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5. Evidence for Plate Tectonics from Magnetics William Wilcock OCEAN/ESS 410.

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Presentation on theme: "5. Evidence for Plate Tectonics from Magnetics William Wilcock OCEAN/ESS 410."— Presentation transcript:

1 5. Evidence for Plate Tectonics from Magnetics William Wilcock OCEAN/ESS 410

2 Lecture/Lab Learning Goals Understand the basic characteristics of the Earth’s magnetic field and how one measures its orientation Know the different kinds of rock magnetization and their use in paleomagnetism Be able to explain the historical concept of polar wander and its explanation in terms of continental drift Be able to explain patterns of marine magnetic anomalies in terms of plate spreading and magnetic field reversals Know how to interpret marine magnetic anomalies - LAB

3 south pole north pole Geodynamo 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. Geographic north pole Geographic south Pole Magnetic south Magnetic north After Plummer The 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. Earth’s Magnetic Field

4 The Earth’s magnetic field close to a dipole. The radial (vertical) and tangential (north-south) components a dipole field are given by θ - Colatitude (0º at south pole; 90º at equator; 180º at north pole μ 0 - magnetic permeability of a vacuum 4  x N A -2 r - distance to the center of the earth (6.4 x 10 6 m at the Earth’s surface) M - is the dipole moment which for the earth is 7.95 x A m 2 B - is the magnetic field. It units are Teslas 1 T = 1 kg A -1 s nT = T = 1 Gamma

5 Earth’s Magnetic Field Field is twice as strong at the poles as at the equator. About 60,000  at poles and 30,000  at equator From The way the Earth Works by P. J. Wyllie, Wiley 1976

6 Measuring the Orientation of the Earth’s Magnetic Field D From The way the Earth Works by P. J. Wyllie, Wiley 1976 I D = Declination (angle from geographic north) I = Inclination (dip angle)

7 Measurements of the Earths Magnetic Field in the Oceans Measurements of the Earth’s magnetic field in the oceans were developed in the 2 nd 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 today

8 Rock Magnetization Most 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 (Fe 3 O 4 ). Others include ilmenite (FeTiO 3 ), hematite (Fe 2 O 3 ), and pyrrhotite (FeS). Forms of rock magnetism –Thermo remnant magnetism - rock becomes magnetized when it cools below the Currie temperature (580°C) in a magnetic field –Detrital remnant magnetism - sediments settle in a magnetic field –Chemical remnant magnetism - Hematite precipitates from a fluid circulating through a rock.

9 Paleomagnetism In 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 Geochronology In the 1950’s scientists also developed reliable techniques of dating rocks using radioactive isotopes The potassium isotope 40 K decays to 40 Ar with a half-life of 1.3x10 9 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 40 K to 40 Ar 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” (a)Position of the North pole relative to Europe and Asia (b)Position of the North pole relative to Eurasia and North America

12 Opening of the Atlantic

13 Polar Wander and Continental Drift K Myr; Tru Myr; Cu Myr; € Myr (a)Polar wander for North America and Eurasia (b)Polar wander corrected for the opening of the Atlantic

14 Evidence for Continental Drift - pre1960’s Evidence Fit of the Atlantic Coastlines and Geology Paleontology (Fossils) Paleoclimate Paleomagnetism Why wasn’t this evidence accepted? Physical impossibility of drift (the mantle is solid - it transmits seismic waves) Difficulties of magnetic measurements - scatter, reversals Conservatism

15 The mechanism of polarity reversals is poorly understood but they happen quickly (within no more than ~1000 years) Polarity Reversals

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 Zone Jurassic 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

22 Magnetic Stripes Raff and Mason, 1961

23 Normally magnetized crust Magma dikes oceanic crust Reversely magnetized crust Normally magnetized crust N N N N N N Magma Vine and Matthews’ Magnetic Tape Recorder

24 Vine and Matthews’ magnetic tape recorder

25 East Pacific Rise Mid-Atlantic Ridge Map shown in next slide Global bathymetry, showing ocean ridge system

26 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. 19 Location of the Eltanin-19 profile

27 The 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. Ocean depth, km Magnetic anomaly, gamma Eltanin 19 Magnetic Anomaly Profile

28 ESE WNW ESE measured profile of total intensity anomalies mirror image of measured profile to show symmetry Symmetry of the Eltanin 19 profile

29 Polarity Reversals and Spreading Rate

30 Polarity Reversals and Sedimentation Rates Depth, m

31 Age of the Seafloor


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