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During explosive eruptions, tephra particles are injected into the atmosphere and undergo different fates: while larger particles settle close to the volcano, smaller ones remain suspended, forming volcanic clouds. In all cases, tephra poses significant hazards to human activities both near the volcano and hundreds of kilometers away.
A new article in Reviews of Geophysics explores our current understanding of tephra plumes and clouds, including their generation, characteristics, and monitoring strategies. Here, we asked the lead author to give an overview of tephra plumes, recent advances in modeling them, and what questions remain.
How does tephra form and spread?
Tephra forms through a process called fragmentation within volcanic conduits and is then expelled into the atmosphere by volcanic plumes. These fragments are classified based on their size: blocks and bombs (greater than 16 mm in diameter), lapilli (ranging from 2 mm to 16 mm), and ash (less than 2 mm in diameter). Once airborne, larger tephra particles typically settle near the volcanic vent, while finer particles (ash) can be carried by the wind over vast distances, forming what are known as volcanic clouds.
What kinds of hazards does tephra pose both in the air and on the ground?
Volcanic clouds pose a significant threat to aviation safety. When aircraft encounter these clouds, tephra particles can be ingested by jet engines, leading to performance degradation and, in severe cases, catastrophic engine failure. In addition, airborne tephra poses serious risks to public health. Studies on populations exposed to volcanic ash have documented increases in both acute and chronic respiratory conditions. The most dangerous particles are those smaller than 4 micrometers in diameter as they can penetrate deep into the lungs’ alveolar region, potentially triggering toxic reactions.
Tephra fallout can cause extensive damage to critical infrastructure, leading to substantial economic losses across multiple sectors.
On the ground, tephra fallout can cause extensive damage to critical infrastructure, leading to substantial economic losses across multiple sectors. These include energy systems, water and wastewater services, transportation networks (aviation, land, and maritime), food and agriculture, manufacturing, and communications. Rural communities, particularly those reliant on agriculture and livestock, are especially vulnerable. Tephra fallout can disrupt livelihoods not only in the immediate aftermath of an eruption but also over the long term. This is because tephra deposits can be remobilized by wind, generating ash storms that resemble the effects of the original eruption. These recurring events can persist for years, hindering economic recovery and prolonging hardship for affected communities.
What factors influence how far tephra spreads?
Tephra can be dispersed over vast distances, and in some cases, it may even travel around the globe. The extent of tephra dispersal is influenced by several factors, including the magnitude of the eruption, the size of the tephra particles (with smaller fragments remaining suspended in the atmosphere for longer periods), the volcano’s geographic location, and atmospheric conditions (particularly wind strength and direction). For example, the 1991 eruption of Mount Pinatubo in the Philippines, one of the major eruptions of the 20th century, injected massive amounts of volcanic ash into the stratosphere, which were carried by high-altitude winds and circled the globe in just 22 days.
Even relatively small eruptions can have major impacts when atmospheric and geographic conditions are unfavorable.
Similarly, the 2010 eruption of Eyjafjallajökull in Iceland, although moderate in size, caused significant disruption to European air travel. The fine-grained ash particles were carried thousands of kilometers by the jet stream, grounding flights across much of Europe for several days. This highlights how even relatively small eruptions can have major impacts when atmospheric and geographic conditions are unfavorable.
How do scientists monitor tephra plumes and clouds?
Scientists monitor tephra plumes and volcanic ash clouds using a combination of ground-based instruments and satellite observations. These data are essential for characterizing key aspects of volcanic activity, including plume extent, eruption column height, umbrella cloud spread, ash cloud altitude and thickness, tephra particle properties (such as size, shape, and settling velocity), mass eruption rate, sedimentation rate, and eruption duration. Ground-based tools include visible and thermal cameras, lidar, radar, infrasound microphones, and lightning detection antennas, each optimized for specific types of observations and deployed at varying distances from the volcanic vent.
Satellite sensors support global monitoring efforts through both active and passive remote sensing across a wide range of wavelengths, from ultraviolet to microwave. Modern ash cloud detection relies heavily on geostationary satellites, which provide high-temporal-resolution imagery (every 1 to 10 minutes), ideal for continuous real-time observation. However, these systems have trade-offs, such as coarse spatial resolution (approximately 4 km² at nadir) and limited coverage at high latitudes due to their equatorial orbital positioning.
