Image credit: Scott Brande
Vesicles in igneous rocks
Molten rock may include various amounts of gas molecules dissolved in the silicate melt. When volcanoes erupt at the earth's surface, pressure is rapidly released and dissolved gas forms bubbles of vapor that may move through liquid lava. Rapid loss of heat leads to rapid cooling and solidification of the lava into rock, trapping the gas bubbles and forming vesicles - holes or cavities.
The density of vesicles may vary greatly, with some rock resembling a foam (or perhaps like a loaf of fluffy white bread) that is mostly vesicles, and other rocks more dense with a smattering of vesicles. The variation among such rocks is continuous, so any distinction among categories is arbitrary. The term scoria is used for a highly vesicular basalt which is more vesicle and less rock (and so quite lightweight, low density), while vesicular basalt is often used for a sample that is more dense with a smaller fraction of vesicles than rock.
Watch this short video to learn how to observe vesicles in scoria and pumice.