Pumice

Pumice is a volcanic rock that consists of highly vesicular rough textured volcanic glass, which may or may not contain crystals. It is typically light colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles and thicker vesicle walls and being dark colored and denser.

Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases (including water and CO₂) that are 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 depressurization freezes the bubbles in the matrix.

Small crystals of various minerals occur in many pumices; the most common are feldspar, augite, hornblende, and zircon. The cavities (vesicles) of pumice are sometimes rounded and may also be elongated or tubular, depending on the flow of the solidifying lava. In pumice occurring among old volcanic rocks, the cavities are usually filled with deposits of secondary minerals introduced by percolating water. The glass itself forms threads, fibres, and thin partitions between the vesicles.
Rhyolite and trachyte pumices are white, andesite pumices often yellow or brown, and pumiceous basalts (such as occur in the Hawaiian Islands) pitch black. Pumices are most abundant and most typically developed from felsic (silica-rich) igneous rocks; accordingly, they commonly accompany obsidian. The major producers are countries that ring the Mediterranean, particularly Italy, Turkey, Greece, and Spain. In the United States it is mined mainly in Rocky Mountain and Pacific Coast states.

In minute fragments, it has an exceedingly wide distribution over the Earth’s surface. It occurs in all the deposits that cover the floor of the deepest portion of the oceans and is especially abundant in the abyssal red clay. In some measure this pumice has been derived from submarine volcanic eruptions, but its presence is also accounted for by the fact that it will float on water for months and is thus distributed over the sea by winds and currents. After a long time it becomes waterlogged and sinks to the bottom, where it gradually disintegrates and is incorporated in the muds and oozes of the ocean floor.

Pumice rafts of frothy volcanic rock, Bequ Lagoon, Fiji. Credit: David Fleetham/Alamy.

Pumice raft from the Krakatau Caldera eruption in Indonesia. (Photo credit to swisseduc.ch)

Pumice sample. From R.Weller-Cochise College.

Pumice sample from Santorini volcano. From Siim Sepp (www.sandatlas.org).

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Biotite and flattened vesicles in a Pumice from New Zeland. PPL image, 10x (Field of view = 2mm)	Plagioclase and flattened vesicles in a Pumice from New Zeland. PPL image, 10x (Field of view = 2mm)	Biotite and flattened vesicles in a Pumice from New Zeland. PPL image, 10x (Field of view = 2mm)
Flattened vesicles in a Pumice from New Zeland. PPL image, 10x (Field of view = 2mm)	Flattened vesicles in a Pumice from New Zeland. PPL image, 10x (Field of view = 2mm)	Biotite and flattened vesicles in a Pumice from New Zeland. PPL image, 10x (Field of view = 2mm)
Flattened vesicles in a Pumice with brown glass from New Zeland. PPL image, 2x (Field of view = 7mm)	Flattened vesicles in a Pumice with brown glass from New Zeland. PPL image, 2x (Field of view = 7mm)	Flattened vesicles in a Pumice with brown glass from New Zeland. PPL image, 2x (Field of view = 7mm)
Flattened vesicles in a Pumice with brown glass from New Zeland. PPL image, 2x (Field of view = 7mm)	Flattened vesicles in a Pumice with brown glass from New Zeland. PPL image, 2x (Field of view = 7mm)	Flattened vesicles in a Pumice from New Zeland. The brown zone is a Spherulite. PPL image, 2x (Field of view = 7mm)
Flattened vesicles in a Pumice from New Zeland. The brown zone is a Spherulite. PPL image, 2x (Field of view = 7mm)	Flattened vesicles in a Pumice from New Zeland. The brown zone is a Spherulite. PPL image, 2x (Field of view = 7mm)	Flattened vesicles in a Pumice from New Zeland. The gray fibrous zone is a Spherulite. XPL image, 2x (Field of view = 7mm)
Brown Spherulite. PPL image, 2x (Field of view = 7mm)	Spherulite. XPL image, 2x (Field of view = 7mm)