A thunder egg is a type of nodule that forms within specific volcanic rock layers. It begins as a simple, rough sphere that appears unremarkable on the outside, looking much like any other rock found in volcanic fields. These formations can range from the size of a marble to over a meter across, representing geological conditions that occurred millions of years ago. Its dense, colorful, and highly patterned core is only revealed when the rock is carefully cut and polished.
Physical Characteristics and Composition
Thunder eggs are typically spherical or oblong nodules encased in an outer shell, or rind, composed of volcanic rock, most often rhyolite or perlite. This exterior is usually a rough, gray-brown surface. The interior, however, is a dense, colorful mass of silica minerals, which is the defining feature of a thunder egg.
The core is primarily chalcedony, a microcrystalline form of quartz, sometimes mixed with agate, jasper, or opal. The vibrant colors and intricate patterns result from various mineral impurities, such as iron, which imparts reds and oranges, or other elements that produce blues and yellows. A thunder egg is generally distinguished from a geode by its interior structure; while a geode is defined as being hollow, a thunder egg is typically a solid nodule.
Step-by-Step Formation
The formation of a thunder egg begins within silica-rich volcanic flows, such as rhyolite or perlite. As the lava cools after an eruption, trapped gases and steam create pockets or cavities, known as vesicles, which serve as the initial molds. These vesicles are often lined with a radiating crystal structure of feldspar and silica minerals, creating a structure called a spherulite.
Groundwater carrying dissolved silica, often in the form of silicic acid, begins to percolate through the porous volcanic rock. This silica-rich solution slowly seeps into the pre-existing gas cavities, depositing layers of material on the inner walls. This process takes thousands to millions of years to complete as the solution fills the void.
Initially, the silica solution often gels, forming a silica gel within the cavity. This gel slowly dehydrates and crystallizes inward, laying down concentric bands of chalcedony or agate. The difference in mineral content and trace elements determines the color banding and patterning of the final core. This inward crystallization process results in the dense, solid nodule characteristic of a thunder egg.
Notable Locations and Finding Them
Thunder eggs are found globally in regions where explosive volcanic activity involving silica-rich lava occurred millions of years ago. Oregon is particularly renowned for its abundance of thunder eggs, designated as the state rock.
Other major sources include:
- Germany
- Poland
- Mexico
- Australia
They are typically found in areas with eroded volcanic ash beds or weathered rhyolite lava flows. The surrounding volcanic rock, which is softer, decays over time into a clay or soil, leaving the much harder, more resistant thunder eggs exposed on the surface or near the topsoil. Collectors search for the generally spherical shape and rough outer surface that makes them stand out. Popular collecting sites in the US include the Ochoco National Forest and Richardson’s Rock Ranch in Oregon, as well as Rockhound State Park in New Mexico.