Environmental Science
Rock Weathering as a Natural Carbon Sink
Quick fact
Over millions of years, natural rock weathering removes billions of tons of CO2 from the atmosphere—about 100-200 million tons per year—equal to the carbon emitted by roughly 20-40 million cars.
Why this is interesting
Have you ever wondered how mountains help keep the planet cool? Over geologic time, rocks slowly eat carbon dioxide from the air.
Read the full explanation
Understanding Rock Weathering as a Natural Carbon Sink
Imagine raindrops falling on a granite mountain. The rainwater is slightly acidic, like a weak lemonade, because it absorbs carbon dioxide from the air. That acid reacts with the minerals in the rock, slowly breaking them down. This reaction consumes the CO2 that made the rain acidic and converts it into dissolved chemicals (bicarbonate ions) that wash into rivers and eventually into the ocean. In the ocean, tiny organisms take up these chemicals to build shells, and when they die, their shells sink to the seafloor, trapping the carbon in limestone. Over tens of thousands to millions of years, this process pulls carbon dioxide out of the air and locks it away in rocks, acting as a giant natural carbon sink.
A deeper explanation
The process starts with atmospheric CO2 dissolving in water to form carbonic acid (H2CO3). This weak acid attacks silicate minerals like feldspar in rocks, dissolving them into clay minerals (e.g., kaolinite), dissolved silica, and bicarbonate ions (HCO3-). The key chemical reaction is: 2KAlSi3O8 + 2CO2 + 11H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-. Importantly, this reaction transfers a carbon atom from air to the ocean in the form of bicarbonate. The bicarbonate ions are transported to the ocean, where marine organisms use them to build calcium carbonate shells. When these organisms die, the shells settle to the deep sea and become limestone. This burial removes carbon from the active carbon cycle for millions of years. The rate of weathering is highly sensitive to temperature and rainfall: warmer, wetter climates accelerate it. This creates a negative feedback loop—when CO2 levels rise and the planet warms, weathering speeds up, pulling more CO2 out and cooling the climate. This feedback has helped regulate Earth's climate for billions of years and is central to Earth's long-term habitability.