How do magnetic reversals in ocean-floor basalt help build a geologic time scale?

Magnetic reversals in ocean-floor basalt are fundamental to building a geologic time scale because the seafloor acts as a permanent magnetic record of Earth’s history.12

The process relies on the theory of seafloor spreading, where molten basalt rises at mid-ocean ridges and cools. As these rocks cool below a specific temperature, they lock in the orientation of Earth’s magnetic field at that time. When the field reverses polarity, the newly forming crust records this opposite orientation.1345

Because the seafloor spreads laterally, these magnetized strips move away from the ridge, creating a symmetrical, "tape-recorder" pattern of normal and reversed magnetic anomalies on both sides of the ridge axis. By correlating these magnetic patterns with absolute ages determined through isotopic dating of lava flows on land, scientists have constructed a magnetic-polarity time scale. This integrated approach allows researchers to date oceanic crust and reconstruct the spreading history of ocean basins over hundreds of millions of years.13264