Cinder cones, also known as scoria cones, are the most common and simplest type of volcano found worldwide. They represent the classic, idealized conical shape, but are significantly smaller than other volcanic forms like stratovolcanoes or shield volcanoes. Their formation is a rapid and relatively brief geological process driven by the violent expansion of gas within magma rising to the surface. Understanding how these distinctive hills are built requires an examination of the specific magma and materials involved, the explosive eruption sequence, and the resulting physical characteristics.
Magma Composition and Eruptive Materials
The formation of a cinder cone begins with a specific type of magma, typically basaltic, which is low in silica and therefore has a relatively low viscosity, meaning it is fluid. This magma, however, is rich in dissolved gases, such as water vapor and carbon dioxide, a feature that powers the subsequent explosive eruption. As this gas-charged magma rises toward the Earth’s surface, the confining pressure rapidly decreases, which allows the dissolved gases to expand dramatically. This rapid, forceful expansion causes the liquid magma to fragment violently into small, porous pieces in the air, solidifying into what are called pyroclasts. The primary material that builds the cone is known as scoria, which is dark, highly vesicular rock containing numerous bubble-shaped cavities, or vesicles, frozen into place.
Constructing the Cone: The Eruption Sequence
The driving force behind the cone’s construction is the explosive release of gas, which propels the fragmented magma high into the air in a style known as a Strombolian eruption. This eruption style is characterized by discrete, rhythmic explosions from a single, typically cylindrical, central vent, rather than a continuous fountain of lava. The ejected pieces of scoria follow ballistic trajectories, raining down around the central opening, accumulating layer upon layer to form the cone’s characteristic steep sides and symmetrical shape. The fragments of scoria are loose and granular, piling up to the steepest possible angle at which they remain stable, a geological measurement called the angle of repose (typically 30 to 40 degrees). During the final stages of eruption, the remaining gas-depleted magma may ooze out from the base of the cone as a lava flow, which is too dense to rise through the loose cinders.
Size, Shape, and Monogenetic Nature
Cinder cones are modest in size compared to other volcano types, typically ranging from a few dozen to a few hundred meters (100 to 1,300 feet) in height above the surrounding landscape. Their profile is distinctively steep, defined by the angle of repose of the loose scoria. At the summit, a deep, bowl-shaped crater is usually present, formed by the explosive vent. Most cinder cones are classified as monogenetic, meaning they are the product of a single, short-lived eruptive episode. Once the eruption ceases, the conduit connecting the magma source to the vent is commonly blocked by solidified magma, sealing the system and preventing any future eruptions from that specific vent.