Some newly erupted lava flows contain abundant dissolved gas. The gas bubbles in the flow move upwards towards the surface in an attempt to escape while the lava is still molten. However, once the lava starts to solidify, the bubbles are trapped in the rock. These trapped gas bubbles are known as vesicles.

If the upper portion of a lava flow contains a large concentration of vesicles, it is often called “scoria” or “vesicular basalt.” This material often has fewer vesicles and a higher specific gravity than the scoria of lapilli.