DEFINITION OF PALEOMAGNETISM

The study of natural remnant magnetism

in rocks to understand the intensity and direction of the

Earth’s magnetic field in the geologic past, and to understand

the history of plate motion. The Earth’s magnetic field can be

divided into two components at any location, including the

declination and the inclination. The declination measures the

angular difference between the Earth’s rotational north pole

and the magnetic north pole. The inclination measures the

angle at which the magnetic field lines plunge into the Earth.

The inclination is 90° at the magnetic poles, and 0° halfway

between the poles.

Studies of paleomagnetism in young rocks have revealed

that the Earth’s magnetic poles may flip suddenly, over a period

of thousands or even hundreds of years. The magnetic

poles also wander by about 10°–20° around the rotational

poles. On average, however, the magnetic poles are coincident

with the Earth’s rotational poles. This coincidence can

be used to estimate the direction to the north and south poles

in ancient rocks that have drifted or rotated in response to

plate tectonics. Determination of the natural remnant magnetism

in rock samples can, under special circumstances,

reveal the paleo-inclination and paleo-declination, which can

be used to estimate the direction and distance to the pole at

the time the rock acquired the magnetism. If these parameters

can be determined for a number of rocks of different ages on

a tectonic plate, then an apparent polar wander path for that

plate can be constructed. These show how the magnetic pole

has apparently wandered with respect to (artificially) holding

the plate fixed—when the reference frame is switched, and

the pole is held fixed, the apparent polar wander curve shows

how the plate has drifted on the spherical Earth.

Paleomagnetism played an enormous role in the confirmation

of seafloor spreading, through the discovery and

understanding of seafloor magnetic anomalies. In the 1960s

geophysicists surveyed the magnetic properties of the ocean

floor and began to discover some amazing properties. The

seafloor has a system of linear magnetic anomalies where one

“stripe” has its magnetic minerals all orientated the same

way as the present magnetic field, and the alternate stripes

have all their magnetic minerals orientated in completely the

opposite sense. These stripes are orientated parallel to the

mid-ocean ridge system; where the ridges are “offset” by

transform faults, the anomalies are also “offset.” The anomalies

are symmetric on either side of the ridge, and the same

symmetry is found across ridges worldwide.

Understanding the origin of seafloor magnetic stripes

was paramount in acceptance of the plate tectonic paradigm.

The magnetic stripes form in the following way. As oceanic

crust is continuously formed as on a conveyor belt, all the

magnetic minerals tend to align with the present magnetic

field when the new crust forms. The oceanic crust thus contains

a record of when and for how long the Earth’s magnetic

field has been in the “normal” position, and when and for

how long it has been “reversed.” Similar reversals of the

Earth’s magnetic field are known from rock sequences on

land, and many of these have been dated. Using these data

geologists have now established a magnetic polarity reversal

timescale. The last reversal was about 700,000 years ago, and

the one before that, about 2.2 million years ago. Oceanic

crust is as old as Jurassic, and documentation of the age of

seafloor magnetic stripes has led to the construction of the

magnetic polarity timescale back to 170 million years ago.

See also PLATE TECTONICS.

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