Nothing is sure but death and taxes, and of course that north is north and…
2025
Nothing is sure but death and taxes, and of course that north is north and south is south, and thus it has always been, so they say. But they’d be wrong. You can perhaps be sure about death and taxes, but you might want to reconsider the rest of it. In fact, at many times in our planet’s history, north has become south and south has become north, in a process called magnetic reversal.
Paleogeologists have discovered the existence of these mysterious phenomena (in a field study known as paleomagnetism) by investigating rocks. When rocks are being formed from magmas, atoms within their crystals respond to the earth’s magnetic field by “pointing” towards the magnetic alignment, scientists can determine where on earth the north pole was located at that time because as the rocks solidified, they trapped that information within them. The study of ancient lava flows has revealed that at certain periods in the earth’s history magnetic north was directly opposite to its present location. In fact, it has been determined that the north/south reversal has occurred on average every 500,000 years and that the last reversal took place about 700,000 years ago. Scientists call those periods of “normal” polarity (the magnetic orientation of our modern er(1) and “reversed” polarity (the magnetic orientation of reverse situation) by the name “magnetic chrons.”
Although the fad : of such reversals is clear, why and how they happen and their effects on the planet are subjects of considerable debate. Because no one knows precisely how the earth’s magnetic field is produced, it becomes difficult so say how it might be reversed. Among explanations proposed are a reversal of the direction of convection currents in the liquid outer core of the earth and a collision between the earth and a meteorite or comet. And while the precise effects of a reversal are not known, there can be little doubt that the earth would receive during the process a great deal more damaging ultraviolet radiation than it now does and that such occurrences have been correlated with the extinction of certain species in the geologic past
The main purpose of the passage is to
- A.
present opposing hypotheses concerning the earth’s magnetic field argue that one of them is adequate
- B.
explain what is meant by ‘normal’ polarity
- C.
set forth a time table for magnetic reversal
- D.
explain the process of magnetic reversal and how it was discovered
Attempted by 3 students.
Show answer & explanation
Correct answer: D
Correct response: the choice that explains the process of magnetic reversal and how it was discovered
Explanation:
The passage describes what magnetic reversal is and how scientists found evidence for it using paleomagnetism: atoms in cooling rocks align with Earth’s magnetic field and preserve its direction, and studies of lava flows show past reversals. It also gives supporting information about the frequency of reversals (about every 500,000 years, with the last about 700,000 years ago), suggests possible causes (changes in outer-core convection or meteorite impact), and discusses potential effects (increased ultraviolet radiation and correlations with extinctions).
Why the choice about presenting opposing hypotheses is wrong: The passage mentions possible explanations but does not compare them or argue that one is correct; it remains descriptive.
Why the choice about explaining ‘normal’ polarity is too narrow: The passage briefly defines ‘normal’ polarity, but that is a detail used to explain reversals rather than the main purpose.
Why the choice about setting a timetable is wrong: The passage supplies some timing information (average interval and last reversal) but does not present a detailed timetable of specific events.
Therefore, the main purpose is to explain the phenomenon of magnetic reversal and how it was discovered, supported by evidence, possible causes, and potential effects.