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2. Geomagnetism and Paleomagnetism 2. GEOMAGNETISM AND PALEOMAGNETISM https://physicalgeology.pressbooks.com/chapter/4-3-geological-renaissance-of-the-mid-20th-century/ Click for audio Topic 2a 1 Topic 2a 2 q Q ELECTRIC FIELD q -Q Topic 2a 3 MAGNETIC DIPOLE Although magnetic fields have a similar form to electric fields, they differ because there are no single magnetic "charges," known as magnetic poles. Hence the fundamental entity is the magnetic dipole arising from an electric current I circulating in a conducting loop, such as a wire, with area A . The field is described as resulting from a magnetic dipole characterized by a dipole moment m Magnetic dipoles can arise from electric currents - which are moving electric charges - on scales ranging from wire loops to the hot fluid moving in the core that generates the earth’s magnetic field. They also arise at the atomic level, where they are intrinsic properties of charged particles like protons and electrons. As a result, rocks can be magnetized, much like familiar bar magnets. Although the magnetism of a bar magnet arises from the electrons within it, it can be viewed as a magnetic dipole, with north and south magnetic poles at opposite ends. Topic 2a 4 Magnetic field B Units of B Tesla (T) = kg/s 2 -A A = Ampere (unit of current) Gauss (G) = 10-4 Tesla Gamma (! )= 10-9 Tesla = 1 nanoTesla (nT) Earth’s field is about 50 "T = 50 x 10-6 Tesla Topic 2a 5 We visualize the magnetic field of a dipole in terms of magnetic field lines pointing outward from the north pole of a bar magnet and in toward the south. The lines point in the direction another bar magnet, such as a compass needle, would point. At any point, the north pole of the compass needle would point along the DIPOLE field line, toward the south pole MAGNETIC of the bar magnet. The earth’s magnetic field, as discussed FIELD shortly, looks essentially like a dipole field. A tricky point is that the earth’s field is that of a dipole pointing south along the earth’s axis, so the magnetic pole near the geographic North pole is a south magnetic pole. We call this the North magnetic pole. because the north pole of a compass needle, which is a north magnetic pole, points toward it Topic 2a 6 DYNAMO GENERATES MAGNETIC FIELD Current produced by Lorentz force F on particles with charge q moving with velocity v in magnetic field B F = qv x B https://12ibenergy.weebly.com/ Davidson et al Topic 2a 7 The solid inner core is roughly GEODYNAMO the size of the moon but at the temperature of the surface of the sun. Convection in the fluid outer core is thought to be driven by both thermal and compositional buoyancy sources at the inner core boundary that are produced as the Earth slowly cools Iron in the iron-rich fluid alloy solidifies onto the inner core giving off latent heat and the light constituent of the alloy. These buoyancy forces cause fluid to rise and Coriolis forces, due to the Earth's rotation, cause the fluid flows http://www.abc.net.au/science/basics/img/earth_spins.jpg to be helical. Topic 2a 8 3D magnetic field structure simulated with the Glatzmaier-Roberts geodynamo model. Magnetic field lines are blue where the field is directed inward and yellow where directed outward. The rotation axis of the model Earth is vertical and through the center. A transition occurs at the core-mantle boundary from the intense, complicated field structure in the fluid core, where the field is generated, to the smooth, potential field structure outside the core. The field lines are drawn out to two Earth radii. https://websites.pmc.ucsc.edu/~glatz /geodynamo.html Topic 2a 9 About 36,000 years into the simulation the magnetic field underwent a reversal of its dipole moment, over a period of a little more than a thousand years. The intensity of the dipole moment decreased by about a factor of ten during the reversal and recovered immediately after, similar to what is seen in the Earth's paleomagnetic reversal record. The solution shows how convection in the fluid outer core is continually trying to reverse the field but that the solid inner core inhibits magnetic reversals because the field in the inner core can only change on the much longer time scale of diffusion. Only once in many attempts is a reversal successful, which is probably why the times between reversals of the https://websites.pmc.ucsc.edu/~glatz Earth's field are long and /geodynamo.html Topic 2a randomly distributed. 