Molten rock deep beneath the Earth’s surface is known as magma. When this material erupts onto the surface, it is called lava. While the common image is a bright, fiery orange or red river of liquid rock, the actual color depends entirely on its temperature. That signature glow is not a pigment inherent to the rock itself, but a direct visual indicator of the extreme conditions under which the rock exists.
The Science Behind the Glow
The characteristic light emitted by magma and lava is called incandescence, a process where an object radiates visible light due to its intense heat. This is a form of blackbody radiation, meaning the color observed is directly proportional to the object’s absolute temperature. The glow is not caused by any particular mineral or chemical compound, but by atoms vibrating at such a high frequency that they emit photons of light.
At the lowest temperatures where molten rock is still flowing, around 475 degrees Celsius, the material emits a dark, dull red light. As the temperature rises, the peak energy of the emitted light shifts toward shorter wavelengths, causing a visible change in color. Lava flowing around 900 degrees Celsius appears as a bright, vibrant orange.
If the temperature reaches 1,000 to 1,150 degrees Celsius, the color progresses to a brilliant yellow-orange. At temperatures exceeding 1,150 degrees Celsius, the rock can appear white-hot, indicating the greatest amount of thermal energy. This continuous spectrum of color provides geologists with a quick way to estimate the heat of a flow from a distance.
How Composition Changes the Color
While temperature directly causes the glow, the chemical composition of the magma dictates the temperature range at which it exists and erupts. Magmas are primarily classified by their silica content, which influences their physical properties. High-silica, or felsic, magmas like rhyolite are thick and viscous because the silica molecules easily link together.
This high viscosity means felsic magmas tend to solidify at lower temperatures, typically erupting around 700 to 850 degrees Celsius. Consequently, the resulting lava flows display a duller red incandescence when visible. When felsic rock cools completely, it often results in lighter-colored solid rock, such as light grey or pink rhyolite.
Conversely, low-silica, or mafic, magmas like basalt have a low viscosity because their mineral components (rich in iron and magnesium) do not bond easily. These magmas can exist at much higher temperatures, generally erupting between 1,000 and 1,200 degrees Celsius. Their higher heat causes them to display the brighter orange and yellow glows associated with lava.
The solidified product of mafic magma is typically the dark, heavy rock basalt, which is black or dark grey when fresh. Variations in composition cause differences in the observed glow color by controlling the maximum temperature the molten rock can maintain. In extremely rare instances, ultramafic magmas, which are very low in silica, can approach 1,600 degrees Celsius, theoretically producing a very bright white incandescence.
Magma Versus Lava Appearance
The common perception of molten rock comes from observing lava, the material that has reached the surface, while true magma remains beneath the crust. Magma is rarely seen outside of a volcanic vent or deep fissure, where it is under immense pressure and different lighting conditions. The appearance of lava changes dramatically upon contact with the cooler atmosphere.
As lava flows, it rapidly forms a thin, solid crust on its exterior due to heat radiating into the air. This crust, generally black or dark gray, acts as an insulator, obscuring the bright, incandescent liquid rock beneath. This phenomenon explains why many lava flows appear dark on the surface even as they continue to flow.
The glow often emerges only through cracks or when the dark crust is broken. As the lava flow continues to cool, the color of the exposed liquid rock transitions down the incandescence scale. It shifts from bright yellow-orange to a deep, dark red, before losing its glow entirely as it solidifies into a dark, unlit mass of rock.
The final color of the cooled, solid rock is not the color of the glow, but the color of the minerals within it, sometimes altered by oxidation. The dark crust often weathers over time, with the iron content oxidizing to give the rock a reddish or brownish hue. The visual color of molten rock is a temporary spectacle of heat, while the final color of the rock is a permanent reflection of its chemistry and cooling history.