Novaculite (metachert) (Arkansas Novaculite, Devonian to Mississippian; Arkansas, USA) 6 : 無料・フリー素材/写真
Novaculite (metachert) (Arkansas Novaculite, Devonian to Mississippian; Arkansas, USA) 6 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
|---|---|
| 説明 | 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 MississippianLocality: unrecorded site in Arkansas, USA |
| 撮影日 | 2021-02-21 21:27:42 |
| 撮影者 | James St. John |
| タグ | |
| 撮影地 |