IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Oral Presentation 1991
Understanding Electromagnetic Properties of Dry and Wet, Stressed and
Unstressed Igneous Rocks: Progress Report
Prof. Friedemann Freund
Earth Science Division, Code SGE NASA Ames Research Center IAGA
Vern Vanderbilt, Akihiro Takeuchi, Bobby W.S. Lau, Milton Bose
The discovery of a new type of electric currents that can be stress-activated in igneous rocks (1) opens
a window of opportunity for controlled laboratory experiments and for considering source generation
mechanisms. Laboratory experiments demonstrate that the propagation of the electronic charge carriers
is not impeded by water films in water-soaked rock, though cm-thick layers of liquid water reverse the
direction of the current flow. The charge carriers that propagate through rocks are defect electrons in
the oxygen anion sublattice (positive holes or pholes for short). They are introduced into the matrix of
nominally anhydrous minerals during cooling from magmatic or high-grade metamorphic temperatures
when hydroxyl pairs (traces of dissolved H2O) split off H2 molecules and convert to peroxy links. The
oxygens in the peroxy links thereby change from the 2- to the 1- valence state. Stress causes
dislocations to move, breaking peroxy links and activating pholes alongside with loosely bound
electrons. The electron-phole pairs are long-lived (hrs to days). Our hypothesis is that the boundary
between stressed and unstressed rock acts analogous to a semiconductor pn junction, impeding
electron flow across the boundary in one direction while promoting flow of pholes. Whereas pholes can
stream out of the stressed rock into the unstressed rock, electrons need an n-type contact to flow. We
report on the magnitude of the transient currents that can be expected, when pholes and electrons both
flow out of the stressed rock volume and reconnect, thereby closing the electric circuit. (1) Freund et al.
2006, Phys. Chem. Earth 31, 389-396; Freund & Sornette, 2007, Tectonophys. 431, 33-47.
Keywords: pre earthquake signals, telluric currents, electromagnetic emissions