Rock magnetism
Rock magnetism
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作者：
J. Clegg
9月4日，在英国协会A分会的会议上，PMS Blackett教授以“霍克磁力”为主题展开讨论，简要介绍了该主题的历史和现状。他指出，虽然在过去的一百年里人们已经知道，某些岩石在形成时会在地球磁场的方向上获得微弱但确定的磁性，但直到最近，这一事实才被用来推断出两类具有最高科学重要性的信息：第一，追溯数百万年地磁场方向和强度的历史；第二，追溯地磁场方向和强度的历史。其次，试图解决关于大陆相对于彼此以及地理两极是否始终占据相同位置的古老而尖锐的争论。在讨论古地磁学的这两个方面中的第一个方面时，埃拉克特教授说，他对这个主题的兴趣最初是由约翰逊、墨菲和托雷森在美国的工作引起的，他们于 1948 年检查了残留物 新英格兰一系列变质粘土的磁化。这些粘土沉积在河床中，含有少量细颗粒形式的碎屑磁性物质，这些物质在沉积时沿着地球磁场定向。通过测量连续垂直序列上磁化方向和强度随深度的变化，可以追踪可追溯到公元前 15,000 年的地磁场历史 结果表明，在整个时间里，它的地理北方方向发生了有规律的变化。最近的研究证实了这一观点，现在看来，至少几百万年以来，地球主磁场的平均水平与位于地球中心的轴向磁偶极子产生的磁场相对应；但平均方向存在连续的长期变化，时间周期为 500-1,000 年。
T a meeting of Section A of the British Associa-tion on September 4, Prof. PMS Blackett opened a discussion on “Hock Magnetism” by giving a brief account of the history and the present state of the subject. He pointed out that although it has been kıown for the past hundred years that certain rocks, when formed, acquire a weak but definite magnetism in the direction of the Earth's field, it is only recently that this fact has been used for deducing two types of information of the highest scientific importance: first, to trace back for many millions of years the history of the direction and intensity of the geomagnetic field; and secondly, to attempt to settle the old and acute controversy as to whether the continents have always occupied the same positions relative to one another and to the geographical poles.Discussing the first of these two aspects of palæo-magnetism, Prof. Elackett said that his interest in the subject was originally aroused by the work of Johnson, Murphy and Torreson" in the United States, who, in 1948, examined the remanent magnetization of a series of varved clays of New England. These clays were laid down in stream beds, and contain small quantities of detrital magnetic material in the form of fine grains, which were oriented along the Earth's magnetic field at the time of deposition. By measuring changes in the direction and intensity of magnetization with depth, over a continuous vertical sequence, it was possible to trace the history of the geomagnetic field over a period extending back to 15,000 BC The results showed that during the whole of this time it has undergone regular variations in direction about geographic north. More recent investigations have confirmed this view, and it now appears that for at least several million years the main Earth's field has corresponded on the average to that which would be produced by an axial magnetic dipole situated at the centre of the Earth; but that there has been a continuous secular variation about the mean direction, with a time period of 500–1,000 years.