It’s hard to study extinct volcanoes; we never see them erupt. Using a unique experimental technique, we were able to recreate a particular type of extinct volcano in the lab and learn more about the magma these volcanoes produce. We found that certain types of magma are surprisingly effective at concentrating rare earth elements, a group of metals that have important applications in a variety of high-tech industries, such as magnets for electric cars and wind turbines.

As society moves away from fossil fuels and electrifies energy production and transportation, demand for rare earths is rapidly increasing. Despite their name, rare earths are not very rare. The biggest challenge is finding rocks where these metals are concentrated enough to be economically viable for mining. A study published in the journal Geochemical Perspective LettersSome extinct volcanoes are amazing places to explore.

Iron-rich magma in extinct volcanoes

There is a mysterious type of magma that contains extremely high amounts of iron. It is so rare that no such magma has ever erupted in history. Instead, only extinct volcanoes that were active millions of years ago are known.

The most famous example of such a volcano is El Laco in Chile. Another striking example is Kiruna in Sweden, where iron ore has been mined for decades. Last year, the operating company LKAB declared Kiruna to be Europe’s largest source of rare earth metals.

The Kiruna discovery has made us (and many others) wonder why rare earths are found in volcanic iron ore. We already know of many other types of rocks that contain rare earths, and none of them look like Kiruna or other extinct iron-rich volcanoes.

Was this just a geological coincidence, or is there something inherent in iron-rich magmas that makes them rich in rare earths? After all, many of these extinct volcanoes are known to be rich in iron, but no one has bothered to test whether they also contain rare earth resources. In addition, despite their rarity, iron-rich rocks are often easy to find because of their strong magnetic signals. Should they be added to the target list of rare earth explorers?

Reproducing volcanism in a bottle

To test this hypothesis, we used a machine called a piston cylinder. We put a synthetic material similar to volcanic rocks and magma into small capsules or “bottles” made of precious metals such as platinum. We then compressed them to a depth of 15 kilometers (9.3 miles) into the Earth’s crust and heated them to 1,100°C (3,000°F).

We found that under these extreme conditions, iron-rich magma bubbles within a more common type of magma known from nearly all modern active volcanoes. The iron-rich magma absorbs rare earth elements from the surrounding liquid.

These iron-rich bubbles will have different densities and viscosities and will separate from iron-poor environments, just as a mixture of water and oil eventually separates into separate layers.

Iron-rich magmas absorb rare earth elements so efficiently that their content is almost 200 times greater than that of the normal magmas around them. This means that the discovery at Kiruna is no coincidence. It is what we would expect from most, if not all, iron-rich volcanoes.

Why do we need more rare earth deposits?

Production of rare earths is concentrated in just a few countries, primarily China, as well as the United States, Myanmar, and Australia. For this reason, rare earths are classified as “important minerals”: they have important uses but are subject to supply chain risk due to geopolitical factors.

The significant increase in demand for rare earth elements has led to significant investment in exploration and discovery for additional deposits. The more deposits that are known, the better the industry can select those that will produce rare earths with the lowest financial, environmental and societal costs.

Extinct iron-rich volcanoes frequently produce iron ore. Our results suggest that existing mines in such locations could potentially be modified for rare earth production.

This would be a positive outcome; existing mining operations could gain additional value. In some cases, tailings could be processed to recover these critical metals. This could mean that new rare earth mines would not be needed and unnecessary damage to the natural environment would be avoided.