Helwan University
Faculty of Science
Department of Geology
Paleomagnetism (GPH4107)
4th Level Year
Lectures by:
Nouran S. Salama, PhD in Geophysics
Paleomagnetism
Magnetism
Magnetic Force field:
• The region around a
magnetic object in which its
magnetic forces act on
other magnetic objects.
Magnetism
Magnetic field about a simple
bar magnet:
• North pole attracts the
south poles of magnetic
objects within the field.
• South pole attracts the
north pole of magnetic
objects within the field.
Magnetism
Magnetic field orientation:
• Parallel to the magnetic
axis at the midpoint of the
magnet.
• Curves strongly towards
the poles.
Magnetic field strength:
• Strongest at the poles.
• Weakest at the midpoint.
Paleomagnetism
• Paleomagnetism is the study of the ancient magnetism of rocks, which has an
ultimate objective of reading and interpreting the direction and strength of the
geomagnetic field over the geologic history.
• This ancient magnetism of rocks has been acquired in different ways through the
geologic history of the rock.
• Rocks containing magnetic minerals become magnetized during their formation,
irrespective of the age of the rock, and can acquire a primary magnetization
parallel to the ambient magnetic field and so record its ancient direction and
intensity.
Paleomagnetism
• The study of their present magnetic properties frequently allows the primary
(original) component to be isolated.
• Measurements of this primary magnetization can be used to determine the
nature of the ancient geomagnetic field.
• These measurements are the only geophysical observations which allow a
detailed examination of a physical property (magnetization) of the Earth through
the geological time.
• Paleomagnetic studies have a variety of geological applications including dating,
tectonic, paleo-geography.
How Does Paleomagnetism Work?
• When rocks form, magnetic minerals align with Earth's magnetic field.
• Rocks preserve a record of the Earth's magnetic field when they formed. By
studying these records, scientists can figure out where the magnetic poles were
in the past.
• Over time, rocks can move due to tectonic activity or erosion.
• Scientists measure the orientation of magnetic minerals to determine past
magnetic poles.
Relationship between paleomagnetism and plate tectonic
• The connection between paleomagnetism and plate tectonics is that the
movement of tectonic plates affects the rocks' magnetic properties. For example,
when new oceanic crust forms at mid-ocean ridges, it records the Earth's current
magnetic field. As this crust spreads away from the ridge, its magnetic properties
can provide information about its past positions and movements.
How does the magnetic poles change
from time to another?
• scientists sailed back and forth across the world’s oceans, measuring the magnetic signatures emanating from the oceanic crust
beneath their ships.
• These surveys revealed a series of invisible magnetic “stripes” of normal and reversed polarity in the sea floor, The patterns reflect
the creation and spreading of oceanic crust along the mid-oceanic ridges.
• Basalt forming at the ridge crest picks up the existing magnetic polarity. Divergence then moves the swath of fresh crust away from
the ridge. As long as the magnetic field remains constant, the polarity “stripe” widens.
• The Earth's magnetic poles can change over time due to the movement of molten iron in its outer core.
• This molten iron generates the planet's magnetic field through a process called the Geodynamo.
• Over thousands of years, the flow of this molten iron can shift, causing the magnetic poles to move. This phenomenon is known as
geomagnetic reversal or magnetic pole reversal.
• During a reversal, the magnetic north and south poles may swap places or undergo significant shifts in their positions.
• These reversals have occurred numerous times throughout Earth's history, as evidenced by magnetic signatures recorded in rocks
and sedimentary layers.
Key Discoveries
• Reversals of Earth's magnetic field: Evidence from rocks with
alternating magnetic orientations.
• Magnetic stripes on the ocean floor: Support for seafloor spreading
and plate tectonics theory.