Gabbro is a mafic rock; it is also phaneritic, intrusive, and igneous.
In the article that follows, you’ll learn more about what makes gabbro a mafic rock.
Is Gabbro Felsic or Mafic? (Explained)
As noted, gabbro is mafic, and more.
Phaneritic means its coarse-grained crystal microstructure can be seen with the naked eye.
Intrusive refers to magma penetrating existing rock, and “igneous” means it was formed by magma or lava.
Metamorphic rock, by contrast, has had time to change because of high pressure and/or temperature, while sedimentary rock is the accumulation of air or water-borne deposits.
While all rocks were once magma or lava, the mafic gabbro is characterized by not having been acted upon by pressure, heat, or erosion.
Mafic is a term for various intrusive rocks that share its characteristics; modern geologists add further specificity to Gabbro by referring to it as “metagabbro” or Metamorphosed Gabbro.
“Mafic,” also called ferromagnesium, is a combination of the words magnesium and ferric.
Ferric refers to iron(III), iron in its +3 oxidation state, which is commonly, though not precisely, known as rust; the reddish-brown crust that forms on metal contains hydrous iron(III).
Mafic rocks are lower in silica but richer in iron, magnesium, and calcium.
These substances have a higher melting point than the elements found in felsic rocks, which is why they have a lower viscosity when forming.
For this reason, magic volcanoes are less explosive than bombastic felsic eruptions.
What does Gabbro look like?
Most are on the dark side in terms of coloration and include basalt and diabase.
Gabbro ranges from black to dark green.
The speckled appearance results from its varied constitution and slowly formed crystals, though almost always tending toward darker shades.
Often, mafic rocks include calcium-rich plagioclase feldspar, usually labradorite or bytownite.
Unlike most other igneous rock, gabbro contains very little quartz.
While gabbro is like basalt in composition, it is rough, whereas basalt is known for its smooth texture.
Gabbro’s larger crystals give it a rugged feel: the extra time it had to cool, coupled with the overall slower viscosity of its magma, means the crystalline structure had time to set up.
So what does felsic mean?
Like mafic, felsic refers to a group; Felsic minerals must have at least 75% quartz, orthoclase, and plagioclase.
Felsic rocks form at a higher viscosity. Its magma flows faster than the slow-moving mafic rocks.
Knowing which is which is helpful in any earthmoving project, as the high density gives it a hardness that is difficult to break.
As hard as gabbro is to work with, that very density makes it great as a fill for construction projects and railroad ballast, or anywhere a tough, rough rockfill is needed.
How does gabbro form?
Gabbro is formed when slow-moving low-silica magma has a chance to cool gradually.
Water and other volatile elements have an opportunity to vent off, as the matierial cools slow enough to crystalize.
Mafic rock cools slowly, usually in mid-ocean ridges or as plutons connected with continental volcanism.
The word “basalt” comes from rocks of the oceanic crust being “basaltic” in composition, but in reality, basalt is only a thin layer on top of the deeper gabbro rocks.
Don’t let the ocean thing fool you: over the eons, the continents have crashed around, while volcanism and earthquakes have pushed, pulled, and ejected gabbro all over the planet.
You can find gabbro on mountain ridges or any continental divide.
If Gabbro is not an element, what is it?
Composed primarily of pyroxene and high-calcium plagioclase, gabbro also contains smaller amounts of hornblende, olivine, and the usual traces of random others.
On the other hand, a “felsic” rock is typically lighter in color due to the higher amounts of aluminum and silicon and is lower in density.
Again, you can see how the elements in felsic have a lower melting point, resulting in a faster-moving, quicker cooling magma—knowing which is crucial to mining, excavation, and any career which requires digging in the Earth.
It could mean the difference between needing demolitions versus the usual earthmovers.
Geologists use stringent standards to classify coarse-grained igneous rock, and the exacting precision of petrology has given us quantitative tools with which to identify rocks with accuracy.
Petrology is related to geology but focuses on the origin, microstructure, and composition of rocks, while geology is more large-scale, concerning itself more with the planet’s composition.
For more detailed analysis, one can refer to the QAPF Diagram; one can plot any igneous rock-based on mineralogical composition.
Standing for Quartz, Alkali feldspars, Plagioclase feldspars, and Feldspathoids, the QAPF Diagram does not include all plutonic rocks.
The essential ultramafic plutonic group has a separate set of diagrams.
Plutonic refers to intrusive rocks which are coarse-grained, usually forming deep in the Earth’s crust but visible as batholiths and stocks when they protrude onto the surface.
In the case of gabbro, a silica content between 45 and 55% is required; any more or less and it is technically not classifiable as gabbro.
What is it for?
Beyond industrial and commercial crushed stone and aggregate (the technical term for gravel), gabbro has a variety of decorative uses.
Highly polished slabs have a black fleck luster, making it popular on countertops, as flooring, facing stones, tomb markers, or any place a resilient classic black rock is called for.
Standing up to wear and weather alike, it is often sold under the name “black granite.”
You will also see gabbro used rough-cut as curbing, stone brick, pavers, and more.
Sometimes you will find economically viable amounts of metals, with significant quantities of ilmenite in gabbro, making it valuable to titanium mining.
Other deposits might be high in nickel, chromium, or even platinum, but processing and extraction are expensive given the high density.
After all of this information, I hope that the humble gabbro has been revealed to be no mere rock but a stone worth considering.
After all, it is not gneiss to take gabbro for granite.
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