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Paleomagnetic Records: Discovery and Correlation. Jeannie Bryson. Outline. Discovery of Magnetic Field Origin of Magnetic Field Properties of Magnetic Field Types of Paleomagnetic Records Volcanic Seafloor magnetic anomalies Archaeomagnetic Marine Sedimentary Sequences - PowerPoint PPT Presentation
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Paleomagnetic Records:Discovery and Correlation
Jeannie Bryson
Outline
Discovery of Magnetic Field Origin of Magnetic Field Properties of Magnetic Field Types of Paleomagnetic
Records– Volcanic– Seafloor magnetic anomalies– Archaeomagnetic– Marine Sedimentary
Sequences Use and Implications of
Magnetic RecordsSource: http://geomag.usgs.gov/program.html
Discovery of Magnetic Field
Hans Christian Oersted-1821– Accidentally discovered that passing an electric
current over a compass caused the arrow to move
Andre-Marie Ampere– Further concluded that magnetism was “ a force
between electric currents”; parallel currents attract and opposite currents repel
Origin of Earth’s Magnetic Field
Source: “motion of electrical charges”– Magnetic field produced by electrical currents but
it is not fully understood how “dynamo effect”- One possibility
– electrical currents produced by the coupling of convective effects
– rotation in the spinning liquid metallic outer core of iron and nickel
Source:http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html
Properties of Earth’s Magnetic Field
Magnetosphere:– Earth’s Surface: Neutral– ~100km above surface:
Solar Rays cause charged ions
– Hinders the ability of solar rays to enter atmosphere, i.e. “bow shock”
Source: http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html
Properties of Earth’s Magnetic Field
Field Lines and Van Allen Radiation Belts
Source:http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html
Properties of Earth’s Magnetic Field
Magnetic Field Components– Declination: angle between
horizontal component of magnetic-field vector in relation to true north
– Inclination: angle between horizontal plane and total field vector
Properties of Earth’s Magnetic Field
Dipole Component– Magnetic field lines come out of south pole and
converge at north pole Non-Dipole Component
– Poles are not antipodal-asymmetrical– Axis is tilted ~ 11 degrees relative to the rotational
axis
Properties of Earth’s Magnetic Field
Geodynamo: a “dynamo” in core generates electrical currents– “self-sustaining” : electrically-conducting fluid flow
across magnetic-field lines and generate electrical current to support existing field
– Earth’s Magnetic Field ~ 3.5 billion years old Rocks dated by paleomagnetic methods Proves that Earth’s magnetic field must be regenerative
Properties of Earth’s Magnetic Field
Geodynamo: alpha-omega dynamo cycle
Source:http://geomag.usgs.gov/intro.html
Types of Paleomagnetic Records
Magnetization– Thermoremnant Magnetization:
Lava and clays contain small amounts of iron and when heated obtain magnetization that is parallel to Earth’s magnetic field after cooling (unless reheated)
– Depositional Remnant Magnetization: Lake and ocean sediments are deposited and settle,
iron particles record Earth’s magnetic field (unless disturbed)
Source: http://www.archserve.id.ucsb.edu/anth3/Courseware/Chronology/11_Paleomag_Archaeomag.html
Types of Paleomagnetic Records
Volcanic Records– Thermoremnant magnetization– 50 kyr record– Lava cools and iron in lava obtains magnetization
of the time and place of cooling– Magnetization can then be measured and dated– Lava records are not continuous
Types of Paleomagnetic Records
Archaeomagnetic Records– Remnant magnetism: heating above Curie point
sets magnetization– Virtual Geomagnetic Pole (VGP): measures
location of magnetic north at a certain time Uses magnetization of object and another dating tool;
i.e. 14C or dendrochronology VGP’s can be used to create a curve representing
paleomagnetic variations
Types of Paleomagnetic Records
Marine Sediment Records– Yohan Guyodo-1996,1999: 200 kyr and 800 kyr – 800 kyr: 33 marine records stacked to create a
composite curve Sediments provide continuous record with few
disturbances Shows non-dipole moments Does not support periodicity thought to be caused by
Earth’s orbital influence Data correlated with O18 reference curves
Sint-800: 800 kyr marine sediment record
Types of Paleomagnetic Records
Sea-Floor anomalies– 780 kyr record– East Pacific Rise– Supported by seafloor glass paleomagnetic
record for past 50 kyr– Similar to marine sedimentary record– Can be used to understand both ridge crest
accretion and geomagnetic field behavior
Concluding Thoughts
Variations in geomagnetic intensity do not appear to show any signs of periodicity– Due to non-dipole component
Mean fluctuations in amplitude appear to be constant