Pumice and obsidian are distinct geological materials formed by volcanic activity, yet they present a striking visual contrast: pumice is light, frothy, and often buoyant, while obsidian is dark, dense, and glass-like. These rocks are fundamentally connected because they are both classified as volcanic glass, a term that speaks directly to their shared origin and internal structure. The differences in their appearance, density, and texture arise from a single variable in their formation process.
Shared Volcanic Origin
Both pumice and obsidian are products of extrusive igneous processes, meaning they form from magma that has erupted onto the Earth’s surface as lava. They are derived from highly viscous, high-silica magma, categorized as felsic in composition. This parent material is rich in silicon dioxide (SiO2), often exceeding 70% by weight, which is the same chemical basis for the crystalline rock rhyolite. This establishes their fundamental chemical commonality.
The magma’s high silica content contributes to its high viscosity, or thickness, making it difficult for dissolved gases to escape easily. Both pumice and obsidian form when this high-silica lava is brought to the surface and cools extremely rapidly.
Amorphous Internal Structure
The rapid cooling that occurs when the lava is ejected or flows onto the surface is responsible for their glassy structure. Because cooling happens so quickly, the atoms within the molten material do not have sufficient time to arrange themselves into an ordered, repetitive structure. This lack of organization prevents the formation of mineral crystals.
The resulting material is described as amorphous, meaning it lacks the regular, three-dimensional geometric arrangement that defines a true mineral. This non-crystalline solid is volcanic glass, a term that applies equally to the solid mass of obsidian and the porous foam of pumice. The shared glassy matrix confirms that both rocks solidify in the same manner.
How Gas Content Creates Their Differences
The single factor that differentiates pumice from obsidian is the amount of trapped volcanic gas, or volatiles, present when the lava solidifies. These volatiles, primarily water vapor, sulfur dioxide, and carbon dioxide, are dissolved in the magma under high pressure beneath the earth.
If the high-silica lava is rich in these dissolved gases and is quickly erupted under low pressure, the gases rapidly exsolve, or bubble out. This rapid expansion causes the lava to froth, inflating the melt into a foam. When the lava cools instantaneously, the walls of the gas bubbles—known as vesicles—are preserved in the solid glass, creating the extremely light, porous texture of pumice. Pumice can have a porosity ranging from 64% to 85% by volume, which allows it to float on water.
Conversely, obsidian forms when the high-silica lava degasses, or releases its volatiles, before or during its rapid cooling. This loss of gas means the melt solidifies into a dense, massive glass without the internal foam structure. Obsidian typically has a very low water content, averaging around 0.3 weight percent. The absence of numerous gas bubbles results in the characteristic smooth, uniform, and dense texture of obsidian, which often exhibits a sharp, curved fracture known as conchoidal fracture.