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Silver, P.G., 2008, Intermittent plate tetonics?: Science, 319, 85-88 Korenaga, et al., 2008, Comment on “Intermittent plate tectonics?”: Science, 320,

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Presentation on theme: "Silver, P.G., 2008, Intermittent plate tetonics?: Science, 319, 85-88 Korenaga, et al., 2008, Comment on “Intermittent plate tectonics?”: Science, 320,"— Presentation transcript:

1 Silver, P.G., 2008, Intermittent plate tetonics?: Science, 319, Korenaga, et al., 2008, Comment on “Intermittent plate tectonics?”: Science, 320, 1291a Silver, P.G., 2008, Response to comment on “Intermittent plate tectonics?”: Science, 320, 1291b

2 The Earth’s Magnetic Field To a first approximation the Earth’s magnetic field is a dipole field – Sir William Gilbert, 1600 The best fitting dipole field is aligned ~11.5° to the spin axis Geomagnetic poles are at 79 N, 71 W and 79 S, 109 E Magnetic poles are at 76 N, 101 W and 66 S, 141 E Wikipedia.com

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4 The Earth’s Magnetic Field Difference between the best fitting dipole field and the Earth’s magnetic field is called the non-dipole field The magnetic field is changing through time Averaged over geologic time the geomagnetic pole is aligned with the Earth’s spin axis, but occasionally reverses polarization – We can use this phenomena to date oceanic lithosphere and reconstruct past plate motion

5 Full nameShort nameValid forDefinitive for IGRF 10 th generation (revised 2005) IGRF IGRF 9 th generation (revised 2003) IGRF IGRF 8 th generation (revised 1999) IGRF IGRF 7 th generation (revised 1995) IGRF IGRF 6 th generation (revised 1991) IGRF IGRF 5 th generation (revised 1987) IGRF IGRF 4 th generation (revised 1985) IGRF IGRF 3 rd generation (revised 1981) IGRF IGRF 2 nd generation (revised 1975) IGRF IGRF 1 st generation (revised 1969) IGRF

6 The Earth’s Magnetic Field

7 The Earth’s Magnetic Field Solar wind (charged particles emitted by the Sun constrains the Earth’s magnetic field to the magnetosphere Major sunspot activity causes substantial disturbances of Earth’s magnetic field – Communications disturbances – Problems for magnetic surveying (nasa movie)

8 The Earth’s Magnetic Field Edmond HalleyEdmond Halley's New and Correct Chart Showing the Variations of the Compass (1701)

9 Magnetization of Rocks As volcanic rock cools is passes through a series of temperatures at which grains of iron minerals acquire spontaneous magnetization – Curie points (580C for magnetite, 680C for hematite At T < blocking temperature, the magnetization is locked in and aligned with the Earth’s magnetic field at that time – 10s of degrees less than the Curie point – Occurs after lavas solidify – Known as Thermoremnant magnetization (TRM)

10 Magnetization of Rocks Sedimentary rocks can acquire magnetization in 2 ways – Depositional remnant magnetization Small previously magnetized grains align with Earth’s field as they settle through still water – Chemical remnant magnetization Acquired in-situ through growth of iron-oxide grains Magnetization of sedimentary rocks is much weaker than igneous rocks

11 Magnetization of Rocks Induced magnetization is response of rock placed in a magnetic field – Determined by magnetic susceptibility – χ is the magnetic susceptibility – Ratio of remnant magnetization to the induced magnetization is called the Konigsberger ratio, Q Q = for oceanic basalts so TRM is usually much stronger

12 Magnetization of Rocks Measure of the angle of inclination for remnant magnetization gives the magnetic paleolatitude With measured remnant inclination and declination we can determine the latitude and longitude of the paleomagnetic pole Measurements made on rocks with different ages defines Apparent polar wander path

13 Dating the Oceanic Plates Raff and Mason, 1961

14 Dating the Oceanic Plates Repeated reversal of Earth’s magnetic poles causes highs and lows in magnitude of magnetic field measured on oceanic plates – ± 500 nT anomalies in the total field – Causes striping in magnetic field that is parallel to spreading centers – Highs when TRM is aligned with current field (constructive interference) and lows when opposed to current field – Width of magnetic stripe is determined by rate of spreading – To understand the pattern we must know either the rate of spreading or the date of field reversals

15 Dating the Oceanic Plates

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18 Reconstructing Plate Motions 1.Assume latitude and orientation then calculate remnant magnetization 2.Calculate field produced by blocks 3.Add to Earth’s magnetic field at blocks present-day location 4.Calculate difference between modeled and measured field 5.Repeat as necessary

19 Atlantic Ocean Simplest of major oceans – No major subduction zones – Relatively constant rate of spreading along mid- atlantic ridge despite different plate pairs and poles of rotation Oldest anomaly in south Atlantic (M11 – 135 Ma, west coast of S Afr) Central Atlantic (M25 – 180 Ma, E N. Am., NW Afr) Rifting between N Am and Eur began ~ Ma

20 Northwest Pacific Ocean Great Magnetic Bight near Aleutian Islands shows that there was a RRR triple junction Northern plate was called the Kula and southern plate was called the Farallon – Rate of subduction was greater than rate of spreading – Juan de Fuca, Explorer, Gorda, Cocos and Nazca plates are remnants of the Farallon – Kula plate has been completely subducted


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