Where on Earth?
Where on Earth?
In some ways the pebble is like one of the newer computer chips, tightly packed with more information than one could ever surmise from gazing on its smooth surface. That stored information can relate to any episode in the history of the pebble, and could be derived from nearby—a microbial mat growing on the exact spot on the sea floor where the pebble sediment accumulated, perhaps. But it could come from afar, such as a micrometeorite landing in the ocean and drifting slowly down to land on that very same spot (there are likely a few of those in the pebble, too). Some information is as pristine as the day it was written, in its own particular code, into the pebble fabric; some, on the other hand, has been almost completely overwritten, when yet further information was imprinted at some later point in time. We might consider here some information that has most likely been all but erased by the pebble’s tumultuous subsequent history—not that that should stop us trying to recover what we can of it. Nevertheless, when it was written into the fabric of the pebble, it provided a clear signal that travelled easily through some 4000 miles of solid rock, straight from the centre of the Earth. This signal gently nudged and guided certain of the flakes of sediment falling on to that sea floor. It made them line up, with almost military precision, to point polewards. They form a memory of latitude. The Earth’s magnetic field is a mysterious thing. What is magnetism? As a child, I used to push together the north poles of two toy magnets, and remember even now how frustratingly difficult it was to make them touch—or how tricky it was to prevent the north and south poles from locking together when I tried to keep them just a tiny bit apart. A few years later, I looked on, impressed but with incomprehension, as a physics teacher sprinkled iron filings around a magnet, to show how they lined up along the invisible lines of force.
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