However, the general trend of the boundary between two spreading plates need not be, and in fact usually is not, perpendicular to the motion of the plates. Along transform faults, the two plates are sliding past each other going in opposite directions. New sea floor is constantly being created along spreading centers. Obviously somewhere else old sea floor must be going away. This occurs in trenches, also called subduction zones.
Trenches occur along the boundary between two plates that are moving towards each other.
As the subducting plate is heated back up to mantle temperatures, certain minerals in the plate melt sooner than others. Minerals that melt at lower temperatures and are lighter than the surrounding material tend to rise, melting their way up through the overriding plate to erupt as volcanoes on the ocean floor. As these volcanoes grow, they rise above the ocean surface to form lines of islands along the leading edge of the overriding plate. Numerous islands of Micronesia and Melanesia in the western Pacific were created in this way. The entire Pacific margin of the Americas provides abundant examples of this process.