The most important gases in lavas are water and carbon dioxide. We say the solubility of gases in liquids is low at low pressures. We say the solubility of gas in liquids is high at high pressures. Let's consider a couple of everyday examples. Look at a clear plastic bottle of soda pop that hasn't been opened or recently shaken.
Most soda pop contains a few per cent by weight of dissolved carbon dioxide.
If you carefully open the bottle, many small bubbles quickly form and rise to the top and escape. If enough bubbles form, they take part of the liquid with them as a foam that flows out of the top. If you shake the bottle vigorously just before opening it, you trigger the formation of so many bubbles so quickly that the soda pop bursts explosively from the top, and you and everyone around you are sprayed with fine droplets of liquid. On the other hand, if the bottle of soda pop is left open for a long time, virtually all of the dissolved gas escapes, and no amount of shaking can cause the remaining liquid to spray out. It can only be poured out.