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- ... since the Second World War ... the proton magnetometer has revolutionized measurements, satellites have given new impetus to magnetic surveying, discoveries of large mineral deposits have maintained activity in conventional magnetic prospecting, and geomagnetism has continued to play a central role in major scientific developments such as plate tectonics (where the time scale of reversals of the polarity of the Earth's magnetic field still provides the main source of information on the movement of the plates.)
- Two recent studies of the geomagnetic field in the last 1Myr have found 14 excursions, large changes in direction lasting 5–10kyr each, six of which are established as global phenomena by correlation between different sites. The older picture of the geomagnetic field enjoying long periods of stable polarity may not therefore be correct; instead, the field appears to suffer many dramatic changes in direction and concomitant reduction in intensity for 10–20 per cent of the time. During excursions the field may reverse in the liquid outer core, which has timescales of 500yr or less, but not in the solid inner core, where the field must change by diffusion with a timescale of 3kyr. This timescale is consistent with the remarkably uniform duration of well-dated excursions. The disparity of dynamical timescales between the inner and outer cores, a factor of 10, is consistent with the 10 excursions between full reversals.
Seismology and Plate Tectonics (1990)
- Earthquakes radiate waves with periods of tenths of seconds to several minutes. Rocks behave like elastic solids at these frequencies. Elastic solids allow a variety of wave types and this makes the ground motion after an earthquake or explosion (called an event) quite complex. There are two basic types of elastic wave: one involving compression and rarefaction of the elastic material in the direction of propagation of the wave, and one involving no compression but shear of the elastic material perpendicular to its direction of propagation. These are called P and S waves respectively, for primary and secondary since the P wave travels fastest and arrives first.
- Seismology and plate tectonics. Cambridge, UK; New York: Cambridge University Press. 1990. (pp. 4–5)
- After a large earthquake the earth "rings" like a bell; this motion can be observed on sensitive instruments up to a month after a large event. These oscillations have specific frequencies which are properties of the whole earth and which can be measured very accurately indeed. The lowest frequency oscillation has a period of about one hour. Any combination of seismic waves can be represented as an equivalent combination of normal modes. In practice the mode representation is most useful at low frequency — for seismic waves above about 40 s — since at higher frequencies the number of modes becomes prohibitively large.
- Seismology and plate tectonics. 1990. (p. 5)
- Earthquakes generate elastic waves when one block of material slides against another; the break between the two blocks being called a fault. Explosions generate elastic waves by an impulsive change in volume in the material. Small explosive charges are used in controlled-source seismic experiments in which the waves penetrate only a few kilometres into the earth.
- Seismology and plate tectonics. 1990. (p. 12)