Lava Flows during Basaltic Eruptions

12 Observations in the USA, Iceland, and Tenerife, Canary Islands, reveal how processes occurring 13 during basaltic eruptions can result in complex physical and stratigraphic relationships between lava 14 and proximal tephra fall deposits around vents. Observations illustrate how basaltic lavas can 15 disrupt, dissect (spatially and temporally) and alter sheet-form fall deposits. Complexity arises 16 through synchronous and alternating effusive and explosive activity that results in intercalated lavas 17 and tephra deposits. Tephra deposits can become disrupted into mounds and ridges by lateral and 18 vertical displacement caused by movement (including inflation) of underlying pāhoehoe lavas and 19 clastogenic lavas. Mounds of tephra can be rafted away over distances of 100s to 1000s m from 20 proximal pyroclastic constructs on top of lava flows. Draping of irregular topography by fall 21 deposits and subsequent partial burial of topographic depressions by later lavas can result in 22 apparent complexity of tephra layers. These processes, deduced from field relationships, have 23 resulted in considerable stratigraphic complexity in the studied proximal regions where fallout was 24 synchronous or alternated with inflation of subjacent lava sheets. These mechanisms may lead to 25 diachronous contact relationships between fall deposits and lava flows. Such complexities may 26 remain cryptic due to textural and geochemical quasi-homogeneity within sequences of interbedded 27 basaltic fall deposits and lavas. The net effect of these processes may be to reduce the usefulness of 28 data collected from proximal fall deposits for reconstructing basaltic eruption dynamics. 29 30