These mysterious deep-sea rocks grow like tree rings over millions of years

Nature · Published 3 October 2026 Swipe more like this →

In short: Scattered across the vast, dark abyssal plains of our oceans lie millions of round, mineral-rich stones known as ferromanganese nodules. These peculiar formations accrete layer by layer around a central nucleus, holding potential as a future mineral resource, yet they remain one of the seafloor's most enigmatic geological puzzles.

Geologically, a nodule is a rounded, discrete concretion of mineral matter that stands apart from the surrounding sediment. While nodules exist in various terrestrial environments—such as chert or flint found within limestone—the marine variety, specifically manganese nodules, are defined by their unique growth process. They form through the slow precipitation of manganese-oxide minerals, often mixed with iron, cobalt, nickel, and copper, accreting in concentric layers around a central nucleus of rock or biological material. When sliced open, these layers reveal a pattern strikingly similar to the rings of a tree.

The formation of these nodules is a process spanning millions of years. They are most famously found on the deep abyssal plains, where they can cover vast, uniform areas of the seafloor. A persistent mystery is how these objects remain at the sediment surface despite the constant, slow accumulation of seafloor debris. Because they concentrate valuable metals, they have become a focal point for potential deep-sea mining exploration, particularly in the Pacific and Indian Oceans. However, the science is still evolving; researchers are only beginning to understand the differences between these abyssal nodules and those found on seamounts.

Seamount-hosted nodules, such as those recently discovered on the Gosnold Seamount during the 2021 North Atlantic Stepping Stones expedition, present a different profile. They typically feature larger nuclei and lower concentrations of manganese and iron compared to their abyssal counterparts, often resembling ferromanganese crusts. While some seamount nodules, like those from the Pali seamount, mirror the structure of abyssal types, the broader spectrum of their composition and formation remains a subject of active study. As exploration continues, these charismatic rocks serve as vital indicators of the complex geologic and oceanographic conditions governing our planet's hidden underwater landscapes.

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