Question

of movement causes
two plates collide, a(n) forms.
that is denser slides under the less - dense plate

Explanation:

When tectonic plates collide, the denser plate slides beneath the less-dense one, and this geological process forms a subduction zone. The term that fits the blank, describing the structure/process formed here, is subduction (or subduction zone, but the context aligns with "subduction" as the key process/feature formed).

Answer:

subduction