An intrusive rock is formed when magma solidifies below the surface of the Earth. With the insulation of the ground, the cooling process is a lot slower, allowing for the formation of larger mineral crystals. Granite is a great example of this kind of stone, as you can usually see the variation of unique minerals clearly.
A post shared by Icelandair icelandair on. Apr 28, at pm PDT. Meanwhile, extrusive rock forms much more rapidly, above the surface of the Earth and often underwater. The swift formation of the stone means that most extrusive rocks have tiny crystals, barely distinguishable by the human eye — this texture is described as aphanitic.
Take obsidian, for example, which cools so quickly that crystals do not have time to form at all and a natural glass is made instead.
Because extrusive rocks generally result from volcanic activity, they are also commonly called volcanic rocks. Quite often, the magma which forms extrusive rock will have already cooled somewhat before erupting, meaning some larger crystals have already formed.
This is described as porphyritic extrusive rock. Mar 18, at pm PDT. The outcrops in the foreground and in the distance of this photo are all made up of layered basalt flows. Although basalt is typically a dark black rock, it often weathers to a yellow-brown color similar to the rocks shown here.
Public domain image by Williamborg. The area shown is what has not yet been eroded away - the original extent of these basalt flows was much greater. Over individual flows have been identified, and several hundred meters of basalt underlies much of the area shown in the map above. The third basalt-forming environment is a continental environment where a mantle plume or hotspot delivers enormous amounts of basaltic lava through the continental crust and up to Earth's surface.
These eruptions can be from either vents or fissures. They have produced the largest basalt flows on the continents. The eruptions can occur repeatedly over millions of years, producing layer after layer of basalt stacked in a vertical sequence see outcrop photo. The word "traps" is derived from the Swedish word for "stairs," which describes the outcrop profile of these layered basalt deposits, as shown in the outcrop photo.
The best way to learn about rocks is to have specimens available for testing and examination. The Roman theatre: left in Bosra, Syria. The dark building stone is basalt. Basalt paving stones: right on a city street in Rome, Italy. Basalt pavers were often used in areas close to volcanoes.
Basalt is used for a wide variety of purposes. It is most commonly crushed for use as an aggregate in construction projects. Crushed basalt is used for road base, concrete aggregate, asphalt pavement aggregate, railroad ballast, filter stone in drain fields, and may other purposes. Basalt is also cut into dimension stone. Thin slabs of basalt are cut and sometimes polished for use as floor tiles, building veneer, monuments, and other stone objects. Article by: Hobart M. Find Other Topics on Geology.
Maps Volcanoes World Maps. Rock, Mineral and Fossil Collections. Hardness Picks. Flint, Chert, and Jasper. Tumbled Stones. Fluorescent Minerals. Lapis Lazuli. Rocks: Galleries of igneous, sedimentary and metamorphic rock photos with descriptions. Minerals: Information about ore minerals, gem materials and rock-forming minerals. Volcanoes: Articles about volcanoes, volcanic hazards and eruptions past and present. Gemstones: Colorful images and articles about diamonds and colored stones.
So why do we see the slowly-cooled intrusive version on the continents, but the rapidly-cooled extrusive basalt in the oceans? Well, again there are several contributing factors.
One relates to the heat involved. Mafic magma is much hotter than felsic magma. Because of this, it is easier for the basaltic lavas to reach the surface while still in the liquid phase.
Felsic magma, starting much cooler, generally crystallizes before reaching the surface. Therefore, more basalt than gabbro, and more granite than rhyolite.
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