THERMAL STRUCTURE OF DOWNGOING SLAB

COLD OCEANIC PLATE SUBDUCTS INTO WARMER MANTLE AND HEATS UP SLOWLY

COLD SLAB IS DENSER THAN SURROUNDING MANTLE AND HENCE PROVIDES "SLAB PULL" DRIVING FORCE

FORCE IS GREATEST FOR OLD SLABS SUBDUCTING FAST THAT ARE THE COLDEST AND THUS DENSEST