Andesite is another extrusive volcanic rock that is usually light gray and sometimes has red or green color. This fine-grained rock comes primarily from stratovolcanoes such as Mount Fuji in Japan. These are tall, cone-shaped volcanoes also referred to as composite volcanoes. Andesite erupts around to 1, degrees Celsius, according to the U. Geological Survey Cascades Volcano Observatory website.
The lava flows are often very long and thick. The rock is found commonly in the Andes Mountains in South America. The composition of andesite includes large amounts of silica and plagioclase feldspar as well as various levels of pyroxene, horneblende and olivine. Andesite may also contain bubbles and quartz. Dacite is an extrusive volcanic rock made up of two-thirds silica. The rock is light gray in most instances and is named for the Roman province called Dacia, where most of this type of rock comes from near the Danube Rive, according to the U.
Dacite and the pumice stones it produces are composed of plagioclase feldspar, quartz, biotite and hornblende, according to the Encyclopedia Britannica website. It erupts between and 1, degrees Celsius and is most commonly associated with the devastating eruptions known as Plinians, the type of eruption that occurred at Mount Vesuvius in 79 AD and Krakatoa in Remember, you can also make your own, personal feed by combining tags from around OpenLearn.
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Search for free courses, interactives, videos and more! Free Learning from The Open University. Featured content. Free courses. All content. Pumice Updated Thursday, 28th September A brief description of the nature of pumice.
Copyright: Weldon Schloneger Dreamstime. Get closer to geology. Geology toolkit Discover the unique landscape of the British Isles with our interactive Geology Toolkit, featuring a geology timeline, rock analyser, rock cycle, landscape features and safety tips. Pumice is a light-colored volcanic rock. It's extremely porous, with a foamy appearance.
Crushing pumice rock into a powder produces a substance called pumicite or simply volcanic ash. Pumice forms when super-heated, pressurized molten rock violently erupts from a volcano. Gases dissolved in magma mainly water and carbon dioxide form bubbles when pressure suddenly decreases, in much the same way carbon dioxide bubbles form upon opening a carbonated drink. The magma swiftly cools, producing a solid foam. While pumicite may be produced by crushing pumice, it also occurs naturally.
Fine-grained pumicite forms when magma containing a high concentration of dissolved gases suddenly depressurizes and cools. Pumice forms so quickly that its atoms often don't have time to organize into crystals. Sometimes there are crystals present in pumice, but most of the structure is amorphous, producing a volcanic glass called a mineraloid.
Pumice consists of silicates and aluminates. The silicic and felsic matter may include rhyolite, dactite, andesite, phonolite, pantellerite, trachyte, and less commonly basalt. While pumice occurs in a variety of colors, it is almost always pale.
Colors include white, gray, blue, cream, green, and brown. The pores or vesicles in the rock take two forms. Some vesicles are roughly spherical, while others are tubular. Probably the most significant property of pumice is its low density. Pumice tends to be so light that it floats on water until its vesicles fill and it eventually sinks. Before it sinks, pumice can float for years, potentially forming huge floating islands.
Pumice rafts from the eruption of Krakatoa drifted around 20 years. Pumice rafting disrupts shipping and is important in the dispersal of marine organisms to new locations. Pumice occurs in everyday products and has many commercial uses.
Stone-washed jeans are made by washing the denim with pumice rocks. The Greeks and Romans rubbed the rocks on their skin to remove unwanted hair.
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