Paleomagnetism

Paleomagnetism ( colloquially fossil magnetism ) is to maintain the direction of the geomagnetic field in rocks at the time of deposition or formation. The measurement of the remanent magnetism of rocks is an important geophysical tool and a research direction of the historical geology. Since both the Earth's magnetic field is constantly changing in geological periods ( pole shift ), as well as the tectonic plates on which the rocks were formed, changing its position in the earth's magnetic field as a result of continental drift constantly, can the traditional in rocks magnetic orientations information about the time and place of rock formation give or deposition.

Paleomagnetic evidence verified in conjunction with other studies in the 1960s, the theory of continental drift and plate tectonics Alfred Wegener and later the Wilson cycle of supercontinent - emergence.

Causes of rock magnetization

The magnetization of minerals can occur both in igneous rocks and in sedimentary rocks. In igneous rocks it occurs in slow cooling basalt or other igneous rocks and is called the thermal remanent magnetization. During cooling of the molten rock below the Curie temperature, the magnetization of ferromagnetic minerals takes the direction of the earth's magnetic field and the current just keeps them if the mineral is not re-heated above the Curie temperature.

Also on sedimentary rocks, the remanent magnetization can be measured. During the slow subsidence of sediments in the ocean iron-containing magnetizable such as magnetite crystals are aligned with the magnetism and acting on them in the mud - which is compressed later to rock and possibly further transformed by metamorphosis - fixed. This weathering and erosion mechanism is called detritus or Sedimentationsremanenz.

In both cases the direction of the magnetic field of the earth is determined at the time of rock formation. If the rock then is not subject to a greater metamorphosis or even melted again, the paleomagnetism with appropriate methods can be found today.

Applications in science

With measurement of the remanent magnetization, the direction of latitude are determined to magnetic north, and by measuring the strength of magnetization of the crystals. The former longitude is not measurable. With these measurements, therefore, a possible migration of the continental plate on which the rock was formed, between north and south poles can be determined, or its rotation. The hike in east-west direction can paläomagnetisch not be measured. The resulting measurements by the apparent change in position of the geomagnetic pole ( polar wander ) can be used to construct Polwanderkurven (English Apparent polar wander path, APWP ). They describe the motion of a continent in the course of the earth.

A second application is the magnetostratigraphy, wherein the sequence of the normal, that is coincident with the current direction of the earth's magnetic field, and magnetization of inverse temporal classification of rocks is used.