Scoria is a common volcanic rock known for its highly porous texture, which results from explosive eruptions. As an extrusive igneous rock, its chemical makeup is key to understanding the volcanic processes that created it. Classifying scoria as mafic or felsic depends entirely on its silica content and the minerals formed from the original magma.
Defining Mafic and Felsic Rocks
Geologists classify igneous rocks based primarily on the rock’s silica (\(\text{SiO}_2\)) concentration. Mafic and felsic represent the two extremes of this chemical spectrum.
Felsic rocks are characterized by a high silica content, typically greater than 65 percent. The name “felsic” is derived from light-colored minerals like feldspar and quartz, resulting in rocks that are generally white, pink, or light gray with a lower density.
In contrast, mafic rocks contain a lower percentage of silica, usually ranging between 45 and 55 percent. The term “mafic” is an acronym for magnesium and ferric (iron), reflecting the high concentration of these heavier elements. This abundance leads to the formation of darker minerals, such as pyroxene and olivine. Mafic rocks are typically black or dark green and possess a higher density.
Characteristics of Scoria
Scoria is an extrusive igneous rock that solidified rapidly after being ejected from a volcano. Its most distinctive feature is its highly vesicular texture, characterized by numerous bubble-like cavities called vesicles. These vesicles give scoria a rough, sponge-like appearance.
The color of scoria is typically dark, ranging from black or dark gray to deep reddish-brown, often due to the oxidation of iron. Although lightweight due to its many holes, scoria is generally dense enough to sink in water, differentiating it from pumice. Scoria is often found as fragmented ejecta (lapilli) or as the crust on certain lava flows.
Determining Scoria’s Chemical Composition
Scoria is classified as a mafic rock, a classification indicated by its dark color and chemical makeup. The dark color signifies a high concentration of the iron and magnesium silicate minerals characteristic of mafic compositions. Its chemical composition is similar to that of basalt, and it is sometimes referred to as scoriaceous basalt.
The silica content in scoria typically falls within the mafic to intermediate range, between 45 and 63 percent. This lower silica level confirms the original magma contained more iron and magnesium, resulting in a darker and denser material.
How Scoria Forms
Scoria formation is driven by gas-rich, low-viscosity magma ejected during volcanic activity. The parent magma is typically basaltic or andesitic, characteristic of mafic to intermediate compositions.
As this molten rock rises toward the surface, the pressure rapidly decreases. This pressure drop allows dissolved gases, such as water vapor and carbon dioxide, to rapidly escape and form bubbles (vesiculation). Because the mafic magma is fluid, the gas bubbles expand and coalesce easily.
When the lava is thrown into the air during an eruption, it cools extremely quickly. This rapid cooling traps the expanding gas bubbles within the solidifying rock, creating scoria’s characteristic porous texture. Scoria fragments are often the main component of cinder cones.