Geomagnetism Geophysics(Geol409).pptx

4.Geomagnetism
4.1. Origin of earth’s magnetism and magnetic field
1. Introduction
Earth’s magnetic field is generated in the fluid outer core by a self-
exciting dynamo process.
Electrical currents flowing in the slowly moving molten iron generate
the magnetic field.
In addition to sources in Earth’s core, the magnetic field observable
at the planet’s surface has sources in the crust and in the
ionosphere and magnetosphere.
The geomagnetic field varies on a range of scales, and a description
of these variations is now made—ordered from low- to high-
frequency variations—in both the space and time domains.
 The final section describes how Earth’s magnetic field can be both a
tool and a hazard to the modern world.
First of all, however, methods of observing the magnetic field are
described.

What's geomagnetism ?
The Earth is associated with the huge
magnet that has an S (N)-pole of a
magnet near the North (South) Pole.
N-pole of a magnetic compass is
attracted to S-pole of near the North
Pole of the earth.
The magnet generates the magnetic field
around it. The magnetic field is caused
by the earth, and we call it
"geomagnetism".

True north and Magnetic north
A compass points approximate north, but
strictly, it doesn't point true north.
There is a slight difference between true
north and magnetic north. The difference
is called "declination". Declination has
temporal and spatial change.
You can see the distribution of
declination by "Declination index". And
declination is listed on Topographic map
of 1:50,000, 1:25,000, 1:10,000 which
have been published by Geospatial
Information Authority of Japan.

ORIGIN OF GEOMAGNETIC FIELD
 The Magnetic field of Earth has two origin
1. Internal origin ( main field)
2. External origin

Main field
 The main geomagnetic field is produced in the fluid outer core
of the Earth
 The major constituent material of the outer core is liquid iron
 For the generation of the magnetic field the important
parameter of the core are its temperature , viscosity and
electrical conductivity
 The origin of the Earth's magnetic field is not
completely understood, but is thought to be
associated with electrical currents produced by the
coupling of convective effects and rotation in the
spinning liquid metallic outer core of iron and nickel.
 This mechanism is termed the dynamo effect.

Conti………
 The main magnetic field of the earth is thought to be
produced by electrical current in the conductive core
 Field lines form outside the Earth connecting from
southern to northern hemisphere
 In the northern hemisphere the field lines are oriented
downward in to the Earth and vice-versa in the
southern hemisphere

Elements of Earth’s Magnetic field.
• A vector is used to represent the Earth's magnetic field at an
observation site. The vector is described by a combination of
seven quantities known as the magnetic elements. The total
field vector B has a vertical component Z and a horizontal
component H in the direction of the magnetic north. Inclination
I is the angle between the directions of the B-field and the
horizontal plane. Magnetic declination D is the horizontal
angle between the geographic north and the magnetic north
that is indicated by a compass.
• The horizontal component H can be further decomposed into a
component X in the geographical north direction and a
component Y in the geographical east direction.

Change of geomagnetism
Geomagnetism changes with time.
 Geomagnetism has daily change in a regular way.
You can see the aspect of its change in graph on
Geodetic Observatories (every 1 min) and graph on
Continuous Geomagnetic Stations (every 1 min).
Also, geomagnetism changes slowly and smoothly on a
long-term. It's called "the geomagnetic secular variation".
Animation of the secular variation since 1970.
Graph of the secular variation at geodetic observatories.
Elements of the geomagnetic field
Geomagnetic field have direction and intensity, it
is vector.
To describe geomagnetism, three independent
elements need to be used.
o For example, combination of (F,D,I), (H,D,Z),
(X,Y,Z) are used.

