A monogenetic volcanic field is a type of volcanic field consisting of a group of small monogenetic volcanoes , each of which erupts only once, as opposed to polygenetic volcanoes , which erupt repeatedly over a period of time. The small monogenetic volcanoes of these fields are the most common subaerial volcanic landform.
29-756: The Navajo volcanic field is a monogenetic volcanic field located in the Four Corners region of the United States , in the central part of the Colorado Plateau . The volcanic field consists of over 80 volcanoes and associated intrusions of unusual potassium -rich compositions, with an age range of 26.2 to 24.7 million years ( Ma ). In Navajo Nation tradition, the vents are known as tsézhiin ‘íí ‘áhí , "black rocks protruding up", and they play an important role in Navajo creation stories . Geologists have studied
58-687: A discussion of the possible importance of the mineral as a significant reservoir of water in the Earth's mantle. Titanium is a minor constituent of clinohumite in most such occurrences. Clinohumite is stable throughout the upper mantle to depths of at least 410 km (250 mi) and is a potential host phase for H (water) in this region of the Earth's interior. Minerals associated with humite include grossular , wollastonite , forsterite , monticellite , cuspidine , fluoborite , ludwigite , dolomite , calcite , talc , biotite , spinel , vesuvianite , sanidine , meionite and nepheline . The structure
87-442: A hardness slightly greater than 6, a lower specific gravity (3.18), and higher maximum birefringence (0.036). Phillip Youngman, master faceter of Los Osos , California , noticed not only that Pamir material is harder than expected, but also that it is less brittle than expected. Youngman observed that clinohumite reacted like beryl to cutting and polishing, and that it reminded him of polishing diopside . Like other members of
116-474: A magma intrusion into the mantle. Stable isotope ratios in Navajo volcanic field rocks suggest that the magmas formed by melting of mica- or pyroxene-rich mantle rock, with the melt subsequently interacting with metasomatized mantle peridotite. The Navajo people refer to the outcrops of the Navajo volcanic field as tsézhiin ‘íí ‘áhí , "black rocks protruding up", and they play an important role in Navajo creation stories . Navajo ethnogeology revolving around
145-473: A matrix of minerals typical of serpentine : serpentinite , chlorite , clay minerals , and talc . The xenocrysts (individual crystals entrained by the magma from surrounding rock) are composed of minerals such as olivine, enstatite , chrome diopside, chlorite, garnet, titanclinohumite , various oxide minerals, and apatite. The xenoliths include both mantle and crust compositions. These rocks were originally identified as kimberlites , but in normal kimberlites,
174-468: Is monoclinic with space group P 2 1 / b ( a -unique). The unit cell has a = 4.7488 Å; b = 10.2875 Å; c = 13.6967 Å; and alpha = 100.63°; V = 667.65 Å ; Z = 2 for pure Mg hydroxyl-clinohumite. The odd setting of space group P 2 1 / c is chosen to preserve the a and b axes of olivine. The structure is closely related to that of olivine as well as the other humite minerals. Mg and Fe are in octahedral coordination with oxygen and silicon (Si)
203-555: Is a product of contact metamorphism and is commonly found as indistinct grains embedded in limestone . Its type occurrence is within the limestone ejecta of the Mount Vesuvius volcano complex near Naples , Italy , where clinohumite was discovered in 1876. The aforementioned gem-quality occurrences of Pamir and Taymyr were discovered only recently: the former in the early 1980s, and the latter in 2000. These deposits are scarce and only sporadically mined, so clinohumite remains one of
232-457: Is as follows: α 1.631; β 1.638–1.647; γ 1.668;, with a maximum birefringence of 0.028 (biaxial positive). Under shortwave ultraviolet light, some clinohumite may fluoresce an orangy yellow; there is little to no response under longwave UV. The Taymyr material is reported to be a dark reddish brown while the Pamir material is a bright yellow to orange or brownish orange. The Pamir material also has
261-418: Is common, resulting in a highly variable habit. Clinohumite is brittle with a hardness of 6 and a poor basal cleavage . Its specific gravity is 3.2–3.4, and its fracture is conchoidal to uneven; its streak is white. Clinohumite's transparency ranges from transparent to translucent; its luster ranges from a dull vitreous to resinous. Its refractive index (as measured via sodium light , 589.3 nm )
