fraction of a rock’s overall magnetization that is a viscous remanent magnetization is dependent on the magnetic mineralogy. Paleomagnetic procedure Collecting samples on land The oldest rocks on the ocean floor are 200 mya – very young when compared with the oldest continental rocks, which date from 3.8 billion years ago. In order to collect paleomagnetic data dating beyond 200 mya, scientists turn to magnetite-bearing samples on land to reconstruct the Earth's ancient field orientation. Paleomagnetists, like many geologists, gravitate towards outcrops because layers of rock are exposed. Road cuts are a convenient man-made source of outcrops. "And everywhere, in profusion along this half mile of [roadcut], there are small, neatly cored holes ... appears to be a Hilton for wrens and purple martins." There are two main goals of sampling: Retrieve samples with accurate orientations, and Reduce statistical uncertainty. One way to achieve the first goal is to use a rock coring drill that has a pipe tipped with diamond bits. The drill cuts a cylindrical space around some rock. This can be messy – the drill must be cooled with water, and the result is mud spewing out of the hole. Into this space is inserted another pipe with compass and inclinometer attached. These provide the orientations. Before this device is removed, a mark is scratched on the sample. After the sample is broken off, the mark can be augmented for clarity. Applications Paleomagnetic evidence, both reversals and polar wandering data, was instrumental in verifying the theories of continental drift and plate tectonics in the 1960s and 1970s. Some applications of paleomagnetic evidence to reconstruct histories of terranes have continued to arouse controversies. Paleomagnetic evidence is also used in constraining possible ages for rocks and processes and in reconstructions of the deformational histories of parts of the crust. Reversal magnetostratigraphy is often used to estimate the age of sites bearing fossils and hominin remains. Conversely, for a fossil of known age, the paleomagnetic data can fix the latitude at which the fossil was laid down. Such a paleolatitude provides information about the geological environment at the time of deposition. Paleomagnetic studies are combined with geochronological methods to determine absolute ages for rocks in which the magnetic record is preserved. For igneous rocks such as basalt, commonly used methods include potassium–argon and argon–argon geochronology. See also Notes and references Notes Citations Sources Further reading External links Geomagnetism & Paleomagnetism