How Does the Earth Recycle Elements?

The rocks that make up our planet may seem old and indestructible. However, given enough time, they change and break down. Rocks are like chocolate chip cookies. Cookies (and rocks) are made of various ingredients. A cookie contains chocolate chips, sugar, flour, and salt. These can be compared to the minerals that make up rocks. The minerals are made of elements. Think of it like this: elements (such as oxygen, carbon, sodium) make up minerals (flour, sugar, salt, chocolate chips) which make up rocks (chocolate chip cookies). The minerals in a rock change when heated, just as cookie batter changes when you bake it. Other changes happen when minerals react with hot water or are squeezed at high-pressure conditions. In nature, this change is called metamorphism. If you could watch a mineral for millions or billions of years, you would see these changes. You would need more than time, though. The Earth’s recycling system is difficult to see because it occurs deep underground. To observe it, you would need to be able to see under the oceans and into the ground, because many of the changes happen inside the Earth.

Recycling occurs because our planet is in motion. The Earth is covered in large moving pieces, called tectonic plates. Sometimes these are just called plates. The crust of continents and oceans is the top part of these plates. Plates can move toward each other or away from each other, and both types of plate motion drive the planet’s recycling system. How does it work? Where plates move apart, the gap, called a rift, is filled by hot rocks and lava that rise up from below. Where two plates move toward each other, the older, denser plate will dive under the other. This is called subduction. This cycle of formation (at rifts) and destruction (subduction) of plates renews the Earth’s supply of minerals, water, and other essential materials. This takes many millions of years, so humans can never see the whole recycling process.

Subduction can be seen as a “factory” because raw materials (minerals) go in, the Earth works on them, and then something different comes out (Figure 1) [1, 2]. The process starts under the oceans, where the raw material of the “factory” is formed at rifts in a plate. In the rift, new rock is created and added to the plate. When the plate eventually becomes old and dense, it subducts into the Earth, where minerals are buried and heated (metamorphosed) until they break down. When the minerals break down, they release trapped water and other elements. Some minerals in the rocks melt. These products of the “factory” make their way back to the Earth’s surface in melted rock and gases.

One way to study this recycling process is to analyze the lava and gases erupted by volcanoes. Another way is to find rocks that were subducted and then brought back to the surface. These rocks escaped melting and other breakdown processes. The minerals in these rocks contain clues about the recycling process. We can analyze minerals that still contain water and other elements. One of the most useful minerals for investigating the subduction factory is lawsonite. Lawsonite is a water-rich mineral that only forms in subducting rocks.

Studying Lawsonite to Learn About Earth Recycling

Newly-formed rocks in rifts deep in the ocean react with hot seawater, forming new minerals that contain water. Later, when these minerals are subducted, they undergo more reactions. This is how water-rich lawsonite forms during subduction (Figure 1). Lawsonite is important because it contains a lot of water and other important elements, including tiny amounts of uranium and lead. Each of these elements is important in Earth recycling, even in small amounts. Lawsonite carries these elements tens to hundreds of kilometers into the Earth. When lawsonite reacts with other minerals and starts to break down, it releases water and other elements. Water is the ingredient needed to melt the hot rocks that are above the subducting plate. This triggers the next step in the element recycling process, as magma (melted rock) transfers elements and water from the inside of the Earth to the surface. Since lawsonite is an important contributor to the Earth’s recycling process, we can study it to understand how the deep parts of the “factory” work.

Geologists still do not know a lot about lawsonite because it is only stable in subducting plates. No other place in the world has the right combination of pressure and temperature for lawsonite to be stable. Also, lawsonite easily breaks down to form other minerals. So, despite its great abundance deep underground in subducting plates, we almost never see it in rocks at the Earth’s surface [3, 4]. This is good for the planet, but not so good for geologists who want to study it! Fortunately, lawsonite does occur in some rocks, so we can collect it and study it. Lawsonite mainly occurs in rocks from plates that were subducted to low or medium depths in the Earth. These rocks are called blueschists because they contain a distinctive blue mineral (Figure 2). Lawsonite can also occur in rocks called eclogite that form at greater depths (Figure 2). There are <20 places in the world where lawsonite occurs in the deep rocks [5], including Turkey, Corsica (France), British Columbia (Canada), the Dominican Republic, Guatemala, Australia, and the Alps. In addition, there is an interesting place in Arizona (USA) where pieces of lawsonite eclogite from a subducted plate have erupted from a now-extinct volcano.

What Can We Learn From the Composition of Lawsonite?

As lawsonite forms, it inherits the elements of the rock it grows in. However, lawsonite can also change in composition as it grows. As a result, one part of the lawsonite crystal may have different amounts of particular elements than another part. This is called zoning. Like the tree rings that grow during the lifespan of a tree, mineral zoning provides a record of information about the chemical reactions, fluids, and elements involved in the recycling process. For example, zones resulting from different amounts of the element chromium in a lawsonite crystal might indicate that a fluid containing dissolved chromium interacted with the lawsonite crystal at certain times, but not at other times (Figure 3). We can try to figure out where possible sources of chromium in the deep Earth recycling factory might be, such as the hot, dense rock called the mantle, which is located directly above the subducted plate.

Why would the amount of chromium have fluctuated as the lawsonite crystal grew? Geochemists do not yet know the answer to this question. One idea is that pulses of fluid move through the rock when there is an earthquake deep in the Earth. If this is true, then lawsonite composition and zoning is a record of dramatic chemical and physical processes that occurred in the Earth’s past.

Summary

Life on Earth is possible because of the recycling of water and elements through the tectonic plate system. As plates subduct, elements from the Earth’s surface move to the interior of the planet in minerals. When these minerals break down, the released elements make their way to the Earth’s surface through magma and gases that erupt at volcanoes. Studying the composition of minerals such as lawsonite is important because we can discover more about the recycling process that takes place deep underground where we can not see it directly.

How the Earth recycles affects the composition of the Earth’s rocks, air, and ocean. If the Earth recycled faster or slower–or not at all–the planet would look very different. For the same reason, minerals that make up the outermost solid layer of the Earth, called the crust, would have different compositions depending on the recycling process. These minerals are the raw materials for technology, medicine, agriculture, and many other uses. For example, a mineral like copper is used in coins and electrical wire. Just like vitamins, calcium, and iron are important nutrients for your body and also for plant growth. No matter where you are when you read this article, you are using and surrounded by minerals, a result of the ongoing recycling process of our planet.

Glossary

Metamorphism: ↑ A change in the minerals, texture, or composition of a solid rock. This process may occur due to pressure or temperature changes or the introduction of water or other fluids.

Tectonic Plate: ↑ A large slab of solid rock that floats on top of Earth’s mantle. Tectonic plates form the world’s continents and oceans and are between 15 and 200 km thick.

Rift: ↑ A linear gap in oceanic or continental crust where tectonic plates move apart.

Subduction: ↑ The process by which one tectonic plate sinks below another plate.

Lawsonite: ↑ A metamorphic mineral that forms in subduction zones. It contains high amounts of water and the elements Ca, Al, Si, O, and small amounts of other elements.

Zoning: ↑ Variation in the composition of a crystal of a mineral, resulting from changing conditions during its growth.

Mantle: ↑ A large volume of hot, dense rock that lies between the crust and the iron-rich core of the Earth.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.