Visible (a and c) and thermal (b and d) images of Mount Etna (Italy) plumes, acquired by the monitoring network of the Italian Institute of Geophysics and Volcanology, Osservatorio Etneo (INGV-OE). Courtesy of INGV-OE. Credit: Pardini et al. [2024], Figure 9
What are some recent advances in modeling tephra dispersal?
The movement of volcanic clouds and the deposition of tephra on the ground can be simulated using specialized numerical tools known as Tephra Transport and Dispersal Models (TTDMs). These models first emerged in the 1980s and have since undergone significant advancements in model physics, numerical solvers, and computational efficiency.
TTDMs require two main types of input data: meteorological information (such as wind speed, temperature, and pressure) and volcanic source parameters, which define what is emitted, how much is emitted, how particles are injected into the atmosphere (including their height and distribution), and the duration of the emission (start and end times). These models produce outputs that describe both the distribution of tephra suspended in the atmosphere and the patterns of tephra deposition on the ground. Modern TTDMs are capable of simulating complex atmospheric processes affecting tephra transport, such as particle aggregation and wet deposition (removal of ash particles by precipitation).
A recent development in the field is the emergence of in-line modeling approaches, which couple TTDMs directly with numerical weather prediction (NWP) models. In this integrated setup, the atmospheric evolution and tephra transport are computed simultaneously, eliminating the need to interpolate meteorological data between separate models. This approach improves the accuracy of tephra dispersal simulations, particularly under rapidly changing weather conditions. However, it comes at the cost of increased computational demand and is currently used primarily in research settings rather than for operational forecasting.
How have models contributed to improved forecasting and risk mitigation?
TTDMs play a crucial role in volcanic risk mitigation by providing forecasts of volcanic cloud movement and tephra deposition during eruptions. These models are especially valuable for early warning systems, enabling timely decisions to protect public health, aviation safety, and critical infrastructure.
One of the key operational users of TTDMs are the Volcanic Ash Advisory Centers (VAACs), which are a network of nine specialized agencies distributed globally under the mandate of the International Civil Aviation Organization (ICAO). VAACs are responsible for monitoring volcanic ash clouds and issuing advisories to aviation authorities. To do so, they routinely run TTDMs to predict the spatial and temporal extent of ash clouds, helping to prevent aircraft encounters with hazardous volcanic plumes. In addition, TTDMs are used by national meteorological and civil protection agencies to forecast and manage the ground-level impacts of tephra fallout. For example, the Japan Meteorological Agency (JMA) issues real-time forecasts of tephra dispersal and deposition following eruptions to support public safety measures. Similar practices are implemented in other volcanically active countries, such as Iceland and Italy.
Example output from the TTDM Ash3d, used at the Alaska Volcano Observatory to forecast the movement of volcanic clouds during periods of unrest. The example shown simulates a hypothetical volcanic cloud from Shishaldin volcano on 7 August 2024, using eruption source parameters that are considered realistic for that volcano. Results are publicly available at the Alaska Volcano Observatory website. Credit: Pardini et al. [2024], Figure 18
What are some of the remaining questions where additional modeling, data, or research efforts are needed?
In recent decades, there has been significant progress in our conceptual understanding of the processes that drive tephra plumes and the behavior of volcanic clouds. However, the inherent variability of explosive eruptions (ranging in style, location, and unique characteristics) continues to pose major challenges for both comprehensive understanding and effective monitoring.
Improving observational capabilities represents a critical frontier in volcanology.
One persistent difficulty lies in connecting model predictions with real-world observations. Large eruption plumes are rare, and even smaller events are difficult to characterize due to the limitations of current satellite systems, ground-based instruments, and visual data. As a result, improving observational capabilities represents a critical frontier in volcanology. Integrating these improved observations into modeling frameworks is essential, also to better understand underexplored processes such as particle aggregation and in-plume phase-change of water.
The emerging potential of artificial intelligence in the detection and forecasting of tephra is increasingly recognized, although its current application remains limited, primarily to a few ash retrieval algorithms. In contrast, the use of large synthetic datasets generated by TTDMs to train data-driven models remains largely unexplored, despite the encouraging results achieved in other atmospheric dispersion contexts, where machine learning models have demonstrated strong generalization capabilities even under previously unseen conditions not represented in the training data.
—Federica Pardini (federica.pardini@ingv.it; 0000-0001-6049-5920), Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Pisa, Pisa, Italy
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.
Citation: Pardini, F. (2025), Inside volcanic clouds: where tephra goes and why it matters,
Eos, 106, https://doi.org/10.1029/2025EO255020. Published on 16 June 2025.
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