Obsidian and Pumice Megan Breiter – Geology CE
Definition of Obsidian Obsidian is an extrusive igneous rock formed by the rapid cooling of high-silica lava floes. It possesses a glassy look and texture with an extremely dark black color similar to basalts while being a felsic-type rock.
Definition of Pumice Pumice is an extrusive igneous rock formed from escaped gas through silica-heavy lavas to form light, frothy solid pieces. These gas bubble chambers can be obvious or more glassy and entwined in texture. Because of these bubbles pumice’s density is extremely low, allowing it to float on water.
Obsidian Scientific Properties • Formula : 70-75% SiO2 + MgO, Fe3O4 • Color : Black • Fracture : Conchoidal • Hardness : 5-6 • Luster : Vitreous • Optical Properties : Translucent
Pumice Scientific Properties • Formula : SiO2 • Color : varies [white, green, blue, grey, brown] • Hardness : 6 • Luster : Vitreous
Occurrences of Obsidian Obsidian is found in volcanically active regions with rhyolitic eruptions, typically around the Ring of Fire on the edges of the Pacific plate, and has also been observed in the upper Arctic plate around Iceland and Scotland. Many areas in the continental United States exhibit obsidian, usually in calderas in the western U.S.
Occurrences of Pumice Similarly to obsidian, pumice tends to originate around the area of the Ring of Fire, though it is not exclusive to any one area. An extremely large deposit of pumice was created as a direct result of the 1883 eruption of Krakatoa, sending rafts of pumice as far as Tonga and Fiji. Oftentimes pumice is farmed in manufacturing areas.
Obsidian Uses + Forms Obsidian has been actively used since prehistoric times, the earliest evidence of which dating back to 700,000 BCE. Due to its arrangement of ions, it delivers an extremely fine, sharp edge when cut, which allowed it to be used for cutting utensils and arrowheads for millennia. Its glassy texture and high reflectivity also allowed for its use as a mirror when cut and polished. In other areas, large slabs of obsidian were carved into bowls and plates, while smaller pieces were polished and inset for decorative purposes.
Pumice Uses + Forms The light-density properties of pumice lends itself to use in concrete and cinder block mixes to make them lighter and insulative. This use has been implemented since Roman times when it was used in the construction of aqueducts as well as the Pantheon. In modern days, pumice is used as an abrasive material in cosmetics, erasers, and some cleaners like toothpastes and hand soaps. When ground into powder, pumice is used for substrates in crop-growing and in dust baths for heavy-furred animals.
Bibliography • http://volcano.oregonstate.edu/education/facts/obsidian.html • http://en.wikipedia.org/wiki/Obsidian • http://en.wikipedia.org/wiki/Pumice Obsidian pictures: http://geology.com/rock-tumbler/gemstones/obsidian.shtml http://geology.com/rocks/obsidian.shtml http://www.nps.gov/gaar/historyculture/obsidian-research.htm Pumice pictures: http://www.proprofs.com/quiz-school/story.php?title=ch-3-rocks-unit-2--igneous-rocks http://geology.about.com/od/rocks/ig/igrockindex/rocpicpumice.htm http://geology.com/rocks/pumice.shtml