|
真题典例 专题三 │ 真题典例 [2007·湖南卷] The north magnetic pole (磁极) is no longer a resident of Canada. It has drifted across the Canadian Arctic and is now angling toward Siberia. Not to be confused with the geographic North Pole—the northern extremity of the Earth’s axis(地轴)，the north magnetic pole (where the Earth’s magnetic field is directed downward) is moving due to natural changes in the Earth’s magnetic field, a process that originates about 3, 000 kilometers below our feet, in the outer core(核心)of the planet. But scientists are now noticing that the magnetic 专题三 │ 真题典例 pole has picked up its pace. Over the past century, the pole has been migrating at an average speed of about 10 kilometers per year, says Larry Newitt, head of Geomagnetic Laboratory at Natural Resources Canada in Ottawa. Since the 1970s, this speed has increased to about 50 kilometers per year. Scientists are wondering why the speed is increasing, says Newitt. One hypothesis (假说) relates it to “magnetic jerks”， sudden shifts (变化) in the rate of change of the magnetic field. There have been three jerks in the past four decades, each one correlating to an increase in speed. 专题三 │ 真题典例 If the north magnetic pole continues at its current rate, it could reach Siberia by 2056. | 
