In the laboratory, a mag-netometer is used to measure the intensity and direc-tion of the magnetic vector of the rock sample.. These younger magnetizations need to be removed; in a sense
Trang 1mathematician Karl Gauss) In the laboratory, a
mag-netometer is used to measure the intensity and
direc-tion of the magnetic vector of the rock sample At this
point, all the magnetizations acquired by the rock
since it was formed are present The goal is to identify
the original magnetization at the time of rock
forma-tion (i.e the natural remnant magnetizaforma-tion)
Subse-quent (younger) magnetizations are referred to as
overprints These younger magnetizations need to be
removed; in a sense we can think of the process as
cleaning up the magnetization history of the rock so
that only the natural remnant magnetization remains
(or at least can be identified) This is done in one of
two ways, depending on which kind of magnetometer
is used Alternating-field demagnetization subjects
the rock samples to a strong alternating magnetic
field that destroys the weaker magnetizations in the
rock, so that only the stronger natural remnant
mag-netization remains Thermal demagmag-netization heats
the rock; during this process weaker magnetizations
tend to disappear first, at lower temperatures than the
stronger natural remnant magnetization Neither
method is usually strong enough to realign the
pri-mary magnetic mineral grains in the rock, and thus
remagnetize it
Magnetostratigraphical Correlation
Once the magnetic polarity of a succession of rocks has
been determined, a magnetostratigraphy can be
estab-lished (Figure 3) The basic unit of such a
magnetostrati-graphy is the magnetostratigraphical polarity zone
(magnetozone for short), a body of rock with normal
or reversed polarity Now, the problem is to establish
whether the observed succession of reversed and normal
intervals has a pattern that can be correlated with
that of another succession and/or with one or more
segments of the global polarity time-scale In other
words, this piece (preserved by a local rock succession)
needs to be matched with another piece or with a piece
of the geomagnetic-polarity time-scale This matching
is termed ‘correlation’ In correlation, a signature – a
distinctive pattern of magnetic-polarity reversals – is
looked for in order to establish a match (Figure 3)
The global pattern of magnetic reversals is irregular
and nonperiodic, so that distinctive long intervals of
magnetic reversals can be recognized Reversals took
place wordwide, independently of rock types and
en-vironments, and were geologically instantaneous (in
rocks millions of years old, the 5000 or so years it
takes for the reversal to happen is insignificant)
How-ever, polarity events are not unique, so that only a long
succession of polarity intervals of distinctive lengths
can be correlated Furthermore, sedimentary hiatuses
(unconformities) and changes in sedimentation rate can
confuse the picture Because of these problems, an independent method of correlation is often needed to provide a tie point (datum) against which to correlate the magnetostratigraphy In other words, some other idea of the general age of the local rock succession – either derived from an index fossil or a numerical age – is usually needed to help narrow the possible correlation of magnetic-reversal histories This means that magnetic-polarity-based correlations are typically not an independent means of correlating strata, al-though, once an index fossil or numerical age places the local slice of magnetic-polarity history ‘in the ballpark’, the matching of magnetic signatures often provides a more exact correlation than can be obtained from fossils or numerical ages alone
Secular Variation The Earth’s magnetic north pole is close to, but not the same as, the geographical north pole This means that, in most places, there is a small east-west difference between true north and magnetic north The angle of this east-west deviation, measured from anywhere on Earth, is called the declination For example, in California the declination is about
20to the east, whereas in New York it is about 10to the west of true north The angle that the magnetic field makes with the Earth’s surface is called the in-clination At the equator, the inclination is nearly horizontal, whereas at the magnetic pole it is vertical The magnetic field varies globally on geologically short time-scales of a few hundred years These vari-ations in declination, inclination, and field intensity are called secular variation Secular variations are not magnetic reversals, but they are well documented over at least the last 10 000 years, and such palaeose-cular variation can provide a succession of magnetic events that may be useful in correlation, particularly
in archaeological research
See Also
Analytical Methods: Geochronological Techniques Lava Palaeomagnetism Sedimentary Rocks: Iron-stones Stratigraphical Principles Tectonics: Mid-Ocean Ridges
Further Reading
Butler RF (1992) Paleomagnetism: Magnetic Domains to Geologic Terranes Boston: Blackwell
Kennett JP (ed.) (1966) Magnetic Stratigraphy of Sedi ments Stroudsburg, PA: Dowden, Hutchinson and Ross Khramov AN (1958) Paleomagnetism and Stratigraphic Correlation Canberra: Australian National University [English translation, published 1960]