Such a rapid decay could not have been going on continuously for millions of years, because the field would have to have been impossibly strong in the past in order for it to still exist today. Creationists of the 1970s extrapolated today's decay back into the past, showing that the field could not be more than about 10,000 years old, assuming a constant decay of intensity. Then it began a slowly accelerating decrease. By about 1000 A.D., the decrease was nearly as fast as it is today. Paleomagnetic data show that while the geologic strata were being laid down, the earth's magnetic field reversed its direction hundreds of times.
Reversals are a very severe departure from steady decay of intensity.
Both archaeomagnetic and paleomagnetic data contradict the early creationist assumption of constant decay of intensity. The validity of the data required a new theory to explain them. The resulting disturbances in the core would cause the field intensity at the earth's surface to fluctuate up and down for thousands of years afterwards. Dynamic decay explains the main features of the data, especially several features evolutionists find puzzling. In fact, the energy loss during reversals and fluctuations would have been even faster than today's rate.