Maars and Tuff Rings: Wide Volcanic Craters Where Magma Met Water

The largest known maars on Earth are in Alaska, and they are as wide as they are because the magma that made them met thick permafrost. A maar is one of two kinds of low volcano, with a wide, bowl-shaped crater, that form when magma interacts with ground or surface water.

Devil Mountain Maar, Bering Land Bridge National Preserve, Alaska. It has a northern and a southern crater, separated by a small spit. The maar is nearly 5 miles (8 km) long, 3.7 miles (6 km) wide and 328 feet (100 m) deep. Image from NPS's page.

Two kinds of crater

Maars and tuff rings have low edifices of shallowly dipping tuff, made mostly of ash and angular, pebble-sized pyroclasts called lapilli. They commonly have a donut-like profile and may be breached.

Both are usually associated with diatremes, funnel-shaped breccia pipes formed by phreatomagmatic (hydrovolcanic) explosions. The type is also called a maar-diatreme volcano. The word “maar” comes from a German dialect word for the circular lakes of these volcanoes in the Rhineland.

How they form

The eruptions come from magma-water interaction at or near the surface. They are classed as Surtseyan, the wet equivalent of Strombolian eruptions. Mixing magma and water produces steam explosions that shatter the magma into fine, angular fragments and excavate a large crater. It is thought that the explosions that form tuff rings happen closer to the surface than those that form maars.

The eruptions are complex and episodic, with several pulses and repeated small explosions. They are usually low on the Volcanic Explosivity Index, 1 to 2, but rarely may reach 3 or 4. Like cinder-cone eruptions, they are short, frequently lasting only a few days to a few months. Most maars and tuff rings come from low-viscosity magma of basaltic or other mafic or ultramafic composition, and lava flows rarely accompany them.

They may occur anywhere on land where surface water, shallow groundwater or permafrost can meet magma: on volcanic islands, in some coastal areas and in continental volcanic fields with wet low ground or a high water table. They are small and typically monogenetic, because they form where the magma supply is limited.

What they look like

Ubehebe Crater, Death Valley National Park, California and Nevada. The light-colored area is preexisting country rock; the pyroclastic deposits from the maar eruption are darker. Image from NPS's page.

• Wide craters. Craters start as blast craters as explosions remove country rock, and become wider and deeper during eruptions, partly through lateral collapse of the walls. Some pyroclastic material fills them in. Most maars and tuff rings are between 0.6 and 1.2 miles (1 and 2 km) across. Devil Mountain maar, the largest of the Espenberg maars, is more than 5 miles (8 km) across, only slightly smaller than the Crater Lake caldera.
• Lakes. Because maar craters lie below the surrounding land, they commonly hold lakes.
• Surge deposits. Each explosion sends out a base surge, a dilute pyroclastic density current that leaves layered deposits, some with cross-bedding like that in sedimentary rocks laid down by currents.

Base surge deposits on the rim of Devil Mountain maar. The plane-bedded layers were formed by base surges, which are low-density pyroclastic flows. The explosion breccia holds rock fragments brought up from depth, and the scoria layer is vesicular chunks of lava. NPS photograph by Chad Hults (NPS Alaska Regional Office).

In the National Park System

At least six units contain maars or tuff rings.

• Bering Land Bridge National Preserve, Alaska. The Espenberg maars are the largest known on Earth; thick permafrost helped make their eruptions prolonged and highly explosive. Their lakes approach 1,000 feet (300 m) deep. The oldest, Whitefish maar, is between 100,000 and 200,000 years old; the youngest, Devil Mountain maar, about 17,500 years. North and South Killeak Maars are approximately 50,000 and 40,000 years old.
• Death Valley National Park. The Ubehebe Craters are a group of 12 maars, and Ubehebe Crater itself is the youngest and largest.
• Petrified Forest National Park, Arizona. Pintado Maar is the only maar of the Hopi Buttes volcanic field within the park. The field holds approximately 300 maars, tuff rings, diatremes and cinder cones, one of the most extensive maar-diatreme fields on the planet. Its maars formed in water-saturated sediments and playas between 8.7 and 6 million years ago.
• Capulin Volcano National Monument, New Mexico. None lies inside the boundary, but the Rim Trail gives views of Mud Hill, a nearby tuff ring in the Raton-Clayton Volcanic Field.

The NPS list also names Aniakchak National Monument and Preserve, Bandelier National Monument and Big Bend National Park.

Left: Mud Hill viewed from Capulin Volcano. Right: feature labels; the yellow line marks the crest of the tuff ring. Credit: NPS photos.

North and South Killeak Maars, Bering Land Bridge National Preserve. NPS photo.

Hazards

Because tuff rings and maars are usually monogenetic, there should be no risk of eruption at existing ones, but a new one may form nearby if an eruption starts. Rockfall is a geohazard on existing tuff rings and maars.

Sources

• National Park Service, Maars and Tuff Rings; rewritten in hubnx's own words.