Geology

rough craggy black lava covers the land with a volcanic cinder cone in the background beneath clear blue skies
Sunset Crater Volcano rises over part of the Bonito Lava Flow created during its eruption.

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Volcanoes and Lava Flows


Sunset Crater Volcano National Monument preserves two volcanoes. The first is the park’s namesake, a 1000-foot-tall (305 m) cinder cone that erupted sometime around 1085 CE. The second is Lenox Crater Volcano, another cinder cone that is much older. Lenox Crater Volcano is approximately 300 feet or 91 meters tall.

Cinder cones form when runny basalt magma is mixed with water beneath the ground. As the rock-water soup reaches the surface of the earth, the water flashes into steam, causing the liquid rock to explode from the ground. When Sunset Crater Volcano erupted, it created a curtain of fire 850 feet (259 m) tall! For comparison, the Statue of Liberty is only 305 feet (93 m) tall.

As these tiny bits of lava cooled off, they fell back to earth as tiny rocks called cinders or lapilli. These cinders piled up around the volcano’s vent, creating a cone-shaped pile we know today as Sunset Crater Volcano.

Along with the explosion of cinders, the volcano ejected larger chunks of material called lava blocks and lava bombs. And even more lava poured over the land to create the Bonito and Ka’naa lava flows. This slow-moving liquid rock cooled and hardened on the surface, but the liquid lava underneath it broke through, only to cool and dry again. Over and over, this repeated process created the jagged landscapes of a’a lava seen today.

Sunset Crater Volcano’s eruption incinerated the landscape. For over 400 years, the area surrounding the volcano was barren, sterile, and utterly devoid of plant life. The first plants did not return until around 500 years ago. The land you see today is an outdoor laboratory, teaching geologists and biologists about the recovery of plants and forests after volcanic events.
 
a large crack in the earth is surrounded by struggling vegetation with a volcano in the background
A giant crack, called a fissure, opened in the ground as Sunset Crater Volcano began to erupt. Part of that fissure is preserved in the Bonito Lava Flow today.

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Lava Features


Within the Bonito lava flow today are the remains of several unique features left behind after the eruption.

Part of the fissure that broke open at the beginning of the eruption is preserved. Nearby is a squeeze-up, where thick lava pushed up through a crack in the ground and dried in a sharp fin of basalt rock. A spatter cone looks like a miniature volcano of its own near Sunset Crater Volcano’s base. Cool air rises from the remains of a collapsed ice cave.
 
tiny clumps of yellow flowers struggle to grow among volcanic cinders
Newberry's Twinpods struggle to grow in cindery volcanic soils.

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Soils


Before the volcano erupted, there was fertile soil here. Ancestral people indigenous to northern Arizona farmed here with their families. Today, their fields are buried beneath hundreds of feet of cinders and lava, and the landscape has changed forever.

New soil is forming, but it is a slow and precarious process. Weathered particles and bits of organic matter must accumulate between cinders for any plants to germinate, survive, and successfully reproduce.

In the early stages of soil formation, this process is easily disrupted. Any disturbance can dislodge the particles and cause them to sift deeper into the cinders, where they may be out of reach for use by plants. Plants already established may also be dislodged. For this reason, it is important for visitors to stay on designated trails.

Last updated: April 25, 2017

Contact the Park

Mailing Address:

6400 U.S. 89
Flagstaff, AZ 86004

Phone:

(928) 526-0502

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