Elements of Earth’s Magnetic field.
Any combination of three magnetic elements is sufficient to completely
describe the Earth's magnetic field vector at an observation site.
Principal equations relating the values of the elements are as
follows:
F=(X2+Y2+Z2)1/2 =(H2+Z2)1/2
H=F*cos(I), Z=F*sin(I), X=H*cos (D), Y=H*sin(D)

F Total force the total intensity of Geomagnetic field Intensity
D Declination the angle between true north and magnetic
north
clockwise "+"
I Inclination the angle between the field vector and the
horizontal plane
under the horizo
ntal plane "+"
H Horizontal
intensity
the intensity of the horizontal part of the
magnetic field
magnetic north
direction "+"
Z Vertical
intensity
the intensity of the vertical part of the
magnetic field
downward "+"
X true north
component
the intensity of the north-south part of the
magnetic field
true north
direction "+"
Y true east
component
the intensity of the east-west part of the
magnetic field
true east
direction "+"

The External Magnetic Field
 Most of the very short wave length fraction of the
solar radiation does not reach the Earth's surface.
 Energetic γ-ray and x-ray and UV-radiations causes•
ionization of molecules of N2 and O2 in the thin upper
atmosphere in altitude between 50km-1500km
forming an ionized region called the ionosphere.

 Electric current in the ionosphere arise from
systematic motion of the ions, which are affected by
different factors such as, the daily and monthly tides
and the periodic fluctuation in ionization related to
the 11yr sunspot cycle, thus the current induce
varying magnetic field which is associated with the
external origin of Earth's magnetic field
• The physical basis of magnetism is fundamental to
the geophysical topics of geomagnetism, rock
magnetism and paleomagnetism.

Unit of the magnetic field
The unit of the magnetic intensity is Tesla(T) that is magnetic flux in SI unit system.
Usually unit of nT ( 10-9T) is used because geomagnetism is so weak.
In a past, Gauss(G) unit is used CGS unit system. And also used gamma(γ), 10-5 of G,
had been used.
The unit of nT and γ are correspondent.
The unit of geomagnetic direction is degree or minute, that is generally used.
Geomagnetic survey conducted by GSI
GSI mainly conducts the following
1. Making geomagnetic chart and declination index : To clarify the
geographical distribution, and to monitor secular variation of geomagnetic
field in Japan, geomagnetic chart and declination index has been updated at
10 yearly epochs. Next time, we will make it every 5 years. We have
released geomagnetic charts for 1970.0, 1980.0, 1990.0, 2000.0, 2010.0
epoch.
2. Geomagnetic observation around volcanoes : To monitor the volcanic
activity.
3. The observation of subterranean electromagnetism : To monitor the change
of conductivity which are accompanied by crustal movement.(MT
continuous observation)

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.
Magnetic field orientation:
Parallel to the magnetic axis at
the midpoint of the magnet.
Curves strongly towards the
poles.
The region around a magnetic object in which its magnetic forces
act on other magnetic objects.

Magnetic field strength:
Strongest at the poles.
Weakest at the midpoint.

Earth’s Magnetic Field
Generated by the convective motion the fluid outer core
about the solid inner core.
Geodynamo: the conversion, within
the Earth, of mechanical energy
(convection of metals) to electrical energy
which produces the magnetic field.
A magnetic field produced by
such fluid motion is inherently
unstable and not as uniform as
about a simple bar magnet.

What we call the “North geographic pole” corresponds to the “south
pole” of the imaginary bar magnetic so that the north needle on a
compass points towards the north geographic pole!
We can visualize the Earth’s magnetic field as being produced by a
giant bar magnet within the Earth.

Angle of magnetic pole – angle of geographic pole = magnetic declination
North and south poles are the points of intersection of the axis of the
magnetic field and the surface of the Earth.
The axis of the magnetic field is at a
small angle to the axis of rotation:
termed the magnetic declination.
The magnetic poles moves about
the geographic poles: termed
secular variation in the magnetic
pole position

Changes in declination
reflect secular variation in
pole position over time.
Due to the inherent
instability of the field
produced by the
Geodynamo.

Orientation (inclination) of the magnetic force field:
Parallel to Earth surface at equator.
Perpendicular to Earth surface at poles.
Field points downward in the
Northern Hemisphere.
Field points upward in the
Southern Hemisphere.
Are Australian and Canadian
compasses different?

Regular increase in the inclination of the Earth’s magenetic field from the Equator (0° latitude)
to the poles (90° latitude).

Force field intensity varies from a maximum at the poles to a minimum
at the equator.