290-406: Is exceptionally large. Monogenetic fields occur only where the magma supply to the volcano is low or where vents are not close enough or large enough to develop plumbing systems for continuous feeding of magma. Monogenetic volcanic fields can provide snapshots of the underlying region beneath the surface, and may be useful in studying the generation of magma and the composition of the mantle since
319-479: The Chuska and Lukachukai Mountains are chemically equivalent to the minette. Other rare rock types found in a few locations in the Navajo volcanic field include monchiquites (sodium-rich lamprophyres), olivine melilites , and katungite , the latter having a silica content as low as 33.6 percent. The eruptions brought xenoliths , fragments of mantle rock, to the surface. Geologists have studied these for clues to
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#1732845150393348-568: The Michoacán-Guanajuato volcanic field , which erupted from 1943 to 1952. Some monogenetic volcanoes are small lava shields , such as Rangitoto Island in the Auckland volcanic field . Other monogenetic volcanoes are tuff rings or maars . A monogenetic field typically contains between ten and a hundred volcanoes. The Michoacán-Guanajuato field in Mexico contains more than a thousand volcanoes and
377-632: The Pamir Mountains of Tajikistan , and the Taymyr region of northern Siberia . It is one of two humite group minerals that have been cut into gems, the other being the much more common chondrodite . A monoclinic mineral, clinohumite is typically a dark to light brownish or orangy yellow, somewhat resembling the hessonite variety of grossular . Clinohumite's crystal habit is usually granular, but may also be prismatic ; crystals are almost always small. Simple and multiple crystal twinning (on {001})
406-596: The Pirin Mountains of Bulgaria ; Bancroft, Ontario , Notre Dame du Laus , Wakefield , and Villedieu Township , Quebec , Canada ; Southern and Western Finland ; Bavaria and Saxony , Germany ; eastern Greenland ; Ambasamudram in Tamil Nadu , India ; Honshū , Japan ; Suan, North Korea ; Nordland , Norway ; KwaZulu-Natal and Northern Cape Province , South Africa ; Andalusia , Spain ; Värmland and Västmanland , Sweden ; Isle of Skye , Scotland ; and
435-574: The humite group , a magnesium silicate according to the chemical formula ( Mg , Fe ) 9 ( Si O 4 ) 4 ( F ,O H ) 2 . The formula can be thought of as four olivine (Mg 2 SiO 4 ), plus one brucite (Mg(OH) 2 ). Indeed, the mineral is essentially a hydrated olivine and occurs in altered ultramafic rocks and carbonatites . Most commonly found as tiny indistinct grains, large euhedral clinohumite crystals are sought by collectors and occasionally fashioned into bright, yellow-orange gemstones . Only two sources of gem-quality material are known:
464-465: The tsézhiin ‘íí ‘áhí has been used to enhance scientific education in Navajo Nation educational institutions. The Chacoan civilization included vents of the Navajo volcanic field, such as Shiprock, Bennett Peak , and Ford Butte , in their system of sacred geography. Monogenetic volcanic field Many monogenetic volcanoes are cinder cones , often with lava flows, such as Parícutin in
493-470: The Colorado Plateau had risen into the overlying mantle wedge . This resulted in metasomatism (replacement of some minerals in the rock with new minerals of different composition) that lowered the rock density and contributed to the uplift of the Colorado Plateau. Dating of monazite emplaced by the metasomatic fluids supports an age of about 28 million years for the metasomatism. Most of the vents of
522-489: The Navajo volcanic field erupted minette lamprophyre , an unusual volcanic rock highly enriched in potassium that contains sizable crystals ( phenocrysts ) of the minerals phlogopite mica , diopside , and sometimes olivine . These are embedded in a very fine-grained matrix of alkali feldspar , diopside, phlogopite, and apatite . The next most common eruptive material is serpentinized ultramafic microbreccia (SUM). This rock consists of xenocrysts and xenoliths embedded in
551-514: The Navajo volcanic field for clues to the geologic history of the Colorado Plateau and the timing and cause of its uplift. The Navajo volcanic field consists of over 80 volcanoes and associated intrusions. Geologically, these erupted in a short time interval, between 26.2 and 24.7 million years ago, based on high-precision Ar-Ar dating of samples from the field. Most of the vents are maar-diatreme volcanoes , formed when magma came into contact with groundwater with explosive results. At some vents,
