Original: Because the Pacific plate is the largest of all the tectonic plates on Earth , crustal material at its western edge has had a long time since formation ( up to 170 million years ) to compact and become very dense ; hence its great height-difference relative to the higher-riding Mariana Plate , at the point where the Pacific Plate crust is subducted .
Simplified: Because the Pacific plate is the largest of all the tectonic plates on Earth , crustal material at its western edge has had up to 170 million years to compact and become very dense ; hence its great height-difference relative to the higher-riding Mariana Plate , at the point where the Pacific Plate crust is subducted .