Geoscience Reference
In-Depth Information
fl uctuations is unknown. They may be due to coupling, mediated by an internal electric
fi eld, between the electron and phole currents fl owing out of the stressed rock volume
in opposite directions.
Figure 5.
Outflow currents from a gabbro tile, 30 x 30 x 1cm
3
, loaded at the center (
10 cm
3
) from
0 to 48 MPa and kept at constant load for 30 min. Currents increase rapidly at beginning, reach a
steady state value and decay slowly. Upon unloading currents return to zero.
∼
Another interesting feature in Figure 5 is that, when stress is removed slowly, the
currents return to zero. The process is repeatable. We have applied the same load-hold-
unload-hold program 22 times over a period of 12 hr and observed that the currents
always return to the same level.
Rocks Turn into Batteries
The most important conclusion to be draw from this series of experiments is that, when
rocks such as granite or gabbro are subjected to deviatoric stress, they behave as if they
were batteries. When stresses are applied, electronic charge carriers are activated, both
electrons and pholes. In other words, the application of stress charges the battery.
Removing the stress causes the battery to return to an inactive state, ready to be
charged again. So far we have not reached a limit of how often we can repeat the pro-
cess. So long as the stress is well below the threshold of macroscopic damage, in our
case about 1/4th the failure strength of the unconstrained rock, the process is repeat-
able many times.
