Currently, you and I (and everyone in North America) are shifting farther from Eurasia at a rate of about an inch every year. A mid-ocean ridge stretches from above Antarctica through Iceland and is constantly oozing fresh lava, causing that gradual spreading of the sea floor. As the ridge produces fresh rock on either side, it forms a series of stripes running roughly parallel to the ridge. The stripes correspond to the ages of each subsequent formation, with the innermost stripes being the youngest. Lining either side of the new rocks are parallel bands of the same age, getting older as distance from the ridge increases.
Global seafloor ages. Reds are the youngest rocks and always run along mid-ocean ridges.
The theory of plate tectonics is relatively new, coming into the mainstream in the early 1900s. Even then, it wasn’t the most readily accepted theory – a good deal of science was inhibited by the popular beliefs of a perfect and static Earth. Enter: Pierre Curie. Before becoming heavily irradiated with his wife Marie, Curie spent years researching the magnetic properties of different materials, leading him to develop Curie’s Law.
Curie’s Law states material in a magnetic field will have an induced magnetic orientation, and the strength of the induced field decreases with increasing temperature. Basically, if you heat a substance, its magnetic field strength will be eventually reduce to effectively zero. The temperature when this occurs is called the Curie point.
So what does that mean for the mid-ocean ridge? As the newly formed rock cools, its magnetic field lines become oriented with the current magnetic north, and as you might’ve guessed, the parallel strips on either side of the ridge have the same orientation.
A spreading seafloor along a mid-ocean ridge. The similarly colored stripes are of similar age.
On a large timescale, the magnetic field of Earth reverses after a few millennia. Though the intervals vary, the actual process of flipping takes place over a few thousand years. In geological time, that is nearly a blink of an eye. New rocks formed after a reversal would show an opposite orientation to the surrounding rocks. Another reversal in geomagnetism causes another reversal in the rock’s alignment. Over time, a “tiger stripe” pattern forms on the seafloor.
There’s no telling exactly when the magnetic fields of Earth will reverse. People won’t even recognize when it does happen as the reversal is imperceptibly gradual. However, we can tell that the process is, and has been, ongoing for eons – noticed only by the rocks.
