Plant and Soil Sciences eLibrary
Learn about gabbro, a dark colored, mafic igneous rock that is the intrusive equivalent to extrusive basalt. It has uses as a building and art. Basalt, on the other hand, is an extrusive igneous rock. It is formed much closer to the What is the difference between layered and foliated gabbro? 84 Views. Basalt is a volcanic equivalent of gabbro. Basalt pebbles near the southern tip of La Palma slowly transforming into black sand typical to volcanic oceanic.
Black is pyroxene augite, orange is olivine or more precisely what is left of it. Orange patches are former olivine crystals that are now composed of a mixture of silicates and iron oxides which is known as iddingsite. Olivine is a common mineral in many basaltic rocks.
Width of view 10 cm. Another basalt chemically probably picrobasalt with lots of olivine fresh olivine is bright green, but it gets more and more yellow as it weathers. Width of sample 6 cm. Basalt in the field Subaerial basalt forms lava flows or pyroclastic fields and cones.
Two main types of basaltic lava flows are aa lava and pahoehoe lava. Aa lava has rough rubbly irregular crust while pahoehoe is smooth. Lava crust of aa type is broken into pieces while pahoehoe retains its continuity.
Both lava flow types are massive beneath the crust and this massive interior may be columnar. Columns are separated from each other by narrow cracks which form because cooling basaltic magma contracts. Cracks start to form at the surface and propagate deeper as lava cools. Submarine basalt usually forms pillows. Pillow basalt forms as a result of very rapid cooling. Outer part of forming pillow cools very quickly in contact with cold seawater while the interior still fills with molten lava.
Basalt mostly forms lava flows because it is among the least viscous magma types and therefore does not generate explosive volcanic eruptions, but sometimes pyroclastic material is formed when magma contains more volcanic gases.
Basalt - Igneous rocks
Basaltic rocks can be thrown out of volcanic vents as lapilli singular: Basaltic volcanoes are fed by dikes planar intrusive rock bodies when solidified that cut through other rocks and sills similar to dike but generally parallel to preexisting bedding planes.
Aa lava in the foreground. La Palma, Canary Islands. Pahoehoe lava ropy lava. Pillow lava near Fasoula, Troodos ophiolite, Cyprus. Pillow lava is very common on Earth, but difficult to find because almost all of it is on the ocean floor. Examples can be found on land usually where former ocean floor is tectonically squeezed between two blocks of continental crust. Scoriaceous lapillus from Etna, Italy. Despite being 5 cm in width it weighs only 15 grams because it is filled with gas bubbles vesicules.
Similar rock type with a felsic composition is pumice. Sometimes dikes are so close to each other that the whole outcrop is composed of them. These sheeted dikes in Cyprus once fed volcanoes on the ocean floor. Dikes are composed of basalt and diabase.
A chart that illustrates the generalized mineral composition of igneous rocks. By studying this chart, you can see that gabbros and basalts are composed mainly of plagioclase feldspar, micas, amphiboles, and olivine.
What Minerals are in Gabbro? Gabbro is composed mainly of calcium-rich plagioclase feldspar usually labradorite or bytownite and clinopyroxene augite. Minor amounts of olivine and orthopyroxene might also be present in the rock. See composition chart on this page.Igneous Rocks
This mineral composition usually gives gabbro a black to very dark green color. A minor amount of light-colored mineral grains may also be present. Unlike many other igneous rocks, gabbro usually contains very little quartz. You can see a close-up view of gabbro toward the bottom of this page. Gabbro and Basalt are Related Gabbros are equivalent in composition to basalts. The difference between the two rock types is their grain size.
Basalts are extrusive igneous rocks that cool quickly and have fine-grained crystals.
Soil Genesis and Development, Lesson 1 - Rocks, Minerals, and Soils
Gabbros are intrusive igneous rocks that cool slowly and have coarse-grained crystals. In the oceanic crust, basalt forms near the surface at a divergent boundary, but gabbro forms at depth from slow crystallization. Learn about teaching plate tectonics. Gabbro in Oceanic Crust It is often stated that Earth's oceanic crust is made up of basalt. The word "basalt" is used because the rocks of the oceanic crust have a "basaltic" composition.
Gabbro: Igneous Rock - Pictures, Definition & More
However, only a thin surface veneer of oceanic crust is basalt. The deeper rocks of the oceanic crust are generally coarser-grained gabbro. Basalt occurs at the surface of the crust because the rocks there have cooled quickly. At greater depth the cooling rate is slower, and large crystals have time to develop.
A view of polished gabbro labradorite.