Explain why it may be difficult to precisely date ocean crust aged

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Homework Statement

Explain why it may be difficult to precisely date ocean crust aged between 85 and 125 million years old using magnetic anomalies on the sea floor. (2 marks)

The Attempt at a Solution

At first I thought about how long it takes for the Earth's magnetic field to reverse. I looked it up, and it was something like thousands of years. So I thought that wasn't the answer because the crust is aged millions of years. I have gone through my notes but I really can't think of anything. Please help.

 

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It may be difficult to precisely date ocean crust aged between 85 and 125 million years old using magnetic anomalies on the sea floor because the rate of change of the Earth's magnetic field is not constant. As such, it becomes increasingly difficult to determine the age of a sample further back in time due to the increasingly unreliable accuracy of the magnetic field measurements.