Rock samples are taken from cores drilled
into magnetized igneous rocks.
The ages of the samples are determined
and their RMS is measured.
The inclination of the RMS reflects the
latitude of the sample at the time of
crystallization.
30° downward, North of equator. 0°, at the equator.
30° upward, South of equator.

Differences and similarities with Gravity
Similarities are:
The magnetic and gravity fields are both potential fields, the
fields are the gradient of some potential V , and Laplace’s and
Poisson’s equations apply.
For the description and analysis of these fields, spherical
harmonics is the most convenient tool, which will be used to
illustrate important properties of the geomagnetic field.
In both cases we will use a reference field to reduce the
observations of the field.
Both fields are dominated by a simple geometry, but the higher
degree components are required to get a complete picture of the
field.
In gravity, the major component of the field is that of a point
mass M in the center of the Earth; in geomagnetism, we will see
that the field is dominated by that of an axial dipole in the center
of the Earth and approximately aligned along the rotational axis.

Differences are:
• In gravity the attracting mass m is positive; there is no such thing
as negative mass. In magnetism, there are positive and negative
poles.
In gravity, every mass element dM acts as a monopole; in contrast, in
magnetism isolated sources and sinks of the magnetic field H don’t exist (∇·H =
0) and one must always consider a pair of opposite poles.
Opposite poles attract and like poles repel each other.
If the distance d between the poles is (infinitesimally) small → dipole.
Gravitational potential (or any potential due to a monopole) falls of as 1over r,
and the gravitational attraction as 1 over r2 .
In contrast, the potential due to a dipole falls of as 1 over r2 and the field of a
dipole as 1over r3 .
This follows directly from analysis of the spherical harmonic expansion of the
potential and the assumption that magnetic monopoles, if they exist at all, are
not relevant for geomagnetism (so that the l =0 component is zero).

The direction and the strength of the magnetic field varies with
time due to external and internal processes.
 As a result, the reference field has to be determined at regular
intervals of time (and not only when better measurements
become available as is the case with the International Gravity
Field).
The variation of the field with time is documented, i.e. there is a
historic record available to us. Rocks have a ’memory’ of the
magnetic field through a process known as magnetization.
The then current magnetic field is ’frozen’ in a rock if the rock
sample cools (for instance, after eruption) beneath the so called
Curie temperature, which is different for different minerals, but
about 500-600◦C for the most important minerals such as
magnetite.
•This is the basis for paleomagnetism. (There is no such thing as
paleogravity!)

4.2.Magnetism and plate motions
Magnetism is a property of materials that respond to an applied magnetic
field.
Magnetism refers to physical phenomena arising from the force between
magnets, objects that produce fields that attract or repel other objects.
 Permanent magnets have persistent magnetic fields caused by
ferromagnetism.
That is the strongest and most familiar type of magnetism. However, all
materials are influenced varyingly by the presence of a magnetic field.
Some are attracted to a magnetic field (paramagnetism); others are repulsed by
a magnetic field (diamagnetism); others have a much more complex relationship
with an applied magnetic field (spin glass behavior and antiferromagnetism).
Substances that are negligibly affected by magnetic fields are known as non-
magnetic substances.
They include copper, aluminium, gases, and plastic. Pure oxygen exhibits
magnetic properties when cooled to a liquid state.
The magnetic state (or phase) of a material depends on temperature (and other
variables such as pressure and applied magnetic field) so that a material may
exhibit more than one form of magnetism depending on its temperature, etc.

Cause of magnetic susceptibility
• At the atomic level, materials have a net magnetic
moment due to:
– Rotation of electrons in various shells around
nucleus
– The spin of the electrons
– Number of electrons in each shell
– i.e., it’s a quantum effect
• All of above result that each atomic nucleus can
be though of as a small magnetic dipole with its
own moment

Classifications of magnetic materials
• Diamagnetic
All electron shells are full, thus there is no net moment.
 In the presence of an external field, the net moment
opposes the external field, i.e., slightly negative
susceptibility.
• Paramagnetic
 Materials contain unpaired electrons in incomplete
electron shells.
 However magnetic moment of each atom is uncoupled
from others so they all behave independently.
 Results in weakly magnetic materials, i.e. small
susceptibility

Ferromagnetic
Materials contain unpaired electrons in
incomplete electron shells.
 Magnetic moment of each atom is coupled to
others in surrounding ‘domain” such they all
become parallel.
Caused by overlapping electron orbits.
Gives rise to a spontaneous magnetization even
in absence of an external field.
 Magnets are ferromagnetic.
Examples: Cobalt, iron and nickel.

