North Cascades National Park: Paleomagnetism: Finding a Rock’s Place of Birth |
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Mountain travelers, especially those in a fog, become all too familiar with the needle of a compass as it swings back and forth until it lines up with the Earth’s magnetic field, pointing towards the north magnetic pole. If the needle were positioned so that it could swing vertically, it would pivot until it reaches an angle that depends on the latitude; that is, on how far away the pole is. Such information might be useful to a hopelessly lost explorer. In the tropics the needle would remain nearly horizontal. At high northern latitudes, the compass needle would incline steeply towards the north. Sensitive compasses used by surveyors in the northern hemisphere are weighted on the south end of the needle to counteract this tendency. (Near the south pole the north end of the needle tends to point steeply up, and southern-hemisphere surveyors move the weights to the north ends of their needles.)
Some iron-rich minerals are magnetic, and during their crystallization and cooling their magnetism aligns itself with the Earth’s magnetic field. With suitable instruments, a paleomagnetist can measure this magnetism, and read some rocks as if they were compasses that had recorded the direction (and inclination) of the Earth’s field when the rocks formed.
Wholesale reversal of the Earth’s magnetic field swapping of north and south magnetic poles causes alternating directions of magnetization in rocks of differing ages and gives rise to the the magnetic stripes on the ocean floor. But a closer reading of the magnetic alignment of certain rocks, described above, reveals smaller changes in the direction to the north magnetic pole,. These changes are due to movement either of the pole or of the rocks themselves with respect to the pole. Both sorts of movement have happened.
Geologist and paleomagnetist, Professor Myrl Beck and his students at Western Washington University first reported evidence of large northward migration of stitching plutons in 1972. Since then, other paleomagnetists have measured other rocks about 100 million years old and found similar results. Measurements on arc-root plutons of the Cascade Volcanic Arc and on volcanic rocks associated with the arc indicate that these younger rocks formed more or less where they are today: terranes of the North Cascades were definitely in place at their present latitude by about 35 million years ago.
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http://www.nature.nps.gov/grd/usgsnps/noca/sb10paleomag.html
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| Material in this site has been adapted from a new book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS and published by The Mountaineers, Seattle |