Hydrovolcanic explosions; II, Evolution of basaltic tuff rings and tuff cones

Abstract

Eleven basaltic volcanoes from a variety of geologic settings were studied in order to compare vent morphology, deposit stratigraphy, and emplacement mechanisms at tuff rings with those at tuff cones. The tuff rings consist of thinly bedded, poorly indurated, relatively fresh pyroclasts deposited with bedding angles less than 12 degrees. Highly inflated pyroclastic surges are the dominant means of emplacement, as evidenced by the abundance of sandwave bed forms. The tuff cones consist of massive, thickly bedded, highly indurated, and hydrated pyroclasts deposited at bedding angles up to 30 degrees. This massive tuff contains ash-fall layers interspersed with nearly equal volumes of base-surge beds, as a result of intermittent Strombolian and Surtseyan base-surge activity. A complete spectrum of explosive volcanic activity, from phreatic to magmatic, results from the shallow-to-moderate depth interaction of magma with external water sources. The venting phenomena range from a single burst that produces an explosion breccia to a prolonged series of explosions of various energies that produce a complex deposit of interbedded pyroclastic fall and surge beds. Stratigraphic data and models of pyroclastic surge suggest that the massive bedding of tuff cones results from a cool (below 100 degrees C), wet emplacement. In contrast, the thinly-bedded deposits of tuff rings are typically emplaced while hot (above 100 degrees C) and relatively dry. As the volume of water that explosively mixes with magma increases, the amount of steam produced increases, but the level of superheating of that steam decreases. This leads to an increased wetness of the resulting surge blasts. With increasing ratios of water to magma, activity changes from lava fountains to dry surges to wet surges. The resulting volcanic landforms are respectively cinder cones, tuff rings, and tuff cones. Accordingly, a dry environment would contain cinder cones; abundant ground water promotes tuff rings; and a shallow body of standing water favors development of tuff cones.