Navajo Volcanic Field

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Map of the Navajo Volcanic Field

The Navajo Volcanic Field , in the Navajo language tsézhiin 'íí' áhí ("black protruding rocks"), is a 30,000 square kilometer volcanic field on the Colorado Plateau at an altitude of around 1,800 meters . The volcanic forms consist of more than 80 larger vents , as well as a variety of smaller intrusions, transitions , sills , lava flows and merchandize igneous origin some cases only by the erosion of the surrounding relatively soft sandstone were uncovered. They were formed around 28 to 19 million years ago.

The geological significance of the Navajo Volcanic Field lies in the fact that the volcanic phenomena penetrated the earth's crust relatively shortly after the end of the laramic mountain formation and carried away numerous rocks from the earth's mantle in the magma flows . These are now embedded in the exposed rocks. Since the Colorado Plateau has remained relatively calm geologically since then, these fragments, known as xenolites , provide an insight into the region's mantle, which has remained largely unchanged to this day.

geography

The Navajo Volcanic Field is located in the Four Corners border area of ​​the four US states of Utah , Colorado , Arizona and New Mexico and extends to all four states. It has a roughly arched structure from northwest to south of around 300 km in length and around 100 km in width, with a detached area in the northeast near the Mesa Verde highlands . The north-western limit of the volcanic field is formed by the eastern edge of the Monument Upwarp , its northern part extends into the Paradox Basin , the Carrizo Mountains rise near the center and its southern part includes the Defiance Uplift and the Chuska Mountains . In the east, parts of the field are already in the San Juan Basin .

The subsoil and surroundings of the Navajo Volcanic Field are predominantly sandstones from the Mesozoic Era with an age of 251 to 65 million years ( mya ) such as the Chinle Formation , in the area of ​​the Chuska Mountains but also younger rocks from the Paleogene with an age of 65 to 28 mya like the Chuska Sandstone .

The Colorado Plateau was raised as a whole about 80 to 40 million years ago as part of the Laramian orogeny . Individual flexures occurred, differences in elevation, which occurred without breaks known as faults . If the surface is eroded by later erosion , the steep stratification of the pushed- off side wing is cut horizontally or with slight differences in height at the flexures . These structures are called monoclines and can extend over 100 km and more. The volcanic phenomena visible on the surface of the Navajo Volcanic Fields are concentrated along these monoclines because magma rises more easily at the transition zones between different rock layers than it can penetrate. In the case of the volcanic vents located away from the monoclines, it can be assumed that they penetrated deeper underground at fracture zones.

The Shiprock , the most famous example of a volcanic neck from the Navajo Volcanic Field
Neck of Agathla Peak , also called El Capitan
In the crater of the Narbona Pass volcano in the Chuska Mountains

Volcanism

A further uplift of the Colorado Plateau by up to 3,000 m only took place after the end of volcanic activity in the period of 20 to 6 million years. Since then, the erosion on the Colorado Plateau has been particularly effective due to the highland location. At some volcanic vents of the Navajo Volcanic Field , the sandstone surrounding the volcanic rocks was removed, the harder volcanic rock was exposed and has since dominated today's terrain. A prime example of this is the Shiprock - a neck made of tuff breccia, which is roughly the middle section of a diatreme . At its summit, the effects of falling falls, as are typical of the upper section of a diatreme, can still be seen. Other well-known examples are the Agathla Peak and the Thumb .

Emergence

The volcanoes of the Navajo Volcanic Field are predominantly maar diatrem volcanoes. They were created by so-called phreatomagmatic eruptions when magma rising up in tunnels met groundwater . At such a meeting, steam explosions are triggered, which achieve a particularly high eruption energy. With multiple eruptions, the chimney known as diatreme breaks through to the surface and a shallow maar crater is blasted out, which is surrounded by a tuff ring made of the pyroclastic material ejected during the eruptions . With the gradual depletion of the groundwater supply, the sources of the explosion move deeper and what remains is a funnel-shaped chimney, filled with breccia of solidified magma and debris. After the end of the explosive activity, magma can rise in the vent breccias and fill the maar crater as a lava flow. Occasionally, lava flows overflow and leave the crater. In the final phase, the upper chimney area often collapses and falls.

