Presentation on theme: "5. Evidence for Plate Tectonics from Magnetics William Wilcock"— Presentation transcript:
1 5. Evidence for Plate Tectonics from Magnetics William Wilcock OCEAN/ESS 4105. Evidence for Plate Tectonics from Magnetics William Wilcock
2 Lecture/Lab Learning Goals Understand the basic characteristics of the Earth’s magnetic field and how one measures its orientationKnow the different kinds of rock magnetization and their use in paleomagnetismBe able to explain the historical concept of polar wander and its explanation in terms of continental driftBe able to explain patterns of marine magnetic anomalies in terms of plate spreading and magnetic field reversalsKnow how to interpret marine magnetic anomalies - LAB
3 Earth’s Magnetic Field Magnetic northEarth’s Magnetic Fieldnorth poleGeographic north poleThe Earth is surrounded by a magnetic field that is strongest near the poles. The magnetic poles are displaced ~ 11.5° from the geographic poles about which the Earth rotates.Discuss Faraday disk generatorGeodynamo Theory: The magnetic field is generated in the liquid metal region of the outer core. The outer core is extremely hot and flows at a rate of several km/yr in large convection currents. Convecting metal (Fe) creates electrical currents, which in turn create the magnetic field.Magnetic southsouth poleGeographic south Polesouth poleAfter Plummer
4 Earth’s Magnetic Field The Earth’s magnetic field close to a dipole. The radial (vertical) and tangential (north-south) components a dipole field are given byθ - Colatitude (0º at south pole; 90º at equator; 180º at north poleμ0 - magnetic permeability of a vacuum 4π x 10-7 N A-2r - distance to the center of the earth (6.4 x 106 m at the Earth’s surface)M - is the dipole moment which for the earth is 7.95 x 1022 A m2B - is the magnetic field. It units are Teslas 1 T = 1 kg A-1 s nT = 10-9 T = 1 GammaUp at south pole, horizontal and north at equator, down and north pole. Twice as big at poles
5 Earth’s Magnetic Field From The way the Earth Works by P. J. Wyllie, Wiley 1976Field is twice as strong at the poles as at the equator. About 60,000 γ at poles and 30,000 γ at equator
6 Measuring the Orientation of the Earth’s Magnetic Field I = -90 at south pole, 0 at equator and 90 at north poleID = Declination (angle from geographic north)I = Inclination (dip angle)From The way the Earth Works by P. J. Wyllie, Wiley 1976
7 Measurements of the Earths Magnetic Field in the Oceans Measurements of the Earth’s magnetic field in the oceans were developed in the 2nd World War as a way to detect submarines (and later mines)Measurements of the magnetic field were first made with a fluxgate magnetometer. Such instruments are still in use todayVictor Vaquier – SIO professor who died aged 102 in 2009.AC current in opposite windings create changing magnetic field that saturates and create equal and opposite voltage in secondary coil – no signalIn presence of magnetic field, one core saturates first so there is a net voltage. Field in particular directionProton procession magnetometer – rate at which protons in water process around earth’s field after strong applied field is removed. Absolute value but not direction
8 Rock MagnetizationMost minerals either repel or concentrate the Earth’s magnetic field lines but do not themselves become magnetized.A few ferromagnetic minerals retain magnetization. In the oceanic crust the most important is magnetite (Fe3O4). Others include ilmenite (FeTiO3), hematite (Fe2O3), and pyrrhotite (FeS).Forms of rock magnetismThermo remnant magnetism - rock becomes magnetized when it cools below the Currie temperature (580°C) in a magnetic fieldDetrital remnant magnetism - sediments settle in a magnetic fieldChemical remnant magnetism - Hematite precipitates from a fluid circulating through a rock.
9 PaleomagnetismIn the 1950’s scientists learned how to measure the remnant magnetism of rock samples. If one can be sure that the rock has not been rotated by tectonic processes then:The Declination of the remnant magnetism gives the apparent direction of the North Pole at the time the rock formed.The Inclination gives the latitude of the rock when it formed
10 GeochronologyIn the 1950’s scientists also developed reliable techniques of dating rocks using radioactive isotopesThe potassium isotope 40K decays to 40Ar with a half-life of 1.3x109 years. As argon is a gas any traces of that element will escape from rocks when they are molten. Therefore, any argon found in solid rocks must have been produced since that molten state ended and the rock solidified. The ratio of 40K to 40Ar can be analyzed and a numerical date since the last molten state can be assigned.By combining paleomagnetic data from lava flows with the lava ages, scientists were able to look at changes in the apparent position of the Earth’s magnetic pole with time.
11 “Polar Wander” Position of the North pole relative to Europe and Asia Position of the North pole relative to Eurasia and North America
13 Polar Wander and Continental Drift K Myr; Tru Myr; Cu Myr; € MyrPolar wander for North America and EurasiaPolar wander corrected for the opening of the Atlantic
14 Evidence for Continental Drift - pre1960’s Fit of the Atlantic Coastlines and GeologyPaleontology (Fossils)PaleoclimatePaleomagnetismWhy wasn’t this evidence accepted?Physical impossibility of drift (the mantle is solid - it transmits seismic waves)Difficulties of magnetic measurements - scatter, reversalsConservatism
15 Polarity ReversalsThe mechanism of polarity reversals is poorly understood but they happen quickly (within no more than ~1000 years)
16 Using volcanic rocks to develop a polarity timescale Most geoscientists were initially skeptical of magnetic reversals but interest increased once it was realized that they provided a means to date events
17 Polarity timescale from magnetized lava flows The first timescales were obtained in the early 1960’s
18 History of Polarity Reversals Cretaceous Quiet ZoneJurassic Quiet Zone (a period of very rapid reversals?)
19 Marine magnetic anomalies The magnetization of the oceanic crust leads to small variations in the intensity of the magnetic field measured at the sea surface
20 Marine Magnetic Anomalies If we remove the background Earth’s magnetic field from the total magnetic intensity, we obtain the magnetic anomaly
21 Relationship Between Magnetic Anomalies and the Polarity of the Crust
25 Global bathymetry, showing ocean ridge system East Pacific RiseMid-Atlantic RidgeMap shown in next slide
26 Location of the Eltanin-19 profile Ship track across the East Pacific Rise which obtained the magnetic anomaly profile shown in the next slide. The measurements were made in the 1960’s by the Columbia University research vessel Eltanin.
27 Eltanin 19 Magnetic Anomaly Profile Ocean depth, kmMagnetic anomaly, gammaThe vertical scale for total intensity anomaly is shown in “gammas”. This is the same as nanoTeslas or nT. The horizontal lines are at zero anomaly; the scale is thus minus 500 to plus 500 nT.
28 Symmetry of the Eltanin 19 profile ESEWNWWNWESEmeasured profile of total intensity anomaliesmirror image of measured profile to show symmetry