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Geomagnetism (I). The Earth’s magnetic field. Magnetic field of the Earth measured at the surface comes from three sources:
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The Earth’s magnetic field • Magnetic field of the Earth measured at the surface comes from three sources: • 97-99% represents main field generated by dynamo action in the outer core. Main field varies significantly with time (secular variation means variations along geological time) • 1-2% represents external field generated in space in the magnetosphere.External field also varies on time scales of seconds to days. • 1-2% represents crustal field from remnant magnetizationabove the Curie depth. Ali K. Abdel Fattah
The Earth’s magnetic field • The Earth’s magnetic field would be: • vertical at the poles • horizontal at the equator • Today, the best-fit dipole is currently oriented 11.5° from the rotation axis of the geographic north pole, but this has varied with time. Ali K. Abdel Fattah
Describing the Earth’s magnetic field • Declination (D) • Inclination (I) • Horizontal Intensity (H) • Vertical Intensity (Z) • North-South Intensity (X) • East-West Intensity (Y) • Total Intensity (B) X Y Ali K. Abdel Fattah
Describing the Earth’s magnetic field • This first order simple model of the field allows to use the paleomagnetic observations to determine past plate motions • Magnetic potential is given by The Earth’s best fit dipole moment (m) equals to 7.94x1022 Am2 in magnitude • Magnetic field is determined by the differentiating the magnetic potential given the magnetic permeability of free space, μ0 = 4x10-7 kg m A-2 s-2 Ali K. Abdel Fattah
Spherical polar coordinates • Conversion from/spherical into Cartesian coordinates: • Gradient operator Ali K. Abdel Fattah
Describing the Earth’s field • If the Earth’s magnetic dipole moment is aligned along the z-axis: • At a latitude of θ and longitude , Magnetic field in spherical polar coordinates can show three components: • Radial Component Br, • Southerly Component B, and • Easterly Components B Ali K. Abdel Fattah
Describing the Earth’s field • For the best fit dipole, Three components are given by • Total field is given by Ali K. Abdel Fattah
Describing the Earth’s field • Then, the magmatic inclination (I) can be computed from the following equation • At the North Pole, θ = 90° which gives I = 90° • At the Equator, θ = 0° which gives I = 0° Ali K. Abdel Fattah
Describing the Earth’s field Ali K. Abdel Fattah
Describing the Earth’s field Ali K. Abdel Fattah
Describing the Earth’s field • The equation of the magnetic inclination is important because it allows use to use a measurement of inclination (I) to determine latitude (θ). This was once used by mariners, but is most important in paleomagnetism. • A rock can record the magnetic field present when it crystallized (temperature fell below the Curie temperature). • Thus we can find the latitude of a continent at some time in the past. • This was the idea of Apparent Polar Wandering.
Diamagnetism and paramagnetism • The magnetic behaviour of minerals is due to atoms behaving as small magnetic dipoles. • If a uniform magnetic field (H) is applied to a mineral, there are two possible responses. • Diamagnetic behaviour • Paramagnetic behaviour
Diamagnetic behaviour • This effect arises from the orbital motion of electrons in atoms. • The atom develops a magnetic field that is opposite direction to the applied magnetic field • Magnetic susceptibility is negative • All minerals diamagnetic but will be masked by paramagnetism
Paramagnetic behaviour • This phenomena arises when the atoms have a net magnetic dipole moment due to unpaired electrons. • The atoms align parallel to the applied magnetic field H and increase the local magnetic field. • For paramagnetic materials Magnetic susceptibility is positive. • Paramagnetic elements include iron, nickel and cobalt.
Geomagnetism (II) Rock magnetization and translation
Magnetizing Igneous Rocks • Curie Temperature • Temperature above which a mineral cannot be permanently magnetized • spontaneous magnetization when temperature drops below Curie temperature • Curie Depth • Is the depth at which magnetic behaviour ceases since temperature exceeds curie temprature. Thermal vibrations of atoms prevents domain formation. • Blocking Temperature • Tens degrees less than the Curie point for most minerals • Temperature below which the orientation of the rock’s magnetization cannot change • magnetization cannot change once below blocking temperature • Both temperatures are much lower than that at which lavas crystallize. • The magnetization becomes permanent some time after lavas solidify. • This type of permanent residual magnetization is called thermoremanent magnetization (TRM); atoms align when molten and freeze • The magnetism of TRM is larger in magnitude than that induced in the basalt by the earth’s present field.
Magnetizing sedimentary rocks • Sedimentary rocks can acquire magnetization in through: • Depositional or detrital remanent magnetization (DRM); acquiring during the deposition of sedimentary rocks. • Chemical remanent magnetization (CRM); acquiring after deposition during the chemical growth of iron oxide grains as the case in sandstones. • Strength of DRM and CRM fields typically 1-2 orders of magnitude smaller than TRM
Detrital remnant magnetization • Detrital magnetization can produce a weak remnant magnetization in sedimentary grains • Grains being deposited contain some magnetite or other magnetic minerals • Preferred orientation as they are deposited
Chemical remnant magnetization • Can occur during alteration • Example from oil field in Gibson and Millegan (1988)
Example • A rock sample was found at latitude of 34°N. Remnant magnetization in the sample was found to have an inclination I = 40 ° from the horizontal. Was the rock magnetized at the location where it was sampled?
Magnetic stripes (Dating the oceans) • Using magnetometer with overseas vessel • Measure the total field intensity • Subtract regional value • Produce magnetic anomaly map
Magnetic stripes (Dating the oceans) • Raff and Mason, 1961 • First magnetic field map • Off the western coast of North America • Magnetic anomaly map • Take total magnetic field intensity and subtract regional average • Black Stripes: positive intensity • White Stripes: negative intensity • Which is normal/reversed polarity? • Coupled stripes with sea-floor spreading and magnetic pole reversals
Origin of Seafloor magnetic anomalies formed at mid-ocean ridges • Important evidence to support the hypothesis of continental drift came from observations of magnetic fields measured by survey ships on profiles that crossed the world’s oceans. • Basalt erupted and when cools it is permanently magnetized in direction of Earth’s magnetic field at that time. • Sea floor spreading moves rocks away from ridge. • Magnetic field reverses direction
Magnetic stripes anomalies • Magnetic stripes anomalies are considered for two cases of magnetic stripes anomalies: • High magnetic latitude • Low magnetic latitude • Magnetic stripes anomalies of high magnetic latitude are characterized by: • Earth’s magnetic field is close to vertical. • Remnant magnetization at the ridge is in the same direction as the Earth’s field. • Positive magnetic anomaly at the ridge crest Magnetic stripes at High magnetic latitudes
