Geological research shows that....
1. Some volcanoes erupt Clouds+Ashes+Debris, with very small amount of Lava.
2.Some volcanoes erupt Lava, with very minor amount of Clouds+Ashes+Debris.
Which type of material will be erupted by Volcanoes, totally depend upon its "Eruptive Styles".
There are 2 types of eruptive styles of volcanoes.
a) Effusive eruption:
These eruptions produce mainly (Lava Flows), Lava flows are mainly of Basalt, having very low viscosity.
Such type of eruptions result in the formation of "Shield Volcanoes".
b) Explosive eruption:
These eruptions produce clouds and avalanches of Pyroclastic debris.
Such type of eruptions yield "Cinder-Cone Volcanoes" & "Composite Volcanoes"