The Earth is cooling down! This has nothing to do with contemporary global warming of the atmosphere and surface. I refer instead to the Earth's interior – the source of the molten rocks erupted by volcanoes throughout the planet's 4.567 billion year history. Aeons before the continents drifted to anything like their familiar positions, and as early as 3.34 billion years ago, parts of the Earth were already colonised by primitive bacterial life forms. At this time, volcanoes erupted lavas with a much higher content of an abundant green mineral called olivine than found in most modern volcanic rocks. This testifies to much higher eruption temperatures for the ancient lavas compared with present-day eruptions from Mt Etna or the Hawaiian volcanoes. In turn, it reveals that the Earth's largest internal shell, the olivine-rich mantle, which comprises about 84% of the Earth's volume, used to be considerably hotter, too (anywhere between 100 and 500 °C depending on whom you believe).

Teotihuacán – now located some 45 kilometres from México City – was one of the greatest cities of the ancient world, rising in the first or second centuries BCE and lasting into the seventh or eighth century CE. At its apogee in the fourth century CE it was home to around 125,000 people and covered an area of 30 square kilometres. By the third century CE, a construction boom had established an astonishing complex of pyramids – including the Sun and Moon pyramids, and the Pyramid of the Feathered Serpent. Despite this grandeur, we know rather little about the society that built and sustained this extraordinary civilisation, in part because, unlike the contemporary Mayan glyphs, the inscriptions found at Teotihuacán have yet to be deciphered.

The signatures of many famous eruptions have been recognised in the ice-core records. For instance, the sulphate layers sourced from the eruptions of Mount Mazama (~5677 BCE), Vesuvius (79 CE), Huaynaputina (1600), Krakatau (1883) and Pinatubo (1991) have all been reported. But even a perfunctory review of the ice-core sulphate archives indicates that there are many more acid layers from as yet unidentified eruptions. The responsible volcanoes have thus far evaded detection. What is more, tree rings and, for the historical period, assorted documents, give testimony to a number of major explosive volcanic episodes and their associated climatic impacts. In the case of one of these mysteries, taking place around 536 CE, much has been written, including excessive claims that the consequences of a great eruption marked a turning point in history between the ‘ancient’ and ‘modern’ worlds.

In truth, it is rather difficult to substantiate some of these more startling or fanciful hypotheses – the ‘Dark Ages’ did not get their moniker without reason (though since the term is considered so tendentious by medieval historians today I promise not use it again).

The 25 largest documented Holocene eruptions (magnitude Me of 6.5 and above). Based partly on reference 270 and the Smithsonian Institution's Global Volcanism Program database (http://www.volcano.si.edu).

Mass extinctions punctuate the palaeontological record. These overturns of species and groups of species are so monumental they help to define the framework of the geological timescale, with many smaller extinction events providing finer subdivisions. Perhaps the most famous mass extinction is that which occurred at the boundary between the Cretaceous Period (of the Mesozoic Era) and the Palaeogene Period (of the Cenozoic Era), 65.5 million years ago when, among countless other creatures, the dinosaurs finally went to the wall. The cause of these biological mega-crises has been sought in several directions but there are two main competing mechanisms – bolide (meteorite or comet) impacts versus volcanism on a massive scale. The extra-terrestrial theory argues that especially large bolides – perhaps 10 kilometres or more across – strike Earth from time to time and the collisions would generate sufficiently vast dust clouds to chill the surface, killing off organisms in the oceans and on land. For a long time, the Cretaceous–Palaeogene boundary and associated extinction have been linked to the impact that formed a 170-km-diameter crater at Chicxulub in México.

The eruption of the Younger Toba Tuff in northern Sumatra 73,000 years ago expelled seven thousand billion tonnes of silicic magma (Me 8.8) and, by some estimates, as much as three gigatonnes of sulphur (more than 300 times as much as Pinatubo's 1991 eruption). It made a sizeable contribution to a 100-kilometre-long by 30-kilometre-wide hole in the ground that is today occupied by Lake Toba. This enormous magnitude ranks Toba as one of the largest single eruptions ever documented, along with the 27.8-million-year-old Fish Canyon Tuff associated with La Garita caldera in the San Juan Mountains of Colorado (Me 9.1–9.2) and the two-million-year-old Huckleberry Ridge Tuff of Yellowstone (Me 8.7–8.9). The much more recent Younger Toba Tuff eruption took place close to the time Middle Stone Age Homo sapiens left Africa prompting far-reaching debate concerning the impacts and consequences for modern human adaptation, cognition and migration. Using geological and modelling evidence, this chapter reconstructs the cataclysmic events, and scrutinises the evidence for Toba's impact on the environment, climate and our human ancestors.

THE ERUPTION

Today, the Toba region is a place of rugged peaks, lush forests and bamboo groves, and emerald rice terraces enclosing the traditional villages of the Batak people. And then there are the inviting blue waters of Lake Toba itself.

Since large eruptions are rare, to understand the wider spectrum of volcanic activity requires delving into the past. The preceding chapter outlined how the rock record, combined with assorted proxies for climatic and environmental conditions such as tree rings and ice cores, yields tremendous information on the nature and atmospheric impacts of past eruptions. This chapter extends the theme of ‘forensic volcanology’ by examining what can be learnt of the complex and long-term interactions and intersections between people, volcanoes and volcanic activity. In this regard, both archaeology and memories and records of eruptions in oral or inscribed forms offer a mine of information since they offer insights into both the physical aspects of past eruptions and social responses to living on volcanoes. The investigation and scope of these sources is introduced here.

Not only can impacts of some of the very largest eruptions be examined – events more extreme than anything seen in the historic period – but it is possible to evaluate how different societies have flourished, coped, declined or collapsed in the face of eruptions of different style and scale.