Here's why:

* Plate Tectonics: Earth's crust is broken into large plates that move and interact.

* Mid-Ocean Ridges: These are underwater mountain ranges where new oceanic crust is formed. Magma rises from the mantle, cools, and solidifies, creating new crust.

* Thin Crust: At these ridges, the crust is extremely thin, only a few kilometers thick. This is because the crust is constantly being generated and pushed away from the ridge, making the new crust very thin.

In contrast, the thickest parts of Earth's crust are found under the continents, particularly under the oldest and most stable regions.