Cobalt-Rich Ferromanganese Crusts: The Overlooked Deep-Sea Mining Frontier

The Mineral Nobody Talks About

When journalists cover deep-sea mining, they almost always focus on polymetallic nodules — those potato-sized lumps scattered across the Clarion-Clipperton Zone. Yet by sheer number of ISA exploration contracts, cobalt-rich ferromanganese crusts dwarf every other resource category. The Abyssal Claims database tracks 751 active concessions targeting crusts, compared to 504 for polymetallic sulphides and just 63 for nodules.

Why so little attention? Partly because crusts are harder to visualise, and partly because the technology to mine them at commercial scale doesn't yet exist. But the environmental stakes are just as high — and in some ways higher.

What Are Ferromanganese Crusts?

Cobalt-rich ferromanganese crusts form by a process called hydrogenous precipitation: manganese, cobalt, nickel, iron, and trace elements slowly precipitate from seawater onto the bare rock surfaces of seamounts, ridges, and plateaus over millions of years. Growth rates are extraordinarily slow — as little as 1–5 mm per million years. A crust 5 cm thick may represent 50 million years of accumulation.

This makes them fundamentally different from polymetallic nodules. Nodules sit loosely on soft sediment and can theoretically be vacuumed up. Crusts are cemented onto hard rock. Mining them requires cutting, scraping, or blasting the substrate — and taking the crust inevitably means taking the rock beneath it.

Where They Are Found

Crusts occur at depths of roughly 800 to 2,500 metres — shallower than the abyssal nodule fields (4,000–6,000 m) but still well beyond continental shelf boundaries. The richest deposits concentrate in three ocean regions:

• Central and Western Pacific — particularly around seamounts in the Marshall Islands, Johnston Atoll, and the Line Islands
• Central Atlantic — the Mid-Atlantic Ridge and associated seamount chains
• Indian Ocean — especially around the Afanasy-Nikitin seamount cluster

The ISA has granted exploration licences across all three regions. Korea (KIOST), China (COMRA), Japan (JOGMEC), France (IFREMER), Germany (BGR), and Russia hold the largest crust portfolios.

Why Cobalt?

The name says it. Crust cobalt grades of 0.3–1% are typically two to three times richer than terrestrial cobalt ores. With cobalt central to lithium-ion battery cathodes — and the Democratic Republic of Congo supplying roughly 70% of global production amid persistent human rights concerns — ocean cobalt has attracted strategic interest from multiple governments.

But crusts also carry platinum-group elements (PGEs), tellurium (critical for thin-film solar cells), and rare earth elements in concentrations that make them interesting beyond cobalt alone. Japan's interest in particular is driven by tellurium and REEs for its electronics and energy industries.

The Seamount Problem

Here is where the ecological concern sharpens. Seamounts are not bare rock. They are among the most biodiverse habitats in the deep ocean — isolated volcanic peaks that concentrate zooplankton, fish, and filter-feeders in mid-water, while their flanks host cold-water coral gardens, sponge fields, crinoid beds, and a diversity of invertebrates found nowhere on the abyssal plain.

The same hard substrate that makes crusts valuable is the attachment point for these communities. Crust mining removes both at once. Unlike sediment-dwelling fauna on the abyssal plain, which might theoretically recolonise disturbed areas given centuries of time, organisms anchored to bare rock — including the rock itself — cannot return once the surface is gone.

The Abyssal Claims map shows thousands of hydrothermal vents and biodiversity hotspots within 50 km of active crust concessions. The combination of seamount ecology and slow-growing crusts means that recovery timescales, if recovery is possible at all, are measured in geological rather than human time.

What Comes Next

The ISA Mining Code — the regulatory framework that would govern commercial crust extraction — remains incomplete and contested. Unlike nodule mining, which at least has several companies in advanced trial stages, commercial crust technology is further from deployment.

But the concessions are in place. The licences exist. And when (or if) the technology matures, 751 active contracts cover areas that took tens of millions of years to form.

Understanding what cobalt-rich ferromanganese crusts are, and what removing them means, is the first step toward any informed conversation about whether mining them should happen at all.

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Explore all 751 crust concessions on the Abyssal Claims interactive map. Filter by resource type to see contractor portfolios, vent proximity, and species risk scores.