Daily RC Article 44

Earth’s Magnetic Field


Paragraph 1

It is a fundamental tenet of geophysics that the Earth’s magnetic field can exist in either of two polarity states: a “normal” state, in which north-seeking compass needles point to the geographic north, and a “reverse” state, in which they point to the geographic south.

Geological evidence shows that periodically the field’s polarity reverses, and that these reversals have been taking place at an increasing rate.

Evidence also indicates that the field does not reverse instantaneously from one polarity state to another; rather, the process involves a transition period that typically spans a few thousand years.

Paragraph 2

Though this much is known, the underlying causes of the reversal phenomenon are not well understood.

It is generally accepted that the magnetic field itself is generated by the motion of free electrons in the outer core, a slowly churning mass of molten metal sandwiched between the Earth’s mantle (the region of the Earth’s interior lying below the crust) and its solid inner core.

In some way that is not completely understood, gravity and the Earth’s rotation, acting on temperature and density differences within the outer core fluid, provide the driving forces behind the generation of the field.

The reversal phenomenon may be triggered when something disturbs the heat circulation pattern of the outer core fluid, and with it the magnetic field.

Paragraph 3

Several explanations for this phenomenon have been proposed. One proposal, the “heat-transfer hypothesis,” is that the triggering process is intimately related to the way the outer core vents its heat into the mantle. 

For example, such heat transfer could create hotter (rising) or cooler (descending) blobs of material from the inner and outer boundaries of the fluid core, thereby perturbing the main heat-circulation pattern. 

A more controversial alternative proposal is the asteroid-impact hypothesis. In this scenario an extended period of cold and darkness results from the impact of an asteroid large enough to send a great cloud of dust into the atmosphere.

Following this climatic change, ocean temperatures drop and the polar ice caps grow, redistributing the Earth’s seawater.

This redistribution increases the rotational acceleration of the mantle, causing friction and turbulence near the outer core-mantle boundary and initiating reversal of the magnetic field.

Paragraph 4

How well do these hypotheses account for such observations as the long-term increase in the frequency of reversal?

In support of the asteroid-impact model, it had been argued that the gradual cooling of the average ocean temperature would enable progressively smaller asteroid impacts (which are known to occur more frequently than larger impacts) to cool the Earth’s climate sufficiently to induce ice-cap growth and reversals.

But theories that depend on extraterrestrial intervention seem less convincing than theories like the first, which account for the phenomenon solely by means of the thermodynamic state of the outer core and its effect on the mantle.

Topic and Scope:

Earth’s magnetic field; specifically, reversals in the magnetic field’s polarity.

Purpose and Main Idea:

The author’s purpose is to discuss the process that results in reversals of the magnetic field’s polarity, as well as two distinct hypotheses that try to explain these reversals; since this text is essentially descriptive, there really isn’t a specific main idea, though the author does say (in the last paragraph ) that the “heat-transfer hypothesis” offers a better explanation than the “asteroid-impact hypothesis.”

Paragraph Structure:

Paragraph 1 introduces the topic and scope of the passage. The rest of this paragraph  and all of paragraph 2 provide a lot of scientific factology about the Earth’s magnetic field, particularly facts about what is known (not much) about the process of polarity reversal. paragraph 3 outlines both the “heat-transfer hypothesis” and the “asteroid-impact hypothesis” of polarity reversal. In paragraph 4, on the other hand, the author asserts that the “heat-transfer hypothesis” is a more compelling explanation of polarity reversal.

The Big Picture:

A good grasp of the passage doesn’t mean assimilating all of the details (they can be looked up should this become necessary). Rather, it means comprehending what the author’s doing in the text—in this case, describing a scientific process and two hypotheses that purport to explain it.

Although this isn’t a very difficult science passage, it’s still not a good place to begin work on the section. Why? Because the author’s purpose isn’t entirely clear until late in the passage. Passages that begin with a mass of facts—instead of a clear statement of authorial intent—are generally best left for later in the section. Moreover, a brief scan of the question set suggests that it’s not going to be an especially easy one.

Note, however, that this passage conforms to a structure that’s common in science passages: a scientific process is described and competing explanations of it are then evaluated.


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