Why the Mariana Trench Is So Deep

President Bush acted Tuesday to set aside some large tracts of American-managed territory in the Pacific, including the waters around the Northern Mariana Islands and, nearby, the Mariana Trench. At nearly 11 kilometers, it's the world's deepest ocean trench, a prime example of a subduction zone. What makes it so deep is the particular plates involved there. The Pacific plate is moving westward and plunging below the Philippine plate. The Pacific plate is old and cold at this point, and where it subducts along the western Pacific it sinks into the mantle very fast. The Philippine plate is young and soft, and it gets pulled down along with the sinking Pacific slab. This is not a continental collision like the ones pushing up the Himalayas or the Alps. Oceanic plates do not interact by colliding. Instead, along the whole western Pacific the slab affects adjoining lands (the Kermadec Islands, Philippines, Japan, the Kurile Islands, Kamchatka, the Aleutians) the way a sinking ocean liner affects the lifeboats around it—by sucking them toward it. The combination of plates at the Mariana Trench is what makes it extreme. Then too, as Ron points out in the comments, the trench is very far from land, so it doesn't fill with sediment like many other trenches do.