another method of dating and more
Earth’s magnetic field is internally generated by the rotation
of the iron-rich core. For reasons that are not fully understood,
the position of the magnetic pole with relation to the surface of
the Earth is constantly changing. Furthermore, at intervals through
Earth history, the polarity of the magnetic field “flips”
such that a compass needle that now points north would point south
instead. These changes in the polarity of the magnetic field are
called “magnetic reversals,” and there have been 171
of them in the last 76 million years. The current polarity of the
magnetic field (in which a compass needle points north) is called
“normal” and the alternate state is called “reversed.”
The study of how Earth’s magnetic field has changed through
geologic time is called paleomagnetics.
When the magma from which igneous rocks
form is still molten, iron-rich minerals can orient themselves in
line with the local magnetic field in the same way that a compass
needle does. As the magma cools, the tiny iron-rich crystals are
“frozen” in position, recording the orientation of the
local magnetic field at that time. Iron minerals in sediments can
also align themselves with the magnetic field, then become fixed
as the sediment turns to rock. The orientation of the magnetic crystals
in rocks can be used to infer two pieces of information—the
direction of the magnetic pole from the point where the rock formed,
and the polarity (reversed or normal) of the magnetic field at the
time the rock formed.
The orientation of the magnetic crystals in a rock can tell us the
latitude at which the rock formed because the magnetic field is
oriented almost parallel to the surface of the Earth near the equator,
and almost perpendicular to it at the poles. By determining the
positions of many rocks with respect to the magnetic pole, paleomagnetists
can calculate the past positions of the continents. From that, they
can deduce the directions in which continents have moved over millions
of years. Adding in the age of the rocks, they can also calculate
the speed at which continental plates have moved.
The polarity of the magnetic crystals in a rock also tells us if
it was formed during a period with a reversed or normal magnetic
field. By measuring the magnetic polarity of a succession of rocks,
scientists can determine a sequence of polarity intervals of varying
durations. Sometimes this sequence of polarity intervals can be
distinctive—for example, a long interval of reversed polarity
might be followed by a series of five quick alternations between
normal and reversed, then another long interval of reversed polarity.
Such local sequences can be compared with a globally compiled record
of magnetic reversals, at least for the late Mesozoic and Cenozoic.
Some magnetic reversal events have been dated radiometrically, so
if the local sequence can be matched to the global record of magnetic
reversals, the age of the local rocks can be determined.