Why do gases come spewing out of volcanoes during eruptions. where do these gases come from?

Question

Why do gases come spewing out of volcanoes during eruptions. where do these gases come from?

Answer 1

Most of the gasses were dissolved in the lava. When the pressure is lowered, the gasses come out of the hot solution.

The pressure becomes lower when rock above molten lava gets pushed out of the way and the smaller amount of ambient pressure is released. With less pressure the gasses cannot stay dissolved and come out of solution with violent results.

This is similar to a warm, shaken soda releasing its carbon dioxide when the pressure lowers by opening the cap.