Quartzite (Sioux Quartzite, Paleoproterozoic, 1.65 to 1.70 Ga; city of Sioux Falls, Transcontinental Arch, southeastern South Dakota, USA)
Quartzite from the Precambrian of South Dakota, USA. (5.9 cm 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.
Quartzite is a common, crystalline-textured, intermediate- to high-grade metamorphic rock. It forms by metamorphism of quartzose sandstones or siltstones. Quartzite can be entirely composed of interlocking quartz crystals, or the original sand grains may still be visible. This rock is hard (H = 7), will not bubble in acid (unlike marble), and can be almost any color.
The term “quartzite” has been used in geology to refer to crystalline, quartzose metamorphic rocks and to hard, well-cemented quartzose sandstones that have not been subjected to metamorphism. It is difficult to not call hard, well-cemented sandstones “quartzite” (for example, the Clinch Quartzite in the Appalachian Mountains & the Eureka Quartzite of the Great Basin in western USA). But the Clinch and Eureka aren’t metamorphic rocks. The term metaquartzite has been used by some geologists to refer to crystalline-textured, quartzose rocks that have been metamorphosed. This implies that “quartzite” be restricted to well-cemented, non-metamorphosed sandstones. I don’t often see the term metaquartzite used in the geologic literature.
The quartzite sample shown above is from South Dakota's Sioux Quartzite. Extensive outcrops of pinkish, Paleoproterozoic-aged quartzites are present at Falls Park along the Big Sioux River in the city of Sioux Falls. The quartzites here have nicely water-worn, sculpted surfaces with good, fluvially abraded polish in places. These rocks are part of the Sioux Quartzite (upper Paleoproterozoic, 1.65-1.70 Ga). Despite being subjected to regional metamorphism, this unit’s original sedimentary features, such as horizontal stratification and ripple marks, are still preserved.
The Sioux Quartzite is a famous erosion-resistant unit in America’s midcontinent. It has formed a long-lived paleotopographic high since Precambrian times - the Sioux tectonic core. This high is part of a NE-SW trending series of paleotopographic highs & depressions known as the Transcontinental Arch, which extends from Arizona to Minnesota (see Carlson, 1999).
Stratigraphy: Sioux Quartzite, upper Paleoproterozoic, 1.65 to 1.70 Ga
Locality: Falls Park along the Big Sioux River in the city of Sioux Falls, southeastern South Dakota, USA
Carlson (1999) - Transcontinental Arch - a pattern formed by rejuvenation of local features across central North America. Tectonophysics 305: 225-233.