How is rhyolite made

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Much earlier, between to million years ago, this rhyolite rock formed from magma spewing out of volcanoes that made mountains of rhyolitic lava and ash. Place: These rhyolite cobbles are common in ancient Eocene sand and gravel deposits of western San Diego County.

But a mystery puzzled geologists—where is the bedrock source for these rocks? Description: Rhyolite cobbles are river-worn cobbles of igneous rock. They are usually reddish-purple, and sometimes gray.

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Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Granitic magmas have produced some of the most explosive volcanic eruptions in Earth's history.

The sites of their eruption are often marked by large calderas. Lava Dome: Photo of a lava dome in the caldera of Mount St. Activity at St. Helens slowly extrudes thick lavas that gradually build domes in the caldera. This dome is composed of dacite , a rock that is intermediate in composition between rhyolite and andesite. Photo by the United States Geological Survey. 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. Lava domes can be dangerous. As additional magma extrudes, the brittle dome can become highly fractured and unstable.

The ground can also change slope as the volcano inflates and contracts. This activity can trigger a dome collapse. A dome collapse can lower the pressure on the extruding magma. This sudden lowering of pressure can result in an explosion. It can also result in a debris avalanche of material falling from the tall collapsing dome. Many pyroclastic flows and volcanic debris avalanches have been triggered by a lava dome collapse.

Fire Opal is sometimes found filling cavities in rhyolite. This specimen of rhyolite has multiple vugs filled with gemmy transparent orange fire opal. This material can be cut into beautiful cabochons and is sometimes faceted when it is transparent or even translucent.

Famous deposits of this type of fire-opal-in-rhyolite are found in Mexico.