1. Obsidian Lava Flow Mckenzie Pass Age Chart
2. Volcanic Obsidian
3. Obsidian Lava Flow Mckenzie Pass Age Chart

Black obsidian is the rock formed by lava when it is cooled down or when it gets in contact with water. It forms whenever the lava cools down quickly and apparently did not have enough time to turn into a glass (although it is believed to be a form of volcanic glass). Obsidian is formed in the latest stage of volcanic eruptions, the silica left over after most of the other elements and water have been used up are ejected or flow out and rapidly chilled at surface temperatures.

'Apache tears' is the popular term for rounded pebbles of obsidian or 'obsidianites' composed of black or dark-colored natural volcanic glass, usually of rhyolite composition and bearing conchoidal fracture. Also known by the lithologic term 'marekanite', this variety of obsidian occurs as subrounded to subangular bodies up to about 2 inches in diameter, often bearing indented surfaces.^[1] Internally the pebbles sometimes contain fine bands or microlites and though in reflected light they appear black and opaque, they may be translucent in transmitted light. Apache tears fall between 5 and 5.5 in hardness on the Mohs scale.^[2]

Geology[edit]

Apache tears originate from siliceous lava flows, lava domes or ash-flow tuffs, often in close association with or embedded in, gray perlite. The spherules occur as cores within perlite masses that typically exhibit texture of concentrically curved, onion-skin fractures. Formation is apparently related to differential cooling and various alkali and water contents. Excessive water present during cooling and quenching of rhyolitic lava causes obsidian to hydrate (i.e., water entering the obsidian glass converts it to perlite). Where perlite is incompletely hydrated, fresh obsidian cores remain as pebbles of marekanite, or Apache tears; this origin has been occasionally described in the geologic literature (for example,^[3]).

Obsidian Lava Flow Mckenzie Pass Age Chart

Apache tears are well known from tertiary volcanic terrain in numerous localities throughout the western United States, particularly Arizona, from where specimens were widely collected and sold in the lapidary and specimen trade.^[4] Several districts in western Nevada also have yielded abundant Apache tears eroding from tuff beds; such areas have been popularized in the lapidary trade through guides for rockhounds.^[5] Specimens from many of these sites have been avidly collected by rockhounds and lapidary enthusiasts, are often tumbled and may be considered semi-precious gemstones; locations are noted in the section 'Gemstones of Nevada' by Rose and Ferdock.^[6]

Culture[edit]

The name 'Apache tear' comes from a legend of the Apache tribe: about 75 Apaches and the US Cavalry fought on a mountain overlooking what is now Superior, Arizona, in the 1870s. Facing defeat, the outnumbered Apache warriors rode their horses off the mountain to their deaths rather than be killed. The wives and families of the warriors cried when they heard of the tragedy; their tears turned into stone upon hitting the ground.^[7][8]

Volcanic Obsidian

American singer songwriter Johnny Cash wrote a song entitled 'Apache Tears' for his 1964 album Bitter Tears: Ballads of the American Indian.

See also[edit]

References[edit]

1. ^American Geological Institute, Gary, Margaret, McAfee, Robert, Jr., Wolf, Carol L., eds., 1972, Glossary of geology: Washington, D.C
2. ^Apache Tears Obsidian Education and InformationArchived June 3, 2012, at the Wayback Machine
3. ^Mrazova, Stepanka & Gadas, Petr, 2011, Obsidian balls (marekanite) from Cerro Tijerina, central Nicaragua: petrographic investigations: Jour. Geosci., v. 56, p. 43-49
4. ^Sinkankas, John, 1959, Gemstones of North America: Van Nostrand; p. 503-508
5. ^Kappele, William A., 1998, Rockhounding Nevada: A guide to the state’s best rockhounding sites: Falcon Press
6. ^Castor, Stephen B. & Ferdock, Gregory C., 2004, Minerals of Nevada: Univ. of Nevada Press, NV Bur. Mines & Geol. Special Publication 31, p. 83
7. ^'Archived copy'(PDF). Archived from the original(PDF) on 2014-02-09. Retrieved 2015-05-23.CS1 maint: discouraged parameter (link) CS1 maint: archived copy as title (link)
8. ^Native American Legends

Thirteen vents along a northwest-southeast trending fissure were active during this eruption, which occurred about 950 years ago. Tephra produced by explosions early in the eruption deposited pumice and ash several meters thick near the vents, but the deposit thin rapidly to <1 cm some 40 km (25 mi) to the northeast. Ten northwestern vents and the southeastern-most vent produced domes, while lavas of the intervening three vents coalesced to form the Glass Mountain flow, one cubic kilometer (0.25 cubic mile) in volume. The flow consists of three dacitic eastern lobes which grade westward to rhyolite and are overlain by rhyolite lobes. Glass Mountain first erupted dacite containing mafic magmatic inclusions and last erupted rhyolite without inclusions. Granitic inclusions are rare in Glass Mountain, but were probably derived from underlying rock related to Medicine Lake volcano itself.

Aerial view west across the upper part of Medicine Lake Volcano toward Mount Shasta on the horizon. Glass Mountain flow, erupted ~950 years ago, is outlined in the foreground. Medicine Lake lies within the shallow basin of Medicine Lake caldera. Glass Mountain flow, draped over the east side of the volcano, is the youngest lava flow at the volcano. The northeasternmost dacite tongue extends nearly to the bottom of the photo (distal lobes are outlined to enhance visibility).

Obsidian Lava Flow Mckenzie Pass Age Chart

(Public domain.)