A cinder cone is made of scoria, a low-density type of hardened lava. As gases escape a volcanic vent, they push rock and ash several hundred feet into the air. When this rock and ash falls and hardens, it often forms cinder cones. Scoria is popular for use in landscaping.
Cinder cones form late in the eruption of a shield volcano, as the number of escape routes for the gas and debris decreases. The force of the explosion fills the lava with air channels. As gravity pulls the lava back toward the ground, it hardens to form scoria that drops in a circular pattern around the vent. Because the ash solidifies in the air, it does not stick together when it lands.
While the formation looks conical from a distance, it often has a depression in the center surrounding the vent. When cinder cones form in strong winds, the air currents can cause distortion of the cones. In subsequent less violent eruptions, lava sometimes escapes through walls of the cinder cone. Even though cinder cones contain loosely packed materials, they usually retain their conical shape for many years. Instead of causing erosion by flowing down the sides of the volcano, rain soaks into the permeable scoria.