Natural Hazards Editors' Vox

Caught on Camera: Volcanic Bombs in Flight

A recent paper in Reviews of Geophysics revealed new insights into the flight patterns of solid and molten debris flung out of volcanos during explosive eruptions.


When volcanos erupt they can spew out a range of substances from viscous lava and hot gas to ash clouds and rock fragments, each of which behave in different ways. A recent review article published in Reviews of Geophysics focused on the larger fragments of solid and molten material ejected from volcanos, and described an innovative new study of the flight paths of these projectiles. The editors asked one of the authors to explain more about this scientific research and its practical relevance.

What different things come out of volcanos when they erupt?

Effusive volcanic eruptions produce flows of molten rock, or “lava,” whereas explosive volcanic eruptions eject a mixture of gas and solids. The solid fragments range in size from micron-sized ash particles, through millimeter- to centimeter-sized, often vesicular fragments (pumice and scoriae lapilli), to meter-sized molten bombs and brittle blocks. All these fragments are ejected from the volcano by expanding gas but, after leaving the volcanic vent, they have different fates depending mainly on their size. Ash and lapilli are often engulfed with hot gas and air in a volcanic plume, where they may travel a considerable distance in the atmosphere. Larger fragments, called “volcanic ballistic projectiles,” are too heavy to be carried by the plume and fall to ground relatively close to the volcanic vent.

What kind of volcanoes produce ballistic projectiles and how dangerous are they?

Ballistic projectiles are a constant of all explosive volcanic eruptions, from current small ones such as Stromboli, Italy, to past super-eruptions such as Yellowstone, USA.

Explosive volcanic eruptions can eject molten and solid debris, as well as gas and ash, as at Yasur volcano, Vanuatu. Credit: Marco Albano

What changes from case to case is the nature, range and size of the projectiles. Small eruptions can launch small projectiles hundreds of meters away, while larger eruptions can produce projectiles several meters in size that travel many kilometers.

While projectile fallouts are more confined than other volcanic processes, they represent a constant threat to life and property in many places. Smaller projectile “showers” may come from small but unexpected explosions, including at active volcanoes that are popular tourist sites.

How is it possible to study ballistic projectiles when you don’t know when a volcano will erupt?

So far, most research on volcanic ballistic projectiles focused on two aspects: the deposits of past eruptions and theoretical studies. On the one hand, mapping the size, weight and location of landed projectiles around volcanic sites helps understand how violent an eruption was and from where it occurred. On the other hand, new experiments and simulations now enable the reconstruction of the trajectory of projectiles with increasing detail.

However, the gap between field deposits and theoretical simulations can only be bridged by direct observation of projectiles being erupted and in-flight. This motivated our study [Taddeucci et al, 2017] with high-speed, high-definition cameras, carried out over the last five years at several volcanoes worldwide, including in Italy, Vanuatu, Japan, Guatemala and Indonesia. You can view more than 40 short videos of flying projectiles from our different research sites for free in the Supporting Information which accompanies our article.

What have your direct observations of flying projectiles contributed to this research field?

The researchers deployed high-speed cameras to capture the trajectory, rotation, collision and deformation of volcanic ballistic projectiles in-flight. Credit: Marco Albano

First, we found new in-flight dynamics such as the breaking of projectiles on collision.

Second, we measured processes that were only assumed or scarcely documented. For instance, by measuring the rotation rate of projectiles we demonstrated that their trajectory can be significantly modified by spinning.

Third, correct modeling of the trajectory of projectiles requires accurate input parameters and we provided the first estimates of these parameters based on the direct observation.

How is your research of wider use?

The only protection we have against volcanic ballistic projectiles are hazard maps and early warning systems. We need to link the features of a certain eruption to the range and size of projectiles, but this requires an understanding of the laws that control the motion of projectiles. These laws are more complex and seem to involve a larger number of variables than previously thought. Thus our advancements may lead to better hazard planning and mitigation. Our study may also help interpreting other types of ballistic projectiles, such as those related to meteoric impacts, and the ballistic motion of projectiles on other planetary bodies.

What are some of the unanswered or unresolved questions in this field of research?

As in most cases, the use of a new observation technique provides more questions than answers. The largest outstanding question is why some of the parameters we measured deviated so much from what theory and experiments suggested. We suspect that the fact that the projectiles are very hot may play a role, and it is even possible that they may still be releasing volcanic gases while flying. Another important point is that our study covers only the small to medium range of explosive volcanic eruptions; larger eruptions are harder to study because they are less frequent and predictable, as well as more dangerous to approach. I definitely plan more research on this exciting topic, with the use of new technology and the involvement of colleagues from all over the world.

—Jacopo Taddeucci, Istituto Nazionale di Geofisica e Vulcanologia, Italy; email: [email protected]