Geological Structures Known as Thundereggs

What is a Thunderegg?

A thunderegg is a sphere-like geological structure that forms in lava flows.

The centre eventually splits to form a cavity which, over time, fills with either chalcedony, agate, opal, jasper or quartz.

When the ordinary-looking outer rock is broken open, it reveals minerals within a self-contained centre.

Thundereggs form when gas bubbles or cavities develop in cooling volcanic lava. As the lava solidifies, mineral-rich fluids seep into these cavities, gradually depositing layers of silica. Over long periods of time, these deposits harden to form the solid or hollow centres found inside thundereggs.

Thundereggs vs Geodes

Thundereggs are often confused with geodes, but they're not the same. 

A geode forms when crystals grow inward from the walls of a hollow cavity, leaving an open centre. Thundereggs usually form as solid or partially solid masses within volcanic rock, with the internal minerals filling the cavity completely or almost completely.

When cut open, thundereggs often reveal complex patterns of agate, chalcedony, jasper, or opal rather than the crystal-lined interiors commonly seen in geodes.

Article Picture

Courtesy of Captain Tenneal.