Subducting plate margins
At subducting plate margins, one plate is pushed under a neighbouring plate as they squeeze together. In addition to the old, weathered plate being forced down and melted, wet sediment and seawater is forced down creating andesitic lava and more violent eruptions containing ash. These volcanoes form classic cone shapes.
Some volcanoes are found at great distances from plate boundaries and are referred to as intraplate, within plate or hot spot volcanoes. These form above hot mantle upwellings or plumes which rise from great depths. As the plate overlying the plume moves away from the hot spot and a new volcano is formed, the previous one cools to become dormant and eventually extinct. This sequence forms a volcanic chain such as that currently found in the Hawaiian Islands. Hotspot volcanism forms very large, low gradient shield volcanoes and are similar in composition and eruption style to those found at divergent plate boundaries.