English: Magmas melt the silica rocks above them, evolving from basalt to andesite, then granite, ever higher in silica content. Above 1,100 degrees C the excess silica crystalizes into cristobalite needles. Convection brings the needles together and they form spheres with high pressure water trapped between the needles. Crystallization generates heat and that allows the displaced feldspar minerals to coat the sphere with a layer of perthite.

Repeated layers build up as the magma heats and cools for hundreds and thousands of times, allowing the spheroids to grow ever larger. Some have been found as large as 10-12 feet in diameter.

When ejected to the surface as rhyolite or obsidian lava, the slow release of pressure allows the trapped water to expand into steam, rupturing the spheroid and transforming it into a Lithophysa "Blown Rock" commonly known as a thunderegg.