Volcanic eruption columns that fail to entrain and heat enough air to become buoyant collapse and generate devastating pyroclastic flows. Turbulent eddies along the column margins entrain and mix air into the column interior, allowing the column to become buoyant. Currently, the turbulent velocity fields of volcanic eruption columns are unknown, and therefore numerical models of eruption columns remain untested against geologic observations. Through extensive reevaluation of video and photographs of the 18 May 1980 eruption of Mount St. Helens (United States), we report the first measurements of the turbulent velocity field of a volcanic column. During the buoyant, B2, phase of eruption, eddies along the column margins were larger and the fluctuating (turbulent) component of velocity was lower than during partial column collapse in the B3 phase of eruption. We propose that the turbulent structure of the column margins reflects the thickness of and velocity gradients within the column boundary layer in communication with the atmosphere. Thus eddy size scales with the column radius when the entire column becomes buoyant, whereas eddy size is controlled by the thickness of a buoyant annulus surrounding a dense, collapsing core during periods of partial column collapse.

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