Convergent boundaries: where two plates are colliding. Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. Divergent boundaries – where two plates are moving apart. Transform boundaries – where plates slide passed each other.

Convergent boundaries: where two plates are colliding.

Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes.

c. seafloor spreading occurs.

d. crust becomes molten rock.

e. denser plates slide under lighter plates.