Silicate Weathering Drives the Carbon Cycle

Rocks break down through weathering or erosion, but did you know there’s a cycle from air to land to ocean that loops this process?

By Helen Petre

When mountains weather and erode, rocks from the tops of mountains become sand on the beach. Most rocks are silicates, which are made of silicon and oxygen. Silicates do more than just break down into smaller physical pieces when they weather. Chemical processes occur that take apart the elements and redistribute them with other elements in the air, land, and ocean. 

In a recent paper published in Nature Geoscience, Gerrit Trapp-Müller and colleagues propose that chemical silicate weathering cannot be studied as a wholly terrestrial process. Instead it should be studied as one continuous system, as products move from mountains, to oceans, to the atmosphere. Chemical silicate weathering is a globally dynamic process that drives the carbon cycle and affects the oceans, atmosphere, and climate. The interactions of weathered and eroded silicate rocks from mountaintops drive ocean acid base chemistry, and they modulate atmospheric carbon dioxide levels through negative feedback loops. 

Silicate rocks

Ninety-five percent of rocks on Earth are silicate rock—igneous rocks made by cooling magma. Sometimes these rocks are transformed into sedimentary rocks, by physically breaking down and cementing together, or metamorphic rocks, by high heat and pressure. Olivine, muscovite and biotite mica, quartz, and feldspar are silicate rocks. When mountains weather, the elements in the rocks break down, interact with other elements, and erode, eventually moving silica and other elements to the ocean. In the ocean, the elements interact again, in ways that affect the air, water, and plants on land. This in turn affects climate, and has for millions, or billions, of years. 

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Weathering and erosion

Weathering is the process of breaking down big things, like rocks, to little things, like grains of sand. A white beach is quartz sand; big pieces of mountain physically broken down into small pieces of mountain. Physical weathering of mountains occurs by freezing and thawing. Chemical weathering occurs by water dissolving elements. 

Erosion is the process of moving the weathered things somewhere else, usually by gravity or hydrologic processes. 

Chemical silicate weathering 

Over geological time, over millions of years, elements move from the mountains, through water to the ocean. As mountain rocks weather, they physically and chemically break down. The elements that were in the rock dissolve in water, and sometimes precipitate out as solids, or percolate out as gas, or get taken up by living things. So, how does this happen, and how does it relate to silicates? This is complicated. Physical weathering breaks big things into smaller of the same things, but chemical weathering changes things into something else. Since most rocks on Earth are silicates, chemical weathering of rocks on Earth is chemical weathering of silicates. Silicates are made mostly of silicon and oxygen, but they contain other elements. So, how do silicates chemically weather? Easy. Water. Water dissolves silicates by forming carbonic acid. Well, water is hydrogen and oxygen, so where does the carbonic acid come from? Easy. Air has lots of carbon dioxide in it and when carbon dioxide dissolves in water the result is carbonated water, like the fizzy stuff in soda, or carbonic acid.

Acid being acid, it dissolves stuff, like silicates. It is not really that easy. It takes millions, or billions, of years, and very specific conditions, but climatic conditions, specific minerals at Earth’s surface, temperature, and humidity define global weathering and drive climate cycles on Earth. Weathering of silicates and carbon cycles are interdependent. Slight changes make big differences, which is why they must be studied as a continuum. 

The researchers on this study represent many different locations and areas of expertise. They worked together to understand how rocks, plants, air, and temperature interact to affect each other. Their paper suggests that biological, chemical, and physical processes that drive the recycling of silica and carbon are interrelated. They are absolutely correct. Studying how mountains weather and erode and skipping the part where the dissolved products enter the ocean would be missing a big part of science. 

Interactions of carbon and the ocean

When carbonated water flows into the ocean, it changes the pH of the water. Most sea water is basic, or has a high pH. Water and carbon dioxide form carbonic acid. They can also form bicarbonate, which is a base, and has a high pH. At high pH, the carbon is stable and calcium carbonate can form shells of sea animals. If more carbon dioxide enters the water, the carbon can shift back to carbonic acid, which has a lower pH, and calcium can dissolve from shells of sea animals. Dead animals can fall to the bottom of the ocean and collect to form limestone, or sedimentary rocks, over millions of years. This is how carbon is sequestered. Carbon cannot go back out to the atmosphere, because it is locked in the rocks. If the pH goes down, the limestone can dissolve and the carbon goes back into the water, is used to make calcium carbonate shells, and the pH goes back up. Sometimes the sedimentary rock can be heated by hydrothermal vents and new igneous rock can form through sea floor spreading. 

