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Eruptions of Granitic Magma
Eruptions of granitic magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions. Explosive eruptions produce tuff or pumice. Effusive eruptions produce rhyolite or obsidian if the lava cools rapidly. These different rock types can all be found in the products of a single eruption. Eruptions of granitic magma are rare. Since 1900…
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What is Rhyolite?
Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite. Trapped gases often produce vugs in the rock. These often contain crystals, opal, or glassy…
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Imported Pumice and Substitutes
All of the pumice production in the United States occurs west of the Mississippi River. In 2011, most of the pumice for consumption in the eastern United States was imported from Greece. In the eastern United States, expanded aggregate, produced by heating specific types of shale under controlled conditions, is used as a substitute for…
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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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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…