10 EARTH’S FIELD Topic 2a 11 EARTH’S FIELD GEOCENTRIC AXIAL DIPOLE (GAD) Topic 2a 12 INCLINATION AND LATITUDE Topic 2a 13 CLASS QUESTION #7 a) Derive the dipole equation from the expressions for Fv and Fh b) Use the dipole equation to find the values of the inclination at latitudes 45North and 45South c) Does your answer to b) make sense given the geometry of the dipole field? Topic 2a 14 The real field isn’t a pure dipole Topic 2a 15 ACTUAL FIELD GEOMETRY Although the inclined dipole is a better description of the field, neither it nor any other dipole fully describes the field. The remaining part of the field, the non-dipole field, is about 10% of the total field. Topic 2a 16 The magnetic field changes with time, presumably because of changing fluid motions in the core. Changes on time scales less than 100,000 years are called secular variation. Topic 2a 17 Rocks record earth’s magnetic field Topic 2a 18 Thermal Rocks record Remnant earth’s field Magnetization when they were (TRM) magnetized Thermal Remnent Magnetism (TRM), results when molten volcanic rock cools. Basalt and other volcanic rocks contain iron and titanium bearing minerals like magnetite (Fe3O4), hematite (Fe2O3), and ilmenite (FeTiO3). These are ferromagnetic materials, whose atomic structure causes regions of parallel dipoles called magnetic domains. Normally, the domains are randomly oriented, so the material has no net magnetization. However, applying an external magnetic field aligns the domains and reorganizes them, giving a net magnetization. This ordering persists after the external field is removed. However, the ordering is destroyed if the material is heated above its Curie temperature, because thermal oscillations overcome the domain alignments. The process occurs in reverse as molten igneous rocks cool, first becoming solid at about 800 – 1100 C, and eventually cooling through the Curie temperature of the magnetic minerals, about 600 C. Below the Curie temperature minerals retain a magnetization that records the earth's field when and where they cooled, even when the earth's field changes and the rock is transported on a moving plate. Topic 2a 19 Detrital Remnant Magnetization (DRM) This occurs when sediment is deposited in water still enough for magnetized grains to be aligned by the earth's field. The earth's field exerts a torque that rotates grains' magnetic moments toward the direction of the field. How this actually occurs during the complex process of sediment deposition - that involves a variety of grains with different sizes, only some of which are magnetic, subject to a number of processes including biological perturbations - remains an area of research. Paleomagnetic data from deep sea sediments provides important data about the evolution of the ocean basins. DRM is typically 1-2 orders of magnitude weaker than TRM. Topic 2a 20 Chemical Remnant Magnetization CRM Topic 2a 21 CLASS QUESTION #8 The stripe on the back of a credit card is a magnetic stripe called a magstripe. The magstripe is made up of tiny iron-based magnetic particles in a plastic-like film. Each particle is a tiny bar magnet about 20 millionths of an inch long. The data are stored by modifying the magnetic particles. Using what we’ve learned about rock magnetism, explain how exposing a credit card to a magnet can destroy the information stored on the magnetic strip. Wikipedia Topic 2a 22 Topic 2b 23 Magnetic timescale Showing anomaly numbers, also called chrons Uyeda Topic 2b 24 Seafloor tape recorder Cooling dikes at midocean ridges acquire earth's magnetic field Seafloor’s magnetic anomalies record spreading history Find ages and spreading rate from known magnetic reversal history Davidson 7.7 Topic 2b 25 SPREADING RATES FROM MARINE MAGNETIC ANOMALIES Identify anomalies, and then use magnetic timescale to convert distances from ridge to rate Topic 2b 26 CHRONS Spreading rate dependence Brunhes Matuyama Gauss Gilbert DeMets et al, 1994 Topic 2b 27 Class question #9 Magnetic anomaly Topography Topic 2b 28 Seafloor magnetic anomalies give ages Davidson 7.8 Topic 2b 29 Collecting paleomagnetic data Topic 2b 30 SPINNER MAGNETOMETER Topic 2b 31 SUPERCONDUCTING MAGNETOMETER Topic 2b 32 Paleomagnetic pole, or "apparent pole" Imagine a rock formed at the equator, which thus has a horizontal magnetic field. If it is moved to another place - for example 45N - its magnetization will not be parallel to the field there. The rock's magnetization shows that it is now 45 away from where it was magnetized. The paleomagnetic pole, or "apparent pole", inferred from the rock is 45 degrees from the current North pole. This apparent polar wander (APW) results primarily from the rock, not the pole, having moved. Apparent pole positions from rocks formed at different times, but on the same plate, can be used to construct an APW path through time. Topic 2b 33 Paleomgnetic pole from paleomagnetic data Topic 2b 34 Topic 2b 35 APW paths show opening of the Atlantic Topic 2b 36 APW paths show northward motion of India Topic 2b 37 Topic 2b Tettegouche State Park, MN 38 Midcontinent Rift (MCR) Prominent on gravity & magnetic anomaly maps Long arms of buried dense & highly magnetized 1.1 Ga igneous rocks ~ 3000 km long ~ 2 x 106 km3 magma Outcrops near Lake Superior Topic 2b 39 MCR likely formed as part of the rifting of Amazonia from Laurentia, recorded by APWP cusp & became inactive once seafloor spreading was established APWP for Laurentia poles Stein et al., 2014 Topic 2b 40 Laurentia’s apparent polar wander path (APWP) has abrupt cusp at ~1.15 Ga before maJor MCR igneous activity starts Cusps indicate change in direction associated with rifting Stein et al., 2014 Topic 2b Schettino and Scotese, 2005 41.
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