Geology Reference
In-Depth Information
What they found instead was a bizarre magnetic pattern that was astonishingly regular and
intricate.ClosetothecentralriftvalleyinboththeAtlanticandthePacific,basaltdisplayed
the normal magnetic orientation, faithfully pointing in the direction of the modern north
magnetic pole. But several miles east or west of the rift valley, the magnetic signal flips
a full 180 degrees: the north magnetic pole is almost exactly opposite its present position,
where the south pole is supposed to be, and vice versa. Sail several more miles in either
direction, and the magnetic field flips 180 degrees again to the correct orientation. Over
and over, dozens of times, the magnetic field frozen in the rocks on any given transect is
observed to flip.
Additionalanalysisrevealedthreekeyfacts.First,therockswithreversedmagneticfield
form long, narrow north-south trending bands that exactly parallel the ridges in both the
Atlantic and the Pacific oceans. Where the central rift valley is broken, offset by transform
faults, so too are the magnetic bands. Second, the pattern of these magnetic stripes is sym-
metricalabouttheridgeaxes:saileastorwestfromthecenter,andtheexactsamesequence
ofnormalandreversestripes,somewiderandsomenarrower,occurs.Andthird,radiomet-
ric dating of these basalts from ridge systems around the world confirms that each reversal
occurred simultaneously over a narrow and precisely defined age. Magnetic reversals thus
serve as a kind of ocean-floor time line.
Two logical, if mind-bending, conclusions followed. First, Earth's magnetic field is
wildly variable: it flips 180 degrees on average every half-million years, and it has been
doingsoforatleastthepast150millionyears.Thereasonsforthisficklefieldarenowun-
derstood in some detail. Our planet is a giant electromagnet; its magnetic field arises from
swirling electrical currents in Earth's convecting fluid outer core. Heat drives this convec-
tion;densehotliquidattheinnercoreboundaryexpandsandrises,tobereplacedbycooler,
denser liquid that sinks from above. Geophysicists employ sophisticated computer models
to show that Earth's rotation adds complicated, chaotic twists to the convection—motions
thatresultinaflipinthemagnetic fieldeveryhalf-million yearsorso.Earth'srotationalso
constrains the magnetic poles to spend most of their time aligned close to the stable rota-
tion axis, but during periods of core instability, the magnetic field can wander widely and
flip, perhaps in the span of a century or less.
The second conclusion is that midocean ridges produce new basaltic crust at the rate of
an inch or more every year. Older basalt moves sideways, both eastward and westward,
away from the ridge, as new lava takes its place. The ridge systems are thus grand two-dir-
ectional conveyor belts, spewing out new ocean floor. Fresh basalt generated at the Mid-
Atlantic Ridge is expanding the Atlantic, which grows as much as two inches wider every
year—roughly thirty thousand years per mile of new ocean floor on average. Play the tape
backward 150 million years, and the Atlantic didn't exist. Prior to that time, the Americas
must have been joined to Europe and Africa, just as Alfred Wegener proposed.
