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The cataclysmic pole shift hypothesis suggests that there have been geologically rapid shifts in the relative positions of the modern-day geographic locations of the poles and the axis of rotation of the Earth, creating calamities such as floods and tectonic events. There is evidence of precession and changes in axial tilt, but this change is on much longer time-scales and does not involve relative motion of the spin axis with respect to the planet. However, in what is known as true polar wander, the solid Earth can rotate with respect to a fixed spin axis. Research shows that during the last 200 million years a total true polar wander of some 30° has occurred, but that no super-rapid shifts in the Earth's pole were found during this period.〔 A characteristic rate of true polar wander is 1° or less per million years.〔 Between approximately 790 and 810 million years ago, when the supercontinent Rodinia existed, two geologically rapid phases of true polar wander may have occurred. In each of these, the magnetic poles of the Earth shifted by ~55°. == Definition and clarification == From 1982 to 2005, the pole drifted southeast toward northern Labrador, Canada, at a rate of about 2 milliarcseconds —or roughly 6 centimetres — per year. But in 2005, the pole changed course and began drifting east toward Greenland at a rate over 7 milliarcseconds per year. The geographic poles of the Earth are the points on the surface of the planet that are intersected by the axis of rotation. The pole shift hypothesis describes a change in location of these poles with respect to the underlying surface – a phenomenon distinct from the changes in axial orientation with respect to the plane of the ecliptic that are caused by precession and nutation, and is an amplified event of a true polar wander. Pole shift hypotheses are not connected with plate tectonics, the well-accepted geological theory that the Earth's surface consists of solid plates which shift over a viscous, or semifluid asthenosphere; nor with continental drift, the corollary to plate tectonics which maintains that locations of the continents have moved slowly over the face of the Earth, resulting in the gradual emerging and breakup of continents and oceans over hundreds of millions of years. Pole shift hypotheses are not the same as geomagnetic reversal, the periodic reversal of the Earth's magnetic field (effectively switching the north and south magnetic poles). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pole shift hypothesis」の詳細全文を読む スポンサード リンク
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