A vent or fissure in the Earth's crust where molten lava is erupted onto the surface. The shape of a volcano depends on the composition of the lava. Lower-temperature, viscous, silica-rich lava forms steep-sided cones interbedded with ash, such as Mt Fuji, Japan. Less viscous, silica-poor, basaltic lavas form gentle slopes, as found in Iceland. Most volcanoes are confined to the zones along boundaries between crustal plates, and are closely associated with earthquakes, as in the circum-Pacific ‘ring of fire’; but there are notable exceptions, such as the Hawaiian Is, which have formed on a ‘hot spot’ in the Earth's crust. The scientific study of volcanoes is known as vulcanology.