Volcanic eruptions are powered by a release of pressure as buoyant magma rises towards the surface. As it does so it expands, pressurising the overlying rocks further until a breaking point is reached and the magma comes out onto the surface. How it does so depends on its chemical composition, those with more silica are more viscous and those with less more runny. As the magma rises, its capacity to hold dissolved gases such as water, CO2 or SO2 reduces, and bubbles come out of solution. If the magma is runny, no problem, the bubbles rise up and by the time the magma turns into lava it has mostly degassed, leading to spectacular lava rivers but (usually) not causing too much havoc. For the more viscous lavas, the gases can’t escape, and so the pressure builds up much higher, leading to spectacular explosions of cone shaped peaks like that of Mt St Helens in 1981 or Vesuvio in 79CE.
When a lava of any composition is cooled quickly, a glass such as obsidian or Pelee’s hair results. If it was very gassy you get pumice , a rock that is often so porous and bubble filled that it will float on water, sometimes as vast rafts in the oceans for months on end. Take the gassiness a stage further in a basaltic lava in a huge lava fountain, and you get this rock called reticulite. It consists of a fine mesh network of glassy walls that were once between now burst bubbles, and is very fragile. It is often carried large distances downwind before settling, as it is so light.