What is the role of the magma formed underneath earth in forming volcanoes?

As the denser tectonic plate subducts, or sinks below, or the less-dense tectonic plate, hot rock from below can intrude into the cooler plate above. This process transfers heat and creates magma. Over millions of years, the magma in this subduction zone can create a series of active volcanoes known as a volcanic arc.

Which type of volcanoes are formed by hotspots?

As hotspot material rises, the pressure drops so the hotspot begins to melt producing magma. In an oceanic hotspot environment, for example Hawaii, dark, silica-poor basalt magma is produced. The runny basalt forms broad sloping shield volcanoes (Fig. 6).