Volcanic explosivity index
The Volcanic Explosivity Index (VEI) is a standardized, semi-quantitative scale ranging from 0 to 8 that classifies the relative explosivity of volcanic eruptions based on the volume of ejected material, plume height, and other observable characteristics.[1] Developed in 1982 by volcanologists Christopher G. Newhall of the U.S. Geological Survey and Stephen Self of the University of Hawaii, the VEI provides a simple metric for comparing eruptions across time, from historical events to prehistoric supereruptions, despite variations in data availability.[2] The index operates on a roughly logarithmic basis, where each unit increase typically represents about a tenfold rise in the volume of tephra (pyroclastic debris) and other ejecta, though it also incorporates eruption column height and qualitative descriptors like "gentle" for low VEI values and "mega-colossal" for the highest.[3] VEI 0 denotes non-explosive activity with less than 10,000 cubic meters of material, while VEI 5 eruptions—such as the 1980 Mount St. Helens blast—involve around 1 cubic kilometer of tephra and plume heights exceeding 25 kilometers; VEI 6 events, like the 1991 Mount Pinatubo eruption, scale up to about 10 cubic kilometers; and rare VEI 8 supereruptions, such as the 631,000-year-old Yellowstone event, release over 1,000 cubic kilometers, with plumes surpassing 20 kilometers.[3] This framework aids hazard assessment by linking explosivity to potential impacts, including ash fallout, pyroclastic flows, and global climate effects from large eruptions.[2] Although widely used for its simplicity and applicability to incomplete records, the VEI has recognized limitations: it does not fully account for eruption duration (e.g., prolonged VEI 4 events like Parícutin from 1943–1952 totaled 1.3 cubic kilometers over years), variations in plume behavior due to wind or composition, or the distinction between single blasts and multi-phase events in ancient deposits.[3] Recent analyses, including the preliminary VEI 5 assignment for the 2022 Hunga Tonga–Hunga Haʻapai eruption, highlight ongoing refinements to address these inconsistencies.[3]Definition and History
Definition
The Volcanic Explosivity Index (VEI) is a semiquantitative scale designed to measure the relative explosivity of volcanic eruptions, ranging from 0 (nonexplosive) to 8 (ultra-plinian), with the potential for higher values in exceptionally large events.[2] It primarily classifies eruptions based on the total volume of ejecta—encompassing tephra, pyroclastic flows, and surges—while incorporating secondary parameters such as plume height for lower VEI levels where volume estimates may be imprecise.[3] This logarithmic scale ensures that each increment (from VEI 2 upward) represents approximately an order-of-magnitude increase in ejecta volume, facilitating straightforward comparisons of eruption magnitudes despite variations in data quality across historical and geological records.[2] Key parameters include ejecta volume thresholds that define each level: for example, VEI 0 applies to eruptions with less than 10,000 m³ of ejecta, VEI 5 to those exceeding 1 km³ (10⁹ m³), and VEI 8 to volumes greater than 1,000 km³ (10¹² m³).[3] Qualitative descriptors provide intuitive labels, such as "gentle" for VEI 0–1 eruptions (effusive or weakly explosive) and "mega-colossal" for VEI 8 events (cataclysmic plinian eruptions with global impacts).[3] Plume height serves as a proxy for lower levels (VEI 0–3), where heights below 100 m indicate nonexplosive activity and those exceeding 25 km suggest VEI 5 or higher, though this metric is adjusted for factors like atmospheric conditions.[2] The VEI's purpose is to offer a standardized, accessible metric for volcanologists, hazard assessors, and the public to compare eruption sizes across different volcanoes and time periods, bridging gaps in incomplete datasets while emphasizing explosive potential over other eruption attributes like duration or composition.[2] It is calculated using the basic formula: \text{VEI} = \log_{10} (\text{[ejecta](/page/Ejecta) volume in m}^3) - 4 which is rounded to the nearest integer; in ambiguous cases, especially for VEI ≤ 4, plume height or qualitative observations may refine the assignment.[2] This approach prioritizes bulk ejecta volume as the dominant indicator of explosivity, ensuring the index remains practical for both modern monitoring and paleovolcanic reconstructions.[3]Development
The Volcanic Explosivity Index (VEI) was developed in 1982 by volcanologists Christopher G. Newhall of the U.S. Geological Survey and Stephen Self of the University of Hawaii, as detailed in their seminal paper published in the Journal of Geophysical Research.[4] This scale emerged from the recognition that volcanology lacked a standardized, quantitative measure for comparing the magnitude of explosive eruptions, relying instead on subjective qualitative descriptors such as "large," "major," or "catastrophic," which varied widely among researchers and hindered global assessments.[3] Inspired by the Richter magnitude scale for earthquakes, which provided a simple logarithmic metric for seismic events, Newhall and Self aimed to create an analogous tool that emphasized eruption volume and intensity while remaining accessible for rapid application to both modern and historical data.[5] To validate the VEI, Newhall and Self retrospectively assigned values to well-documented historical eruptions, demonstrating its utility in standardizing past records.