Anti-ferromagnetic
Almost identical to ferromagnetic except that the
moments of neighboring sub lattices are aligned
opposite to each other and cancel out
Thus no net magnetization is measured
• Example: Hematite
Ferrimagnetic
Sub lattices exhibit ferromagnetically but then couple
antiferromagnetically between each other
• Example: Magnetite and ilmenite

Magnetic properties of materials of interest
• Basement: tends to be igneous or metamorphic, thus
greater magnetic properties.
• Soils and other weathered products: because
magnetic minerals tend to weather rather rapidly
compared to quartz, will get reduction of magnetic
materials with weathering.
• Man-made objects: iron and steel
• Ore deposits: many economic ores are either
magnetic, or associated with magnetic minerals.

Types of magnetization
The magnetization of a mineral is controlled by intrinsic (i.e.,
material dependent) magnetic moments of electrons spinning
about their axes (spin dipole moments) or the motion of electrons
in their orbits about the atomic nuclei (orbital dipole moments).
 There are several types of spin interactions that give rise to
different magnetic effects. The following is a brief summary.
Remanent or Induced? (K¨onigsberger ratio)
When we talk about magnetization, we can broadly identify two
types:
1. Induced magnetization, MI , which occurs only if an ambient field
H is present and decays rapidly if this external field is removed.
This induced field is very important in ore exploration.
2. Remanent magnetization, MR , which is the part of initial
magnetization that remains after the external field disappears or
changes in character. R forms the record of the past field and is the
type of magnetization that makes paleomagnetism work.

38
Magnetic theory
Coulomb’s Law
Magnetic Force (F) :
F : Magnetic Force (dynes)
p1 p2 : The poles of strength
 : Magnetic permeability (a property of the medium)
r : Distance (cm)
r1 : Unit vector
1
2
2
1
r
r
p
p
F 









P1 P2
r N
N

39
Magnetizing field (H) – Magnetic Field Strength :
H´ : Oersted (dynes/unit pole) cgs unit
1γ= 10-5 Oersted
1
2
1
2
'
r
r
p
p
F
H 











40
Magnetic dipole moment :
 2l 
p (+) p(-)
m=2l p
m: A vector in the direction of r1
Magnetic polarization (M) – Magnetization intensity :
M=kH
k : Magnetic susceptibility (magnetic response of rocks and minerals)

41
Magnetic induction (B) – total field
B=o(H+M)
B=oH (the Earth’s magnetic induction)+oM (the rock’s magnetic induction)
o: Permeability of free space (4x10-7 Wb/A-m)
B  Tesla=1 Newton/ampere-meter=1 weber/m2 - SI unit
Gauss=10-4 Tesla
1 γ=10-9 Tesla=1nT (nanoTesla) – emu-unit

Magnetization and magnetic susceptibility
Magnetization M is a normalized moment (Am2). We will use the symbol M for volume
normalization (units of Am-1) or Ω for mass normalization (units of Am2kg-1). Volume
normalized magnetization therefore has the same units as H, implying that there is a
current somewhere, even in permanent magnets
The relationship between the magnetization induced in a material MI and the external
field H is defined as:
The parameter χb is known as the bulk magnetic susceptibility of the material; it can be a
complicated function of orientation, temperature, state of stress, time scale of
observation and applied field, but is often treated as a scalar. Because M and H have the
same units, χb is dimensionless.
(1.4)
Relationship of B and H
B and H are closely related and in paleomagnetic practice, both B and H are referred to as the “magnetic field”.
Strictly speaking, B is the induction and H is the field, but the distinction is often blurred. The relationship
between B and H is given by:
where μ is a physical constant known as the permeability. In a vacuum, this is the
permeability of free space, μo. In the SI system, μ has dimensions of henries per
meter and μo is 4π × 10-7H ⋅ m-1. In most cases of paleomagnetic interest, we are
outside the magnetized body so M = 0 and B = μoH.

