Britannica
Sources used in this answer
- oceanic crust - Marine magnetic anomalies
- seafloor spreading
- Earth - The geomagnetic field and magnetosphere
- oceanic crust - Study of ophiolites
- paleogeography - Paleomagnetism
- paleogeography - Linear magnetic anomalies
- Earth sciences - The theory of plate tectonics
- plate tectonics - Determination of plate thickness
Why are magnetic anomalies arranged symmetrically on both sides of mid-ocean ridges?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
oceanic crust
A cross section of Earth's outer layers, from the crust through the lower mantle.
The symmetrical arrangement of magnetic anomalies on both sides of mid-ocean ridges is a fundamental consequence of seafloor spreading and the periodic reversal of Earth’s magnetic field.21
The process occurs as follows:
- Crustal Formation: New oceanic crust is continuously created at mid-ocean spreading centres as molten material wells up from the mantle and cools.23
- Magnetic Recording: As this magma cools below a specific temperature (approximately 200–300 °C), iron-titanium minerals within the rock undergo thermoremanent magnetization, locking in the orientation and polarity of Earth’s magnetic field at that time.2
- Symmetry through Spreading: Because the seafloor spreads laterally away from the ridge axis in both directions, the newly formed crust—carrying its specific magnetic "signature"—is pushed outward. As Earth’s magnetic field periodically reverses polarity, the crust formed at different times records alternating "normal" and "reversed" magnetic stripes.24
This creates a mirror-image pattern of parallel magnetic bands on either side of the ridge, effectively acting as a geological tape recorder of the planet's magnetic history.21