Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz , feldspar, and mica minerals.
The specimen above is about two inches five centimeters across. Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz , or pyroxene. Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization.
It often contains rare minerals that are not found in other parts of the magma chamber. Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification.
Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne.
Fire Opal is sometimes found filling cavities in rhyolite. The edge of the intrusive rock is usually very fine grained because it is here where the most rapid cooling took place.
This edge of the intrusion is called the chill zone. The grain size in the intrusion increases away from the chill zone toward the center, where it remained the hottest for the longest time.
The intrusive rock often contains xenoliths —fragments of the country rock that were torn away during the emplacement of the magma and that are generally most abundant near the contact with the country rock. Plutonic rocks. Mafic, felsic, and intermediate intrusive rocks. Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite , found often in the U. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand.
Rock texture with crystals that are invisible without magnification. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Intrusive Igneous Rock Intrusive, or plutonic, igneous rock forms when magma remains inside the Earth's crust where it cools and solidifies in chambers within pre-existing rock.
Featured Video—Yosemite Granite. Yosemite Nature Notes Granite Many national parks were founded for their geology, and Yosemite is known throughout the world for its exceptional high cliffs and rounded domes. Duration: 7 minutes, 11 seconds.
Igneous Rock Textures. Felsic rhyolite with an aphanitic texture. Photo courtesy of Tina Kuhn. Aphanitic Describes the texture of fine-grained igneous rock in which different components are not distinguishable by the unaided eye.
Obsidian is a volcanic glass with a conchoidal fracture. Photo courtesy of Tina Kuhn Glassy Describes the texture of certain extrusive volcanic igneous rocks that is similar to broken glass and developed as a result of rapid cooling of the lava without distinctive crystallization.
Synonymous with "vitreous. Diorite is a classic "salt and pepper" rock with a phaneritic texture.
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