Tephrochronology is a young science, with most of the advances being developed in the second half of the twentieth century; Sigurdur Thórarinsson was a pioneered in this area of expertise during the 1960s. Tephrochronology uses volcanic ash and tuffaceous deposits, for relative dating of archaeological sites and for absolute age correlations, when their ages can be determined using chronometric techniques such as potassium-argon dating (Bahn 2008: 154-155; Greene 2002:162-165; Cunliffe, Gosden, and Joyce 2009: 162-163).
Over the last three million years, large regions of the world have witnessed extensive volcanic eruptions of the type that produce massive quantities of ash, that can spread over tens or even hundreds of thousands of square kilometres. Without these volcanic deposits inter-bedded with surface sediments and soils, upon which the hominids lived, archaeologists would not be able to provide the relatively tight chronological framework they have for hominid evolution (Bahn 2008: 154-155; Greene 2002:162-165; Cunliffe, Gosden, and Joyce 2009: 162-163).
In addition to supplying datable material, volcanic ash or tephra, (pyroclastic rock) fragments that are ejected from a volcanic vent, provide marker horizons for correlation. The mineralogy of the separate volcanic eruptions is normally distinct enough that, with sufficient petro-logical study ash horizons can be correlated over large regions, establishing a firm tephrostratigraphy. Although erosion strips large quantities of each ash/tephra mantle from exposed areas of the landscape, enough falls in protected crevices and areas of active deposition to provide widespread preservation (Bahn 2008: 154-155; Greene 2002:162-165; Cunliffe, Gosden, and Joyce 2009: 162-163).
The second radiocarbon revolution, BP 1970 – 1980
Electron Spin Resonance:
A dating method that was introduced to archaeology in the 1970s; although electron spin resonance dating is still in a accelerated phase of development, it has confirmed its value by providing new chronological evidence about the evolution of modern humans. Electron spin resonance dating in archaeology has been applied to tooth enamel, speleothems, spring-deposited travertines, shells, and burnt flint. The principal dating range lies between present and 120,000 BP, in exceptional circumstances, samples older than 1,000,000 BP can be successfully analysed (Bahn 2008: 158-159; Greene 2002:173; Rink 1998).
The basic principle for electron spin resonance dating: Radioactive rays expel negatively charged electrons from atoms in the ground state valence band, the electrons are transferred to an elevated energy state conduction band, leaving positively charged holes near the valence band. After a short time of dispersion, most electrons recombine with the holes, and the mineral is unchanged. However, all natural minerals contain defects that can trap electrons when they fall back from the conduction band (Rink 1998).
These trapped electrons can be measured by an electron spin resonance spectrometer, giving rise to characteristic electron spin resonance lines. The intensity of the electron spin resonance line, is proportional to the number of trapped electrons; the number of trapped electrons in turn,