Carbon capture & storage

How can we capture CO2 gas and cycle it naturally on Earth? What impacts could that have on Earth and to humans/wildlife?

Consider preceding this lesson with Gizmo’s Carbon Cycle Virtual Lab.

Check out this article regarding the experiment detailed here: Transforming Carbon Dioxide into Rocks.pdf

How does chemical weathering capture CO₂?

Basalt dust carbon capture relies on a natural process called chemical weathering. Weathering is the set of reactions by which minerals in rocks slowly break down when exposed to water, acids, and atmospheric gases. When certain silicate minerals in basalt react with carbon dioxide (CO₂) and water, the CO₂ is transformed into dissolved bicarbonate (HCO₃–) or solid carbonate minerals (like calcite or magnesite). In either form, the carbon is removed from the atmosphere and stored for long periods — this is called mineralization or carbon sequestration.

What is basalt, and how does it form?

Basalt is one of the most common types of volcanic rock on Earth — it forms the majority of the ocean floor and much of the crust beneath continents. It is a dark-colored, fine-grained igneous rock that solidifies from lava rich in iron and magnesium (mafic lava). Because basaltic lava is relatively low in silica, it is fluid and can flow easily, spreading over large areas before cooling.

Why basalt?

Basalt is a volcanic rock commonly found in the Earth’s crust. It contains silicon (Si), iron (Fe), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), and aluminum (Al) oxides. Those cations react with CO₂ (as dissolved CO₂/HCO₃⁻/CO₃²⁻) to form stable carbonate minerals (e.g., calcite, magnesite, siderite), making basalt a suitable natural mineral carbonation feedstock to form stable carbonate minerals. Because basalt is relatively common and contains the right chemistry, it is a practical candidate for enhanced weathering.

The role of grinding & surface area

Normally, rock weathering takes thousands of years. Scientists have learned that by grinding basalt into fine dust and spreading it over soil or mixing it with water, the surface area increases dramatically, and the reaction happens much faster. Basalt dust reacts with CO₂ in the air or in soil water, gradually turning the gas into harmless carbonate minerals. This process reduces CO₂ concentration in the atmosphere, helping slow global warming.

Benefits beyond carbon removal

• Soil fertility: Weathering improves soil health, because the weathering releases nutrients like calcium, magnesium, and iron that plants can use.
• Ocean chemistry: When bicarbonate is transported to the ocean it increases alkalinity, which can help counteract ocean acidification. Both effects are reasons researchers investigate enhanced weathering as a climate mitigation strategy.

A simplified silicate-to-carbonate reaction:

CaSiO₃ + CO₂ —> CaCO₃ + SiO₂

This shows CO₂ turning into a solid carbonate, with silicate converted to silica.

Olivine (a common basalt mineral) weathering in water (simplified):

Mg₂SiO₄ + 4CO₂ + 4H₂0 —> 2Mg²⁺ + 4HCO^₃- + H₄SiO₄

The magnesium ions and bicarbonate produced can eventually form stable carbonate minerals or be transported to the ocean and stored as dissolved inorganic carbon.

How this produces the classroom observation (bottle shrinkage)

In the student activity, CO₂ is dissolved into the water inside a sealed bottle, and basalt dust is added. If the basalt reacts with the dissolved CO₂, the gas concentration in the bottle decreases and the internal pressure drops, causing the flexible bottle walls to shrink. That shrinkage is a convenient, visual proxy that some of the CO₂ has been taken up by the water-rock reaction.

Responses to common student questions

“If rocks can capture CO₂, why don’t we just spread basalt everywhere?”

While basalt can help remove CO₂, using it on a large scale takes a lot of energy and material. The rock must be mined, crushed into fine dust, and transported, which itself can produce emissions if powered by fossil fuels. Scientists are studying how to make the process more efficient and environmentally safe, so it helps more than it harms. It’s a balance between benefits and practical limits.

“Is the captured CO₂ stored forever?”

When CO₂ reacts with basalt, much of it forms solid minerals like calcium carbonate or magnesium carbonate. These are stable for thousands to millions of years, effectively locking the carbon away. Some CO₂ also turns into bicarbonate dissolved in water, which can stay safely stored in oceans for very long times before eventually being buried in sediments. So yes — in most cases, it’s stored for very long periods.

“Could adding basalt dust hurt plants or animals?”

Basalt dust mainly contains natural minerals such as calcium, magnesium, and iron —— nutrients that many soils actually need. In moderate amounts, basalt can improve soil fertility and help plants grow. However, if too much dust is added or if it’s not well managed, it could change soil chemistry or release fine particles into the air. That’s why scientists test it carefully before using it in real environments.

“Can this really make a difference for climate change?”

Enhanced weathering alone isn’t enough to solve climate change, but it can be one part of the solution. It works best alongside reducing fossil fuel use and protecting forests. Think of it as a supporting tool — a natural chemical process that helps offset some of the CO₂ already in the air.

Real-world connections

• Sign up for Physicists To-Go to have a scientist talk to your students.
• Have students read and reflect on this year’s scientist profile on Eunice Newton Foote.