The basalt wells up at the mid-ocean ridges and gets pushed out on either side, where it cools to form new crust.
As it cools it is magnetised according to the current direction of the Earth's magnetic field.
The theory of ocean floor spreading thus explains the zebra stripes of magnetic anomalies. Author: Tom Brown Copyright: public domain Date last modified: 7th Oct 2011 Peer-review status: Not yet peer-reviewed Earth Story Oceanic.
The land basalt has been dated, which allows us to infer the age of the oceans: the seafloor is all younger than 200 million years old.
This is around ten times younger than the average age of the continents.
The closer to the ridge you are, the younger the crust is.
Ocean floor created recently, during the period when the magnetic field was the same direction as it is today, is magnetised so that it re-enforces the measured magnetic field, creating a positive anomaly.
In effect, the ocean floor has behaved like a giant tape recorder.The pattern of stripes matches exactly the field reversals found in basalt on land.