MAGNETOSTRATIGRAPHY 331 MAGNETOSTRATIGRAPHY S G Lucas, New Mexico Museum of Natural History, Albuquerque, NM, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Most of the dense core of the Earth is iron The outer portion of the core is liquid, and the motion of this liquid produces a magnetic field, so that the Earth behaves like a giant bar magnet This dynamo, however, changes easily, and for unknown reasons the magnetic field periodically reverses itself – the north and south magnetic poles switch positions On average, the magnetic field reverses itself about every 500 000 years, though the pattern of reversals is erratic Flip-flops of the Earth’s magnetic field, when recorded in a stratigraphical succession of rocks, are the basis of magnetostratigraphy (a contraction of ‘magnetic-polarity stratigraphy’) Magnetostratigraphy correlates rocks on the basis of similarities in their magnetic-reversal patterns and is generally used to correlate surface exposures of rocks, though it can also be applied to subsurface cores As explained below, magnetostratigraphy is not an independent method of correlating rocks Nevertheless, it is a powerful tool because magnetostratigraphical correlation is based on matching magnetic reversals, which are geologically simultaneous events worldwide during the Late Carboniferous and most of the Permian, an interval of about 70 Ma, the magnetic field was stable (reversed) The pre-Carboniferous nature of the magnetic field is still not as well understood as its later history Because of plate tectonics and the subduction of oceanic crust, the oldest seafloor preserved on Earth dates from the beginning of the Late Jurassic, about 160 Ma ago Geologists have determined the magnetic polarities of rocks from the seafloor, which are lavas for which some numerical ages have been calculated Bands of cooled lava on the seafloor adjacent to spreading ridges preserve magnetic stripes that are symmetrical about the ridge This seafloor magnetization provides a template that geologists have used The Geomagnetic Polarity Time-Scale Magnetic reversals have occurred frequently but irregularly during Earth history The process of reversal seems to take about 4000–5000 years The current state of the magnetic field (in which a compass needle points towards the north magnetic pole) has persisted for the last 700 000 years and is referred to as an interval of normal polarity Geologists refer to periods when the poles had switched positions (so that a compass needle would have pointed to the south magnetic pole) as intervals of reversed polarity The first attempts at magnetostratigraphy were made in the 1950s, especially by the Russian scientist A N Khramov Since the 1960s, geologists have made a concentrated effort to decipher the history of the Earth’s magnetic field, and this research is ongoing During much of Earth history, the magnetic field reversed frequently (Figure 1) This has been the case throughout most of the Mesozoic and Cenozoic, but Figure The polarity bias superchrons during the last 700 Ma The polarity history is not well understood before about 350 Ma, and since then it has been mixed (many magnetic reversals) except for two long intervals of polarity stability: the Permo Carboniferous reversed and the Cretaceous normal superchrons