Pumice

Pumice is a textural wonder of the volcanic world. It is the only rock that floats on water. This light-colored, frothy material is the product of the most violent explosive eruptions, representing a “frozen foam” of magma.

The Physics of Formation

Pumice forms from high-viscosity magma (typically rhyolite or dacite) that is packed with dissolved gases.

1. Depressurization: As magma rises rapidly in the conduit, pressure drops, causing dissolved gases (water and CO₂) to exsolve and expand violently. This is identical to opening a shaken soda bottle.
2. Fragmentation: The expanding bubbles stretch the molten rock into thin walls. If the expansion is powerful enough, it tears the magma apart into fragments.
3. Quenching: The fragments are ejected into the cold air and solidify almost instantly. The rock hardens around the gas bubbles, preserving the foam structure.

Physical Characteristics

• Vesicularity: Pumice is composed of highly vesicular glass. The “holes” are called vesicles, which can make up over 90% of the rock’s total volume.
• Density: Because of its high porosity, pumice typically has a specific gravity of less than 1.0, allowing it to float on water until it eventually becomes waterlogged and sinks.
• Composition: Chemically, it is usually felsic (high silica), similar to granite or obsidian. In fact, if you melted pumice down to remove the bubbles, it would essentially become obsidian.

Pumice vs. Scoria

While both are bubbly volcanic rocks, they differ significantly:

• Pumice: Light-colored, high silica, floats, vesicles are tiny and often tube-like.
• Scoria: Dark-colored (red/black), low silica (basaltic), sinks, vesicles are larger and thicker-walled.

Pumice Rafts and Ecology

Large submarine eruptions can produce massive “pumice rafts”—floating islands of rock that can drift for thousands of kilometers across oceans.

• Hazard: These rafts can clog the cooling water intakes of ships and damage propellers.
• Ecology: They act as transport vessels for marine life. Corals, barnacles, and crabs hitch rides on floating pumice, allowing species to colonize new islands and coastlines across vast distances.

Significant Eruptions

The presence of thick pumice layers often signals a cataclysmic past event.

• Krakatoa (1883): This eruption blanketed the Indian Ocean with so much pumice that sailors reportedly walked on floating rafts of it, and ships were impeded for months.
• Mount Mazama (~7,700 years ago): The eruption that created Crater Lake in Oregon covered the surrounding landscape in pumice deposits tens of meters thick, which can still be seen today.
• Novarupta (1912): The largest eruption of the 20th century filled the Valley of Ten Thousand Smokes with ash and pumice flows.

Economic and Industrial Uses

Pumice has been mined for millennia for its unique properties:

• Construction: The Romans used lightweight pumice aggregate to build the dome of the Pantheon, allowing it to stand for nearly 2,000 years without collapsing. Today, it is used in lightweight concrete and cinder blocks.
• Horticulture: Added to soil to improve aeration and drainage.
• Abrasives: Used in polishes, pencil erasers, stone-washed jeans production, and cosmetic skin exfoliants.