Earth’s magnetic reversals are likely due to

The Earth's magnetic reversals are likely due to changes in the circulation patterns of materials in the outer core. This phenomenon is also referred to as geomagnetic reversal and involves the interchanging of position of the magnetic north and the magnetic south. The reversal can occur within a span of 0.1 to 1 million years. It is primarily driven by the dynamic movement of the molten iron found on the outer core that then generates electric currents, thereby, producing magnetic fields. 

Its either a) or b),wind and water erosion gradually widen the canyon