Parameter SI unit cgs unit Conversion
Magnetic moment (m) Am2 emu 1 A m2 = 103 emu
Magnetization
by volume (M) Am-1 emu cm-3 1 Am-1 = 10-3 emu cm-3
by mass (Ω) Am2kg-1 emu gm-1 1 Am2kg-1 = 1 emu gm-1
Magnetic Field (H) Am-1 Oersted (oe) 1 Am-1 = 4π x 10-3 oe
Magnetic Induction (B) T Gauss (G) 1 T = 104 G
Permeability
of free space (μo) Hm-1 1 4π x 10-7 Hm-1 = 1
Susceptibility
total (K:mH) m3 emu oe-1 1 m3 = 106 4π emu oe-1
by volume ( χ: M H) - emu cm-3 oe-1
1 S.I. = 1 _ 4π emu cm-3
oe-1
by mass (κ:mm ⋅ 1 _ H) m3kg -1 emu g-1 oe-1
1 m3kg-1 = 103 4π emu g-1
oe-1

Paleomagnetism and Continental
Drift Revived
• Studies of rock magnetism allowed
determination of magnetic pole locations
(close to geographic poles) through time
• Paleomagnetism uses mineral magnetic
alignment direction and dip angle to
determine the direction and distance to
the magnetic pole when rocks formed
– Steeper dip angles indicate rocks
formed closer to the magnetic poles
• Rocks with increasing age point to pole
locations increasingly far from present
magnetic pole positions

Paleomagnetism and Continental
Drift Revived
• Apparent polar wander curves for
different continents suggest real
movement relative to one another
• Reconstruction of supercontinents
using paleomagnetic information
fits Africa and South America like
puzzle pieces
– Improved fit results in rock units (and glacial ice
flow directions) precisely matching up across
continent margins

Seafloor Spreading
• In 1962, Harry Hess proposed
seafloor spreading
– Seafloor moves away from the mid-
oceanic ridge due to mantle convection
– Convection is circulation driven by
rising hot material and/or sinking
cooler material
• Hot mantle rock rises under
mid-oceanic ridge
– Ridge elevation, high heat flow, and
abundant basaltic volcanism are
evidence of this

Plates and Plate Motion
• Tectonic plates are composed of the relatively rigid
lithosphere
– Lithospheric thickness and age of
seafloor increase with distance
from mid-oceanic ridge
• Plates “float” upon ductile asthenosphere
• Plates interact at their boundaries, which are
classified by relative plate motion
– Plates move apart at divergent boundaries, together at convergent
boundaries, and slide past one another at transform boundaries

Evidence for Plate Tectonics
• Fit of the continents
• Similarity of rock sequences
• Location of volcanos
• Location of deep
• earthquakes
• Paleomagnetism:
– apparent polar wandering
– seafloor spreading

Evidence of Plate Motion
• Marine magnetic anomalies - bands of
stronger and weaker than average
magnetic field strength
– Parallel mid-oceanic ridges
– Field strength related to basalts
magnetized with same and opposite
polarities as current magnetic field
– Symmetric “bar-code” anomaly
pattern reflects plate motion away
from ridge coupled with magnetic
field reversals
– Matches pattern of reversals seen in
continental rocks (Vine and
Matthews)

• Seafloor age increases with
distance from mid-oceanic ridge
– Rate of plate motion equals
distance from ridge divided
by age of rocks
– Symmetric age pattern
reflects plate motion away
from ridge

• Mid-oceanic ridges are offset
along fracture zones
– Fracture zone segment between offset ridge
crests is a transform fault
– Relative motion along fault is result of
seafloor spreading from adjacent ridges
• Plate motion can be measured
using satellites, radar, lasers and
global positioning systems
– Measurements accurate to within 1 cm
– Motion rates closely match those predicted
using seafloor magnetic anomalies

