Pompeii debris yields calamity clues

Geologists have used the magnetic characteristics of rocks and debris excavated from Pompeii to estimate the searing temperatures of the cloud of volcanic ash that suddenly smothered the Italian city in A.D. 79.

When molten rocks solidify, any magnetic minerals that are formed can record the direction of Earth’s magnetic field. That record can be modified or completely reset if the material is heated above a characteristic temperature known as its Curie temperature. Therefore, a series of tests can often discern the temperatures that the objects have experienced, says Raffaello Cioni of Italy’s Cagliari University.

He and his colleagues analyzed more than 200 volcanic rocks and pieces of debris, such as roof tiles, from Pompeii. Results suggest that the ash cloud, which had spewed from Mount Vesuvius at a temperature of about 850°C, had cooled below 380°C by the time it swept down on the city. Most materials there experienced temperatures between 240°C and 340°C, says Cioni. However, in some areas—such as the lee sides of homes, where turbulence might have mixed cool air into the cloud more effectively—temperatures rose to only 180°C or so. The researchers report their findings in the February Journal of Geophysical Research (Solid Earth).

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