In‐flight dynamics of volcanic ballistic projectiles

Abstract

Centimeter to meter‐sized volcanic ballistic projectiles from explosive eruptions jeopardize people and properties kilometers from the volcano, but they also provide information about the past eruptions. Traditionally, projectile trajectory is modeled using simplified ballistic theory, accounting for gravity and drag forces only and assuming simply shaped projectiles free moving through air. Recently, collisions between projectiles and interactions with plumes are starting to be considered. Besides theory, experimental studies and field mapping have so far dominated volcanic projectile research, with only limited observations. High‐speed, high‐definition imaging now offers a new spatial and temporal scale of observation that we use to illuminate projectile dynamics. In‐flight collisions commonly affect the size, shape, trajectory, and rotation of projectiles according to both projectile nature (ductile bomb versus brittle block) and the location and timing of collisions. These, in turn, are controlled by ejection pulses occurring at the vent. In‐flight tearing and fragmentation characterize large bombs, which often break on landing, both factors concurring to decrease the average grain size of the resulting deposits. Complex rotation and spinning are ubiquitous features of projectiles, and the related Magnus effect may deviate projectile trajectory by tens of degrees. A new relationship is derived, linking projectile velocity and size with the size of the resulting impact crater. Finally, apparent drag coefficient values, obtained for selected projectiles, mostly range from 1 to 7, higher than expected, reflecting complex projectile dynamics. These new perspectives will impact projectile hazard mitigation and the interpretation of projectile deposits from past eruptions, both on Earth and on other planets.

Rapid hazard assessment of volcanic ballistic projectiles using long-exposure photographs: insights from the 2010 eruptions at Tungurahua volcano, Ecuador

“…Comparing with other studies, metric-size ballistic projectiles are not typical of Strombolian and Sub-Plinian activity and are more common during Vulcanian explosions [Maeno et al 2013;Bombrun et al 2015;Taddeucci et al 2017]. Based on the size/velocity inverse relationship [Bombrun et al 2015], the maximum launching velocities observed for metric-size projectiles at Tungurahua are higher than those expected for Strombolian activity (<100 m s −1 for >0.6-m-diameter blocks) with velocities up to 117-135 m s −1 , 143-162 m s −1 and 144-153 m s −1 for January, May-June and November eruptions respectively.…”

“…In the distribution histogram, the outer values (<-30 m and >+40 m) represents only 11.5% of all trajectories and could be associated to trajectories modified by processes such as block fragmentation and collision processes during flight [Vanderkluysen et al 2012;Tsunematsu et al 2014;Taddeucci et al 2017]. This amount of abnormal trajectories is significantly higher than the value obtained by Dürig et al [2015] for the Eyjafjallajökull 2010 eruption (97 clast-clast collisions over thousands of ejecta) but it is consistent when taking into account the metric-size of the projectiles for Tungurahua's case.…”

“…Corresponding author: bbernard@igepn.edu.ec This information is used to produce deterministic or probabilistic hazard maps [Alatorre-Ibargüengoitia et al 2016;Biass et al 2016]. Despite a relatively straightforward approach, ballistic projectile hazard assessment is not systematic, one of the main issue being the acquisition of field data [Taddeucci et al 2017]. High-speed cameras (visible and infrared) and volcanic radars provide new insights on projectiles and gas escape velocity at the vent as well as in flight dynamics [Gouhier and Donnadieu 2008;Taddeucci et al 2012a;Bombrun et al 2015].…”

Transport of ballistic projectiles during the 2015 Aso Strombolian eruptions

“…However, we cannot estimate the drag coefficient from this process. Taddeucci et al (2017) estimated the drag coefficient value based on their observation using the high-speed camera. The maximum value of the drag coefficient, calculated using the averaged particle velocity and the flow velocity by Taddeucci et al (2017), was very large (e.g., C D > 3.0).…”

Gas‐Pyroclast Motions in Volcanic Conduits During Strombolian Eruptions, in Light of Shock Tube Experiments

“…In Strombolian eruptions, pyroclast ejection shows significant variability in observed velocities and dynamics, which are thought to be strongly influenced by initial conditions, such as depth of slug bursting (Dürig et al, 2015; Taddeucci et al, 2017), size and amount of particles (Bonadonna & Costa, 2013; Carcano et al, 2014), and conduit and vent geometry (Ogden, 2011; Wilson et al, 1980). Linking the conduit processes (e.g., acceleration and velocity of the particles and their coupling with the gas) to the associated source parameters is an important target, since they control eruptive dynamics, including, for example, particle velocity and ejection angle (Houghton et al, 2004; Kennedy et al, 2005; Pioli et al, 2009), and have strong implications for volcanic ballistic hazard assessment.…”