Volcanology II
A maar is a low relief volcanic crater produced by the interaction of rising magma and ground water and the product is shallow explosive eruptions. What happens is the heating and boiling of ground water when magma invades the ground water table and the explosions blast up through the surface. The result is a crater that looks similar to a hole that is often filled with water to form a lake. The range in size is from 60 to 2,000 metres (200 to 6,600 ft) across and form 10 to 200 metres (33 to 660 ft). There are maars in western USA, in the Eifel region of Germany and in geologically young volcanic region of the world, one of them is Hunt’s Hole and Kilbourne Hole located in Potrillo volcanic field, in Dona Ana New Mexico close to El Paso Texas. There are five maars in the Potrillo volcanic field.
A cinder cone also known as scoria cone, is a steep, conical hill formed on top of a vent. The cinder cones often occur in groups usually on the sides and around the shield volcanoes and the strato volcanoes. They are formed form lava fragments called cinders, it is magma that is glassy and has hundreds of gas bubbles that are frozen, it cools quickly after it is ejected into the air. The lava is expelled from the vent and formed around the vent when they fall back after it is ejected. Cinder cones are approximately 250 metres (820 ft) height and 500 metres (1,600 ft) in diameter.
The Potrillo volcanic field covers approximately 4,600 square kilometres (1,800 sq mi) of Doña Ana County. It is a monogenetic volcanic field that can be divided in three volcanic regions; the West Potrillo Field that consists of more than 100 Cinder Cones, two maar volcanoes and associated flows that covers approximately 1,250 square kilometres (480 sq mi). The Aden–Afton field has a number of young flows, three cinder cones and three maar volcanoes, one of them is the Kilbourne Hole; Aden-Afton Field is approximately 230 square kilometres (89 sq mi).
Aden Crater is a small shield cone, and it is the most unusual feature in the volcanic field because it has a lava tube, where a ground sloth skeleton was found. It is 11,000 years old, and is now at Yale's Peabody Museum of Natural History. This ground sloth (Nothrotheriops shastense) is one of the few specimens of this age which have been found with patches of skin and hair preserved.
Black Mountains–Santo Tomas is made up of basalt, it consists of four different eruptive centers near the Rio Grande. The West Potrillo field is the oldest part of the Potrillo volcanic field.
The Potrillo volcanic field has two important xenolith localities that are Kilbourne Hole and Potrillo maar where mantle peridotites, feldspathic granulites and kaersutite occur. Alkali feldspar, olivine and basalt were erupted during the middle and late quaternary. Rock samples collected in the northern part of the pyroclastic deposit of the Potrillo maar, and lava associated with a cinder cone yielded potassium–argon ages, approximately 1.29 and 1.18 million years. The magmatism is entirely basaltic forming scattered scoria cones. Several short lava flows were originated from each cone. There is a diverse rock types beneath the Potrillo volcanics, the range is from Proterozoic granites through a Phanerozoic sedimentary succession to basalt-andesite volcanics of the southern fringes of the Sierra de las Uvas volcanic field. The Potrillo area is generally classified as part of the southern Rio Grande rift and shows the Tertiary tectonic evolution of that structure.
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