![]() Mafic refers to an abundance of ferromagnesian minerals (with magnesium and iron, chemical symbols Mg and Fe) plus plagioclase feldspar.It is intermediate in silica in the 55-60% range. It usually contains roughly-equal amounts of light and dark minerals, including light grains of plagioclase feldspar and dark minerals like amphibole. Intermediate is a composition between felsic and mafic.Felsic igneous rocks are rich in silica (in the 65-75% range, meaning the rock would be 65-75% weight percent SiO 2) and poor in iron and magnesium. Minor amounts of dark-colored (mafic) minerals like amphibole and biotite mica may be present as well. These light-colored minerals have more silica as a proportion of their overall chemical formula. Felsic refers to a predominance of the light-colored (felsic) minerals feldspar and silica in the form of quartz.It usually colours dark, black, grey-black to grey. Result of this formed, glass occupies more than 80 and that have seeming conchoidal fracture and vitreous luster. Rock names at the top include a continuous spectrum of compositions grading from one into another. It is occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes. ![]() The percentage of silica is shown on the horizontal axis. The percentage of minerals is shown on the vertical axis. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand.\): Mineral composition of common igneous rocks. One extreme type of phaneritic rock is called pegmatite, found often in the U.S. Perhaps the best-known phaneritic rock is granite. This surface is known as a phaneritic texture. Examples of such kind of rocks include pumice, basalt, or obsidian. When the lava cools so rapidly that atoms cant form into a. The magma on the surface (lava) cools faster on the surface to form igneous rocks that are fine-grained. Obsidian is a glassy igneous rock that usually forms as an extrusive, fast-flowing lava. They have large crystals that are usually visible without a microscope. Obsidian (pronounced /bsdi.n, b-/) is a felsic volcanic glass that occurs naturally. Extrusive igneous rocks are formed when molten magma spill over to the surface as a result of a volcanic eruption. Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface. For example, Pele’s hair is long, extremely thin strands of volcanic glass, while pahoehoe is smooth lava that forms shiny, rounded piles. There are many other kinds of extrusive igneous rocks. Obsidian: Can we get started already I don't have all day Rondi: Sorry, folks. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an 'intrusive' rock. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian. Obsidian is an 'extrusive rock, which means it is made from magma that erupted out of a volcano. These fine-grained rocks are known as aphanitic-from a Greek word meaning “invisible.” They are given this name because the crystals that form within them are so small that they can be seen only with a microscope. Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet. Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. Felsic: usually light-colored rocks (obsidian is an exception) with a high silica content, and low iron and magnesium. The two main categories of igneous rocks are extrusive and intrusive. These obsidian tools from the end of the Pleistocene (over 10,000 years ago) were found at the Connley Caves in Oregon by the Oregon Museum of Natural and Cultural History. For example, two rocks from identical magma can become either rhyolite or granite, depending on whether they cool quickly or slowly. They can also look different based on their cooling conditions. Igneous rocks can have many different compositions, depending on the magma they cool from. All magma develops underground, in the lower crust or upper mantle, because of the intense heat there. ' The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo). It is an amorphous material known as a ' mineraloid. Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure.
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