Laboratory Study of the Role of Seawater in Basalt Pebble Abrasion

Abstract

Tumbling experiments simulating the movement of basaltic pebbles on marine benches in tropical environments by transgressive sea waves have raised questions about the role of water in rock wear. Rocks tumbled in distilled water wear more rapidly than identical sets run in seawater, suggesting a solution process. A single rock fragment immersed in seawater or tumbled continuously suffers negligible wear; impact between the rocks seems essential if substantial wear is to occur. However, rocks tumbled dry wear much less than rocks tumbled in seawater. Equivalent rock sets were tumbled in seawater and dioxane, a nonpolar cyclical ether, to assess the effect of the polar property of the water molecule in rock wear. The rocks wore more rapidly in dioxane than in seawater. Thus, water facilitates wear of rocks that move against each other, but it apparently is not acting as a solvent. Analysis of encounters between freshly-broken basalt fragments tumbling in seawater indicates surface pressures sufficient to crush the sharpest prominences. As fragments become rounded, impact pressures fall below crushing strength of basalt; some mechanism other than compressive breaking must account for subsequent wear.