6. Maars, tuff rings and tuff cones. Dan Barker March 2009 Fort Rock tuff ring, OR

Maars, tuff rings, and tuff cones are products of monogenetic (single short period of activity, like scoria cones) phreatic and phreatomagmatic explosions. Phreatic explosions are powered by steam, when magma encounters a shallow aquifer or surface water. Phreatomagmatic explosions occur when phreatic blasts remove enough overburden that gases dissolved in magma can expand violently and, combined with the steam, power the explosions. Compared to scoria cones, the ejected material contains a high proportion of non-igneous rocks. As a group, maars, tuff rings, and tuff cones are the second most numerous volcanic landforms above sea level (after scoria cones). Airfall and pyroclastic surge deposits are common but pyroclastic flow deposits and lava flows are not. This is because the magma was rapidly quenched by high water/magma ratios. The three types of features grade into each other, so the distinctions are arbitrary. All three are suspected to be the surface expressions of diatremes in some cases.

Maars are shallow explosion craters with diameters rarely exceeding 3 km, steep inner slopes, gentle outer slopes, and floors that are at a lower level than the surrounding pre-eruption terrain. Lakes commonly fill them, because the floors are below the groundwater table. "Maar" is a German word for lake or pond. Maars were first recognized in southern Germany, so examples of these follow first.

This small maar in Ulmen, Germany, is the youngest volcanic feature recognized in Germany. It is less than 10,000 years old.

The Ulmen maar is a source of civic pride, as shown by this old stone monument.

This is the Weinfelder Maar at Daun, W Eifel district, Germany

W Schalkebrener Maar, Daun, W. Eifel

The floor of the Meerfeld Maar is prized farmland.

This maar in western Uganda is neatly bisected by the equator (not visible).

This is Lake Averno in the Campi Flegrei caldera, Italy This is Lake Averno in the Campi Flegrei caldera, Italy. The emperor Augustus ordered a canal dug to connect the maar with the nearby sea, providing a harbor. The name Averno means "no birds", because Romans believed that birds flying over the lake were killed by toxic gases. As a final bit of trivia, the poets Virgil and Dante placed the entrance to the Underworld here.

The aptly named Hole-in-the-Ground Maar in Oregon has about the same proportions as smaller lunar craters. The Apollo astronauts were sent here to test their backpacks and spacesuits.They fell over backward trying to climb out.

Kilbourne Hole maar west of El Paso shows a dark layer of older basalt lava that was partly removed by steam explosions as new magma came up.

Ground view of Kilbourne Hole and rim deposits.

Surge deposits on the rim of Kilbourne Hole Surge deposits on the rim of Kilbourne Hole. These consist of particles of the older basalt and underlying sedimentary rocks. Mantle xenoliths are abundant.

Tuff rings have steep inward and more gentle outward slopes, and floors that are at higher levels than the surrounding ground. Beds of ejected material are usually thin, dip outward at less than 12 degrees, and were deposited by hot "dry" surges from groundwater-powered blasts.

The most well known tuff ring is Diamond Head, Honolulu, HI

The asymmetry of Diamond Head in profile indicates the wind direction at the time of eruption.

The Punchbowl is another tuff ring within the city of Honolulu.

Fort Rock in Oregon is an excellent example of a tuff ring within a former lake basin. Waves eroded the eastern portion of the ring.

This is a closeup of the lapilli tuff making up Fort Rock.

Wave erosion by the former lake exposed the internal structure of Fort Rock, and also made overhangs used as rock shelters. In one, sandals woven from plant fibers were found under a layer of Mazama Ash (7700 years old) from the collapse of the Crater Lake caldera.

Within the tuff ring at Fort Rock, dips of the layers get steeper toward the central vent.

This is the interior of the Cerro Colorado tuff ring, Pinacate volcanic field, Sonora.

The saucer-shaped feature marks the last vent within Cerro Colorado.

Younger fallout is plastered on the inner slopes of Cerro Colorado.

Normal faults show downward displacement toward the vent Normal faults show downward displacement toward the vent. Crater Hill tuff ring, Auckland, NZ.

This tuff ring was formed in 1783 by steam explosions on the Laki fissure, Iceland.

The Hverdal tuff ring, Iceland, is the dark profile (arrow).

Two post-1974 faults cut Hverdal tuff ring.

This tuff ring on Rapa Nui (Easter Island) was the source of rock from which the mysterious figures were carved.

Tuff cones resemble tuff rings in having steep inner and outer slopes, and floors that are at higher levels than the surrounding ground. However, tuff cones are higher and steeper than tuff rings, with smaller diameters but greater volumes. Beds are thicker, dip outward at angles up to 32 degrees, and are deposited by airfall and by cool "wet" surges. Tuff cones tend to evolve into scoria cones if the groundwater supply is exhausted before the magma supply is.

Koko Crater tuff cone, Oahu

Wave-cut section of tuff cone, Tule Lake CA. Katia and Maurice Krafft for scale.

Tuff cone, Fort Portal, Uganda

Maars, tuff rings, and tuff cones tend to be situated at coastlines, in lakes, or along the courses of buried river valleys. Why?