Twice the blow, twice the impact?

Volcanic eruptions have occurred throughout geological time. Although direct observation of volcanic eruptions has only taken place during a very short chunk of this time, traces of past volcanism can be teased from ice cores, lake and ocean sediments, and the rocks that shape the landscapes we see today. These traces allow for estimates of eruption frequency to be calculated: how often an eruption of a given size is likely to occur. However, is important to remember that eruptions don’t occur periodically. That is, they are random events, and so the time between two large eruptions would never be identical as the two large eruptions before them, and so on.

Eruption frequency generally decreases with size, meaning that the largest eruptions are generally the rarest. Eruption size, or magnitude (M), is calculated based on the amount of rock/ash/pumice produced by the event, and is presented on a scale defined by equal numerical increments. The magnitude scale is logarithmic, meaning that each successive number represents a factor of 10 increase. For example, a magnitude 4 eruption (e.g., Eyjafjajallajökull, 2010 CE*) produced twice as much material as a magnitude 3 eruption (e.g., Fuego, 2018 CE). In the past 100 years, the Mount Pinatubo event in 1991 CE stands as the largest known event with a defined magnitude of 6.1.

If we consider eruption size from a geological perspective rather than a human one, Pinatubo was a ‘small’ event.

At several points in geological history, volcanic eruptions have surpassed magnitudes of 7, 8, and even 9. These events represent the largest, most catastrophic endmember of natural hazards on Earth and are, inevitably, associated with catastrophic impacts on the environment, atmosphere, and ultimately any life unlucky enough to be caught in proximity to the eruption (1).

In the past 2000 years (the ‘Common Era’), there are two known eruptions exceeding magnitude 7 (M>7): Mount Tambora (1815 CE), and Changbaishan volcano (946 CE). Mathematically, this equates to a frequency of roughly one M>7 eruption every 1000 years.

Remember when I said that volcanic eruptions are random, and unevenly spaced in time? Well, what would happen if two M>7 eruptions occurred back-to-back? More worryingly, what about two M>8 eruptions?

This is the question at the heart of our paper in focus (2), where we take a closer look at new evidence that points to the first known instance of such a geological cataclysm: where two M>8 eruptions occur <5-kyr apart.The two eruptions in question are located on opposite sides of the globe, but each left huge scars on the landscape that can be clearly discerned in satellite images (Figure 1).

Figure 1: A map showing the locations of Toba volcano (a), and Atitlán volcano, Guatemala (b).

The first is the Youngest Toba Tuff eruption of Toba volcano, 73.88 ± 0.32 thousand years ago (3). Occurring on the tropical island of Sumatra, Indonesia, this eruption is the largest we know to have occurred during the past 2 million years, with an estimated magnitude of 9.1 (4). The eruption is one of the most ‘famous’ of the known super-eruptions, and so has been subject to a great deal of both scientific and public interest in recent decades. The second eruption is less well known, but similarly colossal in size. This eruption is the magnitude 8.1 Los Chocoyos eruption of Atitlán volcano, Guatemala. This eruption was brought to scientific attention in the 1980’s. Initially, it was ascribed an age of approximately 84,000 years based on the depth of tephra layers found in sediments extracted from the Pacific Ocean (5). However, a new study published in 2021 used new techniques to directly calculate the age of crystals (known as zircons) contained within rocks erupted during the event, and suggest that the eruption actually occurred 75 ± 2 thousand years ago6. This is approximately 9000 years later than previously suggested.

The new eruption date makes the Toba and Atitlán events the first known doublet of super-eruptions occurring within ~1000 years of one another.

Based on current statistics, > M8 eruptions occur approximately 10,000 years, meaning that the probability of two M8 eruptions occurring in such close temporal proximity is extremely low (7). Perhaps even more fascinating is that this ‘super-eruption doublet’ occurs immediately prior to an unusually severe, abrupt climate excursion.

Unlike the relatively mild, warm conditions we experience on our planet today, paleoclimatologists have discovered twenty instances within the past 100,000 years when the Northern Hemisphere was plunged into a severely cold state, lasting for hundreds, sometimes even thousands of years (8). Ice sheets in the high latitudes expanded, continents became drier, and surface temperatures plummeted. These abrupt cooling events are called stadials. The most severe example of such an event began approximately 74,100 years ago, and is named Greenland-Stadial-20 (GS-20). The event impacted ecosystems across the globe, with climate records from Asia (9,10), Borneo (11), the USA (12), and Mediterranean (13,14) all capturing evidence for similarly severe cooling and/or drying.

Despite ongoing research, we still don’t fully know why this event was so severe. Could it be linked to the Toba and Atitlán ‘super-eruptions’?

This question remains to be answered, but no doubt sets the scene for further investigation. Volcanic eruptions emit huge quantities of sulphur dioxide (SO2). When injected into the atmosphere, SO2 prevents radiation and heat from the Sun from reaching the Earth’s surface, and can subsequently result in colder, and drier conditions lasting far longer than the eruption event itself15. Perhaps the most severe stadials (like GS-20) were triggered by several colossal sulfur injections that reinforced each other, and produced longer lasting or more severe effects?

A simple idea to test…in theory. In reality, there are several problems that currently hamper our ability to explore this idea:

(1) Over 70 % of the largest, M>7 eruptions lack precise dates, meaning we do not know exactly when they happened.

(2) At least 61 M>7 eruptions between 100,000 and 10,000 years ago are missing from our record.

Nonetheless, the Toba and Atitlán case study outlined here provides a base from which we can start to consider novel ideas, concepts, and hypotheses. Specifically, how improving eruption dates could dramatically advance our understanding of what causes abrupt climate change, and what the long-term consequences of large volcanic eruptions may be in the future.

Figure 2: Timing of known eruptions exceeding magnitude 7, relative to the Greenland δ18O record. Stadials corresponding to cooler temperatures are marked as blue shaded periods. Known very large eruptions are shown as red (7 <M< 8) and black (M > 8) bars, and stars represent high-quality eruption dates. Inset: A comparison of Toba and Atitlán eruption dates (age errors shown as black bars) relative to the timing of Greenland Stadial 20 and sulphur anomalies (T1-T9; purple points) measured in ice core records from Greenland and Antarctica.

*CE = Common Era

Paper in focus: Paine, A.R., Wadsworth, F.B., Baldini, J.U.L. (2021) Supereruption doublet at a climate transition. Communications Earth & Environment 2(1): 219-221

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