Some form on or above Earth's surface. We describe these two basic types:. Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form.
Examples of intrusive igneous rocks are: diabase , diorite , gabbro , granite , pegmatite , and peridotite. Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include: andesite , basalt , dacite , obsidian , pumice , rhyolite , scoria , and tuff. Dacite is a fine-grained, extrusive igneous rock that is usually light in color.
It has a composition that is intermediate between rhyolite and andesite. The specimen shown is about four inches ten centimeters across. Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. Diabase is an intrusive igneous rock composed mostly of plagioclase feldspar and pyroxene minerals.
The grains in diabase are larger than those in basalt but smaller than those in gabbro. Diabase is used in the construction industry as trap rock or dimension stone. When the diabase contains colorful labradorite crystals, it makes an especially nice architectural stone.
Gabbro is a coarse-grained, dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine. The specimen shown above is about two inches five centimeters across. Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar , pyroxene, hornblende , and sometimes quartz. 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. Intrusive Igneous Rocks: Intrusive, or plutonic, igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface.
Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies.
Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. Extrusive Igneous Rocks: Extrusive, or volcanic, igneous rock is produced when magma exits and cools above or very near the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere.
Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.
Ever wondered what the difference between a rock and a mineral was? This EarthWord should cover it As fall foliage begins to blanket New Hampshire, pleasantly diverting the attention of residents and visitors, scientists are preparing to unveil some of the geologic secrets of the famous yet not-well-known rocks that lie beneath the fiery cover.
The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer. The project is funded by the USGS Mineral Resources Program that is focused on investigating the bedrock geology of the national park and surrounding areas through geologic mapping and supporting analytical work such as geochemistry and. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised.
A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works. This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust. This outcrop is exposed in a rock quarry in Chalfant Valley about 25 km 15 mi southwest of Long Valley Caldera.
The two main units of the Bishop Tuff deposit are visible here: 1 the lower 5 m 16 f of the section consists of the pumice that fell to the ground airfall pumice downwind from the eruption; and 2 the upper m f of the section consists of the. Skip to main content. Search Search. Apply Filter. It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8, feet 2, meters above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized Is glacier ice a type of rock?
Glacier ice, like limestone for example , is a type of rock. 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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