Volcanic activity is difficult to predict, even when it is closely monitored.
But researchers now believe a crater’s age might offer clues, with older volcanoes producing less frequent but larger and more dangerous eruptions.
They gave the example of Mount St. Helens in the US, which became active 40,000 years ago but its last eruption in 2008 was small and not deadly.
On the contrary, the ancient Mount Toba in Indonesia began erupting about 1.2 million years ago and its last eruption 74,000 years ago was cataclysmic.
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![The last eruption of Mount Toba in Indonesia 74,000 years ago was cataclysmic compared to the smaller eruption of younger Mount St. Helens in the US in 2008, scientists at the University of Geneva said. Prior to that St. Helens (pictured) last erupted on May 18 1980, killing 57 people](https://i.dailymail.co.uk/1s/2021/06/22/14/28532116-9712933-The_explosion_on_May_18_1980_was_the_largest_in_US_history_and_l-a-2_1624369599631.jpg)
The last eruption of Mount Toba in Indonesia 74,000 years ago was cataclysmic compared to the smaller eruption of younger Mount St. Helens in the US in 2008, scientists at the University of Geneva said. Prior to that St. Helens (pictured) last erupted on May 18 1980, killing 57 people
‘When a volcano is just starting to be active, its reservoir is rather small and the surrounding crust is relatively cold, which leads to many frequent, but small and rather predictable eruptions’, said the study’s lead author Luca Caricchi, a professor at the University of Geneva in Switzerland.
‘It’s a different story with old volcanoes. Their reservoir is bigger and the rocks around them are hotter. When new magma is injected, it does not generate much over pressure because the rocks around the reservoir deform and the growth continues.’
Eventually, the accumulation of large amounts of magma will lead to large eruptions, the researchers said.
Knowing the age of the volcano, which can be dated by analysing the zircon in the rocks, allows geologists to understand the stage of life of the volcanoes.
‘There are currently 1,500 active volcanoes, and about 50 of them erupt each year,’ Caricchi added.
‘Knowing whether or not to evacuate the population is crucial and we hope that our study will contribute to decrease the impact of volcanic activity on our society.
![Pictured, the Dhaba site, overlooking the Middle Son Valley, India. The archaeological trench can be seen on the left of the photo. The geological record found tool-using populations persisted after the Toba eruption, indicating humans survived the disaster](https://i.dailymail.co.uk/1s/2020/02/25/15/25178428-8042843-image-a-45_1582645250330.jpg)
Pictured, the Dhaba site, overlooking the Middle Son Valley, India. The archaeological trench can be seen on the left of the photo. The geological record found tool-using populations persisted after the Toba eruption, indicating humans survived the disaster
![The eruption, which occurred 74,000 years ago on the island of Sumatra, Indonesia, was about 5,000 times larger than the Mount St Helens eruption in the 1980s](https://i.dailymail.co.uk/1s/2020/02/25/16/25179934-8042843-image-a-56_1582647365696.jpg)
The eruption, which occurred 74,000 years ago on the island of Sumatra, Indonesia, was about 5,000 times larger than the Mount St Helens eruption in the 1980s
‘Hopefully our findings will be tested on volcanoes that have been studied extensively, such as those in Italy, USA and Japan, and transferred to other volcanoes for which there are less data, such as in Indonesia or South America.’
Prior to its 2008 eruption, Mount St. Helens in southwest Washington state last erupted in 1980, killing 57 people, blasting more than 1,300 feet off the top and raining volcanic ash for miles around.
The team reviewed the literature on the internal and external mechanisms that lead to a volcanic eruption.
Analysing the thermo-mechanics of deep volcanic processes and magma propagation to the surface, together with magma chemistry, the geologists also determined that most of the magma rising from depth actually does not cause a volcanic eruption.
‘During its journey, magma can get trapped in reservoirs within the Earth’s crust, where it may stagnate for thousands of years and potentially never erupt’, said Meredith Townsend, of the University of Oregon.
Scientists found that if it is ‘runny enough’ – by not containing too many crystals – magma can rise very quickly by ‘self-propelled fracking’. But if the magma crystallises by more than 50 per cent it becomes too viscous to reach the surface.
‘The chemistry of magma and the crystals it contains provide vital information on the sequence of events leading to a volcanic eruption, which is valuable to better interpret the monitoring signals of active volcanoes and anticipate – whether an eruption might occur,’ Caricchi said.
The study is published in the journal Nature Reviews Earth and Environment.