Determining the vertical scale in videos of lava fountains from gravitational acceleration of single clasts at their zenith

Videography is a popular tool for monitoring and characterising volcanic eruptions. Video records of lava fountaining episodes allow us to infer eruption parameters such as fountain heights, exit velocities, and pulse durations and frequencies, which may inform us on the subsurface processes that operate within the sub-volcanic plumbing system. However, the evolving shape and size of the natural features surrounding eruptive vent make it difficult to convert pixels in an image to meters in reality, due to the lack of fixed reference points with which to compare dimensions. Here we present a new method for determining the vertical scale in videos of lava fountains. We measure the vertical pixel-position of clasts near their zenith, over successive frames, and convert this to an acceleration. By assuming that the only force acting on single clasts near their zenith is gravity, we use the clast motion to determine the scale – mapping pixels to metres. Geometric considerations around the viewing angle and lens distortions are discussed and corrected for. We validate this method with laboratory experiments using water fountains and vertically projected light plastic balls, which act as analogues for lava fountains and single clasts, respectively. An example of field application is then provided from the 2018 fissure eruption at Kilauea (Hawaii, USA). This approach will be useful to physical volcanologists for monitoring the dynamics of eruptions that produce fountains and/or ballistics from video records, which are becoming increasingly available both from scientific teams and from a wider community of tourists and volcano-enthusiasts.