Download Paleomagnetism

Paleomagnetism
• The magnetic axis wobbles
around the Earth’s rotation
axis
• 2500-8000 years to complete
a loop (at current rates)
• When averaged over 10,000
years or more, the direction
of the magnetic field
coincides with the true north
= paleomagnetic pole
Paleomagnetism
• Suppose we are able to measure
paleomagnetic direction in a
succession of lava flows (e.g.
Deccan Traps, India)
• Each lava flow cooled down in a
few years and recorded the
magnetic field of the time
• Each lava flow can be dated
• One can reconstruct the history of
paleomagnetic directions
• Using tan I = 2 tan λ, one can
reconstruct the history of paleolatitudes
Paleomagnetism
• In the field:
– Oriented sample (short core or block
of rock): azimuth and dip (using a sun
compass…)
– Geometry of the layer (for fold tests)
– Several sample at each site (5-10)
within a few meters
• In the lab:
– Spin sample => produce current in a
coil
– Reproduce in 3 directions => get 3D
magnetic vector
Paleomagnetism
• Declination => direction of
north pole
• Inclination => distance to the
north pole
• Example of a rock now at 10oN:
– Inclination = 49o =>
paleolatitude = 30o (= pole was 60o
north of the rock)
– Declination = 20o
• Apparent pole (= relative to the
rock) can be reconstructed =
virtual geomagnetic pole
(VGP)
Paleomagnetism
•
Inclination:
– How far away the pole was located
= angular distance p
– p = radius of a small circle centered
on the sampling site (latitude λs,
longitude φs)
– Small circle = locus of all possible
VGPs
•
Declination
– Defines a meridian (= great circle)
passing through the sampling site
– Angle D with the NS meridian
– latitude λp, longitude φp, of the pole
can be calculated from
trigonometric formulas
sin !p = sin !s cos p + cos !s sin p cos D
"p = "s + #,
for
"p ="s +180 $#,
with
sin ! =
cos p sin !s sin !p
for
sin p sin D
cos " p
cos p < sin !s sin !p
Paleomagnetism
• Paleomagnetic poles for
Plio-Pleistocene and
Permian rocks:
– Clustered => axial
geocentric dipole hypothesis
ok
– Permian pole 45o from PP
pole => something has
moved…
• Geographic pole?
• Continent?
Paleomagnetism
• Trace of the paleo-apparent poles
through time for a given location
= apparent polar wander
(APW) path
• It is NOT the motion of the
Earth’s pole, but the position of
the continent relative to the pole
• As if the paleomagnetic pole had
moved slowly along this path
toward the present pole
• APW path can be determined for
each continent
The blue line shows the apparent polar wander path
for Australia from the mid- to the late Paleozoic
period.
Paleomagnetism
•
•
•
APW paths for Europe and North America, Ordovician to Jurassic
As measured: different APW paths, but similar shapes => continents
move, paleomagnetic poles don’t
After closing the Atlantic (rigid rotation on a sphere by 38o clockwise
about a pole at 88.5oN/27.7oE):
– Coincide until upper Triassic
– Then: relative motion of the 2 continents
Paleomagnetism
• What can we learn from
apparent paleomagnetic
poles?
– If different from present => the
rock has moved
– Declination not due to north =>
the rock has rotated
– Inclination different from
present => the rock has moved
north or south, or has been tilted
• What is not possible to learn?
