Britannica Sources used in this answer

How do magnetic reversals in seafloor rocks support the theory of seafloor spreading?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

World map highlighting major features of the ocean basins, including trenches, ridges, basins, and plateaus labeled throughout the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Continents are shown with shaded terrain, and prominent undersea features such as the Mid-Atlantic Ridge, Mariana Trench, and Kerguelen Plateau are marked across the map. — ocean basin Major features of the ocean basins.

Magnetic reversals in seafloor rocks provide critical evidence for the theory of seafloor spreading by acting as a chronological record of the ocean floor's formation.1

The theory relies on the fact that Earth’s magnetic field periodically undergoes geomagnetic reversals, where the north and south magnetic poles swap positions. As molten material wells up at mid-ocean ridges and cools to form new oceanic crust, iron-titanium minerals within the rock lock in the magnetic polarity of the time.12

Because the crust moves laterally away from the ridge crests, it creates a symmetrical pattern of "normal" and "reversed" magnetic bands on both sides of the ridge. By mapping these marine magnetic anomalies and correlating them with known time scales of past reversals, scientists can determine the age of the seafloor and calculate the rate at which the ocean basin is expanding. This symmetrical, age-progressive record confirms that new crust is continuously created at spreading centers and pushed outward.13