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SHAI HALEVI / ISRAEL ANTIQUITIES AUTHORITY

MOVING THE NEEDLE. When heated, the magnetic minerals contained in many archaeological materials tend to rearrange their inner magnetic orientation. Since this orientation aligns with the earth’s magnetic field, which varies over time, scientists can determine when the material was last heated. Known as archaeomagnetic dating, this method is most useful for dating destruction layers, where a building or object was exposed to high temperatures during a conflagration event. In this photo from the Givati Parking Lot excavation in Jerusalem, author Yoav Vaknin uses a special compass to mark a burnt plaster floor with horizontal lines, as necessary for archaeomagnetic analysis.