Download Earth`s magnetic field

Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 19
Exploring Earth’s Interior
Exploring Earth’s Interior
Structure of the Earth
• Seismic velocity depends on the
composition of material and pressure.
• We can use the behavior of seismic
waves to tell us about the interior of the
Earth.
• When waves move from one material to
another they change speed and
direction.
Refraction
Refraction
and
Reflection
of a
Beam of
Light
Reflection
Fig. 19.1
P-wave
Shadow
Zone
Fig. 19.2a
S-wave
Shadow
Zone
Fig. 19.2b
P-and S-wave Pathways Through
Earth
Fig. 19.3
Seismograph Record of
P, PP, S, and Surface Waves
Fig. 19.4
Changes
in P-and Swave
Velocity
Reveal
Earth’s
Internal
Layers
Fig. 19.5
Structure of the Earth
Study of the behavior of seismic waves tells
us about the shape and composition of the
interior of the Earth:
• Crust: ~10–70 km, intermediate
composition
• Mantle: ~2800 km, mafic composition
• Outer core: ~2200 km, liquid iron
• Inner core: ~1500 km, solid iron
Composition of the Earth
Seismology tells us about the density
of rocks:
• Continental crust: ~2.8 g/cm3
• Oceanic crust: ~3.2 g/cm3
• Asthenosphere: ~3.3 g/cm3
Isostasy
• Buoyancy of low-density rock masses
“floating on” high-density rocks;
accounts for “roots” of mountain belts
• First noted during a survey of India
• Himalayas seemed to affect plumb
• Two hypotheses: Pratt and Airy
The less dense crust “floats” on
the less buoyant, denser mantle
Mohorovicic
Discontinuity
(Moho)
Fig. 19.6
Crust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic
uplift
Return to isostatic equilibrium
Structure
of the
Crust and
Upper
Mantle
Fig. 19.7
Earth’s internal heat
• Original heat
• Subsequent radioactive decay
• Conduction
• Convection
Upper Mantle Convection as a
Possible Mechanism for Plate
Tectonics
Fig. 19.8
Seismic Tomography Scan of a
Section of the Mantle
Subducted slab
Fig. 19.9
Temperature vs. Depth
Fig. 19.10
Paleomagnetism
• Use of the Earth's magnetic field to
investigate past plate motions
• Permanent record of the direction of
the Earth’s magnetic field at the
time the rock was formed
• May not be the same as the present
magnetic field
Magnetic
Field of
the Earth
Fig. 19.11
Magnetic
Field of a
Bar
Magnet
Fig. 19.11
Use of magnetism in geology
Elements that have unpaired
electrons (e.g., Fe, Mn, Cr, Co) are
effected by a magnetic field. If a
mineral containing these minerals cools
below its Currie temperature in the
presence of a magnetic field, the
minerals align in the direction of the
north pole (also true for sediments).
Earth's magnetic field
The Earth behaves as a magnet whose
poles are nearly coincident with the spin
axis (i.e., the geographic poles).
Magnetic lines of force emanate from the
magnetic poles such that a freely
suspended magnet is inclined upward in
the southern hemisphere, horizontal at the
equator, and downward in the northern
hemisphere
Evidence of a Possible Reversal
of the Earth’s Magnetic field
Fig. 19.12
Earth's magnetic field
declination: horizontal angle between
magnetic N and true N
inclination: angle made with horizontal
Earth's magnetic field
It was first thought that the Earth's
magnetic field was caused by a large,
permanently magnetized material deep in
the Earth's interior.
In 1900, Pierre Currie recognized that
permanent magnetism is lost from
magnetizable materials at temperatures
from 500 to 700 °C (Currie point).
The Earth's magnetic field
Since the geothermal gradient in the
Earth is ≈ 25°C/km, nothing can be
permanently magnetized below about 30
km.
Another explanation is needed.
Magnetic
Field of the
Earth
Fig. 19.11
Self-exciting dynamo
A dynamo produces electric current
by moving a conductor in a magnetic
field and vise versa. (i.e., an electric
current in a conductor produces a
magnetic field.
Self-exciting dynamo
It is believed that the outer core is in
convective motion (because it is liquid and
in a temperature gradient).
A "stray" magnetic field (probably from
the Sun) interacts with the moving iron in
the core to produce an electric current that
is moving about the Earth's spin axis
yielding a magnetic field—a self-exciting
dynamo!
Self-exciting dynamo
The theory has this going for it:
• It is plausible.
• It predicts that the magnetic and
geographic poles should be nearly
coincident.
• The polarity is arbitrary.
• The magnetic poles move slowly.
Self-exciting dynamo
If the details seem vague, it is
because we have a poor
understanding of core dynamics.
Magnetic reversals
• The polarity of the Earth's magnetic
field has changed thousands of times
in the Phanerozoic (the last reversal
was about 700,000 years ago).
• These reversals appear to be abrupt
(probably last 1000 years or so).
Magnetic reversals
• A period of time in which magnetism is
dominantly of one polarity is called a
magnetic epoch.
• We call north polarity normal and
south polarity reversed.
Magnetic reversals
• Discovered by looking at magnetic
signature of the seafloor as well as
young (0-2 Ma) lavas in France,
Iceland, Oregon and Japan.
• When first reported, these data were
viewed with great skepticism
Self-reversal theory
• First suggested that it was the rocks
that had changed, not the magnetic
field
• By dating the age of the rocks (usually
by K–Ar) it has been shown that all
rocks of a particular age have the
same magnetic signature.
Recording the Magnetic Field in
Newly Deposited Sediment
Fig. 19.13
Lavas Recording Reversals in
Earth’s Magnetic Field
Fig. 19.14
Magnetic reversals
We can now use the magnetic
properties of a sequence of rocks to
determine their age.
The Geomagnetic
Time Scale
Based on determining
the magnetic
characteristics of rocks
of known age (from both
the oceans and the
continents).
We have a good record
of geomagnetic
reversals back to about
60 Ma.