Geoscience Reference
In-Depth Information
What we have here is, in fact, a battery with electrons fl owing out from the stressed
rock volume through the pistons into the external circuit and reentering the unstressed
rock along the edges. A current through the external circuit implies a current of the
same magnitude through the rock.
Dormant Electronic Charge Carriers
What Figure 2d demonstrates is a mechanism, previously unknown, to generate elec-
trical currents in a rock subjected to deviatoric stress (Freund, 2002, 2003). The mech-
anism is fundamentally different from piezoelectricity. It is based on the fact that a
small but non-zero number of the oxygen anions in the minerals that make up these
rocks are not in their usual 2-valence state (O
2−
) but have converted to the 1-state (O
−
).
From a semiconductor perspective an O
−
in a matrix of O
2−
represents a defect
electron or hole, also known as positive hole or phole for short. The O
−
normally form
positive hole pairs, PHP, chemically equivalent to peroxy links, O
3
Si-OO-SiO
3
. In
the form of PHPs the O
−
are electrically inactive or “dormant”. During deformation,
dislocations are generated in large numbers. When they intersect a PHP, the peroxy
bond breaks, “waking up” a phole. The phole then becomes a highly mobile electronic
charge carrier.
Figure 3.
Experimental set-up with a slab of granite, one end in a press, Cu contacts attached to front
and the back (left); electric circuit with ammeters to measure currents, one between each Cu contact
and ground, capacitive sensor to measure the surface charge (upper right); circuit showing the flow
of electrons through the outer circuit and the flow of holes through the rock (lower right).
When we stress a portion of a block of igneous rock, a grayish-white Sierra Nevada
granite, 10 × 15 × 120 cm
3
, insulate it from the press and ground, as shown in Figure
3 (left), the stressed portion turns into a battery (Freund et al., 2006). Figure 3 (right,
top) shows the electric circuit similar to the one in Figure 1d. Figure 3 (right, bottom)
shows the current fl ow: electrons, e', leave the stressed rock, the “source” S, and re-
enter the rock at the unstressed end. To close the circuit a current of equal magnitude
must fl ow through the rock. This current is carried by pholes. Some are trapped at
