Pumice is a common type of volcanic rock that forms during explosive eruptions. Geologists classify igneous rocks, which are formed from the cooling of magma or lava, based on their chemical composition. This classification primarily uses the terms felsic and mafic to describe the relative amounts of silica and other elements within the rock.
Understanding Felsic and Mafic Composition
The classification of igneous rocks into felsic and mafic categories is fundamentally based on their silica content, which is the amount of silicon dioxide (SiO2) present. Felsic rocks contain a high percentage of silica, typically more than 65% by weight, and are rich in lighter elements such as aluminum, sodium, and potassium. The name “felsic” itself is derived from the words feldspar and silica, which are two of the dominant minerals found in these rocks. This high silica content contributes to their light color, often appearing white, pink, or light gray.
Felsic rocks also tend to have a lower density compared to their counterparts because silicon and oxygen are relatively light elements. Common examples of felsic rocks include granite, which cools slowly underground, and rhyolite, which is its extrusive, rapidly cooled equivalent. The magma that forms these rocks is highly viscous, meaning it is thick and resists flow, a characteristic directly linked to its chain-like silica structure.
Mafic rocks are defined by their lower silica content, generally ranging between 45% and 55%. The term “mafic” is an acronym for magnesium and ferric (iron), indicating that these rocks are enriched in heavier elements like iron and magnesium. This chemical makeup gives mafic rocks a darker coloration, such as black, dark brown, or green, and makes them significantly denser than felsic rocks. Basalt, the dark rock that makes up much of the ocean floor, is the most common extrusive mafic rock, while gabbro is its intrusive equivalent. Mafic magma is much less viscous, flowing more easily and resulting in typically non-explosive, gentle volcanic eruptions.
The Classification of Pumice
Pumice is classified as an extrusive igneous rock, and based on its chemical makeup, it is overwhelmingly felsic in composition. The rock is primarily composed of silicon dioxide and aluminum oxide, with a silica content that places it firmly in the felsic to intermediate range. Pumice is the frothy, glassy equivalent of rhyolite, meaning its non-vesicular, solid counterpart would be a dense, light-colored rock.
The formation of pumice is directly linked to the properties of highly silica-rich magma. Under immense pressure deep within a volcano, water vapor and other gases, known as volatiles, remain dissolved within this viscous magma. As this gas-saturated magma rapidly ascends during an eruption, the pressure drops drastically, causing the dissolved volatiles to exsolve, or bubble out, much like a carbonated drink being opened. The explosive force of the eruption ejects this frothy material into the air, where it cools almost instantaneously, freezing the expanding bubbles in place and resulting in the glass-like matrix of pumice.
Pumice’s Unique Physical Properties
The most notable characteristic is its extreme vesicularity, which describes the presence of countless tiny air pockets or vesicles trapped within the rock. Pumice has a porosity that can range from 64% to over 85% by volume, creating a highly porous structure with very thin walls between the air spaces. This high volume of trapped gas gives pumice an unusually low bulk density. The abundance of air makes the overall rock lighter than water, which is why pumice is one of the few rocks that will float, sometimes forming massive rafts on the ocean surface after a submarine eruption.
The highly porous and glassy texture also makes pumice inherently fragile and abrasive. These qualities lend themselves to a wide range of commercial uses, such as a lightweight aggregate in concrete to reduce weight, and as a mild abrasive in personal care products and for stonewashing denim. Its ability to absorb and hold moisture also makes it valuable in horticulture for soil aeration.