12.4 What is the evidence that subduction occurs at convergent plate boundaries?
Fig 12.25
Lithosphere is less dense when it forms at ridges than when it subducts. It likewise becomes thicker over that time. Both changes are linked to cooling over time. As ocean lithosphere cools, it contracts and becomes denser. Asthenosphere "adds" to the base of the ocean as it loses heat in contact to the cooler material above and solidifies. It is so massive at the subducting end that it acts as an ever-sinking "anchor" that pulls ocean crust downward.