Lava, the molten rock expelled from a volcano, has an odor that is frequently described as sulfurous. This smell does not originate from the superheated rock itself, but from volatile chemical compounds dissolved within the magma deep beneath the surface. As magma rises and erupts as lava, the tremendous drop in pressure causes these dissolved gases to rapidly escape. This process, known as magmatic degassing, creates the plume that carries the powerful scent associated with volcanic activity.
The Source of the Scent
The primary odors defining the volcanic atmosphere come from two main sulfur-containing gases: hydrogen sulfide (\(\text{H}_2\text{S}\)) and sulfur dioxide (\(\text{SO}_2\)). These compounds are the most common sulfur species released from the melt. The specific scent profile depends on which of these gases is dominant at a given location and time.
Hydrogen sulfide (\(\text{H}_2\text{S}\)) is characterized by a foul, unmistakable smell often compared to rotten eggs or decaying organic matter. This gas typically forms in environments where volcanic pathways are cooler and wetter, such as geothermal areas. In contrast, sulfur dioxide (\(\text{SO}_2\)) has a sharp, acrid, and pungent aroma, similar to a struck match or burning fireworks. \(\text{SO}_2\) is the more abundant gas released directly from hot, dry lava flows and high-temperature vents.
The release of these gases is driven by magmatic degassing, where sulfur is held in the magma under high pressure far below the volcano. As the molten rock ascends and the pressure decreases, the sulfur separates from the melt to form gas bubbles. This process is highly sensitive to temperature and pressure, which determines the ratio of \(\text{H}_2\text{S}\) to \(\text{SO}_2\) that ultimately escapes into the air.
Secondary Smells and Environmental Factors
Beyond the primary sulfur gases, the intense heat of lava creates a range of secondary smells as it interacts with the immediate environment. When a lava flow encounters vegetation, the organic material instantly burns. This combustion produces a smoky, acrid smell often mixed with the strong odor of charring wood and soil.
The interaction of the lava with the non-magmatic rock and soil it flows over also contributes to the local aroma. The high temperatures can cause the breakdown or vaporization of non-sulfur minerals and moisture, resulting in a smell sometimes described as hot, dry stone or superheated metallic dust. Furthermore, the \(\text{SO}_2\) released can react with moisture and oxygen in the atmosphere to form fine sulfate aerosols, creating volcanic smog, or “vog.”
The Toxicity Behind the Odor
The characteristic smell of lava-related gases serves as a natural warning sign. Both sulfur dioxide and hydrogen sulfide are toxic, and their health effects vary significantly with concentration. Sulfur dioxide is highly irritating, causing immediate burning sensations in the nose, throat, and eyes. High exposure levels can lead to serious respiratory issues and pulmonary fluid buildup.
Hydrogen sulfide presents a more insidious danger because it is toxic at lower concentrations than \(\text{SO}_2\), and it rapidly dulls the sense of smell. While the rotten egg odor is detectable by the human nose at extremely low levels, exposure to higher concentrations, such as 100 parts per million (ppm), can lead to olfactory fatigue within minutes. This deadening of the sense of smell means a person can no longer detect the warning odor.