At the Narbona Pass Volcano in the Chuska Mountains, this model of the formation of a maar diatrem volcano can largely be traced. The maar crater at Narbona Pass has a diameter of almost 2 km, at its edge the layered tuff ring can be seen, inside both lava flows and smaller chimney fillings exposed by erosion.

Exceptions are eight vents made of serpentinized , ultramafic microbreccias (acronym: SUM, from English serpentinized ultramafic microbrecia ), seven of which are in the immediate vicinity of the monoclines. The formation of these micro-breccias can be explained by the fact that gas-rich magma at relatively low temperatures met water-containing rock during the ascent, which caused the magma to cool and fragment very quickly; there were no major explosions here. The eight SUMs were initially referred to as kimberlites until the new, descriptive name was established.

Rock composition

The typical volcanic rocks in the Navajo Volcanic Field are predominantly minettes . The lava flows created at the end of the eruptions mostly consist of trachy basalts . These are chemically identical to the minettes, but have a different petrological structure because they hardened on the surface and thus under atmospheric pressure; Minettes already solidify in the chimney below the surface and under pressure. In addition to the minettes, very rare types of rock occur occasionally, such as sodium-containing lamprophyres ( monchiquite ), olivine melilithite and katungite .

The minettes are lamprophyres that are classified as mafic because of their high magnesium and iron content . They consist of about 50 percent by weight of SiO 2 in conjunction with a very high K 2 O content of 6% on average. Only a few minettes have higher SiO 2 contents of up to 60% and are therefore classified as Felsic . The higher SiO 2 proportions can be explained by fractional crystallization in later phases of ascent. The temperature of the Minette magma at the eruption is estimated to be 1000 ± 75 ° C or, according to another melt sample, between 1000 and 1200 ° C.

Magmatic evolution

Various approaches are being discussed for the formation of magma and the explanation of its composition. In principle, the conspicuously high magnesium and iron proportions can be explained either by separating the missing proportions or by enriching the common substances. Specifically, it is suggested that the mantle peridotite had previously been affected by potassium metasomatosis before it partially melted , leaving residues behind and a fraction with the observed composition rose along faults in the earth's crust. But it is also possible that the components missing in the chemistry of the volcanic field had already been separated off in an earlier partial melting event and that the renewed melting process and the rise as magma took place much later. Alternatively, it is also assumed that the high iron and magnesium proportions do not originate from the actual mantle rock itself, but instead accumulated much earlier and deeper in the earth's mantle through melting processes and then intrusively got into the mantle rock that was later to melt into the magma. Ultimately, a fractional crystallization of an existing ultramafic magma, which is not described in detail, is also possible .

In all explanations, the eight SUM diatrems arose from a magma with a particularly high gas content, with the few felsic minettes coming from the uppermost part of the melting area, but the majority of the mafic minettes coming from deeper layers.

The xenolites from the earth's mantle embedded in the volcanic rock usually consist of spinel and, more rarely, garnet peridotites ( lherzolites ) and eclogites . These xenolite rocks from the Navajo volcanic field show that the mantle under the Colorado Plateau bears strong similarities to the oceanic crust . So there was presumably a subduction of an oceanic tectonic plate under a continental plate already in the Precambrian , analogous to the explanation of the laramic mountain formation by subduction of the Farallon plate . However, in terms of their chemical composition, the rocks are not completely identical to the peridotite that predominates in the earth's mantle , but fractions are missing, which increases the potassium, iron and magnesium content relatively. From the analysis of the rock distribution within the diatreme it can be determined that the boundary of the earth's crust and mantle known as Moho lies below the Colorado Plateau at a depth of about 43 km.