Divergent Plate Boundaries
• At divergent plate boundaries, plates
move away from each other
– Can occur in the middle of the ocean or
within a continent
– Divergent motion eventually creates a new
ocean basin
• Marked by rifting, basaltic volcanism,
and eventual ridge uplift
– During rifting, crust is stretched and thinned
– Graben valleys mark rift zones
– Volcanism common as magma rises through
thinner crust along normal faults
– Ridge uplift by thermal expansion of hot rock

Transform Plate Boundaries
• At transform plate boundaries, plates slide
horizontally past one another
– Marked by transform faults
– Transform faults may connect:
• Two offset segments of mid-oceanic
ridge
• A mid-oceanic ridge and a trench
• Two trenches
– Transform offsets of mid-oceanic ridges
allow series of straight-line segments to
approximate curved boundaries
required by spheroidal Earth

Convergent Plate Boundaries
• At convergent plate boundaries,
plates move toward one another
• Nature of boundary depends on
plates involved (oceanic vs.
continental)
– Ocean-ocean plate convergence
• Marked by ocean trench, Benioff zone, and
volcanic island arc
– Ocean-continent plate convergence
• Marked by ocean trench, Benioff zone,
volcanic arc, and mountain belt
– Continent-Continent plate convergence
• Marked by mountain belts and thrust faults

What Causes Plate Motions?
• Causes of plate motion are not yet fully
understood, but any proposed mechanism
must explain why:
– Mid-oceanic ridges are hot and
elevated, while trenches are cold and
deep
– Ridge crests have tensional cracks
– The leading edges of some plates are
subducting sea floor, while others are
continents (which cannot subduct)
• Mantle convection may be the cause or an
effect of circulation set up by ridge-push
and/or slab-pull

Movement of Plate Boundaries
• Plate boundaries can move over time
– Mid-oceanic ridge crests can migrate
toward or away from subduction zones or
abruptly jump to new positions
– Convergent boundaries can migrate if
subduction angle steepens or overlying
plate has a trenchward motion of its own
• Back-arc spreading may occur, but is
poorly understood
– Transform boundaries can shift as slivers of
plate shear off
• San Andreas fault shifted eastward
about five million years ago and may do
so again

Mantle Plumes and Hot Spots
• Mantle plumes - narrow columns of hot
mantle rock rise through the mantle
– Stationary with respect to moving
plates
– Large mantle plumes may spread
out and tear apart the overlying
plate
•Flood basalt eruptions
•Rifting apart of continental land
masses
– New divergent boundaries may form

Mantle Plumes and Hot Spots
• Mantle plumes may form “hot
spots” of active volcanism at
Earth’s surface
– Approximately 45 known hotspots
• Hot spots in the interior of a plate
produce volcanic chains
– Orientation of the volcanic chain shows
direction of plate motion over time
– Age of volcanic rocks can be used to
determine rate of plate movement
– Hawaiian islands are a good example

Plate Tectonics and Ore Deposits
• Metallic ore deposits often located
near plate boundaries
– Commonly associated with igneous activity
• Divergent plate boundaries often
marked hot springs on sea floor
– Mineral-rich hot springs (black smokers) deposit metal
ores on sea floor
• Hydrothermal circulation near
island arcs can produce metal-
rich magmatic fluids

4.3.Magnetization of rocks and paleo-magnetism
4.3.1. Magnetization of rocks
Remnant magnetic signature (RMS):
Magnetic field generated by a rock due to the
alignment of magnetic fields of rock forming
minerals. "Remnant" because it formed at the
time of crystallization and cooling (Igneous and
Metamorphic Rocks) or deposition (Sedimentary
Rocks).
Preserves the direction and inclination of the Earth's
magnetic field and is an indicator of field intensity.

RMS in Igneous and metamorphic rocks
RMS develops as the rock cools and its temperature falls
below the Curie Point.
Curie Point: the temperature at which the magnetic fields
develop in minerals (atomic arrangement becomes fixed).
The Curie Point varies with different minerals but is typically
around 580 degrees Celsius.

Above the Curie Point, atoms within crystals vibrate randomly
and have no associated magnetic field.

Below the Curie Point the magnetic fields of the minerals act like tiny
compass needles: they become aligned to the Earth's magnetic field.