580-524: The base of the crust) starting 1400 million years ago. Xenoliths erupted at The Thumb, a small vent near Shiprock , were mantle rock entrained by the magma at a depth close to 130 kilometers (81 mi) and at a temperature of about 930 to 1,230 °C (1,710 to 2,250 °F). They range from a coarse garnet peridotite, thought to be typical of the mantle rock below this part of the volcanic field, to an extremely coarse rock enriched in iron and titanium and depleted in chromium, thought to have crystallized from
609-402: The diatreme-eruptions excavated the country rock to depths as great as 1,000 meters (3,300 ft) below the original ground surface. Erosion has subsequently removed as much as 1,000 meters (3,300 ft) of overlying rock, exposing the deeper parts of the diatremes. These typically consist of lapilli tuffs and tuff breccias , which are consolidated beds of rock fragments produced by
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#1732845150393638-416: The eruptions ( volcaniclastics ) the size of coarse sand grains or larger. In a few locations, erosion has been less extensive, and lava flows and the upper parts of maars are still preserved. The dikes of the field are oriented at random, except for dikes radiating from local eruption centers, such as Shiprock . This indicates that the Colorado Plateau was not experiencing systematic regional stresses at
667-567: The humite group, the relative amounts of hydroxyl and fluorine vary in clinohumite, and iron commonly substitutes for some of the magnesium, bringing about changes in physical and optical properties. Titanium substitution also causes pronounced changes in optical properties, producing the variety titanclinohumite . Consequently, it is relatively easy to determine that a stone is a humite group mineral, but difficult to determine exactly which member. Other common impurities of clinohumite include aluminium , manganese , and calcium . Clinohumite
696-447: The large crystals in the matrix are phenocrysts, formed by solidification out of the magma, rather than xenoliths, entrained in the magma from surrounding solid rock. Normal kimberlites also contain minerals rich in incompatible elements that are not present in the Navajo volcanic field SUMs. The SUMs are thought to have formed through interactions of minette magma with hydrous mantle rock. Flows and plugs of extrusive trachybasalt in
725-483: The nature of the lithosphere under the volcanic field. The xenoliths are mostly spinel peridotite with rarer garnet peridotite. These suggest that the Colorado Plateau has a stable, cold, chemically depleted mantle root similar to those of Archean cratons . Lower crust xenoliths suggest the basement crust of the Colorado Plateau formed 2000 to 1750 million years ago and experienced a prolonged episode of metamorphism and possible underplating (pooling of magma at
754-788: The rarest gemstones with only a few thousand carats known to exist in private collections. Other (non-gem quality) occurrences of clinohumite include: the Sør Rondane and Balchen Mountains of Antarctica ; Mount Bischoff , Waratah, Tasmania ; the Saualpe Mountains of Carinthia , the Koralpe mountains of Styria , and the Vals, Virgen, and Ziller valleys of the Tyrol , Austria ; the Jacupiranga mine of Cajati , São Paulo State , Southeast Region , Brazil ;
783-412: The single eruption produced would match that of the chamber from which it erupted. The magma supplying such fields is thought to have rapidly ascended from its source region, with only short resident times (decades or less) in shallow magma chambers . This volcanology article is a stub . You can help Misplaced Pages by expanding it . Titanclinohumite Clinohumite is an uncommon member of
812-457: The states of California , Colorado , Massachusetts , New Jersey , New Mexico , New York , Oklahoma , Utah , and Washington , US . Clinohumite also occurs as a minor component of some masses of peridotite from the Earth's mantle emplaced into the Earth's crust and as a very rare component of peridotite xenoliths . These occurrences and implications have been summarized by Luth (2003) in
841-586: The time the volcanoes of the field were erupting. This in turn suggests that the Colorado Plateau had already separated from the Basin and Range Province and from the High Plains Province along the Rio Grande rift , which were experiencing significant regional stresses. The high potassium content of the magma erupted in the province suggests that potassium-rich fluids from the disintegrating Farallon Plate beneath
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