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Paleomagnetism
W
e have seen repeatedly how the geologic
record of ancient magnetism, or paleomagnetism, has become a crucial source of information for understanding Earth’s history. Mag­­netic
stripes mapped on oceanic crust confirmed the
existence of seafloor spreading and still provide
the best data to explain how plate motions have
evolved since the breakup of Pangaea 200 million
years ago. The paleomagnetism of old continental
rocks has been essential for establishing the existence of earlier supercontinents, such as Rodinia.
Scientists have also used paleomagnetism to
reconstruct the history of Earth’s magnetic field.
The oldest magnetized rocks found so far formed
around 3,5 billion years ago and indicate that
Earth had a magnetic field at that time similar to
the present one. The presence of magnetism in the
most ancient rocks is consistent with the ideas
of Earth’s differentiation discussed in Chapter
1, which imply that a convecting fluid core must
have been established very early in Earth’s 4,5 billion year history.
Let’s delve a little more deeply into the rockforming pro­cesses that have allowed geologists to
draw these remarkable conclusions.
Recall that high temperatures destroy magnetism. An important property of many magnetizable materials is that, as they cool below about
500°C, they become magnetized in the direction
of the surrounding magnetic field. This happens
because groups of atoms of the material align
themselves in the direction of the magnetic field
when the material is hot. When the material has
cooled, these atoms are locked in place and therefore are always magnetized in the same direction.
This process is called thermo­remanent magnetization, because the magnetization is «remembered» by the rock long after the magnetizing field
has disappeared. Thus, the Australian student
was able to determine the direction of the field
when the stones cooled after the last fire and took
on the magnetization of Earth’s magnetic field of
that time (see figure 1).
The discovery of magnetic reversals and the
means to decipher them was a key ingredient in
formulating the theory of plate tectonics.
Some sedimentary rocks can take on a different type of remanent magnetization. Recall that
marine sedimentary rocks form when particles of
sediment that have settled to the seafloor become
lithified. Magnetic grains among the particles
(chips of the mineral magnetite, for example) be-
Fantini, Monesi, Piazzini - Elementi
come aligned in the direction of Earth’s magnetic
field as they fall through the water, and this orientation can be incorporated into the rock when
the particles becomes lithified. The depositional
remanent magnetization of a sedimentary rock results from the parallel alignment of all these tiny
magnets, as if they were compasses pointing in
the direction of the field prevailing at the time of
deposition (see figure 2).
30 000 years ago
Today
Figure 1 Earth’s magnetic field 30 000 years ago was the reverse
of today’s, as evidenced by the discovery of reversely magnetized
rocks found in the fireplace of an ancient campsite. The rocks,
cooling after the last fire, became magnetized in the direction of
the ancient magnetic field, leaving a permanent record of it, just
as a fossil leaves a record of ancient life.
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SPAZIO CLIL
Figure 2 Newly formed
sedimentary deposits can
become magnetized in the
same direction as the contemporaneous magnetic
field of the Earth.
Magnetic mineral grains
transported to the ocean with
other sediments become aligned
with the Earth’s magnetic field
while settling through the water.
Direction of
magnetic field
Ocean
Magnetic particles
in ocean sediment
This orientation is preserved
in the lithified sediments, which
thus “remember” the field that
existed at the time of deposition.
Fantini, Monesi, Piazzini - Elementi
Reversals of Earth’s magnetic field are clearly
indicated in the fossil magnetic record of layered
lava flows. Each layer of rocks from the top down
represents a progressively earlier period of geologic time, whose age can be determined by radiometric dating methods. The direction of remanent
mag­
netism can be obtained for each layer, and
in this way the time sequence of flip-flops of the
field (that is, the magnetic stratigraphy) can be deduced. Geologists can also get data on Earth’s magnetic reversal history by mapping magnetic stripes
on the seafloor. From a combination of these data,
they have worked out a detailed history of reversals for the last 200 million years. This information is of use to archeologists and anthropologists
as well as geologists. For example, the magnetic
stratigraphy of continental sediments has been
used to date sediments containing the remains of
predecessors of our own species. About half of all
rocks studied are found to be magnetized in a direction opposite that of Earth’s present magnetic
field. Apparently, then, the field has flipped frequently over geologic time, and normal (same as
now) and reversed fields are equally likely.
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