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非常感激验证。
Ancient Magnetic Reversals: Clues to the Geodynamo
Is the earth headed for a reversal of its magnetic field? No one can answer this question yet, but rocks magnetized by ancient fields offer clues to the underlying reversal mechanism in the earth's core
By Kenneth A. Hoffman
For well over a century geophysicists have observed a steady and significant weakening in the strength of the earth's magnetic field. Indeed, if this trend were to continue at the present rate, the field would vanish altogether in a mere 1,500 years. Most investigators are inclined to think that the decay is merely an aspect of the restlessness inherent in the field and that the field will recover its strength. Yet one cannot dismiss out of hand the possibility that the weakening portends a phenomenon that has recurred throughout geologic time: the reversal of the geomagnetic field.
Which of these two scenarios is correct? The answer lies concealed 3,000 kilometers below the earth's surface within the outer core, a slowly churning mass of molten metal sandwiched between the mantle of the earth and the solid inner core. It is now generally accepted that the earth's magnetic field is generated by the motion of free electrons in the convecting outer core. This theory supposes the core behaves like a self-sustaining dynamo, a device that converts mechanical energy into magnetic energy. In the geodynamo the earth's rotation, along with gravitational and thermodynamic effects in and around the core, drives the fluid motions that produce the magnetic field.
Although the basic principles of dynamo action are well established, geophysicists do not yet understand the thermodynamics, fluid mechanics and electrical properties of the earth's interior well enough to construct a universally accepted model of the geodynamo. Yet its workings can be glimpsed indirectly by observing the present-day field. These measurements yield many details of the short-term behavior of the field, such as its shape and "secular variation," or ordinary fluctuation. To study the activity of the dynamo over aeons one must turn to the paleomagnetic record―the ancient magnetism frozen into rocks from the time of their formation.
Indeed, paleomagnetic evidence led to the first proposal that the earth's field has reversed itself, put forward in 1906 by the French physicist Bernard Brunhes. Brunhes was intrigued by the discovery of rocks that were magnetically oriented in the direction opposite to the earth's field. His startling suggestion was furiously debated for more than five decades. It was not until the early 1960's, at about the time J. S. B. Van Zijl and his colleagues published the first detailed study of a paleomagnetically recorded field reversal in lavas from South Africa, that the idea was accepted by the scientific community at large. Today 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.
In the 1960's studies of radiometrically dated lavas yielded a consistent log of past polarity changes, including no fewer than nine major reversals in the past 3.6 million years, the most recent of which occurred 730,000 years ago. The time scale of polarity transitions has since been extended back nearly 170 million years.
Paleomagnetic records show that the geomagnetic field does not reverse instantaneously from one polarity state to the other. Rather, the process involves a transition period that typically spans a few thousand years. Hence for perhaps 98 percent of the time the field is stable and its shape is well understood. But for the remaining 2 percent of the time the field is unstable and its shape is not obvious. The foremost task for geophysicists in my field has been to chronicle the behavior of the reversing field―its shifting shape and fluctuating intensities―based on the sometimes faint and complex record of past events, imprinted in stone. The findings provide an invaluable probe into the hidden mechanisms of the geodynamo.
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发表于 2018-8-19 21:51:43
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