What Are Polymetallic Nodules?

Way out in the middle of the ocean, deep beneath the surface, explorers may encounter a curious sight— thousands of potato-sized rocks littering the ocean floor like a shipment of spilled ping-pong balls. These curious little rocks may appear simple on the surface, but there is more to them than meets the eye. Let’s take a closer look at how these rocks – called polymetallic nodules — form, where we can find them, and why they’re important.

What are polymetallic nodules?

Polymetallic nodules – also called manganese nodules – are small, hard, round rocks that naturally form at the bottom of the ocean. While few people have heard of them before, polymetallic nodules occur in nearly every ocean on Earth. Nodule fields can be immense, covering many thousands of square miles of flat, sandy seafloor, known as abyssal plains. Nodules form in deep water, usually between 13,000 and 19,000 feet (3,960-5790 m) below the surface. They typically measure just shy of one to three inches (2-8 cm) in length and can vary dramatically in appearance: from smooth to rough, and from spheres, to ovals, to discs, to blobs. Nodules are also very porous, meaning they are full of tiny holes – with empty space making up at least 25 to 60 percent of their total volume. For comparison, sponges are about 60 to 90 percent porous, soil is about 50 percent porous, and solid rock, like granite, is only about 1 percent porous. Polymetallic nodules are different from other rocks on the seafloor in that they are not formed from lava, nor are they formed within the Earth’s crust like diamonds or coal – instead, they are formed from the seawater itself!

How do polymetallic nodules form?   

Polymetallic nodules form naturally by “precipitating” out of ocean water. Precipitation, in chemistry, is when something separates from what it was dissolved in.  Leave a container of syrup – which is sugar dissolved in water - alone for a very long time, and sugar crystals can form at the very bottom of the container. These sugar crystals precipitated out of the water they were dissolved in, forming a solid substance.

A similar process occurs at the very bottom of the ocean. Ocean water is not simply a mixture of water and salt, but of many different elements, including manganese, iron, oxygen, calcium, nickel, zinc, lithium, and cobalt, just to name a few. Mollusks, like snails and scallops, build their shells from calcium dissolved in ocean water. Polymetallic nodules are formed from a mixture of different elements (“poly-metallic” meaning “composed of many metals”), but mainly from the elements manganese and iron.  

Polymetallic nodules form over many millions of years, building up layer by layer. In fact, most nodules we find today started forming between eight and 32 million years ago! The first step to growing a polymetallic nodule on the ocean floor requires something on which to grow. This may be a fallen shark tooth, a piece of bone, or even a small rock and can occur through one of two main ways: through hydrogenetic or diagenetic growth.  

“Hydrogenetic growth” is where nodules form from ocean water just above the seafloor. In this process, oxygen reacts with dissolved iron and manganese in seawater, changing them in such a way that they can no longer stay dissolved (called “oxidation”). They then precipitate and form a thin layer of rock on whatever hard surface is nearby. Metals like cobalt, nickel, lithium, and zinc often hitch a ride into solid forms during this process, too. This type of growth is very slow, with hydrogenetic nodules growing about one to five mm per million years.

“Diagenetic growth,” on the other hand, is where nodules are formed from the ocean water trapped in seafloor sediments. In this process, animal and plant waste that has sunk to the bottom of the ocean reacts with oxygen as it decomposes. This process eventually uses up all the oxygen within the seafloor sediments. When that happens, decomposition switches to using other reactive elements —like manganese and iron— instead of oxygen. These minerals get “oxidized,” just like in hydrogenetic growth, and thus precipitate out of seawater and form a layer of rock. Unlike hydrogenetic growth, however, diagenetic growth is much faster, with diagenetic nodules growing up to 250 mm per million years.

Most nodules grow through a mixture of these two processes, depending on how much oxygen is in the seawater at any given time. Nodules may also grow at oceanic hot springs (called hydrothermal vents), or through bacterial activity on the seafloor.

Where do they occur?

Polymetallic nodules can be found around the globe. There are known nodule fields located throughout the Atlantic Ocean, including near the Eastern U.S., Brazil, Argentina, and off Western Africa. There are also fields in the Indian Ocean, the Southern Ocean, and throughout the Central Pacific. Nodules can also be found in freshwater lakes and rivers, but only when there is enough manganese and iron present.

Nodules occur in some regions and not others because they require very specific conditions to grow. First, they usually grow best in areas where the seafloor is sandy and flat. There must be plenty of starting materials, such as pieces of bone or rock, as well as plenty of metals in the environment – particularly manganese and iron. In the deep ocean, the most common source of metal is from microscopic plants (called “phytoplankton”) that live at the surface but sink down to the bottom of the ocean when they die. The ocean floor also can’t move too much. It is unlikely to find a nodule field in regions with a lot of tectonic activity, like areas that are prone to earthquakes or volcanoes. Lastly, water movement at the seafloor must be fast enough to prevent growing nodules from becoming buried by sediment, but slow enough that essential elements aren’t swept away.   

What role do they play in the ocean? 

The bottom of the ocean is very soft and squishy – made up of a thick mud that’s even finer than the sand you find on the beach. To prevent getting stuck in this muck, most deep-sea animals prefer to live on solid ground. Polymetallic nodules, therefore, are important because they provide animals at the bottom of the ocean with much-needed hard surfaces to live on.

In the nodule fields of the Eastern Pacific Ocean – an area called the Clarion-Clipperton Zone – more than 600 different types of animals have been found living on polymetallic nodules. In fact, the concentration of animals found on nodules is about two to three times higher than nearby areas of the seafloor without nodules. Nodule fields host a diversity of life, including glass sponges, corals, crabs, shrimps, snails, brittle-stars, bryozoans, and scavenger fishes like rattails and eelpouts. Within a nodule field, some species only live within a very small area, making them very rare and vulnerable to disturbances.  

Because polymetallic nodule fields are so hard to get to, their exploration is still relatively new! Recent exploration at nodule fields have revealed new species, chemical processes, and even, potentially, a new source of oxygen on our planet. Despite these discoveries, there is still plenty about these unique environments that we have yet to learn.