PALEOMAGNETIC TIME SCALE
MAGNETOSTRATIGRAPHY: study of changes in the earth's magnetic field
HISTORY: based on technology developed since the 1950's.
The Polarity Time Scale Commission (IUGS) first met in 1972
PRINCIPLES:
The Earth's magnetic field is generated by rotation of electrical field of Earth's outer core
The magnetic field acts as bar magnet oriented 11o to the axis of rotation
The magnetic field is made up of two components:
1. dipole field: more stable "bar magnet" part
MEASUREMENT OF THE MAGNETIC FIELD:
Declination angle between magnetic north and true north
Inclination angle between horizontal and compass dip: 0 at equator 90o at poles
Strength (intensity) varies from ½ at equator to 1 at pole
2. non-dipole field: less stable, geographically variable in time and space
(mantel circulation?)
Strength |
 |
COMPONENTS (declination, inclination, strength) CHANGE THROUGH TIME
1. small continuous change (SECULAR VARIATION) in the apparent position of the geomagnetic pole (VGP)
record varies from place to place
chronologic technique for sediment and archeological features
2. complete reversals, change of long duration (DIPOLE)
paleomagnetic timescale
METHODS
Oriented samples collected in the field
In the laboratory magnetism studied in electronic device "magnetometer"
TYPES OF PALEOMAGNETISM
1. TRM = thermoremnant magnetism, Curie point
volcanic rocks, archeological hearths. during cooling process, magnetic carriers align with prevailing magnetic field
2. DRM = depositional remnant magnetism
sedimentary rocks, sediment cores, signal weaker (mineralogy, lithology) hard to sample without effecting remnant magnetism of sample subject to post-depositional digenetic changes
3. CRM = chemical remnant magnetism
TIME SCALE
- based on complete reversals of earth's dipole field
- most work done on lava flows
- major events now independently dated
- reliable chronology to 13 my, looser back to 80 my
PROBLEMS (particularly for 80 my chronology)
1. count-from-the-top matching, sensitive to gaps
2. provisional: more "events" and "excursions" may be found
TERMINOLOGY
- polarity epoch (Chron) chronologic unit characterized by one geomagnetic polarity lasting 105 - 106 years
- named after pioneers in paleomagnetism
- Bernard Brunhes worked with French lavas
- M. Matuyama worked with Korean and Japanese lavas
- polarity event chronologic unit characterized by one geomagnetic orientation: 104 - 105 years
- named after places first discovered; e.g., Olduvai Gorge in Kenya, Africa
- standard error on K-Ar dates often larger than duration of event
- polarity excursions brief events without complete reversal
- VGP virtual geomagnetic poles move < 80o
- named after places first discovered. e.g., "Gothenburg Flip" 12.4-12.35 Ka - but not replicated (Morner, 1977)
SECULAR VARIATION AND DATING
- continuous regional (non-global) variation in the non-dipole component
- regional master chronologies compiled, used in dating of archeological features and lake cores
- regional comparisons require conversion to common latitude and longitude,
based on wiggle matching of troughs (even #'d) and peaks (odd #'d)
ARCHEOMAGNETISM:
measurement of position of Virtual Geomagnetic Pole
(VGP) recorded in archeological samples
Secular Variation is the movement of the VGP through time
Archeomagnetism important in Southwestern Desert Archeology
- no trees for dendrochronology
- 14C ambiguous over the last 600 years due to production variability through LIA,
the industrial increase of 12C, and atmospheric nuclear bomb testing production of 14C
Paleomagnetic (TRM) acquired through
- high temperatures of pottery kilns
- repeatedly re-used hearths
- burning of large adobe-walled buildings
Study of Archeomagnetism began with 1950's study of Roman and Japanese kilns
[technique]
[technique]
In North America
Robert L. Duboise (1960's) produced curve from A.D. 600-1500, later extended chronology
Eighmy and Sternberg (1990) textbook extended technique
Archeomag Web Page
Paleomag Readings
HOMEWORK
