Convective Cooling Cools Rocks Much More Rapidly Than Heat Conduction

Multiple Choice

Convective cooling cools rocks much more rapidly than heat conduction.Hydrothermal circulation represents convective cooling at ocean ridges and is well known from things like black smokes,but only occurs close to the spreading ridge axis.When geophysicists measure the geothermal gradient in areas along ridges where there is no hydrothermal activity,the thermal gradient is far below what you would predict theoretically,but near hydrothermal vents it is far more than you would predict.Why would this be?

A) The measurements are incorrect because it is too hot to measure thermal gradient in molten rock.
B) Most of the heat is carried away by convection as hydrothermal systems,so the average geothermal gradient away from the hydrothermal circulation is depressed.
C) Organisms growing around hydrothermal vents disturb the thermal properties,insulating the surface,to make an apparent high thermal gradient.
D) The thermal gradient in water represented by the hydrothermal system must be different than the rock,so it is measurement artifact.