Paleomagnetism is the study of ancient magnetic fields preserved in rocks. When rocks form, magnetic minerals within them align with the Earth's magnetic field at that time, recording its orientation. Studying the orientation of these magnetic minerals in rocks from different locations and time periods allows scientists to map changes in the Earth's magnetic field through time and reconstruct movements of tectonic plates. Paleomagnetism provides strong evidence for plate tectonic theory, showing that continents have moved over Earth's history by matching the magnetic signatures of separated landmasses and revealing symmetrical magnetic patterns on either side of mid-ocean ridges from sea floor spreading.

1. PALEOMAGNETISM
PRESENTED BY: MUZNA KASHAF
16261514-030

2. CONTENTS
 Introduction
 History
 Magnetization of minerals in
 Igneous rocks
 Sedimentary rocks
 Metamorphic rocks
 Measurement of paleomagnetism
 Applications
 Paleomagnetism and Plate Tectonic Theory
 References

3. INTRODUCTION
• Paleomagnetism= ancient magnetic field.
• It is the study of magnetism in ancient rocks.
• Some rocks and materials contain minerals that respond to
the magnetic field.
• When rocks form, the minerals align with the magnetic field
preserving its position.
• It’s called rock magnetism when rocks record the position of
the magnetic field.
• The magnetic signature of the rocks allows paleomagnetists
to date the rocks and map the position of the field at the time
of their formation.

4. HISTORY
• The phenomenon was first discovered by the French physicist
Achilles Delesse in 1849.
• He observed that certain magnetic minerals in rocks were
aligned parallel to Earth’s magnetic field.
• Bernard Brunhes in 1906, made similar discovery.
• He observed that the magnetic minerals in some rocks are
oriented in exactly the reverse position.
• Direct observations of the geomagnetic field were not recorded
until the magnetic compass became a widespread tool for
navigation.
• Paleomagnetic research draws information of nature and origin
of earth’s magnetic field from rocks that acquire a remnant
magnetization upon formation.

6. Magnetization of minerals
• Minerals can be magnetized and oriented with Earth’s
magnetic field in a variety of ways.
• In molten rock, atoms of minerals are free to move and
align themselves with Earth’s magnetic field.
• When the rocks cool, attain form of igneous rocks the
minerals are then frozen in position and oriented along
Earth’s magnetic north-south axis.
• Magnetic minerals found in rocks today, however, are not
necessarily oriented along Earth’s present magnetic north-
south axis.

7. Continue…
• The deviation of a mineral’s orientation to the present
magnetic field is of value in determining changes in Earth’s
structure in the past.
• Deviation may be inclined or declined.
 Declination is the angle between magnetic north (the direction
the north end of a compass needle points) and true north.
 Magnetic inclination is the angle made by a compass needle
when the compass is held in a vertical orientation.

9. Magnetization In Sedimentary
Rocks
• Magnetic minerals can also be found in sedimentary rocks.
• Sand, silt, clay, and other such materials are moved from place
to place by some agent, the magnetic minerals are constantly
reoriented.
• When these materials finally settle out and form permanent
accumulations, they orient themselves with Earth’s magnetic
axis.
• These sediments, which may eventually become sedimentary
rocks, can preserve the orientation of Earth’s magnetic field.

10. Magnetization In Metamorphic
Rocks
• Magnetization of minerals also occurs within metamorphic
rocks.
• Again, freedom of movement allows the minerals to become
magnetized along Earth’s existing magnetic lines of force.

11. Measurement of Paleomagnetism
• The study of paleomagnetism started in the 1940s when the
British physicist Patrick M.S. Blackett invented a device for
measuring the very small amount of magnetic fields associated
with magnetic minerals.
• The astatic magnetometer consisted of a number of tiny
magnets suspended on a thin fiber.
• The magnetometer was rotated around a sample and the
amount of magnetism measured by changes in the fiber.
• Today, two other devices are more commonly used to study
paleomagnetic materials:
 The spinner magnetometer
 The cryogenic magnetometer.

13. Spinner and Cryogenic magnetometer

14. Applications of Paleomagnetism
• Understanding of Earth history.
• History of Earth’s magnetic field helps us predict its future
behavior.
• Orientation of magnetic minerals in rocks is often very much
out of phase with Earth’s present magnetic field.
• Two possible explanations for this phenomenon are possible
and have been proposed by scientists.
 First;
Earth’s magnetic field itself changes over time.
Differences in orientation result from changes in the magnetic
poles, not in the orientation of the minerals.

15. Continue…
 Second;
Variations in the orientation of magnetic minerals have
been caused by the movement of the minerals themselves.
This theory would suggest that it is the rocks themselves that
are moving across Earth’s surface.
• Earth’s magnetic field has flipped and reversed in the past.
• Based on magnetic records, we know the last magnetic pole
shift occurred 781,000 years ago.
• Polarity of Earth’s magnetic field has shifted at least 171 times
in the past 76 millions years.
• Earth’s magnetic field also fluctuates in due to changes in
temperature and convection currents at the core.

16. Continue
• Earth’s magnetic field has been a dipole field.
• Its shape resembles that of the field of a bar-magnet.
• The field lines emerge at one pole and re-enter at the other
pole.
• Earth’s magnetic field, is not caused by a mass of iron with a
remanent magnetization.
• Its origin lies in the outer fluid core where convective motion
generates the magnetic field in a self-sustaining dynamo
action.
• That’s why its shape and orientation are not constant but
subject to temporal variations on time scales.
• Averaged over time spans greater than 100,000 years, the
dipole axis is parallel with Earth’s spin axis.

17. Paleomagnetism and Plate tectonic
theory
• Lithosphere consists of about 20 large plates that are about 100
km thick and thousands of miles wide.
• These plates move back and forth, collide with each other,
slide past each other, and pull apart from each other.
• Significant geological events, such as volcanoes and
earthquakes, are in most cases the result of plate movements.
• Paleomagnetism is the strongest pieces of evidence for plate
tectonics.

18. Example:
• Some rocks in Alaska have magnetic minerals oriented in such
a way that they must have been laid down at or near the
equator.
• The fact that they are now at 70° north latitude suggests
strongly that the plate on which they are riding must have
migrated a very long distance during Earth history.
• Paleomagnetism can also be used to match up land masses that
are now separated from each other, but which must once have
been joined.
• For example;
The orientation of magnetic minerals along the
eastern coast of South America matches to that of similar
minerals on the western coast of Africa.

19. Continue…
Sea floor spreading;
• Mid-oceanic ridge-rift systems are areas in the oceans where
the edges of two plates, are being forced away from each other
by currents in the underlying asthenosphere.
• Magma from the asthenosphere is pushed up from below the
rift to create new ocean floor.
• Strong evidence for this theory has come from the study of
paleomagnetism on either side of mid-ocean ridges.
• Magnetometers towed by ships sailing above the rifts have
found that the patterns of orientation of magnetic minerals on
either side of a rift form stripes that are mirror images of each
other.

20. REFERENCES
• www.encyclopedia.com/science/encyclopedias-almanacs-
transcripts-and-maps/paleomagnetism-0
• https://www.apexmagnets.com/news-how-tos/what-is-
paleomagnetism/
• https://www.education.psu.edu/earth520/content/l3_p5.html
• https://www.infoplease.com/encyclopedia/earth/geology-
oceanography/info/paleomagnetism