The dazzling sight of “blue lava” is a rare and visually stunning natural phenomenon. This ethereal electric-blue glow, often photographed streaming down a volcanic slope at night, suggests a temperature far beyond that of typical molten rock. Many observers assume the color indicates an even hotter form of lava, based on the understanding that color in fire relates directly to heat. Understanding this spectacle requires exploring the distinct chemical processes at play. The science behind the blue glow involves combustion, high pressure, and extreme temperatures, but the molten rock itself is not radiating the blue light.
The Misconception: Why Lava Appears Blue
The electric-blue flow is not molten rock, but the visible result of burning volcanic gases. This phenomenon is most famously observed at the Kawah Ijen volcano complex on the island of Java, Indonesia. Kawah Ijen is unique due to the extremely high concentration of sulfurous gases that emerge from fissures within the crater. These gases are pressurized and hot, containing an immense volume of elemental sulfur.
When these sulfur-rich gases contact oxygen in the air, they ignite spontaneously. The combustion of sulfur produces a bright, neon-blue flame. Because the gases exit under high pressure, these flames can shoot up to 16 feet (5 meters) into the air. The sulfur gases sometimes condense into liquid sulfur, which continues to burn as it flows down the slopes, creating the illusion of blue molten rock spilling down the mountainside. The effect is only visible at night because the blue flame is not bright enough to be seen against the sun’s light.
Temperature of the Blue Flame
The blue color of the fire is a direct result of the burning sulfur. The sulfur gases emerging from the volcanic vents at Kawah Ijen do so at extremely high temperatures, sometimes reaching up to \(1,112^\circ\text{F}\) (\(600^\circ\text{C}\)). This high temperature is responsible for vaporizing the sulfur and sustaining the combustion. The blue flame itself is caused by the sulfur reacting with atmospheric oxygen, a process that requires a minimum temperature of approximately \(680^\circ\text{F}\) (\(360^\circ\text{C}\)) to initiate.
The actual temperature of the blue flame, resulting from the burning sulfur, is often cited to be around \(960^\circ\text{F}\) to \(1,022^\circ\text{F}\) (\(515^\circ\text{C}\) to \(550^\circ\text{C}\)). This heat is generated by the exothermic reaction of sulfur combustion, which continues as long as gas is supplied and oxygen is present. The flame exhibits a uniform, vibrant blue hue due to the rapid and complete combustion of the sulfur gases. The heat from the volcano’s interior drives the entire process, providing the initial thermal energy necessary for ignition.
Temperature of the Molten Rock
The actual molten rock, or lava, flowing at Kawah Ijen is significantly hotter than the visible blue flame. The temperatures of typical basaltic lava, common in this region, range from approximately \(1,300^\circ\text{F}\) to \(2,200^\circ\text{F}\) (\(700^\circ\text{C}\) to \(1,200^\circ\text{C}\)). This range is much higher than the \(1,112^\circ\text{F}\) (\(600^\circ\text{C}\)) maximum temperature of the sulfur gas or the estimated \(1,022^\circ\text{F}\) (\(550^\circ\text{C}\)) temperature of the blue flame. This substantial difference highlights the separate nature of the two heat sources.
Lava at these temperatures glows a bright red or orange, consistent with the principles of black-body radiation. The blue flame is merely a side effect of the volcanic gases being released from the same vents as the lava. If lava were truly blue, it would need to be heated to an impossible temperature exceeding \(10,830^\circ\text{F}\) (\(6,000^\circ\text{C}\)), far hotter than any known terrestrial volcano. The molten rock provides the heat and pressure that forces the sulfur gas to the surface, creating the visually deceptive blue fire effect.