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Quartzite as a Ridge-Former
Quartzite is one of the most physically durable and chemically resistant rocks found at Earth’s surface. When mountain ranges are worn down by weathering and erosion, less-resistant and less-durable rocks are destroyed, but the quartzite remains. This is why quartzite is so often the rock found at the crests of mountain ranges and covering their…
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Where Does Quartzite Form?
Most quartzite forms during mountain-building events at convergent plate boundaries where sandstone was deposited on a continental plate. There, the sandstone is metamorphosed into quartzite by the intense pressure of a plate collision and often by deep burial. Compressional forces at the plate boundary fold and fault the rocks and thicken the crust into a…
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Physical Properties of Quartzite
The interlocking crystalline structure of quartzite makes it a hard, tough, durable rock. It is so tough that it breaks through the quartz grains rather than breaking along the boundaries between them. This is a characteristic that separates true quartzite from sandstone. Quartzite is usually white to gray in color. Some rock units that are…
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What is Quartzite?
Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. Metamorphism recrystallizes the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength.
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Uses of Rhyolite
Rhyolite is a rock that is rarely used in construction or manufacturing. It is often vuggy or highly fractured. Its composition is variable. When better materials are not locally available, rhyolite is sometimes used to produce crushed stone. People have also used rhyolite to manufacture stone tools, particularly scrapers, blades, and projectile points. It was probably…
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Rhyolite and Gemstones
Many gem deposits are hosted in rhyolite. These occur for a logical reason. The thick granitic lava that forms rhyolite often cools quickly while pockets of gas are still trapped inside of the lava. As the lava quickly cools, the trapped gas is unable to escape and forms cavities known as “vugs.” Later, when the lava flow has cooled…
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Lava Domes
Sluggish rhyolitic lava can slowly exude from a volcano and pile up around the vent. This can produce a mound-shaped structure known as a “lava dome.” Some lava domes have grown to a height of several hundred meters. Lava domes can be dangerous. As additional magma extrudes, the brittle dome can become highly fractured and…
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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…