Dynamics of sand sedimentation resulting from turbidity currents caused by explosive submarine volcanic eruptions

摘要

Deposits of turbidity currents induced by volcanic eruptions under water are increasingly recognized in settings ranging from lakes to the deep sea, and have been referred to as varieties of "eruption-fed density currents." The deposits result from explosive subaqueous eruptions that through condensentation and entrain-ment produce aqueous eruption plumes and then collapse to form turbidity currents or other density flows. These flows emplace layers of typically glassy volcanic ash. Features of deposits from these turbidity currents reflect their specific origin from volcanic eruptions that inject heat and porous volcanic particles into the water column, with both the difference in current temperature, and in particle characteristics, affecting current dynamics and deposit features. Laboratory studies on density currents performed at St. Anthony Falls Laboratory (University of Minnesota) were staged in order to address the effects of variations in grain density and water temperature on flow properties and depositional processes. Laboratory runs utilized natural grain populations from previously studied volcaniclastic deposits for which bedding features, bulk grain size and particle settling-velocity distributions are known. Corresponding runs were performed under the same flow conditions, but utilizing silica sand that was sieved to match the grain size distribution of the volcaniclastic material. Each grain type was used in a series of runs in which a hot density current enters a cold environment, and a series in which a cold current enters a cold environment. A distinction among deposits of the four sets of currents is displayed on the basis of density of the grains driven by the current and the density contrast between the current and ambient water. Experiments for which the inflow contained hot water and low-density sediment tended to place the center of mass of the deposit proximally, whereas experiments for which the inflow contained cold water and high-density sediment tended to deposit mass distally. Results of the study have special relevance to work on submarine volcaniclastic deposits, but fundamental aspects of how grain properties and water temperature affect the driving force of density currents are also illustrated.