Brittle-ductile transition zone
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The brittle-ductile transition zone is the zone in the Earth's crust, at an approximate depth of 10-15 km at which rock becomes less likely to fracture, and more likely to deform ductilely by creep. This happens because the brittle strength of a material is increased by the confining pressure, whilst the ductile strength of a material increases with decreasing temperature. The transition zone occurs at the level in the crust where the downwards increasing brittle strength equals the upwards increasing ductile strength, giving a characteristic "saw-tooth" crustal strength profile. This zone is, therefore, the strongest part of the crust and the depth at which many earthquakes occur. The level of the transition zone depends on both strain-rate and temperature gradient, being shallower for slow deformation and/or high heat flow and deeper for fast deformation and/or low heat flow. Crustal composition will also affect the depth at which this zone occurs.
It is a common misunderstanding that the BDTZ is caused by rock becoming "soft" in the higher temperature of the deep. [1] This is false. Deep rock does not become soft; on the contrary, it becomes harder, more like steel (or prestressed concrete) and less likely to crack under pressure.
