FIRE AND WATER: INTERACTIONS BETWEEN LAVA FLOWS AND RIVERS DURING ERUPTIVE EVENTS

While lava-ice and lava-sea water interactions have been the subject of intensive study, surprisingly little attention has been paid to interactions between lava flows and fluvial systems, particularly during eruptive events. This reflects, in part, the absence of surface water in areas of frequent effusive activity, such as Hawai`i, USA, and Mt. Etna, Italy, where most lava flow studies are done. However, there are historical examples of complex and dramatic interactions between lava flows and the rivers that occupy the valleys they flow into, with one good example being the 1783 eruption of Laki, Iceland. This eruption illustrates a number of processes that can be broadly distinguished in terms of the effects of lava flows on rivers and effects of rivers on lava flows. Lava flows entering rivers can temporarily or permanently dam or dry up mainstem and tributary streams, divert flows, and create large hot or cold syn- or post-eruptive floods. At the same time, river valleys efficiently focus and channelize lava flows, allowing them to thicken and travel further than they might otherwise over rough unchannelized surfaces. Conversely, river flows and ponded water can also quench lava flows, suppressing movement, and promoting rootless, phreatic explosions and overthickened flows.

We couple historical eyewitness accounts from Iceland with field studies of mid- to late-Holocene lava flows that entered the McKenzie River in western Oregon, US, and explore ways to recognize lava-river interactions in the field. From the standpoint of lava flows, field evidence includes well-defined intracanyon flows, hyaloclastites and bedded palagonite structures, rootless explosive vents, and evidence of rapid quenching, including thin and irregular columnar jointing. The record preserved in the fluvial system includes evidence of lava dams (often as spectacular waterfalls), lava-dammed lakes, dam outbreak floods, and channel reestablishment at flow margins. Taken together, this evidence allows us to construct a rough chronology of lava flow and river interactions for recent volcanic events, and expand understanding of hybrid hydrovolcanic hazards accompanying effusive volcanic events.