Volcanic rocks, also known as extrusive igneous rocks, form when molten rock cools and solidifies on or very near the Earth’s surface. This rapid cooling process creates a distinctive texture, but the primary way these rocks are classified is by their chemical makeup. The composition of the original magma dictates the final rock type, with the amount of silica (silicon dioxide) being the most important factor in this classification. This chemical variation results in three main groups of volcanic rocks, each with unique physical characteristics and geological origins.
Defining Extrusive Igneous Rock
Extrusive igneous rocks form external to the Earth’s crust. When magma erupts onto the surface, it becomes lava, which is exposed to relatively cold temperatures. This sudden drop in temperature causes the lava to cool and solidify extremely quickly, often within hours or days. Because the cooling is so rapid, mineral crystals do not have sufficient time to grow to a visible size.
This process results in a very fine-grained texture, which geologists call aphanitic, where individual mineral grains are too small to see without magnification. If the lava cools almost instantly, it prevents crystal formation entirely, forming volcanic glass, such as obsidian. Volcanic rocks may also contain small holes, called vesicles, created by gas bubbles trapped as the lava solidified. This contrasts with intrusive rocks, which cool slowly deep underground and develop large, visible crystals.
The Three Primary Classification Groups
The three main types of volcanic rock are classified along a spectrum based on their silica content, ranging from low to high. This chemical composition determines the magma’s viscosity, which influences how a volcano erupts. The three primary groups are mafic, intermediate, and felsic.
Mafic (Low Silica)
Mafic volcanic rocks have a low silica content, typically ranging from 45% to 52% by weight. They are rich in magnesium (Mg) and iron (Fe), which is the source of the term “mafic.” Mafic magmas have a very low viscosity, meaning they are fluid and flow easily over long distances. This fluidity often leads to non-explosive eruptions and the formation of broad shield volcanoes.
The most common example is Basalt, a fine-grained, dark-colored rock that is the primary component of the ocean floor. Basalt is compositionally equivalent to the intrusive rock gabbro, sharing the same mineral makeup but differing in texture due to cooling rate. It is mostly made of dark minerals like pyroxene and olivine, which contain high amounts of iron and magnesium.
Intermediate (Medium Silica)
Intermediate volcanic rocks have a silica content ranging from 52% to 63% by weight, falling between the mafic and felsic groups. This composition results in a moderate viscosity. The lavas are thicker than basalt but still able to flow, often leading to volcanoes that exhibit both explosive and effusive eruptive styles.
The most representative rock is Andesite, named for the Andes Mountains where it is commonly found. Andesite is typically light gray and is the fine-grained, extrusive equivalent of the intrusive rock diorite. Its mineral assemblage is a blend of light-colored plagioclase feldspar and darker minerals like amphibole.
Felsic (High Silica)
Felsic volcanic rocks contain the highest amount of silica, generally exceeding 63% by weight. The term “felsic” comes from the combination of “feldspar” and “silica.” This high silica content causes the magma to be extremely viscous, making it thick and sticky. Felsic magmas trap gases effectively, often resulting in highly explosive eruptions and the formation of steep-sided stratovolcanoes or lava domes.
The most common example is Rhyolite, a light-colored rock that is the volcanic counterpart to the intrusive rock granite. Other examples of felsic volcanic products include Pumice, a very light, highly vesicular rock, and Obsidian, a dark volcanic glass. Rhyolite is rich in light-colored minerals such as quartz and potassium feldspar.
Observable Differences and Global Distribution
The chemical composition of these three rock types translates directly into visually distinct characteristics, especially color and density. Mafic Basalt is characteristically dark, ranging from black to dark brown. In contrast, felsic Rhyolite is typically light-colored, often appearing white, tan, or pink, while Intermediate Andesite commonly presents as a medium-gray rock. Density also follows the compositional trend, with Basalt having a greater density than Rhyolite due to its iron and magnesium content.
The global distribution of these rocks corresponds to different tectonic settings. Basalt is the most abundant volcanic rock and forms the vast majority of the oceanic crust, often erupting at mid-ocean ridges and within oceanic plates. Andesite is commonly found in areas associated with subduction zones, such as the volcanic arcs along the Pacific Ring of Fire. Rhyolite is primarily associated with continental crust, where its highly viscous magma forms thick lava flows and domes.