The minerals themselves generate a small magnetic field
(the rock's RMS).
The RMS records the orientation and strength of the Earth's field at the time of cooling.
The stronger the Earth's magnetic field, the stronger RMS.
RMS is fixed unless the rock heats up to above the Curie Point at some future time.
RMS in sedimentary rocks
Develops as fine grained sediment deposits from suspension in very
quiet water (no currents).
Individual grains have weak magnetism that causes them to become
aligned to the Earth's magnetic field as they settle (like tiny compass
needles).
When the grains are deposited their RMS parallels the Earth's field.
The stronger the Earth's magnetic field, the stronger the RMS.

RMS remains fixed as the sedimentary deposit becomes cemented to
form a sedimentary rock.
In a rock we can measure:
1. The strength of the RMS (a measure of the Earth's field strength
when the rock formed).
2. The direction of the RMS (the direction to the Earth's magnetic poles
at the time of rock formation).
3. The inclination of the RMS (the inclination of the Earth's field which
reflects the latitude at which the rock formed).

Testing the plate tectonics model
• Paleomagnetism is the study of the magnetic properties
of rocks. It is one of the most broadly applicable
disciplines in geophysics, having uses in diverse fields
such as geomagnetism, tectonics, paleoceanography,
volcanology, paleontology, and sedimentology.
• Paleomagnetism
• Ancient magnetism preserved in rocks at the time
of their formation
• Magnetized minerals in rocks
–Show the direction to Earth’s magnetic poles
–Provide a means of determining their latitude
of origin

• Paleomagnetism
• Palaeomagnetism is the study of the Earth's magnetic field preserved
in rocks. The discovery that some minerals, at the time of their
formation, can become magnetized parallel to the Earth's magnetic
field was made in the nineteenth century.
• Polar wandering
– The apparent movement of the magnetic poles illustrated in
magnetized rocks indicates that the continents have moved
– Polar wandering curves for North America and Europe have
similar paths but are separated by about 24 of longitude
– Different paths can be reconciled if the continents are place
next to one another

Apparent polar-wandering paths for
Eurasia and North America
Apparent polar wander (APW) paths are the usual way to represent the
changing orientation of continents relative to the spin axis.

• Magnetic reversals and seafloor spreading
• Earth's magnetic field periodically reverses
polarity – the north magnetic pole becomes
the south magnetic pole, and vice versa
• Dates when the polarity of Earth’s
magnetism changed were determined from
lava flows
Magnetic reversals and seafloor spreading
Geomagnetic reversals are recorded in the ocean
crust
In 1963 the discovery of magnetic stripes in the
ocean crust near ridge crests was tied to the
concept of seafloor spreading
The Earth’s field is also subject to reversals, the last of which occurred at 0.7
Ma (i.e., 0.7 million years ago).

Paleomagnetic reversals recorded by
basalt at mid-ocean ridges

Testing the plate tectonics model
• Magnetic reversals and seafloor spread-ing
• Paleomagnetism (evidence of past
magnetism recorded in the rocks) was the
most convincing evidence set forth to
support the concept of seafloor spreading
• The Pacific has a faster spreading rate
than the Atlantic

Magnetic Anomalies
An outcome of the magnetization of rocks is that they can
locally change the Earth’s magnetic field strength: increasing
or decreasing the local strength due to strong or weak
magnetization, respectively.
E.g., an Iron Ore body with a strong normal magnetic field
strength can significantly increase the local Earth field
strength.
Magnetic anomaly= local magnetic field strength - average
magnetic field strength

Magnetic anomalies are local deviations from the
regional magnetic field. They and may be positive or
negative relative to the regional field.
It is assumed that these anomalies are caused by buried
structures. There are two main factors:
Increased magnetic susceptibility due to iron objects
Permanent magnetization due to certain rocks
The strength and shape of the anomaly is controlled by
the shape, composition and depth of the structure.

Anomalies are identified using the corrected field over
the survey area.
The correction is used to remove the effect of short term
fluctuations.
Once corrected, the field can then be divided into the
average (regional) field and the local (residual) field:
residual field = corrected field - regional field
The residual field corresponds to the anomaly field.

Geomagnetism Geophysics(Geol409).pptx

Geomagnetism Geophysics(Geol409).pptx

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