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Pumice and Pumicite Production
Pumice is produced in two forms: rock pumice and pumicite. “Pumicite” is a name given to very fine-grained pumice (less than 4 millimeters in diameter down to submillimeter sizes). The word can be used synonymously with “volcanic ash.” It is mined from volcanic ash deposits, or it can be produced by crushing rock pumice. About…
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Uses of Pumice
The largest use of pumice in the United States is the production of lightweight concrete blocks and other lightweight concrete products. When this concrete is mixed, the vesicles remain partially filled with air. That reduces the weight of the block. Lighter blocks can reduce the structural steel requirements of a building or reduce the foundation…
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Pumice Has a Very Low Specific Gravity
The abundant vesicles in pumice and the thin walls between them give the rock a very low specific gravity. It typically has a specific gravity of less than one, giving the rock an ability to float on water. Large amounts of pumice produced by some island and subsea eruptions will float on the surface and…
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Composition of Pumice
Most pumice erupts from magmas that are highly charged with gas and have a rhyolitic composition. Rarely, pumice can erupt from gas-charged magmas of basaltic or andesitic composition.
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Mount Mazama Eruption (Crater Lake)
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Gas and Pumice at the Pinatubo Eruption
The second most powerful volcanic eruption of the 20th century was at Mount Pinatubo in 1991. The description below explains how enormous volumes of dissolved gas powered the eruption and how a cubic mile of ash and pumice lapilli was blasted from the volcano.
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How Does Pumice Form?
The pore spaces (known as vesicles) in pumice are a clue to how it forms. The vesicles are actually gas bubbles that were trapped in the rock during the rapid cooling of a gas-rich frothy magma. The material cools so quickly that atoms in the melt are not able to arrange themselves into a crystalline…
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What is Pumice?
Pumice is a light-colored, extremely porous igneous rock that forms during explosive volcanic eruptions. It is used as aggregate in lightweight concrete, as landscaping aggregate, and as an abrasive in a variety of industrial and consumer products. Many specimens have a high enough porosity that they can float on water until they slowly become waterlogged.
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Prospecting for Peridotite
Peridotite bodies exposed at Earth’s surface are rapidly attacked by weathering. They can then be obscured by soil, sediment, glacial till, and vegetation. Finding a peridotite body as small as a kimberlite pipe, which might be only a few hundred yards across, can be very difficult. Because peridotite often has magnetic properties that are distinctly…
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Chromite in Peridotite
Some peridotites contain significant amounts of chromite. Some of these form when a subsurface magma slowly crystallizes. During the early stages of crystallization, the highest-temperature minerals such as olivine, orthopyroxene, clinopyroxene, and chromite begin to crystallize from the melt. The crystals are heavier than the melt and sink to the bottom of the melt. These…