Fossil lightning, balls and tubes

Fulgarites on the desert floor

On our journey through the Great Sand Sea we encountered fulgarites, fragile straws of fused sand. These fulgarites are formed when lightning hits, and the electrical energy is dispersed directly down into the arid sand. The centre of these tubes is hollow because the sand evaporated here due to the intense heat of the lightning beam, the rim consists of molten and fused sand, giving it a glassy appearance.

Fulgarite tubes in detail

Some hematite concretions we found have similar tube like appearance. Hematite is an iron-based mineral, which leaves a red color when you scratch it (Heamos is blood in Greek, so hematite is blood rock). Because the hematite mineralised rock is stronger than its surrounding, wind erosion will nicely expose the concretions.

Tubes of hematite concretions

Some other concretions start around a nucleus and form round, often hollow, balls. The exact process of this mineralisation, and why it forms these odd shapes is difficult to unravel. One factor is slight differences in rock permeability, creating preferred pathways for mineral-rich fluids, influencing the shape of the concretions. Another factor is grain size. Areas with lower size grains in the rock (which translates in to a higher grain surface area) are places where minerals preferentially precipitate. Fossil root casts, or fossil borrows from animals that lived on the seafloor, can create tube like areas where permeability and grain size is different from the surrounding rock. With the odd balls and tubes we found, we could not determine if the factors mentioned above had an influence on the shape.

Hematite balls