Book contents
- Frontmatter
- Contents
- Preface to the second edition
- Preface to the first edition
- Preface to the English edition
- 1 Heat from within
- 2 At the time of the Earth’s birth
- 3 Formation of the layered structure of the Earth
- 4 Time scale of the Earth’s evolution
- 5 Plate tectonics revolution
- 6 Evolution of the mantle
- 7 Origin of the atmosphere and oceans
- 8 Isotopes as DNA of nature
- 9 The Earth’s magnetism
- 10 The Moon
- 11 The past and future of the evolving Earth
- References
- Index
5 - Plate tectonics revolution
Published online by Cambridge University Press: 05 May 2012
- Frontmatter
- Contents
- Preface to the second edition
- Preface to the first edition
- Preface to the English edition
- 1 Heat from within
- 2 At the time of the Earth’s birth
- 3 Formation of the layered structure of the Earth
- 4 Time scale of the Earth’s evolution
- 5 Plate tectonics revolution
- 6 Evolution of the mantle
- 7 Origin of the atmosphere and oceans
- 8 Isotopes as DNA of nature
- 9 The Earth’s magnetism
- 10 The Moon
- 11 The past and future of the evolving Earth
- References
- Index
Summary
PALEOMAGNETISM AND APPARENT POLAR WANDER
The Earth’s magnetic field can be thought of as generated by an enormous magnet placed at the center of the planet. The points that pass through the Earth’s surface when both poles of this enormous magnet are extended are called the north and south magnetic poles. The present north and south magnetic poles differ a little from those of the geographic poles, the difference being about 10° at present. However, if we take an average of the geomagnetic field direction over a few thousand years, the averaged magnetic poles almost perfectly coincide with the geographic poles. As mentioned in Chapter 3, the origin of the geomagnetic field is the fluid motion in the liquid outer core, and it is expected from the geodynamo theory that the magnetic poles align with the rotation axis of the Earth.
When magma erupts on the surface and cools down, it acquires magnetization in the direction of the ambient magnetic field, i.e. the geomagnetic field. The magnetization thus acquired is called thermo-remanent magnetization and has been shown to be extremely stable (cf. Chapter 9). Therefore, the remanent magnetization of volcanic rocks provides a very faithful record of the geomagnetic field at the time of eruption. Accordingly, by measuring the remanent magnetization of volcanic rocks with various ages, we can infer the direction of the geomagnetic field in the past.
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- Information
- The EarthIts Birth and Growth, pp. 50 - 62Publisher: Cambridge University PressPrint publication year: 2012