The magma escapes through weaknesses in the rock and rises up through a composite volcano. The volcanic eruptions are often violent, with lots of steam, gas and ash.

Most geologic activity stems from ... can also lead to underwater volcanoes that eventually build up into island arcs like Japan. At divergent boundaries in the oceans, magma from deep in the ...

Trigger volcanoes. Create new sea ... an eye on the map to see where in the world the activity takes place. Also known as spreading boundary, a divergent boundary occurs where two plates move ...

This is called a constructive or divergent plate boundary. These are usually found under oceans. As the plate moves apart, magma rises to the surface and cools, forming shallow-sided volcanoes.