Geography
Paleomagnetic Stripes Reveal Seafloor Spreading Rates and Reversals
Quick fact
The stripes on the ocean floor are symmetric around mid-ocean ridges and record the history of Earth's magnetic field flips over millions of years, acting like a tape recorder of spreading.
Why this is interesting
You've probably seen a map of the ocean floors with weird striped patterns. Did you ever wonder why those stripes exist?
Read the full explanation
Understanding Paleomagnetic Stripes Reveal Seafloor Spreading Rates and Reversals
Imagine pouring syrup onto a tray and spreading it with a spatula. If the spatula sometimes left a colored stripe pattern, you could see exactly how the syrup was being pulled outward. Similarly, as magma rises at mid-ocean ridges, it cools into new seafloor. Iron minerals in the lava align with Earth's magnetic field at that moment, freezing a ''magnetic snapshot''. When Earth's magnetic field flips (north becomes south), the minerals align opposite, creating alternating stripes on both sides of the ridge. Measure the stripes and you'll find they are a perfect mirror image across the ridge, evidence that new crust is being added symmetrically.
A deeper explanation
Earth's magnetic field occasionally reverses polarity due to processes in the fluid outer core. These reversals are recorded in the ocean crust as stripes of normal and reversed polarity. By mapping these stripes with magnetometers, scientists like Fred Vine and Drummond Matthews in 1963 showed that the pattern is symmetric about mid-ocean ridges. The width of each stripe equals the time the field stayed in one polarity multiplied by the spreading rate. By calibrating with known magnetic reversal ages (from dated lava sequences on land), we can calculate spreading rates, typically a few centimeters per year. For instance, the Mid-Atlantic Ridge spreads about 2.5 cm/year, while the East Pacific Rise spreads up to 10 cm/year. This provides fundamental evidence for plate tectonics and allows us to date the ocean floor: the oldest seafloor (about 200 million years old) is far from ridges, and at subduction zones it is recycled.