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Electromagnetism and the Earth’s Magnetic Field: • What are its characteristics? • Shape, properties • Does it change? • How is it generated? • What does that tell us about the Earth? • Applications? Effects? Required Readings: 1st edition: Chapter 4 [also: Ch.2 37- 40] 9th US edition: Chapter 17 [also: Ch.19 471-473] Electromagnetic Field Basics • An EM field describes the behaviour of forces that are applied to a moving, charged particle • Electromagnetic fields are generated at the atomic level • all atoms generate magnetic fields (moving electrons) • Large-scale magnetic fields are produced by the alignment of smaller-scale fields Geomagnetism - Characteristics of the Earth’s Field • the Earth’s magnetic field is complex • dominant component (90 %) is a dipole field Static components: a) Intensity b) Declination (azimuth) c) Inclination (dip) • shape and strength of the field provide clues as to the field’s origin Geomagnetism - Characteristics of the Earth’s Field • the Earth’s magnetic field is complex • dominant component (90 %) is a dipole field • the field changes constantly! Time scale of variations a) Short b) Historical c) Geological • variations provide clues as to the field’s origin Geomagnetic Field Characteristics 1) Static: • 90% explained by a dipole slightly tilted from the rotational axis 2) Dynamic: • Secular variation: • dipole field axis moving • dipole axis averages out to the rotational axis • intensity always varying • Reversals: • two stable orientations of the Earth’s magnetic field Clues to what is generating the field Generation of the Earth’s Magnetic Field - Dynamo TheoryWhat is generating the geomagnetic field? Paleomagnetic studies provide a number of clues: • the magnetic field cannot be generated by magnetized minerals 1) Interior is too hot (above Curie temperature) 2) Outer, cool, crustal layer doesn’t produce a strong enough field 3) A static source cannot account for field reversals or secular variation. Must be dynamic! What is generating the geomagnetic field? Paleomag data indicates the field source must be dynamic Requirements: The field source must: 1) create a nearly dipole field by flowing current basis of electromagnetism 2) be a dynamic source - must change with time reversals (2 “stable” states) wobble (secular variation) 3) be self-sustaining over geological time Earth’s oldest rocks indicate a magnetic field was present Source? Likely a “dynamo” operating in the fluid outer core Concept: the electromagnetic dynamo • Complete the circuit so that current can flow mechanical energy electrical energy (a “dynamo”) • Current flowing creates a magnetic field • With the correct path, flowing current can reinforce • or strengthen the initial field • A self-exciting dynamo! Generation of the Geomagnetic Field - Dynamo? 1) Need a conductor • Core composition: mostly iron • iron is a conductor in a rotating Earth electrical current produces a magnetic field • not enough…. Need additional motion • Heat convection in outer core o • outer core: ~6000 C and 106 atmospheres pressure • fluid (as fluid as water!) Convection in fluid outer core: current magnetic field Reversing Geomagnetic Field • fluid iron in outer core (free electrons provide moving charge) • motion: a) earth rotation b) convection within outer core • well organized patterns: high intensity dipole field • poorly organized patterns: low intensity dipole field • chaotic patterns: non-dipole field a chance for a reversal! Generation of the Geomagnetic Field - Dynamo? Convection in the core: • What is the source of the heat?: - conduction of heat: - heated from below (conduction from the inner core) - cooled from above (conduction to base of the mantle) and the mantle convects heat away to the surface - solidification of the core releases heat - inner core is growing… - some radioactive decay Dynamo modelling • understanding improving, but computers not yet powerful enough 3D field snapshot: - primarily dipole Blue: outer core Red: inner core Yellow: region in outer core with fastest fluid flow Modelled reversal: -500 years, 0, +500 years (Glatzmaier & Roberts, 1995, 1996) Review: Field Characteristics and the Dynamo 1) Stable state: • geocentric dipole 2) Dynamic properties: Secular variation: • dipole field axis wobbles around the rotational axis (but averages out to be aligned with it) • field intensity varies Reversals: • dipole field can reverse or “flip” (2 stable orientations) Geomagnetic Field Source - “Dynamo” • fluid iron in outer core (free electrons provide moving charge) • motion: a) earth rotation b) convection within outer core Review: The first half of