WHAT FACTORS CONTROL THE COMPOSITION OF ANDESITIC SAND

The modal composition of andesitic sand and sandstone is not only a fu nction of source-area climate and transport processes typically consid ered for nonvolcanic sediment but is also strongly controlled by volca nic fragmentation and pyroclastic-transport processes. Most volcanicla stic sediment deposited penecontemporaneously with active volcanism is not epiclastic, and therefore its composition is not dependent on cli mate. Crystal-rich andesite sand cannot simply be regarded as the prod uct of weathering in a humid climate. In fact, there is no relationshi p between precipitation and the ratio of crystals to rock fragments. F luvial-transport abrasion demonstrably generates crystal-rich sand onl y in the case of porphyritic glassy rock fragments that are not durabl e during transport; holocrystalline pyroclastic fragments apparently d o not disintegrate during transport to yield crystal-rich sand. Many s and-size primary volcanic deposits are crystal-rich as a result of eru ptive processes that physically fractionate particles of different siz es and densities. Reworking of these deposits results in crystal-rich sand that is not a product of weathering or transport abrasion, The ab undance of unaltered green hornblende is one measure of the importance of pyroclastic material in a volcanic sand because this mineral is no t found in lava flows. Interpretation of volcaniclastic sandstone requ ires consideration of volcanic processes not typically considered by s edimentologists.