Pumice Origin & Properties

The word pumice is derived from the Latin word ‘pumex’ meaning ‘foam’. It is a textural rock formed from volcanic eruptions. pumice is a volcanic rock that consists of highly vesicular rough textural rock glass.

• Texture: Aphanitic and vesicular (contains abundant small gas cavities).
• Composition: Felsic (rhyolitic).
• Color: White to light gray or light tan.
• Cooling rate: Rapid, extrusive.
• Intrusive equivalent: Granite.
• Weight: Very light and will float in water.
• Minerals present: Feldspar, augite, hornblende, zircon.

pumice Composition

pumice is primarily = Silicon Dioxide + some Aluminum Oxide and trace amounts of other oxide. 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.

Occurring among old volcanic rocks, the cavities are usually filled with deposits of secondary minerals introduced by percolating water. It forms so quickly that its atoms often don’t have time to organize into crystals. Sometimes there are crystals present in pumice , but most of the structure is amorphous, producing a volcanic glass called a mineraloid. The glass itself forms threads, fibres, and thin partitions between the vesicles. Rhyolite and trachyte pumice s are white, andesite pumice s often yellow or brown, and pumicious basalts pitch black.

pumice formation

pumice is created when gas-saturated liquid magma erupts like a carbonated drink and cools so rapidly that the resulting foam solidifies into a glass full of gas bubbles. This occurs most commonly near water or underwater volcanoes. When hot magma comes into contact with water, rapid cooling and rapid pressure loss reduce bubble by forming lava.

The cooling of the rock below the melting point of the rock means that the bubbles are trapped inside when the rock changes into a solid immediately after contact with water. Since the pumice is irreversible, it is sometimes like glass and the bubbles are held between the thin translucent bubble walls of the rock. pumice from silica-rich lavas are white, those from lavas with intermediate silica content are often yellow or brown, and rarer silica-poor are black.

The hollows in the froth can be rounded, elongated, or tubular, depending on the flow of the solidifying lava. The glassy material that forms can be in threads, fibres, or thin partitions between the hollows. Athough pumice is mainly composed of glass, small crystals of various minerals occur.

pumice has a low density due to its numerous air filled pores and for this reason can easily float in water.