Windborne Volcanic Ash: A Possible Index to Polar Wandering

Mapping upper air currents of past epochs by studying the distribution and thickness of extensive deposits of volcanic ash may provide an independent approach to the problem of polar wandering. Records left on the ground by accumulating ash are controlled in large part by upper-air circulation. Because upper-air winds are relatively simple in their gross geometry and in their relationship to the earth's rotational axis, an interpretation of their effect can be made which is subject to considerably less doubt than any other approach thus far suggested (e.g., cross-bedded eolian sandstones) as a check on rock magnetic data applied to polar wandering. The principal features which make the proposed method attractive are: (1) eruptions carrying large amounts of ash to high altitudes (15,000-40,000 feet) are frequent, with approximately 5 × 10⁵ eruptions every one million years; (2) compilation of the direction of long axes of Recent ash falls shows an east-west statistical orientation that accurately defines the present-day rotational axis; and (3) volcanic ash and its alteration products are abundant in Paleozoic and Mesozoic rocks on most continents, thus providing ample material for study. Difficulties in interpretation of ancient deposits arising from changes in direction of upper-level winds, erosion and dissection of ash falls, dispersal of ash by marine currents, and chemical alteration are examined in the light of Recent ash deposits and found to be of minor consequence in most instances.