Pumice is an igneous rocks which formed when lava cooled quickly above ground. Little pockets of air can be seen in the rock . This rock is so light, that many pumice rocks actually float in water. Pumice is actually a kind of glass unike other rocks which are mixture of minerals. Because this rock is so light, it is used quite often as a decorative landscape stone.
By origin, Pumice is a is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It is formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid cooling and rapid release of pressure. This release of pressure creates bubbles by lowering the solubility of gases (including water and CO2) dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling and pressure release freezes the bubbles in the matrix.
Economic Uses
Pumice is used extensively to make lightweight concrete or insulative low-density breeze blocks. When used as an additive for cement, a fine-grained version of pumice called pozzolan is mixed with lime to form a light-weight, smooth, plaster-like concrete. This form of concrete was used in Roman times. Roman engineers used it to build the huge dome of the Pantheon and as construction material for many aqueducts.
It is also used as an abrasive, especially in polishes, pencil erasers, cosmetic exfoliants, and the production of stone-washed jeans. “Pumice stones” are often used in beauty salons during the pedicure process to remove dry and excess skin from the bottom of the foot as well as to remove calluses. It was also used in ancient Greek and Roman times to remove excess hair. Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners (such as Lava soap) as a mild abrasive. Pumice is also used as a growing substrate for growing horticultural crops.Its porous nature makes it a natural for filters.
In some aspects Pumice is like scoriain that both are frothy, lightweight volcanic rocks, but the bubbles in pumice are small
and regular and its composition is more felsic than scoria’s.Scoria differs from pumice in being denser. Pumice is glassy and scoria is lava with microscopic crystals.
Pumice differs from obsidian in that obsidian is all glass and lacks the extensive vesicles of pumice. Most pumice is acidic/felsic in composition associated with rhyolite since those lavas tend to have more volatiles, but intermediate and basic varieties are known to occur. Scoria is a much heavier ropey volcanic rock with larger but less prolific vesicles than pumice.
Pumice Islands
Some ocean volcanoes have produced what are known as pumice rafts, which are actual floating mini islands made of rock. These pumice islands can be there for years floating along the ocean currents. Some may have been responsible for the distribution of island hopping animals and plants of the Pacific Ocean. Some pumice islands were found with plants actually growing on them.
Links and Sources:
Wikipedia, Rocks Hounds, Mineral Galleries
Rashid's Blog: A Place for Inquisitive Souls 