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Structure
Gabbro is what is known as an intrusive, or plutonic, igneous rock. In other words, it is formed under conditions of intense heat, such as volcanic action, having been forced between existing rocks while molten and solidifying below the surface of the earth. The structure of gabbro is often layered, with light layers of silicates of calcium, potassium and sodium, known as feldspar, interspersed with darker layers of pyroxene.
Occurrence
The occurrence of gabbro is widespread in the earth̵7;s oceanic and deeper continental crust. Different rates of cooling at the boundaries between tectonic plates -- the plates that make up the surface of the earth -- typically produce a thin layer of basalt on the surface of the oceanic crust with layers of gabbto beneath. In the continental crusts, gabbro is formed by the slow cooling of lava flows. In the United States, extensive quantities of gabbro occur beneath the so-called Columbia River Basalt Group, which erupts mainly in northeast Oregon and southeast Washington and was formed by volcanic activity in the area.
Uses
The name ̶0;black granite̶1; refers to the fact that gabbro can be polished to a bright black luster, making it an attractive option for kitchen counter tops and floor tiles. Indeed, according to the Online Etymology Dictionary, the name ̶0;gabbro̶1; comes from an Italian word and, originally, from the Latin word, ̶0;glaber̶1;, meaning ̶0;bare̶1;, ̶0;smooth̶1; or ̶0;bald.̶1; Although too fragile for use as a construction material, gabbro is resistant to weathering and wear and tear, making it suitable for outdoor applications, such as cemetery headstones, curbing and paving stones.
Gabbro versus Basalt
Gabbro is sometimes referred to as the coarse-grained equivalent of basalt. The composition of the two rocks is almost identical, but basalt is an extrusive igneous rock -- it is forced out onto the surface of the earth in a molten condition -- which cools quickly and forms fine-grained crystals. By contrast, gabbro cools slowly and solidifies below the surface of the earth, producing coarse-grained crystals. Collectively, basalt and gabbro are sometimes referred to as ̶0;mafic̶1; or ̶0;ferromagnesian̶1; rocks, indicating a relatively high concentration of iron and magnesium.