Metachert (novaculite) (Arkansas Novaculite, Devonian to Mississippian; Ouachita Mountains, Arkansas, USA)
Metachert (novaculite; microporous metachert) from the Paleozoic of Arkansas, USA. (3.8 centimeters across at its widest)
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.
Metachert is 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.
The rock shown above is an Arkansas metachert sample called novaculite. 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.
The novaculite shown above is composed of quartz grains that range from about 5 to 20 microns in size.
Stratigraphy: southern facies of the Arkansas Novaculite, Middle Devonian to upper Lower Mississippian
Locality: Ouachita Mountains of west-central Arkansas, USA