Paleomagnetism is the study of the record of the Earth's magnetic field in rocks, sediment, or archaeological materials. This field of geophysics and geochemistry allows scientists to understand the magnetic properties of the Earth's past, which has profound implications for the study of plate tectonics, the Earth's geodynamo, and the history of magnetic reversals.
The study of paleomagnetism began in the 19th century when geologists noticed that some rocks possessed magnetic properties that did not align with the Earth's current magnetic field. The first significant observation was made by the French physicist Jean-Baptiste Biot in 1830 when he noted that certain basalts retained a magnetism opposite to that of the Earth's present field. However, it was not until the mid-20th century, with the advent of plate tectonics theory, that paleomagnetism gained prominence:
Magnetic Reversals: The Earth's magnetic field has undergone numerous reversals throughout geological time. These reversals are recorded in the magnetization of rocks, especially in igneous rocks like basalt, which acquire their magnetic orientation when they cool below the Curie temperature.
Apparent Polar Wander: By studying the magnetic orientation of rocks of different ages from various locations, scientists can trace the movement of continents over time, which appears as if the magnetic poles have wandered. This phenomenon, known as Apparent Polar Wander, is actually a result of continental drift.
Magnetostratigraphy: This is a technique used to date sedimentary layers by correlating their magnetic polarity with the known sequence of Earth's magnetic reversals. It is particularly useful in regions where traditional dating methods like radiometric dating are not applicable.
Techniques in paleomagnetism involve: