geologia: Earth's magnetic field

Evidence: 74 • authority: 72

Paleomagnetism (occasionally palaeomagnetism) is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called paleomagnetists. Certain magnetic minerals in rocks can record the direction and intensity of Earth's magnetic field at the time they formed. This record provides information on the past behavior of the geomagnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Evidence from paleomagnetism led to the revival of…

Evidence: 50 • authority: 72

In geophysics, a magnetic anomaly is a local variation in the magnetic field of a planetary body resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material. The magnetic variation (geomagnetic reversals) in successive bands of ocean floor parallel with mid-ocean ridges was important evidence for seafloor spreading, a concept central to the theory of plate tectonics.

Evidence: 48 • authority: 92

Assessing the reliability of paleomagnetic datasets using the R package PmagDiR.. Paleomagnetism is the most important source of information for determining the position of tectonic plates in the geological past. Over the last few decades significant advancements have been made in improving the reliability of paleomagnetic data, ranging from analytical methods to statistical assessments. Here I present the first version of PmagDiR, an R-based open-source package which displays, assesses the reliability, and, when possible, corrects given paleomagnetic directions distributions. The main functions of the package are to: plot paleomagnetic directions, their averages and confidence boundaries;…

Evidence: 42 • authority: 80

Geomagnetic polarity reversals and the geological record

Evidence: 42 • authority: 80

Geomagnetic polarity reversals and the geological record

Evidence: 41 • authority: 72

A geomagnetic reversal is a change in the Earth's dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's magnetic field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons. Reversal occurrences appear to be statistically random. There have been at least 183 reversals over the last 83 million years (thus on average once every ~450,000 years). The latest, the Brunhes–Matuyama reversal, occurred 780,000 yea…

Evidence: 41 • authority: 72

Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo. The magnitude of Earth's magnetic field varies over its surface and changes with time. In a 2019 survey it ranged around 30,000 to 60,000 nT (0.30 to 0.60 G). As an approximation, it is represented by a field…

Evidence: — • authority: 88

Paleomagnetism and geochronology of Oligocene and Miocene volcanic sections from Ethiopia: geomagnetic variability in the Afro-Arabian region over the past 30 Myr. We report palaeomagnetic and K-Ar geochronologic results of two volcanic sections from Ethiopia. One section, dated around 29-30 Ma and spanning ~1 km in thickness, is related to the Oligocene Afro-Arabian traps. The second ~700-m-thick section was emplaced during the Miocene in two pulses around 10-11 and 14-15 Ma. We sampled 67 flows (550 cores) of predominantly basaltic rocks at the Oligocene section and 59 rhyolitic to trachybasaltic flows (500 cores) at the Miocene section. The Oligocene section was correlated to subchrons C…

Evidence: — • authority: 88

Do lower crustal rocks record reversals of the Earth's magnetic field? Magnetic petrology of oceanic gabbros from Ocean Drilling Program hole 735B. Ocean Drilling Program (ODP) Leg 118 at the Southwest Indian Ridge recovered the first vertically oriented, continuously cored gabbros from in situ ocean crust. Magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the Earth's magnetic field. Primary, igneous magnetite is common only in evolved, Fe‐Ti oxide‐rich gabbros and averages, by volume, 2% of the rock, although substantia…

Evidence: — • authority: 92

High geomagnetic field intensity recorded by anorthosite xenoliths requires a strongly powered late Mesoproterozoic geodynamo.. Obtaining estimates of Earth's magnetic field strength in deep time is complicated by nonideal rock magnetic behavior in many igneous rocks. In this study, we target anorthosite xenoliths that cooled and acquired their magnetization within ca. 1,092 Ma shallowly emplaced diabase intrusions of the North American Midcontinent Rift. In contrast to the diabase which fails to provide reliable paleointensity estimates, the anorthosite xenoliths are unusually high-fidelity recorders yielding high-quality, single-slope paleointensity results that are consistent at specimen…

Evidence: — • authority: 92

Extraordinarily long duration of Eocene geomagnetic polarity reversals.. One of the most intriguing attributes of Earth's magnetic field is that it reverses polarity. Previous palaeomagnetic records mainly from the last ~17 million years indicate that the reversal process typically occurs over ten thousand or fewer years. Here we present an exceptionally resolved deep sea sedimentary palaeomagnetic record that captures two polarity transitions that occurred ~40 million years ago. These transitions occur over 18 and 70 thousand years and are much longer than the typical 10 kyr duration for younger reversals. Longer-duration transitions like these are calculated in numerical geodynamo models,…

Evidence: — • authority: 92

Micromagnetic Tomography for Paleomagnetism and Rock-Magnetism.. Our understanding of the past behavior of the geomagnetic field arises from magnetic signals stored in geological materials, e.g., (volcanic) rocks. Bulk rock samples, however, often contain magnetic grains that differ in chemistry, size, and shape; some of them record the Earth's magnetic field well, others are unreliable. The presence of a small amount of adverse behaved magnetic grains in a sample may already obscure important information on the past state of the geomagnetic field. Recently it was shown that it is possible to determine magnetizations of individual grains in a sample by combining X-ray computed tomography an…