One reason the Mariana Trench is so deep, he added, is because the western Pacific is home to some of the oldest seafloor in the world—about 180 million years old.
Seafloor is formed as lava at mid-ocean ridges. When it's fresh, lava is comparatively warm and buoyant, riding high on the underlying mantle.
But as lava ages and spreads away from its source, it slowly cools and becomes increasingly dense, causing it to settle ever lower—as is the case with the Mariana Trench.
Read the rest at National Geographic News. Link