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.
One can infer from the passage that :
- A.
if the earth collides with a meteorite, the magnetic field will be reversed
- B.
a magnetic reversal could present a damage to humans
- C.
the earth’s magnetic field was produced about 700,000 years ago
- D.
in spite of past reversals, ‘normal’ polarity is now firmly established
Attempted by 1 students.
Show answer & explanation
Correct answer: B
Answer: a magnetic reversal could present a damage to humans.
Explanation: The passage explicitly states that during a reversal the earth would receive a great deal more damaging ultraviolet radiation and that such occurrences have been correlated with the extinction of certain species. From these statements it is reasonable to infer that a magnetic reversal could harm humans.
Why the other choices are not supported:
The passage mentions a meteorite or comet collision only as one proposed explanation for how reversals might happen; it does not state that a collision will definitely cause a reversal.
The passage reports that the last reversal occurred about 700,000 years ago; it does not claim that the earth’s magnetic field was produced at that time.
The passage does not assert that the current 'normal' polarity is permanently or firmly established; it describes reversals as recurring events and gives an average interval, so permanence cannot be inferred.
Note: The original solution merely referenced the final sentence of the passage. This revised solution explains exactly which statements in the passage support the inference and why the other choices are not supported.