GRE Reading Comprehension
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Source: XDF
The Earth's magnetic field is generated as the molten iron of the Earth's outer core revolves around its solid inner core; when surges in the molten iron occur, magnetic tempests are created. At the Earth's surface, these tempests can be detected by changes in the strength of the Earth's magnetic field. For reasons not fully understood, the field itself reverses periodically every million years or so.
Clearly, geophysicists who seek to explain and forecast changes in the field must understand what happens in the outer core. Unlike meteorologists, however, they cannot rely on observations made in their own lifetimes. Whereas atmospheric storms arise in a matter of hours and last for days, magnetic tempests develop over decades and persist for centuries.
Question List: 1 2
Select the sentence in the passage that implies the reason why geophysicists seeking to explain magnetic tempests ought to conduct research on the Earth's outer core.