The “depth” of tectonic plates depends on what part of the plate you mean. The rigid plate, or lithosphere, is generally less than 15 km thick for young oceanic plates and about 200 km or more for old continental plates; some estimates put continental rigid rock around 175 km deep.

Quick Scoop

Tectonic plates are not like flat sheets at one fixed depth. They’re thick slabs of Earth’s outer shell, and their thickness varies a lot by location and age. Under oceans, the plate can be relatively thin, while under old continental interiors it can be much thicker.

What that means

  • Young oceanic lithosphere: less than 15 km thick.
  • Continental lithosphere: about 200 km or more in some old, stable regions.
  • A commonly cited estimate for rigid continental rock is around 175 km deep.
  • Beneath the rigid plate is the warmer, softer asthenosphere, where the rock can deform more easily.

Simple picture

Think of tectonic plates as a thick, stiff shell floating on a softer layer beneath it. The shell is thin in some places and much thicker in others, so there isn’t one single “depth” for all plates.

Extra note

If you meant the depth of earthquake activity at plate boundaries, that can go much deeper in subduction zones, reaching as deep as about 700 km.