Ethnogeology

Today the Navajo settle in the region , who call themselves Diné . Almost all of the Navajo Volcanic Field is in the territory of the Navajo Nation , their self-governing territory. They have a large number of myths and legends that they associate with the volcanic rock formations, for example Shiprock is the setting for a significant episode in their creation story . The Navajo worldview is characterized by an all-embracing dualism : in the Navajo imagination, the earth and all manifestations of animate and inanimate nature owe their creation to the interaction of Nohosdzáán (earth) and Yadihil (sky). The individual processes are divided into creating, feminine, and destructive, masculine factors. In an explanation based on the traditional ethno-geology of Diné called tsé na'alkaah (rock theory), the volcanic formations of the Navajo Volcanic Field can be described as the result of the violent (male) interaction of magma from the earth with water coming from the sky . The rocks in the chimneys were exposed by the interaction of the (female, from the earth) uplift of the Colorado Plateau with the (male, from the sky) erosion. This integration of scientific geology with traditional ideas is used in Diné schools and colleges in order to merge science lessons with the Diné culture and imagination.

literature

  • Steven Semken: Black Rocks Protuding up: The Navajo Volcanic Field . In: New Mexico Geological Society Guidebook . 54th Field Conference, Geology of the Zuni Plateau, 2003, pp. 133-138 (online: Fieldguide for the Navajo Volcanic Field PDF file; 1.14 MB).

Web links

Commons : Navajo Volcanic Field  - Collection of Images, Videos, and Audio Files

Individual evidence

  1. ↑ a b c d Michael F. Roden, Douglas Smith, V. Rama Murthy: Chemical constraints on lithosphere composition and evolution beneath the Colorado Plateau . In: Journal of Geophysical Research . 95, no. B3, year 1990, 1990, p. 2811-2831 .
  2. ^ Douglas Smith, William L. Griffin, et al .: Trace-element zonation in garnets from The Thumb: heating and melt infiltration beneath the Colorado Plateau . In: Contributions to Mineralogy and Petrology . tape 107 , 1991, pp. 60-79 .
  3. ↑ USGS: Geologic Provinces of the United States: Colorado Plateau Province , as of 2004
  4. ^ Paul T. Delaney: Ship Rock, New Mexico - the vent of a violent volcanic eruption . In: SS Beus (Ed.): Geological Society of America Centennial Field Guide, Rocky Mountain Section . tape 2 . Boulder, Co. 1987, pp. 411-415 .
  5. ^ Wohletz, K. & Heiken, G .: Volcanology and geothermal energy . University of California Press, Berkeley 1992, pp. 432 .
  6. ^ A b c d Steven Semken: Black Rocks Protuding up: The Navajo Volcanic Field
  7. ↑ Brittany D. Brand, Amanda B. Clarke, Steven Semken: Eruptive conditions and depositional processes of Narbona Pass Maar volcano, Navajo volcanic field, Navajo Nation, New Mexico (USA) . In: Bulletin of Volcanology . tape 71 , 2009, p. 49-77, 61 f .
  8. ^ A b Douglas Smith, Susan Levy: Petrology of the Green Knobs diatreme and implications for the upper mantle below the Colorado Plateau . In: Earth and Planetary Science Letters . tape 29 , 1976, p. 107-125 .
  9. ↑ a b c Michael F. Roden: Origin of coexisting minette and ultramafic breccia, Navajo volcanic field . In: Contributions to Mineralogy and Petrology . tape 77 , 1981, pp. 195-206 .
  10. ^ All explanations according to Steven Semken: Black Rocks Protuding up: The Navajo Volcanic Field with further evidence
  11. ↑ Thomas R. McGetchin, Leon T. Silver: A crustal-upper mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock dike . In: Journal of Geophysical Research . tape 77 , 1972, p. 7022-7037 .

Coordinates: 36 ° 41 ′  N , 108 ° 50 ′  W