PLINY the Younger, the ancient Roman scholar, was lucky enough to be at a safe distance when Mount Vesuvius blew her top in AD 79. His uncle, Pliny the Elder, was not so fortunate and was suffocated by the eruption, which also buried the cities of Pompeii and Herculaneum. Afterwards Pliny wrote about his experience, describing the volcanic cloud as 鈥渂eing like a pine rather than any other tree, for it rose to a great height on a sort of trunk and then split off into branches鈥�.
An important question for geologists investigating such eruptions is: how do volcanoes accumulate their magma? To Pliny, the lava from Vesuvius looked like one big splurge of molten rock, as if the volcano had binged on magma from below and thrown it up. But geologists have recently discovered that volcanoes have more refined habits, spending years ingesting magma in many small gulps before eventually venting it. As part of a European project called ERUPT, Jon Davidson and colleagues at the University of Durham, UK, have shown that Vesuvius ate the equivalent of breakfast, lunch and tea before erupting, albeit with a few decades between each meal.
鈥淰esuvius ate the equivalent of breakfast, lunch and tea before erupting鈥�
Advertisement
How do you study a volcano鈥檚 digestion? For a beast like Vesuvius, the best way is to trawl through the leftovers. Dan Morgan, working on the ERUPT project with Davidson, collected samples of pumice from the AD 79 eruption and examined their crystals under a powerful microscope. This allowed him to identify the kind of magma diet Vesuvius fed on. If the volcano had had just one large magma meal, then the crystals would all be a similar shape, composition and size. Instead Morgan found a wide variety of crystals, each representing a different course in the volcano鈥檚 feast.
Crystal-gazing can tell us a fair bit about the feeding habits of volcanoes. Crystals grow from the centre outwards and so, like tree rings, they tell a story about the events that occurred as they grew. The youngest of Vesuvius鈥檚 crystals were uniform throughout, but the oldest showed four distinct changes in chemical composition. Each of these rings represented a new injection of magma. By counting rings and working out the proportion of magmas that had accrued, Morgan could construct a feeding history for Vesuvius. Breakfast arrived around 80 years before the eruption, lunch after another 40, and tea 20 years later. Supper in AD 79 finally made her spew.
Geologists have been studying the eating patterns of other volcanoes. Stromboli, for example, a volcanic island north of Sicily, exists on snack meals for several thousand years before disgorging herself into the Tyrrhenian Sea. Knowledge like this could help researchers predict when volcanoes like Vesuvius and Stromboli erupt 鈥� and help local residents avoid Pliny the Elder鈥檚 fate.
