Basalt is the most prominent igneous rock on Earth’s crust. Basalt lava has a relatively low viscosity and generates thin flows that can travel great distances due to its low silica concentration. The dominant minerals present in basalt are plagioclase and pyroxene. The spatio-temporal distribution of basalt is ubiquitous on the planet Earth e.g. from the Archaean Greenstone belts to modern Island Arcs and Mid-ocean ridges. Basalts are generally formed via the decompression melting of the mantle at the ridges and fluid-flux melting at the subduction zones on the tectonic plate margins. Along with it basalt also formed in the intra-plate zones associated with hot spots and mantle plumes e.g. Hawaii Emperor seamount chain and different flood basalts. Flood basalts, such as the Columbia River Basalts in the Northwest United States and the Deccan Traps in India, are thick successions of basalt that erupted onto continents during brief periods.
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Tholeiites, which are silica-saturated to oversaturated, and alkali basalts, which are silica-undersaturated, are the two primary chemical subtypes of basalt. Alkali basalts are frequently found in continental magmatism and on oceanic islands, where tholeiites predominate in the top layers of the oceanic crust. Basalts can be categorized chemically into three major types according to the level of silica saturation. The best way to see this is to first cast the analyses into molecular CIPW norms, which are equivalent to CIPW norms except that the findings are expressed as mole % as opposed to weight %). Because of this, the normative minerals olivine, clinopyroxene, plagioclase, and quartz or nepheline make up the majority of basalts. These minerals belong to the Ol-Ne-Cpx-Qtz four-component normative system, which is shown as a tetrahedron here.
Global warming is generally caused by a rise in global average temperature. The reason behind the rise in this temperature is the surplus amount of carbon dioxide (CO2) in the atmosphere. Every year we humans liberate 40 Gt of CO2 in the atmosphere and if not captured and stored, this can even accelerate the rise in global average temperature. The amount of carbon dioxide in the atmosphere has increased to levels never seen before in human history, topping 280 parts per million (ppm) in preindustrial times in 2023. Over the previous ten years, this growth has happened at a pace of about 2.3 ppm each year, which is almost 100 times quicker than natural changes. Geological systems pose a huge potential for storing carbon dioxide captured directly from the atmosphere and industries.
Compared to other geological formations, basalt formations have a higher CO2 storage efficiency because of their exceptional reactivity, extensive availability, and geological durability. Because basalt contains reactive minerals, mineral carbonation can occur quickly and effectively, allowing for excellent CO2 storage for thousands or even millions of years.
Basalt's chemical characteristic as a carbon-storing rock
Magnesium, calcium, and iron-containing minerals are among the reactive minerals found in basaltic rocks that react easily with CO2. When these minerals come into contact with CO2, they can go through reactions of dissolution and reprecipitation that turn CO2 into solid carbonate molecules and the process is called mineral carbonation. Mineral carbonation requires interactions between water and rock. As a host rock, basalt permits the infiltration of water and dissolved CO2 into its structure. The disintegration of basalt minerals and the subsequent precipitation of carbonate minerals are made possible by this interaction.
The first step in this mineral carbonation process is the dissolving of CO2 in water, which produces carbonic acid. The formation of carbonic acid changes the in situ water's pH and increases its reactivity. Because of this increased reactivity, the hydrogen ions (H+) in the solution interact with the basaltic rock to dissolve the main minerals found in the rock’s mineralogy, releasing cations into the solution such as iron (Fe2+), calcium (Ca2+), and magnesium (Mg2+). As basalt undergoes carbonation, different minerals show different chemical reactions. These include olivine, plagioclase, pyroxene, spinel, and others. Calcite, dolomite, and magnesite are chief carbonate minerals produced chemically by the reaction of calcium and magnesium ions with CO2 infiltrated into basalts. This process is a slow-moving, naturally occurring chemical reaction that happens as rocks weather and erode. Mineral carbonation reactions can occur either in situ, or below earth, and ex-situ, or above ground. While the in-situ approach involves injecting CO2 into porous rock, the ex-situ method requires mining and grinding of the rock (for natural minerals) before the CO2 reaction. A comprehensive study of the geochemistry of basalt is crucial for the efficacy of CO2 storage initiatives. In basaltic rocks, geochemical processes are crucial for establishing the stability and long-term fate of CO2 storage. Several studies have established the potential for storage of long-term carbon dioxide in basalt and even in volcanic edifices.
The various techniques through which basalt can be used for carbon capture and storageÂ
There are several methodologies through which basalt can be used as a storage for carbon dioxide captured from the atmosphere. The major processes involve in-situ mineral carbonation, enhanced rock weathering, direct injection of CO2 into basalt aquifers etc.
This In-Situ Mineral carbonation involves injecting CO2 a great distance into permeable and porous subterranean basalt deposits. A sequence of chemical reactions occurs when the CO2-rich fluids are injected and come into touch with the minerals in the basalt. When CO2 is dissolved in water, carbonic acid is created. This acid then interacts with the feldspar, olivine, and pyroxene minerals found in basalt. Stable carbonate minerals, such as calcite, magnesite, and serpentine, are the products of these interactions. The acquired CO2 is efficiently sequestered by these carbonates, which are safely deposited in the geological formations.
The increased pace of weathering and chemical alteration (mineral reaction) that occurs naturally in geology is replicated by the Enhanced Rock Weathering method. Rainwater becomes slightly acidic (carbonic acid, pH 5–5.5) when carbon dioxide dissolves in water droplets. This solution can react with various minerals at the surface depending on their chemical stability. ERW is a chemical process that involves dispersing finely ground or powdered silicate rocks (rich in calcium and magnesium) with a highly reactive surface to accelerate the chemical interaction between the rocks, water, and CO2 in the air. The bicarbonate that is formed stores the CO2. Eventually, this bicarbonate washes into the seas, where the carbon is either trapped in a soluble form on the sea floor or is conserved.
Direct injection technique involves injecting CO2 into formations that already contain water the process of injecting CO2 into deep subterranean basalt aquifers. Carbonic acid is produced when the injected CO2 dissolves in the water and combines with the minerals in the basalt. As a result of this reaction, minerals dissolve and carbonate minerals precipitate, which are subsequently preserved in the aquifer.
Scaling basaltic rocks for carbon storage are scientific endeavors that require attention. Businesses that address climate change, such as UNDO in the UK and CarbFix in Iceland, are utilizing these technologies to practically store carbon dioxide on a commercial scale. However, additional participation from other businesses and the government is needed. Several mitigation techniques have been proposed by the Intergovernmental Panel on Climate Change (IPCC) to keep global warming below 1.5–2°C of preindustrial levels by the year 2100 (IPCC, 2018) and storing captured CO2 in basalt has a huge potential. Importantly, all mitigation plans ask for the active removal of atmospheric carbon dioxide on the scale of 100 to 1,000 gigatons (billion tons) throughout the next century to keep global warming below 1.5°C (IPCC, 2018).
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