ABSTRACT

The modal composition of andesitic sand and sandstone is not only a function of source-area climate and transport processes typically considered for nonvolcanic sediment but is also strongly controlled by volcanic fragmentation and pyroclastic-transport processes. Most volcaniclastic sediment deposited penecontemporaneously with active volcanism is not epiclastic, and therefore its composition is not dependent on climate. Crystal-rich andesite sand cannot simply be regarded as the product of weathering in a humid climate. In fact, there is no relationship between precipitation and the ratio of crystals to rock fragments. Fluvial-transport abrasion demonstrably generates crystal-rich sand only in the case of porphyritic glassy rock fragments that are not durable during transport; holoc ystalline pyroclastic fragments apparently do not disintegrate during transport to yield crystal-rich sand. Many sand-size primary volcanic deposits are crystal-rich as a result of eruptive processes that physically fractionate particles of different sizes and densities. Reworking of these deposits results in crystal-rich sand that is not a product of weathering or transport abrasion. The abundance of unaltered green hornblende is one measure of the importance of pyroclastic material in a volcanic sand because this mineral is not found in lava flows. Interpretation of volcaniclastic sandstone requires consideration of volcanic processes not typically considered by sedimentologists.