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Earth Sciences

October 19 Polar Wander and Global Seismology 1. Geomagnetism - William Gilbert (1544-1603) [died of plague] o physician to Elizabeth I o book “De Magnete” published o reviews knowledge about magnetism and his own experiments o shows link to electricity o compass was important for navigation at sea, for example to get to colonies... o proposed that Earth is a giant magnet “terella” little Earth: a small magnetized sphere he used to demonstrate his theory to the Queen o indeed: magnetic field at Earth’s surface resembles that of a bar magnet at its centre o indication of field (angle of field lines to vertical) is related to latitude: tan (inclination) = 2 tan (latitude) o Carl Frederich Gauss (1777-1855)  professor in Goettingen, Germany  mathematician, astronomer, physic  established magnetic observatories  developed instrumentation  formulated “spherical harmonic analysis” to describe Earth’s field, ascribed to an internal origin  magnetic reversal detected in horizontal lava flows: • B. Brunhes (Massif Central, France, 1904) • M. Matuyama (Japan, 1929) • about 60 in past 20 million years, last one 250 kyr ago o Keith Runcorn (1922-1995)  researcher in Cambridge 1950-6  department head in Newcastle-upon-Tyne 1956-88  his group (Irving, Creer, Fisher,...) • determined fossilized ancient directions of Earth’s magnetic fields in igneous (and sedimentary) rocks found in Europe and North America • reconstructed the movement of the North Pole • obtained different routes for the two continents  magnetic minerals (eg. magnetite) • a) magmatic rock cools below Curie temperature • b) sediment grains orient themselves when settling or new minerals grow  procedure to determine ancient direction of field: • obtain orientated core samples • demagnetize rock: o last stable magnetic direction o is likely the magnetic direction acquired during formation of the rock  recall: latitude deermines inclination of magnetic fiels • magnetic direction points towards magnetic pole and tells us how far away rock was from the pole • plot these pole positions over time •  polar wander curves  Paleomagnetic Evidence for Continental Drift (1962) observation magnetic pole has wandered • drift differs for Europe and North America • curves could be reconciled if continents were joined note: first palaeomagnetic APWP (for Britain) presented by Ken Creer in 1954 includes suggestion that his m
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