The Core is better than the last. But that’s like saying the end of a spear someone shoved into your head didn’t hurt as much as the tip. - John Larsen What basic concepts did the screenwriters have wrong? What basic concepts did the screenwriters have wrong? Scenario 1) Experiment stops core from rotating 2) Need to start core rotating again or: • No protection from cosmic rays (extinction to all life) • Massive seismic activity Problems 1) Convection (core rotates with Earth) 2) No mass extinctions correlate with field reversals (atmosphere protects) • May have genetic mutations…? • Massive seismic activity Paleomagnetism – The Study of “Fossil” Magnetic Fields • can measure the remanent magnetic field “trapped” in rocks • field orientation • field direction • field strength (or intensity) • can measure the age of a rock • radioisotope dating or other methods • can develop a “magnetic time scale” • description of how the magnetic field has changed through time How can we use the geomagnetic field to learn about geologic history or human history? Key discoveries using Paleomagnetism 1) Magnetic reversals and seafloor spreading • Observed symmetric and parallel magnetic reversals across oceanic ridgecrests • the key evidence “proving” plate tectonics (Vine & Matthews, Morley, 1963) 2) Apparent Polar Wander • the plates are moving, not the poles Magnetic time scale • provides another method of dating ancient events • yields spreading rates in the past Paleomagnetism Measuring the Earth’s field in the past 1) Continents: Lava flows (TRM) • episodic lava flows form a layered sequence 0 Time (My) 0.5 1 1.5 2 Paleomagnetism Measuring the Earth’s field in the past 1) Continents: Lava flows (TRM) • episodic lava flows form a layered sequence 2) Oceanic crust (TRM) • near-continous creation of new oceanic crust at mid-ocean ridges 3) Marine sediments (DRM) • near-continous sediment deposition • high sedimentation rates = detailed magnetic record Paleomagnetic age dates match radioisotope age dates Secular Variation vs. Apparent Polar Wander Secular Variation: • real, historically measurable motion of the magnetic poles • the “wobble” of the dipole field about the rotational axis • averages out to the rotational axis • caused by changes in the convection patterns in the outer core Apparent Polar Wander: • paleomagnetic data could be interpreted to indicate that the magnetic field is not an axial dipole but... • it is actually the plates that are moving! Discoveries Related to Paleomagnetism A. Plate tectonics 2) Apparent Polar Wander • Paleopole: Crustal remanent magnetism indicates location of magnetic pole at the time the rocks cooled • as age increases, paleopoles = rotational pole • and even more bizarre, different plates have different paleopoles! Explanations? 1) in the past, the magnetic field was not an axial dipole??? 2) each plate has its own independent field??? or 1) we have an axial dipole 2) the plates move independently Apparent Polar Wander • Recall: rocks trap the magnetic field at the time they cooled past their Curie temperature • If the rocks (or the plates they comprise) move, then it appears that the pole has moved! Apparent Polar Wander Path Apparent Polar Wander • the plates wander, not the magnetic or rotational poles N.A. Eurasian Apparent polar wander curves for the North American and Eurasian plates a) different plates, different polar wander curves b) if you reconstruct the plate positions • apparent polar wander curves match! Secular Variation vs. Apparent Polar Wander Secular Variation: • real, historically measurable motion of the magnetic poles • the “wobble” of the dipole field about the rotational axis • averages out to the rotational axis • caused by changes in the convection patterns in the outer core Apparent Polar Wander: • paleomagnetic data could be interpreted to indicate that the magnetic field is not an axial dipole but... • it is actually the plates that are moving! Summary: Key discoveries using Paleomagnetism 1) Magnetic reversals and seafloor spreading • Observed symmetric and parallel magnetic reversals across oceanic ridgecrests • the key evidence “proving” plate tectonics (Vine & Matthews, Morley, 1963) 2) Apparent Polar Wander • the plates are moving, not the poles Magnetic time scale • provides another method of dating ancient events • yields spreading rates in the past Effects of the geomagnetic field? • Extinctions? • No, but possible increased genetic mutation rate… • Navigation • human and animal • Interactions with charged particles from the sun generate strong currents in the atmosphere • aurora borealis and aurora australis • major failures of power grids, satellites