Earths magnetic field

1
Earths magnetic field
Earths magnetic field is a dipole field, much
like one generated by a bar magnet
Field lines show the direction of the magnetic
field
2
Earths magnetic field generated in the outer
core
The most spectacularsimulations haverecently
been madeon a super-computer
A dipole field (bar magnetic field) is
generated when electricity runs through a coil.
Flow liquid iron in the corecan be seen as
circuits ofelectric currents that
generateEarths magnetic field We often call
this process The Geodynamo
3
Earths magnetic field inclination
Equator Compass needle hangs horizontally Ann
Arbor Compass needle hangs with an angle North
Pole Compass needle hangs vertically
Magnetic inclination depends on geographic
latitude
4
Earths magnetic field declination
Declination is the angle of themagnetic field
from North
Compass needle points tothe (magnetic) North Pole
5
Earths magnetic field changing rapidly
6
How to make rocks magnetic
In molten rock, these magnetsare randomly
oriented
Fe3O4 magnetite make little magnets within rock
minerals
When rock solidifies,these magnets align with
the magnetic field
7
Rocks carry the orientation of past magnetic
fields

• Paleo magnetists
• measure the magnetic properties of rock.
• This tells them
• The orientation of Earthsmagnetic field in the
  past
• The orientation of the rockin relation to the
  past magnetic field

8
Changes in magnetic field inclination
Steep incidence angle ? near-polar location
Modest incidence angle ? mid-latitude location
Shallow incidence angle ? equatorial location
Changes in the field inclination suggest that the
rock has moved through Earths magnetic field
9
Reversals of Earths magnetic field
10
Earths paleomagnetic time scale (bar code)
Frequent magnetic field reversals (every
300,000-500,000 years or so ...)
65 million years ago (K/T boundary)
Cretaceous quiet zone (no reversals of magnetic
field for nearly 40 million years)
160 million years ago
11
Paleomagnetism and sea floor spreading
Study of Earths magnetic field in the past is
key to understanding sea-floor spreading