You can see a close-up view of gabbro toward the bottom of this page. 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. Gabbros are intrusive igneous rocks that cool slowly and have coarse-grained crystals. Divergent boundary: 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. 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. 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. See illustration. Black granite: A view of polished gabbro labradorite. Polished gabbro is sold under the name "black granite" and is used for cemetery markers, floor tile, kitchen counter tops, facing stone, and other dimension stone uses.
On the continents, gabbro can be found within thick lava flows of basaltic composition, where slow cooling allows large crystals to form. Gabbro will also be present in the deep plutons that form when magma chambers that feed basaltic eruptions crystallize.
Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia River flood basalts of Washington and Oregon and the Deccan Traps of India.
Close-up view of gabbro: Magnified view of the gabbro shown in the photograph at the top of the page. Gabbro can be polished to a brilliant black luster. Plagioclase is striped black-gray-white, pyroxene is colorful, there are some small olivine grains as well. Yellow stripes in blue in the upper part of the image is an intergrowth of two pyroxenes Opx in Cpx.
Gabbro pegmatite from Cyprus the Troodos ophiolite. A sample from Tangen, Norway. Width of sample 10 cm. La Palma is an oceanic island which is largely composed of volcanic rocks.
Deep-seated intrusive rock gabbro is exposed there because of uplift and subsequent deep erosion. A sample from La Palma. Width of sample 5 cm. Gabbro from the Troodos ophiolite in Cyprus. Width of sample 7 cm. A sample from Tangen, Hedmark, Norway.
Norite may be very similar to gabbro but contains orthopyroxene, not clinopyroxene. Rogaland, Norway. Width of sample 8 cm. Leucocratic norite orthopyroxene gabbro. Troctolite is a variety of gabbro which contains almost no pyroxene. Gray is plagioclase, orange is weathered olivine iddingsite. Width of sample 15 cm. Hornblende gabbro pegmatite. This rock was originally normal gabbro, but it has been metamorphosed by hot fluids that altered augite to similar but hydrated silicate mineral hornblende.
Tappeluft, Norway. Width of sample 9 cm. Hornblende gabbro metagabbro with finer structure. Width of view 35 cm. Tremolite-scapolite rock.
The source rock was probably gabbroic, but it has been severely altered to its current form which is composed of tremolite-actinolite series amphibole and framework silicate scapolite. Hornblende-scapolite marialite rock. Metamorphosed gabbro again. Originally the rock was composed of plagioclase and pyroxene. Larvik complex Ekerite Larvikite. Oka complex Niocalite carbonatite Monticellite carbonatite.
Quartz gabbro Gabbro: A coarse-grained plutonic rock composed essentially of calcic plagioclase, pyroxene and iron oxides. The term gabbro was used originally by Italian geologist Tozzetti in and brought into geological terminology by a German geologist Leopold von Buch.
The pyroxene content is mostly clinopyroxene, generally augite, but small amounts of orthopyroxene may also be present. Intermediate rocks are termed gabbronorite. Gabbro may also contain small amounts of olivine olivine gabbro amphibole hornblende gabbro and biotite. Gabbros may also contain moderate amounts of feldspathoid minerals Foid -bearing gabbros like nepheline, cancrinite, analcime and sodalite.
Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.
0コメント