Pompeii debris yields calamity clues

11:21am, March 8, 2004

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.

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