When a trench or volcano comes.
An oceanic plate is a tectonic plate mainly underneath the ocean, and a continental plate is a tectonic plate mainly underneath a continent,A tectonic plate is a giant slab of rock which makes up the...
The denser plate (which is usually the older and so cooler plate) will subduct under the younger less dense plate.
Oceanic is heavier but thinner.
In a head-on collision, the oceanic plate subducts into the mantle.