Cinder cones: the most common type of volcano and called the "most endangered"

Cinder cones are the most common type of volcano in the world, and the source calls them the "most endangered" volcanoes on Earth because they are easily mined for road-construction cinders, icy-road sanding and grill "lava rocks". This page follows the National Park Service's volcano series, which describes what they are, how they build up and which parks protect them.

Cinder Cone in Lassen Volcanic National Park, California (2018). It erupted in approximately 1650 CE. NPS photo.

What a cinder cone is

A cinder cone looks like an idealized volcano: a steep, conical hill, usually with a prominent crater on top. The technical name is scoria cone. Scoria is irregularly shaped, highly vesicular lava, meaning full of bubble-shaped cavities, thrown into the air and typically solid by the time it lands. In the United States, "cinder" is the everyday word for pieces of scoria roughly nut- to fist-size. Dark lava flows often pour out near a cone's base.

Most cinder cones are a few hundred feet tall (100 to 150 m) and rarely exceed 600 to 900 feet (200 to 300 m). Each is usually its own structure built around a central vent. Some are nearly perfect, such as Sunset Crater Volcano, but many are lopsided: cones over a linear fissure are elongated, strong prevailing winds can build the downwind side much taller, a vent that shifts during an eruption can stretch the cone, and a lava flow breaking out near the base can carry off part of the flank.

Wizard Island in Crater Lake National Park, Oregon. Its crater is less than 500 feet (150 m) wide and about 70 feet (20 m) deep. NPS photo.

How they erupt

• Composition: most are basaltic to basaltic andesite, though some are andesitic.
• Style: usually mildly to moderately explosive (Strombolian to Vulcanian), rarely Sub-Plinian. The low explosivity comes from magma that is low in viscosity and volatile content.
• Size: usually 1 to 2 on the Volcanic Explosivity Index, rarely as high as 4.
• One and done: cinder cones are generally monogenetic, the product of a single eruptive episode. Afterward rock, such as solidified magma, blocks the plumbing between the magma source and the vent.
• Duration: half of all historic cinder cone eruptions lasted less than 30 days, and 95 percent less than a year.
• Company: they often sit with other volcanoes, in calderas, near domes or as satellite cones on the flanks of composite and shield volcanoes, and frequently in fields of as many as hundreds of cones.

Cinder cones in the national parks

Cinder cones are the most common volcano in the National Park System, found in at least 24 units.

Ka Moa o Pele, flanked by dark and light lava flows, Haleakalā National Park. NPS photo by Russell Shurtz.

Capulin Volcano from its Boca, or base vent area; at approximately 1,000 feet (300 m) it is an unusually tall cinder cone. Capulin Volcano National Monument, New Mexico. NPS photo.

Features to look for

The crater. Most cones have a pronounced crater at the top. Capulin Volcano has a trail into the crater and around its rim.

Layers. A cone is built of layers of pebble-sized or larger cinders and layers of ash, sometimes with larger volcanic bombs and blocks. Its overall slope is set by the angle of repose, between 25 and 32 degrees for ash and cinders. Cinders piling up near the vent can exceed it from time to time, and then ash and cinders avalanche down the sides and into the crater to restore the slope. Those avalanches are a major reason cones are layered.

Sunset Crater Volcano's crater. The red comes from oxidized cinders. The page notes that hikers are not allowed to climb Sunset Crater because its fragile slopes are easily damaged. NPS photo by C. Schelz.

Cinders exposed by the road cut where the paved road climbs to the rim of Capulin Volcano. USGS photo.

Spatter. As an eruption wanes, it gets less explosive, so cinders and globs of lava are not thrown as high and may still be molten or partly molten when they land. They weld together as spatter, which can look like lava. Spatter can armor a rim and slow erosion; spatter on Capulin Volcano's rim and crater is largely responsible for its youthful, largely uneroded look, though the volcano formed more than 54,000 years ago.

Side vents. Most cones have a vent at the base or side where basaltic lava flows erupt. Such vents are called bocas or boccas (Spanish and Italian for "mouth"). The side vent area can be complex, with lava lakes, spatter deposits, push-ups and even pieces of the cone's flank carried off as rafts on the lava. Capulin's, known as the Boca, is more complex than those at other cinder cones, with ramparts, levees, collapsed lava tubes, lava lakes and chunks of rafted cone.

Geologic map of the Boca area at Capulin Volcano. NPS Geologic Resources Inventory.

Hazards, and threats to the cones

Because cinder cones are usually monogenetic, an existing cone should not erupt again, though a new one may form nearby if an eruption starts. Warning signs include earthquakes and rumblings, which are noises underground. The hazards on an existing cone are rockfall, debris flows and other mass wasting on its slopes.

The bigger threat is human. Cinder cones are easily mined for cinders used in road construction, for sanding icy roads and as "lava rocks" for gas grills.

Sources

• National Park Service, Cinder Cones, part of the Volcano Types series; rewritten in hubnx's own words. Photographs by the National Park Service and the U.S. Geological Survey; two photographs credited to a private photographer are not reproduced.