[4] For instance, the 1815 eruption of Mount Tambora in Indonesia was rated VEI 7, reflecting its immense ejecta volume of approximately 150 cubic kilometers and global climatic impacts, while the 1883 eruption of Krakatoa in Indonesia received a VEI 6 rating, consistent with its 20 cubic kilometers of erupted material and widespread pyroclastic flows.[4][3] Over the subsequent decades, the VEI underwent minor refinements to enhance its compatibility with large-scale eruption databases, particularly in the 1990s as the Smithsonian Institution's Global Volcanism Program (GVP) expanded its cataloging efforts.[6] These adjustments, such as clarifying thresholds for ultra-large eruptions exceeding 1,000 cubic kilometers (assigned VEI 8), facilitated consistent application across thousands of global events without altering the core logarithmic structure.[6] The GVP's adoption of the VEI as a standard metric has since supported systematic analysis of volcanic frequency and patterns, underscoring the scale's enduring role in the field.[7]Classification Scale
Assessment Criteria
The primary criterion for assigning a Volcanic Explosivity Index (VEI) rating is the total volume of tephra and pyroclastic deposits (explosive ejecta), typically measured in cubic kilometers (km³) of dense-rock equivalent.[4] This volume is estimated through methods such as field mapping of deposits, analysis of satellite imagery for dispersal patterns, and calculations based on deposit thickness and extent.[3] The scale is logarithmic, with each integer increase in VEI corresponding to approximately an order of magnitude increase in ejecta volume, providing a standardized measure of eruption magnitude.[4] Secondary criteria, such as eruption plume height, are used to corroborate or estimate VEI when volume data are incomplete, particularly for prehistoric eruptions where direct measurements are unavailable.[4] Plume height is assessed via eyewitness observations, satellite thermal imaging, or modeling of atmospheric dispersion, with thresholds like greater than 25 km indicating VEI 5 or higher.[8] Eruption duration may also factor in qualitatively to refine the assessment.[4] Data for VEI assessment are drawn from geological surveys conducted by organizations like the U.S. Geological Survey (USGS) and the Smithsonian Institution's Global Volcanism Program (GVP), which catalogs over 7,742 Holocene eruptions as of 2025.[7] For historical eruptions, eyewitness accounts provide details on plume dynamics and ejecta distribution, while isotopic dating methods, such as ⁴⁰Ar/³⁹Ar, enable volume estimation for ancient events by establishing eruption timelines and correlating deposits.[4] Challenges in VEI assessment include significant uncertainty in volume estimates for submarine eruptions, where underwater dispersal and fragmentation complicate deposit mapping and quantification.[9] Similarly, ice-covered eruptions pose difficulties, as glacial cover can obscure or alter tephra deposits, hindering accurate thickness measurements and volume calculations.[10] Moreover, the process requires post-eruption analysis, limiting its utility for real-time hazard evaluation during ongoing events.[4] The procedural steps for determining VEI involve: (1) estimating the total ejecta volume using field, remote sensing, or modeling data; (2) applying the logarithmic scale to map the volume to a preliminary VEI value; (3) cross-checking against secondary indicators like plume height and eruption duration; and (4) assigning the final integer VEI based on the composite evaluation.[8][4]VEI Levels
The Volcanic Explosivity Index (VEI) classifies eruptions on a scale from 0 to 8, where each level represents an approximate order-of-magnitude increase in the volume of tephra (ejecta) and corresponding eruption column height, serving as indicators of explosivity. This ordinal scale balances qualitative observations with quantitative metrics to categorize eruption magnitude, emphasizing the potential for widespread dispersal of volcanic products. Lower VEI levels characterize frequent, localized events, while higher levels denote infrequent, cataclysmic occurrences with hemispheric or global consequences. The scale was developed to standardize reporting and enable comparisons, drawing on historical records and geological evidence.[2] The vast majority of volcanic eruptions—over 90% of those documented in the Holocene epoch (the last ~11,700 years)—have a VEI of 3 or lower, reflecting the dominance of smaller-scale activity at most volcanoes. These events typically involve modest plume heights and limited ejecta volumes, resulting in regional rather than global impacts. In contrast, VEI 4 and higher eruptions are progressively rarer, with VEI 7 events occurring roughly once every 1,000 years on average and VEI 8 eruptions approximately once every 50,000 years; consequently, only about a dozen VEI 7 eruptions are confirmed in the Holocene, and none reach VEI 8. Higher-level eruptions often feature ultra-Plinian styles, producing towering plumes that inject aerosols into the stratosphere, potentially causing years-long climatic cooling.[2][5]| VEI | Qualitative Term | Tephra Volume | Plume Height (km) | Typical Eruption Style |
|---|---|---|---|---|
| 0 | Non-explosive | < 10^{-5} km³ | < 0.1 | Hawaiian (effusive lava flows) |
| 1 | Gentle | 10^{-5}–10^{-4} km³ | 0.1–1 | Strombolian (mild explosions) |
| 2 | Explosive | 10^{-4}–10^{-3} km³ | 1–5 | Strombolian to Vulcanian |
| 3 | Severe | 10^{-3}–0.01 km³ | 3–15 | Vulcanian to Surtseyan |
| 4 | Cataclysmic | 0.01–0.1 km³ | 10–25 | Plinian |
| 5 | Paroxysmal | 0.1–1 km³ | >25 | Plinian |
| 6 | Colossal | 1–10 km³ | >25 | Ultra-Plinian |
| 7 | Super-colossal | 10–100 km³ | >25 | Ultra-Plinian |
| 8 | Mega-colossal | >100 km³ | >25 | Ultra-Plinian (global effects) |