MadSci Network: Earth Sciences

Re: Why is there such a concentration of geothermal wells near the faults?

Date: Fri Aug 11 18:50:45 2000
Posted By: James Kopchains, Secondary School Teacher, Earth Sciences, Woodside Intermediate School
Area of science: Earth Sciences
ID: 953231583.Es

   Geothermal energy may be either in the form of heat or hot water.  Deep 
in the planet's core, radioactive atoms such a uranium, potassium, and 
thorium have decayed for many billions of years.  This has generated heat 
intense enough to melt solid rock.  Because the earth's crust is a poor 
conductor of heat, this heat has largely remained deep underground near 
where it was produced.
   Like heat from a wood stove in a large room, the heat produced in the 
core gradually diminishes with distance.  If you could stick a thermometer 
in the earth's core, the temperature may be as high as 6,600 degrees 
Celsius.  If you stuck it into the mantle, the temperature might register 
between 3,700 and 1000 degrees Celsius.  A few kilometers under the 
earth's surface the temperature ranges from a few dozen to several hundred 
degrees Celsius.
   The fault lines throughout the world create weak points in the crust 
that permits this geothermal energy to escape.  The pressure that this 
energy creates causes movement in the magma, or molten rock, which tries 
to push the crust apart.  However, the strength and weight of the crust 
does not allow the heat to emerge, except where the crust is weakest along 
the faults.  For example, along the Pacific Rim there is a great 
concentration of geothermal activity.  That's because they lie along the 
boundary of large, tectonic plates.  Along the fault boundaries magma 
rises to the surface and forms geothermal vents such as volcanoes and lava 
   This heat and pressure also causes geothermal vents created by water.  
Over millions of years, water has trickled through the crust and collected 
in layers of porous rock called aquifers.  The aquifers can be heated by 
geothermal energy to hundreds of degrees.  At sea level the water would 
boil and turn to steam.  Because of the enormous pressures several 
kilometers down, it cannot.  Instead, it is pushed upward through channels 
and fissures, or cracks.  It may bubble steadily out onto the surface; or 
it may rise into the air through geothermal vents as curling clouds of 
steam and condensed water vapor; or it may jet out of the ground as a 

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