DESCRIPTION:
Maars and Tuff Cones

Also called "tuff cones", maars are shallow, flat-floored craters that scientists interpret have formed above diatremes as a result of a violent expansion of magmatic gas or steam; deep erosion of a maar presumably would expose a diatreme. Maars range in size from 200 to 6,500 feet across and from 30 to 650 feet deep, and most are commonly filled with water to form natural lakes. Most maars have low rims composed of a mixture of loose fragments of volcanic rocks and rocks torn from the walls of the diatreme.

Maars occur in the western United States, in the Eifel region of Germany, and in other geologically young volcanic regions of the world. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6,500 feet across and 400 feet deep. Its low rim is composed of loose pieces of basaltic lava and wallrocks (sandstone, shale, limestone) of the underlying diatreme, as well as random chunks of ancient crystalline rocks blasted upward from great depths.

Maar volcanoes are low volcanic cones with broad, bowl-shaped crater. Three general kinds are well-represented in the Fort Rock Basin:

• maar, with a crater floor below original ground level, such as Hole-in-the-Ground;
• tuff ring, with a crater floor at or above original ground level, such as Fort Rock; and
• tuff cone, which is a tall tuff ring, such as Table Rock.

Tuff is a volcanic rock made up of a mixture of volcanic rock and mineral fragments in a volcanic ash matrix. Wherever there are explosive volcanic eruptions you can expect to find tuff. Tuff forms when some combination of ash, rock and mineral fragments (pyroclastics or tephra) are blasted into the air, then fall to the ground as a mixed deposit. Most of the rock fragments tend to be volcanic rocks that were once solidified parts of the volcano that erupted to produce the tuff, but sometimes other types of rock are blasted out and incorporated into the tuff as well. Sometimes erupted material is so hot when it reaches the ground that it fuses together to produce a welded tuff.

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Battle Ground Lake, 1:25,000

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USGS Aerial Photo, Battle Ground Lake

The lake's origin is volcanic, and is believed to have been formed as a "Maar" volcano. This type of volcano is the result of hot lava or magma pushing up near the surface of the earth and then coming into contact with underground water. This is thought to have resulted in a large steam explosion, leaving a crater that later formed a lake.

Battle Ground Lake Menu

Nearly 40 maars, tuff rings, and tuff cones of Pliocene and Pleistocene age occur in the Fort Rock Basin of south-central Oregon. Most are significantly eroded, allowing excellent exposures of their lithology, bedding, and sedimentary structures; a few retain much of their oringial morphology. The Fort Rock Basin is dry, internally drained, and largely filled with lacustrine sediments which accumulated during the episodic existence of pluvial Fort Rock Lake. This area lies within the extensional environment of the Basin and Range Province and is characterized by numerous normal faults of Pliocene and Pleistocene age that cut volcanic rocks of similar age.

Fort Rock

Fort Rock is an isolated tuff ring with spectacular, wave-cut cluffs and terraces. The wave-cut remnant is approximately 1,400 meters in diameter and 60 meters high, and the present crater floor is 6 to 12 meters above the floor of the lake basin. The south rim has been breached by waves of the former Fort Rock Lake providing easy access to the crater. The best developed wave-cut terrace is 20 meters above the floor of the basin.

Hole-in-the-Ground

Hole-in-the-Ground is a nearly circular maar with a floor 150 meters below and a rim 35 to 65 meters above original ground level. Its diameter from rim to rim is 1,600 meters. The volume of the crater below the original surface is only 60 percent of the volume of ejecta. Only 10 percent of the ejecta is juvenile basaltic material. Most of the ejected material is fine grained, but some of the blocks of older rocks reached dimensions of 8 meters. The largest blocks were hurled distances of up to 3.7 kilometers from the center of the crater. Accretionary lapilli, impact sags, and vesiculated tuffs are well developed.

Fort Rock Basin Menu

Kilbourne Hole

Kilbourne Hole, the best known of the Potrillo maar volcanoes, sits astride the north-south-trending Fitzgerald fault, surrounded by the late Cenozoic Afton basalt flow. The maar was formed by steam explosions due to the heating of water-saturated sand and silt strata by rising basaltic magma. From the bottom of the crater to the top of the rim the following units are exposed: (1) Santa Fe Group sediments, (2) olivine basalt (Afton basalt), (3) bedded hydroclastic tuffs (base surge and air fall) and vent breccia, and (4) Holocene wind-blown sand.

Red Hill

The Red Hill maar is interesting as an example of a moderately youthful maar associated with faulting and fissure-type eruptive activity. ... Like the Zuni Salt Lake maar to the north, a small cinder cone occurs on the floor of the Red Hill maar, and is offset from the center of the crater. Cinders interbedded with maar deposits suggest that the eruption started with pyroclastic cinder and ash activity, evolved to more explosive maar-type eruptions, and later returned to the eruption of ash and cinders. The late ash blankets most of the crater rim-forming maar deposits, and ash was apparently dispersed by strong southwesterly winds to the northeast where ash covers the countryside for 5 to 10 kilometers along US Highway 60.

Zuni Maar

Maars occur in the western United States, in the Eifel region of Germany, and in other geologically young volcanic regions of the world. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6,500 feet across and 400 feet deep. Its low rim is composed of loose pieces of basaltic lava and wallrocks (sandstone, shale, limestone) of the underlying diatreme, as well as random chunks of ancient crystalline rocks blasted upward from great depths.

New Mexico Volcanoes and Volcanics Menu

The Ubehebe Craters include over a dozen maar volcanoes formed during hydrovolcanic eruptions of alkali basalt through permeable fanglomerate deposits on the north side of Tin Mountain. The craters formed along a range-bounding fault that marks the western margin of Tin Mountain. The volcanic center is named Ubehebe Crater, the largest tuff ring (0.8 kilomters wide, 235 meters deep) of the volcanic field. Initial eruptive activity at the center developed a scoria cone south of Ubehebe Crater. Subsequent activity was predominantly hydrovolcanic and produced two clusters of explosion craters and tuff rings west of, and south of, Ubehebe Crater. The southern cluster includes Little Hebe Crater, the second youngest crater in the field. The youngest eruptive events at the center were the episodic hydrovolcanic explosions that formed Ubehebe Crater. Ejecta from the crater covers all preexisting craters in the area. The deposits also overlie lake beds of Lake Rogers, approximately 4 kilometers north of the vent. This stratigraphic position, and the lack of erosional modification of the pyroclastic surge apron, suggest the youngest activity was Holocene. The Ubehebe Craters are accessible via a paved road, west of Scotty's Castle, at the north end of the Death Valley National Monument. The road leads to an overlook at the west side of Ubehebe Crater.

California Volcanoes and Volcanics Menu

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