Page 51 - Global Tectonics

38 CHAPTER 2

The climb process
(a) (b) Extra half-plane Extra half-plane

V V

V

Figure 2.24 (a) The diffusion of a vacancy (v) through a crystal; (b) the downward climb of an edge dislocation as
adjacent atoms (crossed) exchange bonds leaving behind a vacancy that moves by diffusion (from Structural Geology
by Robert J. Twiss and Eldridge M. Moores. © 1992 by W.H. Freeman and Company. Used with permission).

(a) Vacancy flux 1 (b) Formation of a vacancy

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Crystal surface
(c ) Vacancy destruction

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Crystal surface

Figure 2.25 Nabarro–Herring creep: (a) vacancies diffuse toward surfaces of high normal stress; (b) creation of a
vacancy (v) at a surface of minimum compressive stress; (c) destruction of a vacancy at a surface of maximum
compressive stress (from Structural Geology by Robert J. Twiss and Eldridge M. Moores. © 1992 by W.H. Freeman and
Company. Used with permission). Solid lines in b and c mark crystal surface, solid circle marks the ion whose position
changes during the creation of a vacancy.