What is the Brittle-Ductile Transition Zone?
It seems certain that the earth gets hotter as you go deeper into the crust, and soon reaches a temperature where the rocks will change from the solid state into a higher energy plastic/fluid state. The place where this change happens is called the Brittle-Ductile Transition Zone. It is also certain that the earth's crust is under quite a bit of stress from a multitude of internal forces. The BDTZ is that depth in the crust where the earth's crustal rocks change from being cold and rigid enough to break when subjected to directed stress, to being warm and plastic enough to bend and/or fold when stressed.
The BDTZ is certainly not at a consistent depth, nor is it marked with red and green flashing lights. It is a highly variable "transition zone" which moves up and down in the crust in response to local conditions. But one thing is certain: start at any place on the surface, dig a hole, and you will reach the Brittle-Ductile Transition Zone long before you get to China. (It is important to note that humans have never managed to dig or drill a hole completely through the crust and into the upper mantle. Both the United States and the old Soviet Union have publicly made the attempt, and both have failed - thwarted in the attempt by the BDTZ!)
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