Geoscience Reference
In-Depth Information
theyfirstcrystallizeandthen,coolingfurther,reachatemperature(calledtheCurie
point, after Pierre Curie) at which the electrons in iron atoms line up and lock
onto the direction of the prevailing magnetic field. Thus a rock's fossil magnet-
ism points to the North Pole at the time the rock formed. Since moving continents
would continually change their position relative to the pole, fossil magnetism, or
let us call it paleomagnetism, offered a test of continental drift: If the paleomag-
netism of rocks of the same age from different continents points to the North Pole
in the same place as it is today, then the location of the continents on the globe has
not changed: they have not drifted.
But before they could apply the test, scientists had to answer a formidable set of
questions:
• Could rock magnetism possibly remain stable for hundreds of millions of years, surviving
chemical reactions, folding, faulting, metamorphism, and the like?
• The magnetism of some rocks (half, it would turn out) points not toward the North Pole but
180 degrees opposite, toward the South Pole. Had the Earth's magnetic field somehow re-
versed its polarity, or had the minerals spontaneously changed their magnetic orientation?
Either possibility appeared to cast doubt on the reliability of rock magnetism.
• Has the Earth always had only two poles, north and south? Though this may seem a strange
question, because scientists did not know why the Earth has a magnetic field, they could
hardly rule out the possibility of multiple poles.
• The Earth rotates around its geographic pole. It also has a magnetic pole where the lines of
magnetic force emerge. Over time, the magnetic pole wanders randomly, giving rise to what
scientists call secular variation. (This is why topographic maps show the deviation between
the magnetic and geographic poles at the time cartographers constructed the map.) How then
can we be sure that the pole has not wandered by larger deviations in the past? If the poles
move, how can paleomagnetism distinguish between wandering poles and drifting continents?
These problems would go away if the position of the magnetic pole averaged out over time to
the position of the geographic pole, but did it?
• As the British scientist and Nobel Prize winner P. M. S. Blackett put it: “Without the study
of rock magnetism we had no possibility of knowing whether the field might not have been
this idea to an extreme, could scientists be sure that hundreds of millions of years ago, the
Earth even had a magnetic field?
Three groups of scientists, two in England and one in America, were to answer
these questions and establish the basis for the acceptance of continental drift.
Blackett, one of the leading British scientists of midcentury, was a key figure.
DuringWorldWarII,Blackett workedontheall-important radar,helpingensure
that Britain had a working system when the war began. He invented an improved
bombsight, used operations research against the German U-boats, and, togeth-
