Hopi Buttes Volcanic Field
Coordinates: 35 ° 27 ′ N , 110 ° 6 ′ W
The Hopi-Buttes-Volcanic-Field is a monogenetic volcanic field in the northeast of the US state Arizona . It is located about 50 to 80 kilometers northeast of Winslow at an altitude of 1,650 to 2080 meters and has a diameter of about 60 kilometers. There are around 300 maars and diatrems in an area of 2500 square kilometers . The main part of the phreatomagmatic activity took place in the Miocene , in the period 8.5 to 6.0 million years BP ( Tortonian to Messinian ), the last explosions occurred in the Pliocene about 4.2 million years ago ( Zancleum ).
Buttes and mesas
The name imparting Buttes ( dt. " Härtlinge ") are usually made limburgitischen established, the Diatremen or Diatremkomplexen Mesas ( "mesas") are generally of lava flows from Monchiquit covered.
Maars and diatrems
The following major maars are located in the volcanic field:
The maars can reach a diameter of up to 800 meters and usually only form very low elevations. Some are also completely hidden under alluvium . They were created by phreatomagmatic explosions in an extensive Playa system and were then in turn filled with very different lake sediments . In addition to fine-grained clastic sediments and carbonates ( travertine ), there are limburgitic tuffs and tuff breccias, agglomerates , sedimentary rocks torn from the chimney walls (usually Wingate Sandstone ), as well as dikes , necks and lava flows made of monchiquite.
There are two possibilities to explain the explosive activity:
- When the magma rose, gas was released from the magma due to the pressure relief .
- Rising magma came into contact with surface water or wet Playas sediments.
Geological overview
The Hopi-Buttes-Volcanic-Field is part of the Painted Desert and geologically belongs to the Colorado Plateau . It is on the extreme southern edge of the Black Mesa Basin , partly on the territories of the Navajo Nation and the Hopi . The alluvial alluvial plains from the Pleistocene and Holocene are overlooked by countless necks and lava-covered mesas by an average of 180 meters. The surrounding alluvial plain terraces are complex, their different levels testify to several erosion and sedimentation cycles.
The oldest rock unit in the subsurface consists of red-brown siltstones with activated siliceous limestone layers of the owl rock member from the Upper Triassic Chinle formation . Above it follows the Wingate Sandstone - red-brown siltstones with diagonally layered, red-brown and white sandstones . These two units are exposed in the southwest part of the volcanic field.
The actual volcanic rocks of the Hopi Buttes are integrated into the Bidahochi Formation . The Bidahochi Formation - calcareous lacustrian siltstones, claystones and tuffs in its lower section - began with a slight discordance about 16 million years ago after a long gap in the middle Miocene .
Tuffles from the 150-meter-thick lower Bidahochi Formation were dated from 15.8 to 13.7 million years BP (Middle Miocene - Langhian ). Sedimentation in the lower Bidahochi Formation ended 8.5 million years ago BP in the Tortonium .
The sediments of the Chinle Formation, the Wingate Sandstone and the lower Bidahochi Formation were then penetrated by basaltic and tuff breccias leading vents and tunnels. The majority of the radiometric ages of this explosive, phreatomagmatic volcanism are between 8.5 and 6.0 million years BP; H. in the period Tortonian to Messinian . At the same time, there was the extrusion of almost horizontally lying lava flows and associated volcanic agglomerates. Also Tuffringe emerged.
After the volcanic activity subsided, mainly fluvial and aeolian sands were sedimented in the upper Bidahochi Formation .
The slopes of the chimneys and the remains of the alluvial terraces are now mostly covered by coarse gravel material, while silts filled the lower alluvial plains.
Petrology
The black lava flows, the necks and the corridors, which cover a large part of the mesas, consist of monchiquit , a lamprophyric rock . The tuffs and tuff breccias are composed of limburgite . Chemically and mineralogically, however, these two types of rock are very similar, the main difference being their petrological structure . The monchiquite is coarser-grained, has microliths of augite in the base mass and forms lava flows. The Limburgite tuffs consist of brown glass , cavities are filled with calcite and framed by black, opaque iron sulfides .
Both rock types contain the following phenocrystals :
They differ from the normal alkaline rocks of the southern Colorado Plateau by the following characteristics:
- Orthopyroxes , hornblende and plagioclase are absent or only present in small quantities.
- Feldspar representatives and zeolites are available. Matrix mineral is mostly analcime , but nepheline can also occur.
- Extreme undersaturation of SiO 2 .
- High contents of TiO 2 , P 2 O 5 , water and carbon dioxide.
The Monchiquite are very rich in incompatible elements such as arsenic , barium , lead , mercury , rubidium , rare earths , silver , strontium , uranium , vanadium , zinc , tin and zirconium . Uranium in particular reaches very high concentrations (up to 5000 ppm) and was therefore mined in places in uranium-bearing travertine deposits in the 1950s.
The non-existent europium anomaly indicates a minimal dwell time of the magma in the crustal area. The place of origin of the magma was an upper mantle located below the continental shield and interspersed with inhomogeneities .
Individual evidence
- ^ Coliseum Maar, Hopi Buttes, Arizona . Volcano World, North Dakota and Oregon Space Grant Consortia. Archived from the original on September 26, 2008. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved May 7, 2007.
- ↑ Charles A. Wood, Jurgen Kienle: Volcanoes of North America . Cambridge University Press , 1993, ISBN 0-512-43811-X , pp. 283-284.
- ^ Morale Claim Maar, Hopi Buttes, Arizona . Volcano World, North Dakota and Oregon Space Grant Consortia. Archived from the original on September 26, 2008. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved May 7, 2007.
- ^ White, JDL (1991). Pliocene subaqueous fans and Gilbert-type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA. Sedimentology, Volume 39 Issue 5, Pages 931 - 946
- ↑ Damon, PE and Spencer, JE (2001). K-Ar Geochronologic Survey of the Hopi Buttes Volcanic Field. In: Spamer, EE & Young, RA (eds.) The Colorado River: Origin and Evolution: Grand Canyon, Arizona. Grand Canyon Association Monograph 12, pp. 53-56
swell
- Wood, CA and Kienle, J .: Volcanoes of North America. Cambridge University Press 1992.
Web links
- K-Ar Age dating of the Hopi Buttes with geological map PDF file; 587 kB
- Bidahochi Formation with Hopi Buttes volcanic rocks PDF file; 937 kB