Why do scientists think that lithosphere has to be destroyed somewhere on or in the earth?

Question

Quizplus · Accepted Answer

Lithosphere is indeed created at divergent boundaries, such as mid-ocean ridges, where tectonic plates move apart and magma rises to fill the gap, cooling to form new crust. Since the Earth's size remains constant, the creation of new lithosphere implies that an equivalent amount must be destroyed elsewhere, typically at convergent boundaries where one plate is forced beneath another into the mantle, a process known as subduction.

Why Do Scientists Think That Lithosphere Has to Be Destroyed