Do Pebbles Smell Weird Because They’re Rotting?

Warning: Not a pebble! (And also smells really bad)

Pick up a pebble after it rains, or break one open (with a hammer!), and there is sometimes a faint smell. Depending on the rock at hand, it may smell metallic, earthy, or even disgustingly reminiscent of eggs. However, rocks consist of non-organic minerals and so cannot undergo biological decomposition. The smell instead comes from small amounts of gas released when minerals react with their surroundings and also when microorganisms interact with the rock surface [1, 4].

But first, where do smells even come from?

Smells are created by volatile molecules which are small and light enough to evaporate from solid or liquid surfaces and stimulate olfactory receptors in the nose. Common examples of volatile compounds include alcohols, aldehydes, ketones, and sulfur-containing compounds, which are especially noticeable because the human body can detect them at very low concentrations. What’s important to know is that a material does not need to be alive, or organic, to produce a smell. It only needs to release enough volatile molecules for our body to perceive them [2].

Weathering as a source of volatile gaseous products

At the Earth’s surface, rocks are altered by three different types of weathering. Physical weathering breaks them into smaller fragments; chemical weathering changes their mineral composition through reactions with oxygen, water, and weak acids; and finally, microorganisms biologically weather mineral surfaces by secreting organic acids and enzymes. During these changes, small quantities of gases are released, including volatile compounds [1, 4].

Aside from weathering, when a rock is manually fractured or cracked, fresh mineral surfaces are exposed to the air. These surfaces are chemically reactive because they have not yet reached equilibrium with the surrounding environment. Iron-bearing minerals, for instance, oxidise when exposed to oxygen and water. Sulfur compounds can also oxidise depending on whether the environment is aerobic or anaerobic. For example, sulfur dioxide (SO₂) has a sour, acrid odour, while hydrogen sulfide (H₂S) has a characteristic ‘rotten egg’ smell [3].

What about stronger metallic or sulfuric smells?

The metallic smell characteristic of some rocks is not caused by iron itself floating in the air. Metallic iron and iron oxides such as Fe₂O₃ are not volatile, so they cannot be directly smelled. Instead, the smell comes from secondary reactions at the rock surface that generate volatile organic compounds.

When iron-containing minerals are exposed to air and moisture, redox reactions occur, which catalyse the oxidation of organic matter present in the environment, such as residual lipids from soil, water films, or even compounds on the skin. One important product of these reactions is ketones such as oct-1-en-3-one (C₈H₁₄O), which have a strong metallic or blood-like odour. These compounds form when unsaturated fatty acids undergo oxidative cleavage (breaking apart) in the presence of catalysing ions Fe²⁺ or Fe³⁺ [7].

Sulfur-containing minerals produce a more potent smell because they release volatile gases directly. When reduced, minerals such as pyrite (FeS₂) can produce hydrogen sulfide (H₂S), giving some rocks their characteristic rotten egg smell. Under more oxidising conditions, sulfur dioxide (SO₂) can form, making a sharp, acrid odour [3].

Variation between rock types

Ultimately, the chemical composition of a rock determines what reactions can occur and which aforementioned gases may be released. Basalt contains iron- and magnesium-rich minerals, so oxidation processes within it would produce earthy scents. Limestone is primarily calcium carbonate (CaCO₃), which reacts mainly through dissolution in acidic water and produces fewer volatile compounds, hence a weaker smell. Sedimentary rocks such as shale may actually contain organic material that releases more complex compounds when exposed. The smell of metamorphic rocks varies widely depending on their mineral composition, since they comprise sedimentary and igneous rocks [5].

Miniature processes, macro timescales!

Microbial activity in rocks may be slow, but it directly determines how minerals are broken down and transformed over time. Lithotrophic microbes use compounds like iron (Fe²⁺, Fe³⁺) or sulfate (SO₄²⁻, H₂S) for energy. In the process, they destabilise mineral structures and release byproducts such as dissolved ions and trace gases, including CO₂ and sulfur compounds [4].

One interesting example is basalt on the ocean floor. Microbes insert themselves into fresh volcanic rock and accelerate its alteration into clay minerals by driving oxidation reactions in iron-rich minerals like olivine. This not only changes the structure of the rock, but also releases elements such as iron and magnesium into seawater. At the same time, these reactions contribute to long-term carbon storage by catalysing processes that remove CO₂ from the ocean and atmosphere and embed it back into the crust [4].

On land, the same process occurs during soil formation. Microbes on newly exposed rock surfaces produce organic acids and carry out redox reactions that weaken minerals and release nutrients such as phosphorus, which would otherwise be locked in insoluble forms. Without this step, many ecosystems would not be able to flourish [4].

To identify volatile gases, scientists use gas chromatography-mass spectrometry (GC-MS), a technique used to separate a vapourised sample into its individual components. This technique can be combined with mass spectrometry which identifies each compound based on its mass-to-charge ratio, allowing very small quantities of gas to be detected and identified [6].

What do…space rocks…smell like?

Most of us reading this article will probably never sniff a lunar rock, but we do have accounts of what they smell like. When samples from the Apollo moon missions were opened inside the spacecraft, astronauts described a burnt, gunpowder-like odour. But why?

Well, the dust had been sitting in a vacuum, exposed to micrometeorite impacts and solar radiation, which left the mineral surfaces highly reactive. Once inside the cabin, those surfaces came into contact with oxygen and moisture for the first time. Rapid reactions at the surface produced volatile compounds, which is what created the smell [8]. If we were to cut open a meteorite, the same reactions could produce a metallic or sulfuric smell much stronger than those we find from terrestrial rocks.

In the end, rocks are teeming with chemical and biological processes, both within their mineral structures and on their surface. Their smell is just one manifestation of this non-stop activity, which continues for millennia after the rock formed. But it sure makes me wonder: why don’t we have any rock-scented perfumes?

References:

1. Insam, H. & Seewald, M. S. A. “Volatile organic compounds (VOCs) in soils.” Biology and Fertility of Soils (2010).

2. United States Environmental Protection Agency (EPA). “Volatile Organic Compounds (VOCs).”

3. Symonds, R. B. et al. “Volcanic gas studies: methods, results, and applications.” Reviews in Mineralogy and Geochemistry (1994).

4. Edwards, K. J., Bach, W. & McCollom, T. M. “Geomicrobiology of ocean crust.” Trends in Microbiology (2005).

5. Lumen Learning. “Putting It Together: Rocks and the Rock Cycle.”

6. Thermo Fisher Scientific. “Gas Chromatography–Mass Spectrometry (GC-MS) Information.”

7. ScienceDaily. “‘Metallic’ odor of iron derives from skin oil decomposition.” (2006).

8. NASA. “Apollo Lunar Surface Journal – Dust and Smell Observations.”