Implications of long-term changes in valley geomorphology on the behavior of small-volume pyroclastic flows

摘要

Stratigraphic mapping in the lower 3km of the Vazcun Valley on the NE flank of Volcan Tungurahua (Ecuador) provides insight into the effects of long-term geomorphic changes on pyroclastic flow behavior. Exposures of deposits in the Vazcun Valley record activity over the last 2000years, during which time significant changes in the geomorphology of the valley have occurred. Two sets of terraces are present in the lower 2-3km of the valley, the older of which grades into a small debris fan at the mouth of the valley. Each terrace formed during a period of frequent activity that was separated by a long period of quiescence during which the Rio Vazcun eroded a channel as deep as 40m reaching the previous base level. The pyroclastic flows from historical eruptions appear to have been largely contained within the channel that is cut through the higher terraces and debris fan. Their surface forms the lower terrace located upstream from the head of the debris fan. Thin pyroclastic deposits exposed within the city of Banos are mostly related to ash cloud surges that detached from the main flows as they slowed down within the channel. The lower reach of the present channel of the Rio Vazcun is very sinuous and deeply incised into the two sets of terraces. The winding channel would severely impede the mobility of future pyroclastic flows resulting in the deposition of thick deposits. Detachment of an overriding ash cloud surge could also occur in this region. Such a surge could be more likely to surmount the channel banks and travel over the surface of the terraces and debris for up to several kilometers from the channel.