Thunder Egg

A Thunder Egg is a nodule-like or geode-like geological structure with some adhering silicified host rock comprising the outer zone of the thunder egg. Thunder Eggs are usually filled with agate, chalcedony; less commonly opal, and/or visibly crystalline quartz. A common feature of thunder eggs is that the infilling material might not have all concentrically formed layers: often an outer zone is concentrically zoned but the central material is flat-layered as in sedimentary layers. The layers may have contrasting colors and degrees of crystallinity.

Thunder eggs are generally lithophysae, largely filled with chalcedony, usually as agate, and often with a quartz-lined geode. As the silica-filled cavities are harder than the surrounding rock they are often weathered out, or partly weathered out, of their source rock, usually a rhyolite.

Lithophysae are spherulites formed in fine grained, felsic igneous rocks that have undergone some expansion during growth to create voids, which are very often (but not always) infilled with silica to form Thunder eggs. Spherulites, to igneous petrologists, are high temperature crystallisation domains forming due to rapid nucleation in glassy, silica rich lavas, and are initially composed mostly of sanidine with cristobilite or tridymite.