IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_3E-P15

Date/Time: July 24 Poster

The effect of pre-existing craters on the initial development of explosive volcanic
eruptions: an experimental investigation
Jacopo Taddeucci1 , Greg A. Valentine2 , Ingo Sonder2 , James D.L. White3 , Pierre-Simon Ross4 ,Piergiorgio Scarlato1
1HP-HT Lab, Istituto Nazionale di geofisica e Vulcanologia, Italy, 2 Department of Geology and Centerfor Geohazards Studies, University at Buffalo, USA, 3 Geology Department, University of Otago, NewZealand, 4 Institut national de la recherche scientifique (INRS), centre Eau Terre Environnement (ETE),
CanadaE-mail: taddeucci@ingv.it

Most volcanic eruptions occur in craters formed by previous activity. The presence of a crater implies specificconfinement geometries, variably filled by loose fragmental deposits, which are expected to exert a strong, yet
poorly studied, control on the violent gas expansion that drives the eruption. Here we analyze patterns of ejection
from buried explosions in analog experiments, in order to investigate how the presence of a crater and changes in
explosion depth and intensity may affect the formation of eruptive ejecta jets. Different explosive charges weredetonated at variable depth in prepared pads of granular material, resulting in variable scaled depth (chargeburial depth divided by the cubic root of charge energy). Explosions were filmed at 600 frames per second, andvideo analysis provided information on: 1) jet morphology and evolution; 2) jet vertical speed; 3) jet spread angle.
Results show that explosions with progressively smaller scaled depth produced exponentially faster-growing jetswith linearly larger spread angles. Deeper scaled depth explosions are marked by more-complex jets, featuring acentral, narrower, faster portion that pierces upward through a larger, slower annulus having a stronger componentof lateral expansion. For a fixed scaled depth, the pattern of expansion of the jet seems to be largely controlled by
the initial presence of a crater, which limits the development of a laterally expanding annulus.
The relationship between jet velocity and scaled depth holds true independently of the presence of a pre-existing
crater infill, while jet spread angle shows, overprinted on a major control exerted by scaled depth, also an effect ofthe presence of a crater. Finally, the presence of a crater around the explosion site does exert a strong control on
the expansion pattern of the jet. The complex development of explosions with both a vertically-expanding central
core and a peripheral, lateral expanding annulus seems promising for characterizing the intra-crater evolution ofan eruption. This distinction has also implications for both hazard assessments and for investigations of eruptiveproducts. The vertical core and radial annulus could be associated with the formation of buoyant jets and ballistic
showers, and dilute pyroclastic density currents, respectively. Since the radial annulus tends to be inhibited by thepresence of a relatively deep crater, this could promote a decrease in the occurrence of coarse-grained pyroclastic
surges (and their deposits) extending beyond a crater as an eruption and the respective crater evolve over time.
Conversely, the fallback of a vertically-focused jet could trigger a fine-grained pyroclastic surge, potentially leaving
fine-grained, bedded deposits.

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