The schematic figure shows how all this works. It is clearly a continuum, as the authors suggest. Adding plants into the cycle contributes more factors. 

Biological weathering

As early as the late 1800s, scientists noticed that rocks in forested areas weather faster than rocks in deserts. Decaying vegetation produces acid. Rainwater percolates through decaying vegetation into physically weathered rocks. Physically weathered rocks have lots of surface area exposed to slightly acidic rain water. Slightly acidic rain water combines with acids produced by decaying vegetation and dissolves silicate rocks faster than silicate rocks that are in areas with no vegetation and no water. So, lots of physical and chemical weathering of rocks from vegetated mountains effectively traps carbon dioxide, because carbon dioxide is in the water, and less carbon dioxide in the atmosphere means lower atmospheric carbon dioxide, thus cooling the Earth. 

Well, yes, but the researchers noted, this does not end here. Where does this carbonated water go? Since we all know it goes to the ocean, the researchers suggest we study silicate weathering and carbon cycling together. They propose that the increased amount of carbonic acid flows to the ocean through rivers, and what does it do in the ocean? It increases the amount of carbon dioxide in the ocean, which lowers the pH, which dissolves calcium carbonate shells of sea animals, which precipitates out, settles, and maybe is incorporated into sedimentary rock. Alternatively, carbon dioxide can escape the ocean as a gas, and effectively add to the atmospheric carbon dioxide. 

Rocks break down to inorganic elements necessary for plant growth

There is more than that. If we add plants, the cycle gets more complex. When mountains form, magma cools, physical weathering begins, and it rains, soil forms. Plants do not grow on rocks. The rocks must be broken down into soils in order for plants to be able to use the nutrients chemically weathered from the rocks, to grow. Chemically weathered rocks contribute iron, magnesium, phosphate, calcium, potassium, boron, and manganese, in addition to silica and oxygen, for plants to use. Nutrients, like fertilizer, induce plant growth. Plant roots push through rock, physically breaking it into smaller pieces, increasing the surface area for more chemical weathering. Earth tends to thrive on negative feedback loops. 

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Relationship between silicates weathering and climate 

Over millions of years, the carbon cycle on Earth is controlled by two main processes and the interactions between them: 1) volcanic eruptions which increase carbon in the atmosphere, or the anthropogenic processes which increase carbon such as burning fossil fuels; and 2) chemical weathering of cooled igneous rocks, or dissolution of silicate rocks by carbonic acid, which results in the sequestering of carbon in the oceans. So dissolving silicate rocks with slightly acidic water can moderate climate by taking carbon dioxide out of the atmosphere, thus reducing temperatures. Today burning fossil fuels has somewhat replaced volcanic eruptions, but not entirely. Still chemical weathering of silicates does reduce carbon dioxide in the atmosphere. 

Chemical weathering and atmospheric carbon dioxide 

When atmospheric carbon dioxide increases, Earth’s temperature increases. This causes chemical silicate weathering that reduces carbon dioxide in the atmosphere and completes the carbon cycle. The researchers are suggesting that the carbon cycle is driven by negative feedback. As carbon increases in the atmosphere, it triggers processes that result in the reduction of carbon in the atmosphere. The cycle transitions between mountain building, and weathering of mountains. Carbon builds in the atmosphere, and then decreases in the atmosphere, through normal physical processes.

Conclusion 

Chemical silicate weathering operates through negative feedback loops that operate as a continuum across mountains, atmosphere, and oceans. Considering that habitability of Earth depends on the stability of feedback loops, it is important to consider terrestrial, atmospheric, and oceanic factors as a continuum. The Earth is dynamic and chemical alteration of surface rocks is continually altering the carbon cycle, just as the carbon cycle is altering the chemical silicate weathering cycle. Nothing on Earth happens independently. The researchers suggest that the good news is that each time conditions change too far in one direction, negative feedback loops drive it back in the other direction.

It seems like Earth is able to stay relatively constant in regard to conditions conducive to life. Considering Earth has maintained relatively stable conditions for billions of years, there is good news suggesting it will continue to do so, within limits. In essence, chemical weathering is the Earth’s method for regulating carbon dioxide. 

This study was published in the peer-reviewed journal Nature Geoscience.

Reference

Trapp-Müller, G., Caves Rugenstein, J., Conley, D. J., … Zhang, X. Y. (2025). Earth’s silicate weathering continuum. Nature Geoscience,18, 691–701. https://doi.org/10.1038/s41561-025-01743-y.