File:Novaculite (metachert) (Arkansas Novaculite, Devonian to Mississippian; Arkansas, USA) 7.jpg
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[edit]DescriptionNovaculite (metachert) (Arkansas Novaculite, Devonian to Mississippian; Arkansas, USA) 7.jpg |
English: Novaculite from the Paleozoic of Arkansas, USA.
Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy. Novaculite is a local name in Arkansas for metachert (= metamorphosed chert). The physical properties of chert and metachert are quite similar (in some cases, not distinctive at all) - hard, dense, relatively smooth surfaces, sharp broken edges, and conchoidal fracture. The metamorphism involves recrystallization of the chert's microscopic quartz grains. Information given about Arkansas novaculite invariably mentions its use as a whetstone, which is a sharpening stone for knives. But, this is not a geologic definition, and so, prima facie, “novaculite” shouldn't be a geologic term. It turns out that metachert in Arkansas is distinctly, but variably, microporous. This is what makes novaculite different from ordinary chert. The texture, look, and feel of most novaculite is indeed different from chert. Some Arkansas novaculite is not microporous, and is exactly like non-metamorphosed chert. Most novaculites, however, are somewhat rough, even on cut surfaces. The roughness is a consequence of the microporosity. All that said, I’d still prefer to ditch the term “novaculite” and use “metachert” instead. A rose by any other name is still a rose. Arkansas novaculites are composed of quartz grains that range from about 5 to 20 microns in size. Stratigraphy: Arkansas Novaculite, Middle Devonian to upper Lower Mississippian Locality: unrecorded site in Arkansas, USA |
Date | |
Source | https://www.flickr.com/photos/47445767@N05/50968300691/ |
Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/50968300691. It was reviewed on 22 February 2021 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
22 February 2021
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current | 17:07, 22 February 2021 | ![]() | 2,579 × 1,380 (2.03 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/50968300691/ with UploadWizard |
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Camera manufacturer | Canon |
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Camera model | Canon PowerShot D10 |
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F-number | f/9 |
ISO speed rating | 80 |
Date and time of data generation | 21:27, 21 February 2021 |
Lens focal length | 8.295 mm |
Width | 4,000 px |
Height | 3,000 px |
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Pixel composition | RGB |
Orientation | Normal |
Number of components | 3 |
Horizontal resolution | 180 dpi |
Vertical resolution | 180 dpi |
Software used | Adobe Photoshop Elements 18.0 (Macintosh) |
File change date and time | 23:03, 21 February 2021 |
Y and C positioning | Co-sited |
Exif version | 2.21 |
Date and time of digitizing | 21:27, 21 February 2021 |
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Image compression mode | 3 |
APEX shutter speed | 5.90625 |
APEX aperture | 6.34375 |
APEX exposure bias | −0.66666666666667 |
Maximum land aperture | 3.34375 APEX (f/3.19) |
Metering mode | Pattern |
Flash | Flash fired, compulsory flash firing, red-eye reduction mode |
Supported Flashpix version | 1 |
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Focal plane resolution unit | inches |
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File source | Digital still camera |
Custom image processing | Normal process |
Exposure mode | Manual exposure |
White balance | Auto white balance |
Digital zoom ratio | 1 |
Scene capture type | Standard |
Lens used | 6.2-18.6 mm |
Date metadata was last modified | 18:03, 21 February 2021 |
Unique ID of original document | 7891A06566FF276B102F47931B7236C2 |
IIM version | 24,576 |