At point x, only the air that is in direct contact with the sand is heated. and at point y, only subsurface sand that is in direct contact with the hot surface sand can gain heat. this means that mostly is occurring at points x and y.

The answer is conduction. This is a form of heat transfer due to temperature difference, through direct contact with objects that are touching, in which the hotter object passes heat to the cooler objects by molecular collisions. Convection is another form of heat transfer in which heat travels through fluids while radiation through empty spaces.

Could you elaborate?