Igneous rocks, formed from the cooling and solidification of magma or lava, are classified primarily by their chemical composition. Geologists use the mafic-felsic spectrum, which categorizes rocks based on the relative abundance of silica. This spectrum ranges from silica-poor to silica-rich, and it is a tool for understanding the original magma’s chemistry. The percentage of silica, or silicon dioxide (SiO2), acts as the defining characteristic within this system.
The Felsic-Mafic Rock Classification System
The terms felsic and mafic represent the two compositional extremes for igneous rocks. Felsic rocks are defined by a high content of silica, typically more than 65% by weight, and a low amount of iron and magnesium. These rocks are generally light in color, such as white, pink, or light gray, and are composed of minerals like quartz and feldspar, which contribute to their lower density. The name “felsic” itself is a blend of feldspar and silica, highlighting their primary mineral components.
In contrast, mafic rocks contain a significantly lower percentage of silica, usually falling between 45% and 55% by weight. They are rich in magnesium and iron, which are denser and give these rocks their characteristic dark coloring, often black, dark gray, or green. Common mafic minerals include pyroxene and olivine. Basalt and gabbro are examples of mafic rocks, forming from magma that is less viscous and tends to erupt less explosively.
Rocks that do not fit neatly into these two categories are labeled as intermediate, possessing a silica content between approximately 55% and 65%. This intermediate group contains a mixture of both light and dark minerals, such as plagioclase feldspar and amphibole. The boundaries between felsic, intermediate, and mafic compositions are defined by these silica percentages.
Defining the Composition of Dacite
Dacite is an extrusive igneous rock, meaning it forms when lava rapidly cools and solidifies, giving it a fine-grained or aphanitic texture. It is the volcanic equivalent of the intrusive rock granodiorite, which forms deep underground and has a much coarser texture. The rock often appears light-colored, ranging from off-white and gray to pinkish or blue-gray, with a low color index indicating a small proportion of dark minerals.
The mineral makeup of dacite is dominated by plagioclase feldspar and quartz, which are both light-colored, silica-rich minerals. Plagioclase feldspar, specifically varieties like oligoclase or andesine, makes up a significant portion of the total feldspar content. Quartz is present either as visible, rounded crystals called phenocrysts, or as minute grains within the fine-grained groundmass.
Accessory minerals in dacite often include biotite, hornblende, and sometimes pyroxene or iron-rich olivine. These minerals introduce the darker elements to the rock but do not change its overall classification. The presence of abundant quartz, which is pure silicon dioxide, is a strong indicator of the rock’s high silica content.
Why Dacite is Classified as Felsic
Dacite’s classification is determined by its chemical composition, specifically its high silica content, which firmly places it in the felsic-to-intermediate range. The silica content of dacite typically ranges between 63% and 69% by weight. This places it well above the threshold for intermediate rocks and often crosses the 65% boundary into the felsic category.
The abundance of quartz, a mineral composed entirely of silica, is the mineralogical evidence supporting this felsic classification. Dacite contains between 20% and 60% quartz by volume of its total quartz and feldspar content, a characteristic it shares with other felsic rocks like rhyolite. The high silica concentration in the original dacitic magma also makes it highly viscous, compared to low-viscosity mafic magmas like basalt.
The high silica content makes the magma highly viscous, as the silica tetrahedra link together to form complex molecular structures. The light color of dacite also visually confirms its felsic nature, resulting from the dominance of light-colored minerals like quartz and plagioclase feldspar. Although its silica content sits near the boundary, dacite is fundamentally a high-silica rock.
Formation Environment and Geological Significance
Dacitic magma commonly forms in geological settings associated with subduction zones, such as the active continental margins found along the Andes Mountains or the Cascade Range. In these environments, an oceanic plate slides beneath a continental plate, and the resulting process of partial melting and differentiation generates silica-rich magma. Water released from the subducting slab facilitates the melting of the overlying mantle and crustal rock, driving the magma’s composition toward a more silicic nature as it rises.
As the magma migrates upward through the continental crust, it undergoes further chemical evolution, leading to the crystallization of sodium-rich plagioclase and quartz. The high viscosity inherent to dacitic magma, a result of its high silica content, traps volcanic gases within the melt. This gas-trapping mechanism is the primary reason why dacitic volcanoes are prone to explosive eruptions, such as the catastrophic events seen at Mount St. Helens.
When dacitic magma erupts, it often forms thick, blocky lava flows or steep-sided volcanic domes rather than flowing freely across the landscape. The study of dacite formation provides important information about the evolution of continental crust. Its presence demonstrates a process where dense, mafic material is transformed into buoyant, silica-rich, felsic rock.