Obsidian Definition and Origin
It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. Because of their high silica content, felsic lavas do not behave like the mafic, or silica-poor, lavas we see on the island of Hawaii.
It is sometimes classified as a mineraloid. Though obsidian is usually dark in color, similar to mafic rocks such as basalt, the composition of obsidian is extremely felsic.
What is obsidian classified?
Rondi: Everyone, meet Obsidian , an igneous rock that from melted rock, or magma. Obsidian is an “extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an “intrusive” rock.
Does obsidian have a felsic composition?
Though obsidian is usually dark in color, similar to mafic rocks such as basalt, the composition of obsidian is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% by weight or more.
Is obsidian fine grained or coarse grained?
Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Granite and gabbro are examples of phaneritic igneous rocks. Fine grained rocks, where the individual grains are too small to see, are called aphanitic. The most common glassy rock is obsidian.
Is obsidian igneous or metamorphic?
Obsidian is an igneous rock occurring as a natural glass that is formed by the rapid cooling of viscous lava from volcanoes.
Is obsidian an aphanitic?
The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
What does felsic stand for?
Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava. Felsic rocks are usually light in color and have specific gravities less than 3.
What minerals are in obsidian?
Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian.
What sedimentary rock does obsidian turn into?
Obsidian can be weathered and eroded into sediments and become black sand. Sediments are naturally occurring and are bits and pieces of weathered and eroded rocks that are moved by wind, water, ice, and gravity. rock is formed as a result of high temperatures and high pressure.
Is obsidian plutonic or volcanic?
Basalt and obsidian are volcanic rocks; granite is plutonic.
Why does obsidian have a fine grained texture?
Extrusive or volcanic rocks crystallize from lava at the earth’s surface. The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals. Volcanic glass is called obsidian.
Is andesite felsic?
Andesite is INTERMEDIATE in composition between MAFIC and FELSIC rocks. The mixture of mafic and felsic minerals gives the rock a “salt and pepper” appearance. DIORITE – phaneritic rock (large crystals) of both mafic and felsic minerals. ANDESITE – aphanitic rock (small crystals) of both mafic and felsic minerals.
Is obsidian organic or inorganic?
All rocks except obsidian and coal are made of minerals. (Obsidian is a volcanic rock made of glass and coal is made of organic carbon.) Most rocks contain several minerals in a mixture characteristic of the particular rock type.
Does obsidian have cleavage or fracture?
Obsidian, a naturally occuring glass, has a conchoidal fracture. Photo by Siim Seep.
How is green obsidian formed?
Obsidian is a naturally occurring volcanic glass. It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. The green coloration can also be achieved as some gas bubbles (vescicles) remained in the lava flow during crystallization.
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