Fold mountains

Before the development of the theory of plate tectonics and before the internal architecture of thrust belts became well understood, the term was used to describe most mountain belts but has otherwise fallen out of use.

When plates and the continents riding on them collide or undergo subduction (that is – ride one over another), the accumulated layers of rock may crumple and fold like a tablecloth that is pushed across a table, particularly if there is a mechanically weak layer such as salt.

Since the less dense continental crust "floats" on the denser mantle rocks beneath, the weight of any crustal material forced upward to form hills, plateaus or mountains must be balanced by the buoyancy force of a much greater volume forced downward into the mantle.

Thus the continental crust is normally much thicker under mountains, compared to lower-lying areas.

Severely folded and faulted rocks are called nappes.