How Do Cinder-Cone Volcanoes Erupt?

Cinder-cone volcanoes, properly called scoria volcanoes, erupt when expanding gas bubbles drive lava to the volcano's surface. Because of this pressure, the lava fountains are usually very high and vertical. By the time the erupted material lands, it is already cool.

When lava erupts from the vent of a cinder-cone volcano, small fragments of lava known as tephra spray into the air and fall back to the earth as loose clumps of glassy basalt. These low-density rocks are known as scoria. As the volcano erupts, the scoria slowly accumulates into a cone shape. Cinder-cone volcanoes are typically circular in shape, but they may have a more unusual shape if the location of the vent is altered during the eruption.

Prevailing winds can also alter the shape of a cinder-cone volcano if they are blowing consistently during an eruption. Horseshoe-shaped cinder cones are created when a major lava flow carries away falling chunks of lava. Cinder cones are usually formed in the later part of an eruption when the volcanic activity becomes localized to a few select fissure points.

Most cinder-cone volcanoes are relatively small. They typically reach heights between 300 feet and 1200 feet. They often form in only a few years, if not months.