Nature
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Every quarter of a million years or so, Earth's magnetic field "flips” direction. North effectively becomes south, and south becomes north. You don’t have to take our word for it; physical evidence of geomagnetic reversal is built into the seafloor in the form of “zebra stripes”.
These vast stripes appear next to underwater mountain ranges known as mid-ocean ridges, which lie along the seafloor where two tectonic plates meet. At these boundaries, molten rock from beneath the Earth's crust rises to fill the gap between plates as they're pulled apart. It eventually cools and solidifies, creating new oceanic crust in a process known as seafloor spreading.
Within this mix of molten rock are magnetic iron oxide crystals (called magnetite) that align themselves with Earth’s geomagnetic field, a bit like tiny compass needles pointing toward magnetic north. Once the lava cools and solidifies, the orientation of these crystals is literally set in stone, preserving a permanent record of the magnetic field at that moment in time.
Each time Earth's geomagnetic poles flip – meaning the North and South Poles switch places – the magnetite settles in the opposite direction, according to the Woods Hole Oceanographic Institution.
After several pole flips and plenty of seafloor spreading, this creates an alternating “zebra stripe” pattern. One stripe is imprinted when Earth’s magnetic field is in its “normal” direction that we see today, while another represents bits of seabed that were solidified when the field was in the opposite direction.
Bear in mind that these stripes are not clearly visible to the naked eye, but they can be tracked using scientific equipment that measures magnetism.
In fact, these stripes have helped the theory of plate tectonics to gain widespread acceptance. This radical idea was first suggested in the early 20th century and gained traction around the 1960s following decades of research and growing evidence from studies of earthquakes and ocean floor mapping.
Around this time, scientists dragged magnetic sensors behind ships and uncovered remarkable patterns in the magnetic properties of seafloor rocks. These patterns reflected the timeline of magnetic reversals already observed in rocks on land.
The magnetic stripes are physically parallel to the ocean ridges, creating a symmetrical design on either side. This pattern could only be explained by new seafloor forming at the ridges as tectonic plates diverge, serving as powerful evidence supporting the theory of plate tectonics. By comparing these striped patterns to the known timeline of geomagnetic reversals, scientists can not only date sections of the seafloor but also calculate the rate at which it spreads.
Geomagnetic field reversal occurs on Earth every 200,000 to 300,000 years on average, yet the last record of a total reversal was (probably) around 780,100 years. This could suggest that our planet is overdue for another pole flip, although some scientists believe an imminent reversal isn’t likely anytime soon.
