The thickest part of a tectonic plate lies below the lithosphere, which is the rigid outermost layer of the Earth.

Here's why:

* Lithosphere: This layer is made up of the crust and the uppermost part of the mantle. It's relatively cold and rigid, which is why it can break into tectonic plates.

* Asthenosphere: Beneath the lithosphere is the asthenosphere, which is a partially molten layer of the upper mantle. The asthenosphere is hot and deformable, allowing the tectonic plates to move over it.

The thicker part of the plate, which includes the crust and the upper part of the mantle within the lithosphere, is where the bulk of the plate's mass resides.