GRE Reading Comprehension
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Source: 1994
The Earth's magnetic field is generatedas 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. During the past million years, for instance, the magnetic north pole has migrated between the Antarctic and the Arctic.
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.Fortunately scientists have been recording changes in the Earth's magnetic field for more than 300 years.
It can be inferred from the passage that geophysicists seeking to explain magnetic tempests ought to conduct research on the Earth's outer core because the Earth's outer core
- A is more fully understood than the Earth's magnetic field
- B is more easily observed than the Earth's magnetic field
- C has been the subject of extensive scientific observation for 300 years
- D is involved in generating the Earth's magnetic field
- E reflects changes in the inner core caused by magnetic tempests