Or it can be large bodies of magma forcing their way to the surface. There are many different things that can be seen during eruptions. Such as lava flows, release of gases, and pyroclastic materials. Lava flows from shield volcanoes are more smooth and runny when compared to that of strato volcanoes. It is a fact that lava from strato volcanoes have more viscosity than that of shield volcanoes.
Viscosity basically means the mobility of something.
The more viscous something is, the harder it is to flow freely. The less viscous something is, the easier it flows. Steam makes up the greatest portion of the gases that are release during eruptions. When the confining pressures of being underground no longer have hold of the steam, it expands rapidly and violently to many times it's previous size. In this swift and explosive action, rocks are often propelled far away from the volcano.