Mean paleolatitude for the Caribbean
plate (from ODP cores)
– Changes in longitude
Paleomagnetism
• Effects of large scale
tectonics
• Late Paleozoic to Late
Cretaceous declinations:
– Large difference between
Europe and Africa =>
relative rotation
– Similar declinations for
central and southern Europe
– Slight difference in CorsicaSardinia…
Paleomagnetism
•
•
19th century: E. Seuss => great Late
Paleozoic super-continent,
Gondwanaland
1912: Wegener
– All the present continents laid close
together during the Late Paleozoic =
Pangea
– Paleoclimatic, paleoenvironments,
paleontology, tectonic evidence
– Continental drift: “dispersion” of
Pangea
– No mechanism, no direct evidence
•
1950’s: development of
paleomagnetism
– Irving: Paleomagnetic poles of southern
continents:
• Incompatible with their present-day
arrangement
• Agree better when the continents are
rearranged in a Gondwanaland cluster
•
Using APW path for each continent,
one can reconstruct their relative
motions
http://jan.ucc.nau.edu/~rcb7/paleogeographic.html
http://jan.ucc.nau.edu/~rcb7/paleogeographic.html
Paleomagnetism
• David and Bruhnes, 1904-1906:
– Clays baked in lavas, Massif
Central, France
– Same direction of TRM as lava
– Changes in magnetization
direction => polarity change
• Matyuama, 1929:
–
–
–
–
Quaternary lava = normal polarity
Pleistocene lava = opposite polarity
Miocene = 3 different polarities
Link between polarity and age
Record of a reverse-to-normal Miocene
Polarity transition, Oregon
Paleomagnetism
•
Secular variations of the Earth’s
magnetic field
– Slow and irregular variations
– When average over several 1000 years =>
coincides with axis of rotation
•
Polarity reversals
– Inversion of the Earth’s magnetic field
– Occurred at regualr intervals
– Inversion occurs in a few 1000 years
•
Excursions
– Magnetic pole moves far from the
geographic pole
– Returns within a few 1000 years
•
•
Definitions: chrons, sub-chrons, excursions
Magnetostratygraphy
Paleomagnetism
•
•
1950’s: improvement of radiometric dating
techniques => correlation between polarity and age is
global
Magnetic time scale:
– Established progressively as number and distribution of
samples improve
– Epochs named after investigators or locations
– Last 5 million years particularly well documented
Paleomagnetism
•
Since the 1960’s, additional data
from deep-sea sediments:
–
–
–
–
–
Quiet sedimentary environment
Rather uniform sedimentation rates
Continuous sedimentary series
Dating: microfossils
Reversal occur at different depths
because of different sedimentation
rates
– Perfect correlation with on-land
studies in lavas
•
•
Since magnetizatin process is
different (TRM vs. DRM) => selfreversal unlikely
Same pattern of reversals found
everywhere => global phenomenon
Geomagnetic time scale
• Based on compilation of magnetic
anomalies in all major oceans
– Cover the last 155-160 Ma
– Older oceanic crust ~180 Ma, but no
clear anomaly
• 1 sequence of polarity reversals:
– Current chron = Bruhnes (=C1N)
– Chrons C1N to C33R
• 1 long interval of constant
polarity = Cretaceous Quiet
Interval, ~35 Ma, C34N
• 1 sequence of polarity reversals:
M0 to M29N
Ocean magnetic anomalies
• Seismic data
– Stratified oceanic crust
– Layer 2A:
• Oceanic basalts (flows and/or
dykes)
• Oceanic basalts:
– Titano-magnetite =>
strongly magnetic
– When cools below Tc (200300oC) => acquires TRM
direction of Earth’s current
magnetic field
Ocean magnetic anomalies
• Observations in the oceans:
– Ocean crust is younger at ridges,
age increases towards continental
margins
– Vine and Matthew, 1963:
• Linear magnetic anomalies
• Parallel to ridges
• Symmetrical pattern w.r.t. the
ridge
Sea-floor spreading
•
Mechanism = sea-floor spreading:
– Continuous input of magma at the
ridge
– Basalt acquires current magnetic
field direction as it cools
– Divergence between plates =>
more magma (records reverse
polarity)
– Etc…
Age of the ocean floor
http://www.es.usyd.edu.au/geology/people/staff/dietmar/Agegrid/digit_isochrons.html
Sea-floor spreading and
continental drift
Continental drift
http://www.scotese.com/Default.htm
Continental drift: another look…
“Continental Drift”, sculpture, 175 cm,
acid-oxidized steel, 2001.
(www.sculptures.freeserve.co.uk/)
Untold tragedies of Continental Drift.
What have we learned?
• Remanent magnetization can be measured in the laboratory on
rock samples => 3D paleomagnetic field direction and strength
• From the paleomagnetic data:
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Virtual Paleomagnetic Pole
Apparent polar wander
Demonstration of continental drift
Paleomagnetic reversals:
• Magnetostratigraphy: global magnetic time scale
• Sea-floor spreading