Temperature dependence of the
electrical conductivity of granite and
quartz-monzonite from south Bulgaria:
geodynamic inferences
Stefan Shanov,*+ Yotzo Yanev*
and Marcela Lastovickova**
* Geological Institute, Bulgarian Academy
of Sciences, 1113 Sofia, Bulgaria
** Geophysical Institute of Czech Academy of Sciences, 14131 Praha,
Czech Republic
+ Corresponding author: E-mail: shanov@geology.bas.bg
(Received 16 November 1999; accepted 28 January 2000)
Abstract: The temperature dependence
of the electrical conductivity for granite and quartz-monzonite specimens
from South Bulgaria has been studied, using equipment of the Geophysical
Institute of Czech Academy of Sciences, which enables the automatic control
of the experiment. Comparisons between electrical conductivity curves measurements
under laboratory and the real local deep geothermal conditions have been
made in order to evaluate the influence of the mineral transitions on the
formation of geophysical layers in the Earth's crust. The abrupt decrease
in electrical conductivity at 200o-330°C correlates with the
beginning of the transition magnetite-maghemite. The maximum of electrical
conductivity in the temperature interval 750-820°C in granite coincides
with the transition of b -quartz to b
2-tridymite. All these mineral transitions are accompanied by volume changes.
The probable fluctuations of the heat flow in the Earth's crust in South
Bulgaria, near the critical phase transition zones, are the cause of the
volume changes in the rocks, the consecutive increase in the stress and
the restoration of the tectonic equilibrium by weak earthquakes sequences.
The hypothesis for sinking of the tectonic block is not acceptable because
the recent vertical movements are positive - 1 to 2 mm/year. What remains
is the hypothesis of progressing of the thermal flow from the deep parts
of the Earth's crust and upper mantle towards the most shallow layers of
the crust. The area of the most active influence of the heat flow from
the asthenosphere uplifting is exactly the zone of the Maritza fault, from
where the studied specimens originate.
Key Words: Electrical Conductivity, Temperature Dependence,
Granite, Monzonite, Phase Transitions, Earthquakes.