Next to asteroid impacts, supervolcanoes are the most catastrophic natural hazard on Earth. On average, supereruptions have occurred on Earth approximately every 100,000 years, blanketing surrounding regions with thousands of cubic kilometers of volcanic material and affecting the global climate. During these eruptions, collapse of the magma chamber roof leaves a caldera (a crater tens of kilometers in diameter). In the following decimillennia, the volcano recovers as magma readjusts to the disturbance (rather like the surface of water when something is dropped into it) causing the ground above to swell (‘uplift’) and deform – a process known as resurgence. Earthquakes, lake tsunamis and fresh eruptions characterise this recovery, posing significant and continuing hazards.
Next to asteroid impacts, supereruptions are the most catastrophic natural hazard on Earth
The potential impacts make understanding supervolcanoes a task of the utmost importance, and one that is being tackled by Professor Shanaka de Silva and his colleagues at the College of Earth, Ocean, and Atmospheric Sciences, Oregon State University. In particular, the group are addressing a number of questions:
- Magma bodies that feed supereruptions are likely at least an order of magnitude larger than the calderas they form and develop over hundreds of thousands to millions of years. Questions remain as to how such large volumes of magma can accumulate in the crust and eventually erupt, rather than cool and solidify into a granite.
- The very conditions that promote the growth of large magma bodies demote the likelihood of eruptions. Why then do these magma systems eventually fail and erupt?
- After catastrophic supereruptions, the system recovers during the ‘resurgence’ and ‘restlessness’ stages (or as Professor de Silva describes it, ‘the afterparty after the big dance’). Why does this happen and what are the driving mechanics and time scales? Since all currently active calderas (e.g., Yellowstone, Campi Flegrei, Long Valley, Toba) are resurgent and restless, how long will this last and what is the hazard posed?
- Since many large calderas erupt repeatedly, going through cycles of eruption and recovery, what is the relationship between supereruptions and resurgence?
Pieces of the puzzle
Professor de Silva and his colleagues are gathering information using different scientific techniques – an approach they have termed Supervolcano Forensics – at calderas around the world. Students and postdoctoral researchers have conducted much of this ground-breaking work, examples of which include:
- Geochronology (led by graduate students Casey Tierney, Chris Folkes, Jamie Kern, Jason Kaiser, Rodrigo Iriarte and collaborators Axel Schmitt and Martin Danišík), which uses the decay of radioactive isotopes in magmatic minerals (i.e., crystals within the magma; for example, zircon) to date volcanic processes. This work has focused on calderas in the Central Andes and has shown that:
- crystals can form in the storage region several 100,000 years before eruption; and
- most magma in the storage region actually remains non-erupted.
- the multi-stage evolution of magma chambers, with distinct changes in volume, composition, and heterogeneity; and
- that thermally and chemically homogenous magmas reside in the storage region both before and after a supereruption, and drive resurgent activity. These magmas do not solidify owing to regular periodic injections of fresh, hot magma from depth.
- the rheology (whether brittle or ductile) of surrounding rock is a controlling factor;
- negative feedbacks between magmas’ thermal energy, rock plasticity, internal pressurisation and likelihood of eruption promotes growth rather than eruption;
- eventual failure of large magma chambers (i.e., eruption onset) is a function of roof rheology and geometry; once reservoir volumes reach 104–105 km3, the crust is unable to support them and the roof collapses, producing calderas of up to 103–104 km2, consistent with the largest calderas on Earth.
The work of Professor de Silva’s group, grounded firmly in field-based observation of the deposits and stratigraphy (the relative temporal and spatial relation of events) is showing that supervolcanoes are surface manifestations of crustal scale magmatic activity. The development and longevity of supervolcano magmatic systems depend on the interplay between heat transfer and the mechanical strength of the crust. Without this feedback, magma could not be stored in large volumes; it would erupt in small events, or solidify too early. This in turn controls the eventual size of the eruptions and calderas.
As an integrative framework and with an eye to hazard assessment, Professor de Silva and his colleagues are developing a simple model that frames calderas behaviour as a reaction to changes in the balance of forces in the crust and magma system. In this model, the caldera cycle is a continuous loop. An exciting possibility is that since the temporal and spatial scales of deformation associated with pre-eruptive development of large magma systems is quite different from those associated with restlessness, the transition from resurgence and restlessness to pre-eruption build-up could, in principle, be detected. Part of the challenge is nailing down the temporal and spatial scales of the different stages and their surface representations.
Professor de Silva and his colleagues are gathering information using different scientific techniques – an approach they have termed Supervolcano Forensics
New research focus
To specifically improve understanding of resurgence and restlessness, Professor de Silva and his team have now turned their attention to Toba, Indonesia. Approximately 74 ka (thousand years ago), Toba experienced the most catastrophic eruption of the last 100,000 years, during which at least 2,800 km2 of magma was erupted (that is 28,000 times the amount erupted during the 1980 eruption of Mt St Helens!), forming a caldera 30 km wide and 100 km long. Since then, the caldera floor has experienced well over 1 km of vertical uplift, forming the island of Samosir. This project, which won the support of the National Science Foundation, aims to test the hypothesis that resurgence is fed by magma left over after the climactic eruption.
So far, graduate student Adonara Mucek has used geochronology to date zircon crystals and lake sediment deposits, revealing that resurgence began at least 30 ka and continued until at least 2.7 ka. Eruptions fed by remnant magma rejuvenated by fresh magma from deep continued for at least 20,000 years after the climactic eruption. New work by graduate students Katharine Solada and Jade Bowers is further constraining lake sediment history, and expanding our understanding of resurgent eruptions, including possible relationships with the actively erupting Sinabung volcano.
Where (and when) will be the next supereruption?
Current statistics suggest that the Earth experiences a supereruption (Magnitude, M 8) approximately every 100,000 years. However, there have been at least two such eruptions in the last 74 ka, and it is likely that our inventory of Earth’s supereruptions is incomplete. Calderas appear to be cyclic, but their periodicity varies rapidly. Our best strategy is to be vigilant at the currently active systems and pay attention to volcanic areas around the Earth that have shown this type of activity in the last two million years or so.
What would be some of the local, regional, and global impacts of a supereruption today?
What is the radius of total destruction for a supereruption?
How far away could you be from a supereruption and still hear it?
What new technologies and/or scientific advances will help us to better understand supervolcanoes?