Here's why:
* Seafloor Spreading: New oceanic crust is constantly being created at mid-ocean ridges. As magma rises and cools, it solidifies, preserving the Earth's magnetic field at the time of its formation.
* Magnetic Reversals: The Earth's magnetic field flips its polarity over time (north becomes south and vice versa). These reversals happen irregularly, but they are recorded in the newly formed rock.
* Magnetic Stripes: As the seafloor spreads, the solidified rock carries the magnetic record. These magnetic "stripes" of normal and reversed polarity are symmetrical on either side of the mid-ocean ridge, providing evidence of seafloor spreading and the age of the ocean floor.
In summary: The magnetic record in the rock on the ocean floor tells us when that specific section of the ocean floor was formed. Scientists can use this information to determine the age of the ocean floor and understand the history of Earth's magnetic field.
