Figure 2.12Magnetic anomalies allow geologists to measure the rate of seafloor spreading. (a) An oceanographic survey over the Mid-Atlantic Ridge just southwest of Iceland revealed a banded pattern of magnetic field intensity. (b) Geologists found and dated similar magnetic anomalies in volcanic lavas on land to construct a magnetic time scale. (c) That magnetic time scale was used to date the magnetic anomalies on the seafloor worldwide.