WHEN IS A SURGE NOT A SURGE? THAT IS THE PERPLEXING QUESTION

James G. Moore (USGS) adopted the term “base surge” to describe phreatic to phreatomagmatic explosions and their fine grained (pulverized, broken clasts), bedded deposits. In Moore’s observations of the eruption of Taal volcano, Philippines, he used the term to describe the ground hugging clouds similar to those seen following underwater and underground nuclear explosions. However, names and applications have proliferated: pyroclastic surge; ground surge; blast; ground based surge; Surtseyan eruption; or pyroclastic density flow, with commensurate variation in usage making precise definition problematic. A “base surge” has been defined as a destructive, dilute, fast moving (30 m/s) turbulent density current (flow) of particles and gas and/or liquid that is the result of an explosion. This definition covers a variety of deposits as well as bedded layers within a single event (pyroclastic flow). From the perspective of a hazard mapper and one concerned with risk assessment, understanding if the deposit represents a single event or multiple events is critical. Originally the term was intended to cover phreatomagmatic explosions; characteristically hard to predict, destructive and highly erosive near vent, with thin distal deposits. These facies belay the magnitude of the destructiveness of event and the erosive near-vent impact may be missed entirely. Facies analysis continues to help unravel laminated strata associated with pyroclastic fall deposits where these inter-unit deposits represent a single complex event from a frequency- magnitude perspective, but careful stratigraphic analysis for phreatomagmatic base-surge events must be carried out around temperate region volcanoes. The 8:32am, May 18, 1980 eruption of Mt. St. Helens, USA, is a case in point of the importance of understanding the two different “surge” endmembers. This phreatomagmatic explosion had immeasurable consequences on the area around the volcano, yet, the deposits were thin and quickly disappeared over subsequent years through erosion and revegetation. Careful mapping to identify erosive horizons or thin deposits of highly commutated rock (often dominated by basement, non-juvenile material) and organic debris is needed in order to accurately assess the hazard associated with future eruptions of the edifice under scrutiny.