Appreciating Earth’s Amenities

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Ancient pottery, like this jar from Iron Age Judea, can record our planet’s magnetic ebb and flow. COURTESY ODED LIPSCHITS

Funny, as we just started carrying a new line of amenities at Chan Chich Lodge (what we had was already earth-friendly, but the new line is even more so), to be reminded of an amenity we never thought of as an amenity:

EARTH’S MYSTERIOUS MAGNETIC FIELD, STORED IN A JAR

Of all the environmental amenities that this hospitable planet provides, the magnetic field is perhaps the strangest and least appreciated. It has existed for more than three and a half billion years but fluctuates daily. It emanates from Earth’s deep interior but extends far out into space. It is intangible and mostly invisible—except when it lights up in ostentatious greens and reds during the auroras—but essential to life. The magnetic field is our protective bubble; it deflects not only the rapacious solar wind, which could otherwise strip away Earth’s atmosphere over time, but also cosmic rays, which dart in from deep space with enough energy to damage living cells.Although sailors have navigated by the magnetic field for a millennium and scientists have monitored it since the eighteen-thirties, it remains a mysterious beast. Albert Einstein himself said that understanding its origin and persistence was one of the great unsolved problems in physics. Today, the scientific consensus is that the field arises in Earth’s outer core, where the movement of liquid iron creates a giant, self-perpetuating electromagnetic dynamo, and that the geometry of the field is approximately dipolar, like a bar magnet, with the two ends coinciding, on average, with the geographic North and South Poles.

In detail, however, things are much messier. The global magnetic field also has quadrupole and octopole components, which make its actual geometry something like a playground jack with extra spikes. The strength and orientation of the field can change without warning, over millions of years or in a few days. Since 1990, the magnetic North Pole has migrated almost nine hundred miles, from Axel Heiberg Island, in the high Canadian Arctic, to a site close to the true North Pole. At the same time, the over-all intensity of the field has been falling at a rate of about six per cent each century. Changes like this do not always happen steadily; since 1969, there have been four well-documented “geomagnetic jerks,” in which the rate of change in the field strength abruptly accelerated before, months later, settling back to normal.

Direct measurements of the magnetic field now span almost two hundred years, and iron-rich volcanic rocks on the ocean floor provide a lower-fidelity chronicle of its erratic behavior—including wholesale reversals in polarity—back about a hundred and fifty million years. But reconstructing the field’s behavior between these two extremes has been difficult. The trick is to find an iron-bearing object that locked in a record of the magnetic field at a well-constrained time in the past, in the way that wine of a given vintage preserves an indirect record of that year’s weather conditions. For this sort of remnant magnetism to form, the object generally must have been heated and then cooled through its Curie temperature—the threshold, named for Pierre, at which iron-oxide particles will align themselves with the ambient magnetic field. At best, however, young volcanic rocks can only be dated to within a few thousand years. Fortunately, natural rocks aren’t the only ones with magnetic memories; archeological materials like fired pottery and even smelting slag may bear similar information. On Monday, in a study published in Proceedings of the National Academy of Sciences, a team of Israeli and American archeologists and geophysicists reports the most detailed reconstruction yet of the magnetic field in pre-instrumental times, using a set of ceramic jars from Iron Age Judea…

Read the whole story here.

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