INCREASE OF PRESSURE WITH DEPTH HAS TWO EFFECTS

 

- STEADY COMPRESSION (SQUISHING) OF MATERIAL MAKES IT STRONGER AND DENSER AND SO INCREASES VELOCITY GRADUALLY WITH DEPTH IN UPPER (100-410 km depth) AND LOWER (660-2900 km depth) MANTLE

- AT APPROPRIATE PRESSURE AND TEMPERATURE CONDITIONS MINERALS TRANSFORM TO DENSER PHASES, CAUSING THE RAPID VELOCITY INCREASES (DISCONTINUITIES) AT 410 AND 660 km depth