Magnetostratigraphy

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  • #198776

    Otis Ranhofer
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    Premium Member

    Dolores asked for the correct spelling of Umtanum. Not sure, http://www.dogpile.com to the rescue!
    From there I wondered to this wiki link that gives a LOT of info about the lands we all travel on. One entry I had never heard of is Magnetostratigraphy that I find fascinating. Especially when they were able to detect a variation that occurred in Earths magnetic field direction of 50⁰ over 15 days.

    This would be an interesting topic at the NWOL rally for a geologist to enlighten us on.

    https://en.wikipedia.org/wiki/Columbia_River_Basalt_Group
    Quote:
    Magnetostratigraphy is also used to determine age. This technique uses the pattern of magnetic polarity zones of CRBG layers by comparison to the magnetic polarity timescale. The samples are analyzed to determine their characteristic remanent magnetization from the Earth’s magnetic field at the time a stratum was deposited. This is possible as magnetic minerals precipitate in the melt (crystallize), they orient themselves with Earth’s magnetic field.

    The Steens Basalt captured a highly detailed record of the earth’s magnetic reversal that occurred roughly 15 million years ago. Over a 10,000-year period, more than 130 flows solidified – roughly one flow every 75 years. As each flow cooled below about 500 °C (932 °F), it captured the magnetic field’s orientation-normal, reversed, or in one of several intermediate positions. Most of the flows froze with a single magnetic orientation. However, several of the flows, which freeze from both the upper and lower surfaces, progressively toward the center, captured substantial variations in magnetic field direction as they froze. The observed change in direction was reported as 50⁰ over 15 days.[15]

    #198789

    John Davies
    Participant
    Basic Member

    It’s interesting stuff. It was one of the confirming methods for plate tectonics in the 60s. They measured the magnetic alignment of basalts in the middle of the Atlantic, and found that the sea floor was spreading outwards east and west evenly  from a central north-south rift. The magma wells up in the center and moves outward, older rocks being further away. The magnetic variations were completely symmetrical on both sides.

    This also demonstrates the periodic reversals of the Earth’s magnetic field (North pole becomes South). For rock that is not migrating, rather is accumulating in situ, the alignment varies and does complete flips throughout the depth of the core sample.

    John Davies

    Spokane WA

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