At constructive plate margins, mid-ocean ridges will be formed.
As the plates move apart (very slowly), magma rises from the mantle. The magma erupts to the surface of the Earth.
This is also accompanied by earthquakes.
When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock.
This process is repeated many times, over a long period of time. Eventually the new rock builds up to form a volcano.
Constructive boundaries tend to be found under the sea, e.g. the Mid Atlantic Ridge.
Here, chains of underwater volcanoes have formed along the plate boundary.
One of these undersea volcanoes may become so large that it erupts out of the sea to form a volcanic island, e.g. Surtsey and the Westman Islands near Iceland.
The Mid-Atlantic Ridge