by Nicole L Cox,
Principal Structural Geologist at Aeris Resources Ltd
Search my handbag, and you’ll find a small, joined pair of Moqui marbles tucked into one of the front pockets. This set was collected and given to me by my engineer and fossicker dad. As he handed them over, he told me they were ‘lucky’. However, I don’t carry them around for superstitious reasons. I carry them as a gentle reminder of something that has shaped who I am and where I am today.
As small as they are—lucky or not—they influenced my life. Collecting Moqui marbles was a piece of my childhood that helped build me into the geologist I am today.
Moqui marbles and their legends
Moqui marbles (Figure 1), also called Thunderballs or Shaman stones, come from the porous and permeable Navajo Sandstone Formation, found mainly in southern Utah and northern Arizona.
They are named after the Moqui (pronounced ‘Mo-Key’) Indians, an ancestral Hopi group of the southwestern United States. According to one source, the word moqui means ‘dearly departed one’ in the Hopi language.
Background—Liesegang rings in a sandstone unit of the Tapley Hill Formation near Arkaroola, South Australia.
One legend tells of Hopi ancestors playing games with these ‘marbles’ in the evenings when spirits were allowed to visit the earth. As the sun rose, the spirits returned to the heavens, leaving the marbles behind to reassure their loved ones that they were happy and well.
Other beliefs suggest Moqui marbles offer protection, shielding their owners from calamities such as floods, fire, and misfortune. Some even claim they bring prosperity or promote fertility. All this explains why my dad called them ‘lucky’.
The science behind Moqui marbles
Moqui marbles are small, generally spherical iron concretions that formed within the Jurassic-age Navajo Sandstone (around 190 million years old). They consist of a sandstone core encased by a hard shell of iron oxide minerals (primarily hematite and goethite; Figures 2 and 3).
Studies estimate that these concretions formed no more than 25 million years ago, when iron-rich minerals precipitated from episodic groundwater flow through the much older, highly permeable eolian sandstone (Chan et al., 2001).
Chan et al. (2005) summarized the formation process as follows:
- Iron source and initial sandstone formation
The iron originally came from the chemical breakdown of iron-bearing silicate minerals. These oxidized and coated the sand grains in the ancient dunes, giving the Navajo Sandstone its characteristic pink-to orange-red hues. - Bleaching and dissolution of iron
During late diagenesis (the process of turning sand into sandstone), reducing fluids (such as hydrocarbons) migrated through the rock, dissolving the iron-rich coatings. This fluid movement bleached portions of the sandstone white, stripping it of iron. - Precipitation and concretion growth
When these iron-rich fluids encountered oxygenated groundwater, the iron oxidized and precipitated. When there is a concentration of precipitated minerals, it is referred to as a concretion. Concretions can range in shape from spheroids and bulbous nodules to pipes and banded structures.
After the concretions formed, the surrounding weaker Navajo Sandstone (Figure 4) slowly broke down and eroded. The hard, erosion-resistant concretions were set free and accumulated on the ground, often in great numbers.
The formation of these iron concretions is a fascinating story of fluid movement and chemical reactions. Understanding how they formed has broader implications, including for the study of mineral deposits and fluid transport in sedimentary basins.
Additionally, Moqui marbles have been studied as a terrestrial analog for Martian spherules, or ‘Martian Blueberries’—iron-rich spheres discovered by the Opportunity rover. Their presence on Mars has helped scientists hypothesize about the past presence of water on the planet.
Lucky marbles
As a child, I loved searching for these small, round, brownish rocks. If only that little girl knew she would grow up to travel the world studying folded rocks, linking them to local and global geologic histories, natural hazards, and mineral deposits.
When choosing a career (or considering a career change), I believe in finding that sweet spot—where passion, talent, and societal need intersect. For me, rocks are that sweet spot.
I consider myself lucky to do what I do each day.
Thank you, Moqui marbles—and thank you, Dad.
References
Chan, M.A., Bowen, B., Parry, W., Ormö, J., & Komatsu, G. (2005). Red rock and red planet diagenesis: Comparisons of Earth and Mars concretions. GSA Today, 15(8), p. 4–10.
Chan, M.A., Parry, W.T., Petersen, E.U., & Hall, C.M. (2001). ⁴⁰Ar/³⁹Ar age and chemistry of manganese mineralization in the Moab to Lisbon fault systems, southeastern Utah. Geology, 29(4), p. 331–334.
For more information: Get in touch with Nicole on LinkedIn
