Regions of the seafloor with positive magnetic anomalies were formed during times when Earth’s magnetic field .

had reversed polarity

Explanation:

The use of magnetometers to investigate the magnetic properties of the seabed, allowed to discover the presence of a system of magnetic bands parallel to the dorsal axis that alternately have normal (the current) and inverse polarity. Thus the particles of the rocks had aligned in one direction or another and had produced different positive or negative magnetic anomalies.

This bandeado allows to deduce that the crust maintains the magnetism of the time in which it was formed. The bands would indicate fragments of bark that have been displaced from the rift pushed by the exit of new materials that have a magnetism inverse to that of the preceding materials.

Regions of the seafloor with positive magnetic anomalies were formed during times when Earth’s magnetic field had normal polarity

Explanation:

In geophysics,  a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks is known as a magnetic anomaly is. Mapping of variation over an area  helps in detecting structures covered by overlying material. The magnetic variation in successive bands of ocean floor parallel with mid-ocean ridges is an important evidence that supports the theory of seafloor spreading, central to plate tectonics.

The positive magnetic anomalies were due to the presence of rocks with root-mean-square ( RMS) of normal polarity.