This feature was first published in early May in My Mensa Weekly, our exclusive newsletter for Mensa members

The discovery of the driving force behind the world’s deadliest volcanic events could help improve forecasting of their devastating effects, according to a team at University of Edinburgh’s School of Geoscience. The findings reveal what enables dense, scorching avalanches of rock, ash and gases to travel at up to 100 miles per hour, unlocking the potential to reduce the devastation wreaked by volcanic eruptions. 

Establishing what makes the pyroclastic flow – the most common and destructive type of volcanoes – move so swiftly and so far could help improve forecasting of their catastrophic effects and prevent many injuries and deaths. The Edinburgh team has discovered that a particular process during eruption enhances fluidity which coupled with the large amounts of volcanic material, facilitates pyroclastic flows travelling for up to 12 miles at high speeds as dense avalanches, destroying everything in their path.  

History of volcanoes 

Think volcanoes and most of us think of Vesuvius erupting in 79AD, destroying Pompeii and Herculaneum and claiming at least 1,500 lives, although the total number of deaths are unknown. Casts made from the impressions of bodies in the ash deposits paint a haunting and tragic picture of the events 2,000 years ago, when people were apparently so swiftly overcome by the eruption that they died where they stood.  

The early history of volcanology was dependent on eyewitness accounts – such as Pliny the Younger’s account of Vesuvius – and written history of the time, with the eruptions of Mount Etna first documented in 1500 BC and more than 200 eruptions recorded since then.  

Much volcano history was believed to be fairly patchy until quite recently, when an unexpected source of information was revealed in the writings of medieval monks, who have long been credited with considerably more than simple lives of religious devotion. They are also responsible for introducing cheesemaking procedures impossible to improve on – more than 600 years later – educating the young families of the aristocracy and producing meticulous and beautifully illustrated books and manuscripts. These detailed all the ordinary day to day activities of their communities, as well as recording any notable events, celebrations and tragedies, providing invaluable records of medieval life for modern historians. 

Linking lunar eclipses 

We can now attribute another extraordinary achievement to these medieval monks, with the discovery that they could have unknowingly recorded the ferocity of volcanic activity. An international team of researchers led by Sébastien Guillet at the University of Geneva has found another way to learn about these historical eruptions, by studying descriptions of lunar eclipses in medieval manuscripts. They compiled hundreds of records of lunar eclipses from across Europe, the Middle East, and Asia, documenting 187 eclipses between 1100 and 1300,  focusing on descriptions detailing the brightness and colour of the moon during the eclipse. The majority of these descriptions came from European monks, writing in Latin, revealing the colour and brightness of the moon as reported in each total eclipse. This allowed the researchers to rank that colour and brightness to reveal how clear the atmosphere was at the time, with darker eclipses indicating a higher level of aerosol particles in the upper atmosphere, which is a marker of recent volcanic activity. 

Combining the data 

The eclipse data was then put together with simulations of how aerosol particles behave in the atmosphere, modern satellite observations and climatic evidence from historical tree ring records. Trees are sensitive to local climate conditions, such as rain and temperature, with tree rings usually growing wider in warm, wet years and thinner in cold and dry years when it is cold and dry. If the tree has experienced stressful conditions, such as a drought, the tree might hardly grow at all in those years and very old trees can help speculation about what the climate was like long before measurements were recorded. 

Little Ice Age 

This meant the researchers could estimate the timing of the culprit eruptions more precisely than from previous ice core records and determine which eruptions reached the stratosphere and would be more likely to generate climatic cooling effects. This in turn could have been the catalyst for the Little Ice Age, which occurred from the early 14th century through to the mid-19th century, though not rated as a true ice age. Mountain glaciers expanded at several locations and the mean annual temperatures across the Northern Hemisphere declined by 0.6 °C 

How it works  

During an eclipse, the Sun, Earth, and Moon align perfectly, with Earth blocking direct sunlight from reaching the Moon’s surface. However, Earth’s atmosphere bends sunlight around the planet so some of it reaches the Moon even during a total eclipse, though having been filtered by Earth’s atmosphere, much of the blue and yellow light is removed. The state of Earth’s atmosphere at that time controls just how much light is filtered, so a smoky or dusty atmosphere renders the moon redder and darker during the eclipse. The largest volcanic eruptions result in vast amounts of material being flung into the Earth’s stratosphere, where it can remain for many months and spread around the Earth. In the months after a large eruption, any lunar eclipse would be markedly darker than normal, thus leading to the monks recording the different colours and brightness of the eclipses. This has resulted in more precise timings of ancient eruptions, the impact on climate and the role they played in the Little Ice Age, adding yet another achievement to the considerable reputation of these holy men from so long ago.