtained allow one to evaluate the distance traveled and time associated with the amount of abrasion. The cement mixer was run in 30-minute increments. The mass and volume of each 'cube' as well as their long- intermediate- and short- axes were measured in order to determine how shape changed over time. Fragmentation, rounding and other abrasion-related features were recorded. Results of this experiment can be used to assist in tectonic interpretation of ancient conglomerates. It could also be used to determine what rock types would be beneficial to place in a stream during stream restoration. Different mineral content in each of the samples was a factor of how fast they were abraded. Rocks that had more resistant minerals in them were abraded more slowly.