Geomagnetic field characteristics in the Paleozoic, based on three very extensive extrusive sequences

基于三个非常广泛的喷出层序的古生代地磁场特征

• 批准号: 0909288
• 负责人: Rob Van der Voo
• 金额: $ 21.5万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-06-15 至 2013-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0909288&HistoricalAwards=false
• 关键词: Geomagnetic; field; characteristics; Paleozoic; based

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The Earth's magnetic field is known to change annually in strength, orientation and in its location of the magnetic poles, which move on average some 3000 feet per month. These changes are readily observed by comparing the directions in which a compass needle points from year to year. Geographic north and the Earth's rotation axis will stay more or less in place, as magnetic north moves considerably. Thus, the compass-needle direction is typically different from the true ("geographical") north-pointing direction, and is called the geomagnetic declination. Records of the magnetic field from 19th-century logs of navigational parameters and from magnetic observatories have revealed, for instance, a shift of some 1500 km in the location of the magnetic northpole in the last 150 years. This shift may be seen in declination changes in the continental United States up to a degree every five or six years. Such changes are called secular variation, derived from one of the meanings of the latin word seculum as "century". For older times, it turns out that ancient lava flows have recorded the direction of the magnetic field that existed when they solidified and cooled. Thus, rocks formed in the last five million years have recorded the geomagnetic field and reveal secular variation. However, this variation is only sparsely documented for the preceding 250 million years of Earth history, whereas before that time, in the Paleozoic and the Precambrian, secular variation characteristics have not been determined in any reliable and robust ways. The is remedying that situation by characterizing the secular variation in three unusually thick lava sequences in central Asia, with ages of about 500, 370 and 270 million years. The project is also assessing whether an evolutionary trend may exist in the magnitude of the geomagnetic field's secular variation as the Earth's solid inner core has grown steadily at the expense of the liquid outer core where most of the Earth's magnetic field is generated. Because the geomagnetic field generation in the outer core is influenced by the thickness of the outer core layer, secular variation studies may, in turn, reveal geodynamo characteristics. A more technical evaluation of the total geomagnetic field observable at the surface of the Earth describes its structure as constituting a main ("dipole") field plus higher-order fields (e.g., quadrupole, octupole etc.). During the last 550 million years the field consisted predominantly of a dipole field, but secular variation implies that on short time-scales higher-order fields exist and may wax or wane at any moment in time. A well-known application of records of the magnetic field has been to determine the ancient latitudes of continents; the method is called paleomagnetism. In order for a given magnetic direction to be interpreted as originating at a given ancient latitude, one must assume that the averaged magnetic field was purely that of a dipole. This assumption is testable for limited geological times, and here too this project will significantly extend the knowledge about the characteristics of the magnetic field. Lastly, the magnetic field is known to reverse its polarity at irregular intervals, in a process whereby the (magnetic) northpole becomes southpole and vice versa, in an otherwise unchanging, steadily rotating Earth. In a few geological periods, very long intervals of constant polarity are known, and it has been speculated that in these intervals secular variation may be significantly reduced. The proposed research is testing this idea for one of these long polarity intervals ending at about 265 million years ago. In addition to the research goals of this project, the award is providing support for the education and training of a graduate and undergraduate student at the University of Michigan.

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。众所周知，地球磁场的强度，方向和磁极位置每年都会发生变化，平均每月移动约3000英尺。 通过比较指南针每年指向的方向，很容易观察到这些变化。地理北和地球自转轴将或多或少地保持在原地，因为磁北移动得很大。因此，罗盘指针的方向通常不同于真正的（“地理”）北向，并被称为地磁下降方向。 例如，19世纪航海参数记录和地磁观测站的磁场记录显示，在过去的150年里，磁北极的位置移动了大约1500公里。这种变化可以在美国大陆的十年变化中看到，每五年或六年一度。这样的变化被称为世俗的变化，来自拉丁语seculum的一个意思是“世纪”。 在更古老的时代，古老的熔岩流记录了它们凝固和冷却时存在的磁场的方向。因此，在过去的五百万年中形成的岩石记录了地磁场，并揭示了长期变化。然而，这种变化在地球历史的前2.5亿年中只有很少的记录，而在此之前，在古生代和前寒武纪，长期变化特征尚未以任何可靠和可靠的方式确定。他们正在通过描述中亚三个异常厚的熔岩序列的长期变化来纠正这种情况，这些熔岩序列的年龄分别为5亿年、3.7亿年和2.7亿年。 该项目还在评估地磁场长期变化的幅度是否存在一种演变趋势，因为地球的固体内核稳步增长，而液体外核则是产生地球大部分磁场的地方。由于外核地磁场的产生受到外核层厚度的影响，长期变化研究反过来可能揭示地球发电机的特征。 对在地球表面可观察到的总地磁场的更技术性的评估将其结构描述为构成主（“偶极”）场加上高阶场（例如，四极、八极等）。 在过去的5.5亿年中，该场主要由偶极场组成，但长期变化意味着在短时间尺度上存在高阶场，并且可能在任何时刻发生变化。 磁场记录的一个众所周知的应用是确定大陆的古代纬度;这种方法被称为古地磁学。为了将一个给定的磁场方向解释为起源于一个给定的古代纬度，我们必须假设平均磁场纯粹是偶极子的磁场。这个假设在有限的地质时期内是可以测试的，而且这个项目也将显着扩展有关磁场特征的知识。最后，我们知道磁场会以不规则的间隔反转其极性，在这个过程中，（磁）北极变成南极，反之亦然，在一个不变的，稳定旋转的地球上。在一些地质时期，已知极性恒定的时间间隔很长，据推测，在这些时间间隔内，长期变化可能会显着减少。拟议中的研究正在测试这些长极性间隔之一的想法，该间隔结束于大约2.65亿年前。除了该项目的研究目标外，该奖项还为密歇根大学的研究生和本科生的教育和培训提供支持。