Volcanic rock cavity, also known as a VESICLE, is a bubble-like cavity in an igneous rock that is formed when gas bubbles become trapped in the molten lava. The size of these cavities can range from microscopic to several centimeters in diameter. They are often filled with secondary minerals such as quartz, calcite, and zeolites. Vesicles are common in basaltic rocks, which are formed from cooled lava flows.