Question

Technological advances had helped reveal that the ocean floor was not flat, as once believed, but instead was marked by 50,000-kilometer-long, 3,000-meter-high ridges and 11-kilometer-deep trenches. How do scientists explain why, if new crust is constantly being formed from volcanic materials at the ridges, Earth's crust stays the same size?

Answer 1

By the late 1960s, scientists had developed the theory of plate tectonics based on a range of new evidence. Technological advances had helped reveal that the ocean floor was not essentially flat, as once assumed, but instead was marked by 50,000-kilometer-long (31,000-mile), 3,000-meter-high (9,800-ft) ridges and 11-kilometer-deep (7-mile) trenches. Scientists found striking patterns related to these features. They found that the youngest oceanic crust is located nearest the mid-ocean ridge and the oldest crust is nearest the trenches. They also detected a pattern of alternating magnetic polarity along the ocean floor, which emanated from the ridge tops. These two pieces of evidence, coupled with the fact that volcanic activity and island-building occurred most commonly at ocean trenches, suggested that new crust was created at mid-ocean ridges and destroyed at ocean trenches. Scientists Harry Hess and Robert Dietz used this evidence to revive and expand Holmes' convection theory into the theory they called "seafloor spreading." Finally, Wegener's notion of continental drift was coupled with a mechanism that could explain the movement of tectonic plates.

Answer 2

long story short the answer is  

Scientists found that the youngest oceanic crust is located nearest the mid-ocean ridge and divergent boundary and the oldest crust is at subduction zones where it sinks back into the Earth.