Fig' 19'11

1
(No Transcript)
2
Magnetic Field of the Earth
Fig. 19.11
3
Earth's magnetic field

• magnetic poles are nearly coincident with the
  spin axis (i.e., the geographic poles).
• A freely suspended magnet will point down at N.
  Pole and up at south pole.

4
Earth's magnetic field

• declination horizontal angle between
  magnetic N and true N
• inclination angle made with vertical

5
Magnetic Field of the Earth
Fig. 19.11
6
Magnetic Field of a Bar Magnet
rubbish
7
The Earth's magnetic field

• Since the geothermal gradient in the Earth is
  25C/km, nothing can be permanently magnetized
  below about 30 km.
• Another explanation is needed.

8
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.

9
Evidence of a Possible Reversal of the Earths
Magnetic field
10
Paleomagnetism

• Use of the Earth's magnetic field to investigate
  past plate motions
• Permanent record of the direction of the Earths
  magnetic field at the time the rock was formed
• May not be the same as the present magnetic field

11
Use of magnetism in geology

• some Fe, Mn, Cr, Co minerals are magnetic
• but only below the Curie point 550C
• lava solidifies at 1100C

12
Self-exciting dynamo

• outer core convection guided by earth's rotation
• A "stray" magnetic field (e.g.from the Sun)
  interacts with the moving iron in the core to
  produce an electric current
• this creates a local magnetic field
• this creates an electric current
• this creates a local magnetic field etc

13
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.

14
Magnetic reversals

• The polarity of the Earth's magnetic field has
  changed thousands of times in the Phanerozoic
  (the last major reversal was about 700,000 years
  ago).
• These reversals appear to be abrupt (probably
  last 1000 years or so).

15
Magnetic reversals

• A period of time when magnetism is dominantly of
  one polarity is called a magnetic epoch.
• We call north polarity normal and south polarity
  reversed.

16
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 KAr)
  it has been shown that all rocks of a particular
  age have the same magnetic signature.

17
Recording the Magnetic Field in Newly Deposited
Sediment
Fig. 19.13
18
Lavas Recording Reversals in Earths Magnetic
Field
Fig. 19.14
19
Magnetic reversals

• because reversals are random they provide a key
  to determining age

numbering of reversals based on epochs C is a
chron C1 toC34 (121 million years) C34
cretaceous magnetic quiet 82-121 million
years Mesozoic chrons numbered M1 125 milion
years to M29 157 million years..
20
The GeomagneticTime Scale
from ocean floor basalt from sediment from lava
flows good record of geomagnetic reversals to
60 Ma.
21
magnetics

• Total field vector B has vertical component Z and
  horizontal component H.
• Inclination, Itan-1 Z/H
• tan I 2 tan (latitude)
• Declination of H measured relative to true north
• B 25000nT at equator 70,000 nT at poles
• Earths field varies 2nT per km.
• fluctuates 1000 nT in magnetic storm (solar
  radiation)
• proton magnetometer (or flux gate) can detect 1
  nT in 50knT
• Magnetic anomaly on sea surface is about 100nT

22
paleomagnetism

• atoms in paramagentic minerals contain an odd
  number of electrons.
• these act as tiny magnets (dipoles) because of
  the spin of the electrons.
• Induced magnetism -when placed in a weak magnetic
  field the atomic dipoles align parallel to the
  field.
• ferromagnetic materials have many unpaired
  electrons which couple together to form magnetic
  domains unless temperature is too high - the
  Curie temperature
• below Curie temperature the magnetic field is
  retained as permanent or natural remanent
  magnetism NRM
• Thermoremanent magnetism in Igneous Rocks TRM
• Detrital remanent magnetism in sedimentary rocks
  DRM
• The above are termed primary Secondary remanent
  magnetism occurs after rock is formed

23
Secondary Remanent magnetism

• Isothermal remanent magnetism IRM Strong field
  eg.lightning strike
• Chemical remanent magnetism CRM (formation of
  new minerals)
• Viscous Remanent Magnetism VRM expose a rock to
  a field for a long time
• VRM adds noise to other magnetic signals in
  rocks, but can be removed by exposing the rock to
  an AC field of increasing strength.

24
Paleolatitude from rock cores

• only declination can be derived from a core (no
  longitude information)
• tan (latitude) tan(inclination)/2
• if paleo pole not aligned with current north pole
  - then polar wander.
• true polar wander or apparent polar wander
  (relative to continent)? Does pole or continent
  move
• half the cores indicated a reversed pole position

25
motions on a sphere - Euler poles

• Eulers fixed point theorem The most general
  displacement of a rigid body with a fixed point
  is equivalent to a rotation about an axis through
  that fixed point
• if a tectonic plate is the rigid body, and the
  center of the earth is chosen as the fixed point
  then Every displacement from one position to
  another on the surface of the earth can be
  regarded as a rotation about a suitably chosen
  axis passing through the center of the earth.

26
Euler poles

• a rotation axis intersects the earth in two
  locations and passes through the center of the
  earth.
• the intersection of the rotation axis with the
  earths surface is termed the rotation pole.
• sign convention is that rotation is positive
  looking out from the center of the earth.
  (positive rotation is thus anticlockwise looking
  down from outsde the earth). (the two poles thus
  have opposite rotations)
• Euler poles are determined from the strike of
  transform faults and spreading centers, from
  earthquake slip vectors, and for instantaneous
  rotations from space geodesy.

27
relative velocites at plate boundaries

• velocity at plate boundary increases with
  distance from the rotation pole
• u??a?sin? ?where aradius of the earth, ? is
  the angular distance between the rotation pole
  and the point, and ??is the angular velocity
• Velocity needs ? to be calculated first.
• Azimuth of the velocity at the plate boundary
  follows using sine rule.