Igneous rocks are those that are formed by the cooling of magma that arises through the crust from the earth’s center. This category include a wide variety of sub-categories, such as felsic, alkic and mafic rocks. Gabbro is named after a town in Tuscany, Italy.
Description
Gabbro is a dense, black or greenish rock with a coarse grain and crystalline structures engrained into material. Small amounts of lighter material may also be present. The composition of the gabbro is roughly half pyroxene and half plagioclase. Generally, the pyroxene is clinopyroxene, though small amounts of orthpyroxene may also occur. When the amount of orthopyroxene is greater than the clinopyroxene, the rock is called norite. Gabbro rocks that have a large amunt of silica result in a type of rock called nepheline. The cleavage surfaces on plagioclase portions are shiny. Cleavage planes on pyroxene portions are duller. Gabbro generally has equally sized grains.
Formation
As the plates of the earth’s surface shift, the hot magma can push up from the center through the earth’s crust. This material gradually cools to form rock such as gabbro and basalt. When the magma cools at the surface, basalt rock is formed. Gabbro forms when it forms below the surface in the crust.
Composition
Gabbro is a mafic type of rock with a specific composition. Mafic rocks contain silicates and are high in magnesium and iron. Gabbro may also contain a number of minerals such as feldspar, calcium, olivine, and quartz. Gabbro rocks can contain up to 5 percent quartz. If they contain more than this amount, they are called quartz gabbro. When the amount of quartz in the gabbro goes above 20 percent, it is no longer gabbro rock. It is then considered granitoid. Gabbro may also contain small amounts of gold, silver, platinum, copper, chromium, cobalt and sulfides.
Where Gabbro Is Found
Because gabbro is formed from magma that arises from the earth’s crust, it is found near mountainous area that emerged from shifts in tectonic plates. This process generally occurs in mountainous areas where the pressure from shifting oceanic plates pushed upward. Mafic rock such as gabbro can be found in areas where volcanic activity has been present in the past or is present currently.
Uses For Gabbro
When polished to a smooth surface, gabbro makes an attractive and durable surface for countertops and vanities. In its rougher state, gabbro is used as paving stone. Tombstones are often made of gabbro, as well.
The formation of gabbro rock gives us a good look at the interesting forces that form the geology of the planet. It provides a dense, hard building material with many uses.
