Pumice is a highly porous rock that is often used as an abrasive in skin treatment.
The grains that make up rocks have spaces between them called pores. A rock’s porosity is the percentage of pore volume or void space. Highly porous rocks are not necessarily highly permeable as the pores may not connect to each other to allow the flow of liquid or gas. The rock’s method of formation and subsequent alteration over time determine its porosity.
Pumice is a light-colored igneous rock formed when frothy lava solidifies. Once out of the volcano and exposed to the atmosphere, rapid cooling means that the bubbles of hot gas within the lava are left as pores. There are so many bubbles that pumice can be said to be mostly trapped air, proof of which is that some pumice samples float on water. Pumice is the most uniformly porous rock. The Science Daily website reports that pumice has an average porosity of 90 percent.
Scoria is a lightweight dark-colored igneous rock. Its vesicles are formed as a result of trapped gas at the time of solidification. Scoria often forms as a frothy crust on the top of flowing lava. Although the pore spaces in scoria can be large, the rock is generally heavier than water and does not float. Samples of scoria studied by scientists from King Abdulaziz University ranged between 41 and 47 percent, with an average of 43 percent.
Basalt is a relatively heavy dark-colored igneous rock formed when magma emerges from beneath the earth’s crust into the sea. It forms a major part of the upper layers of the planet’s ocean floors. Its grains are almost entirely a combination of plagioclase feldspar and pyroxene. The Indian Academy of Sciences reports that basalt has an average porosity of 17 percent.
Sandstone forms a large proportion of the sedimentary rock found around the world. It is mostly composed of rock grains or sand-sized minerals cemented together. Because these grains can’t fit together closely, sandstone has a relatively high porosity of between 10 and 35 percent, according to the Wisconsin Geological Survey.