“Basic” rocks, also called “mafic” or “alkaline” rocks, are some of the most common you can find. Formed in extreme heat and pressure beneath the surface of the earth, they are hard, tough rocks.
In this article, we will look at what makes a rock “basic” and some of the most important types of basic rock out there.
Basic Rocks Examples (Characteristics and Types)
What is a Basic or Alkaline Rock?
In this case, “basic” doesn’t mean “ordinary” – it refers to the opposite of an acid.
However, basic rocks aren’t higher on the Ph scale, and the idea of Ph doesn’t have any real scientific relevancy when it comes to “basic” or “acidic” rocks.
What makes a rock basic is the level of silica content.
A rock with 45-55% silica content is considered basic, while a rock with 60% or more is considered acidic.
In modern terminology, basic rocks are called “mafic” and acidic rocks are called “felsic”.
Basic rocks are typically igneous, meaning that they are formed in volcanic or tectonically active environments by the cooling of liquid substrate rocks.
Magma containing pyroxenes and plagioclase cools as it approaches the earth’s surface and crystallizes into the basic rocks we know and recognize.
As one of the most common rocks in the world, basalt makes up a huge proportion of the world’s bedrock.
The majority of the rock that lies underneath the world’s oceans is basalt, and many volcanic mountain ranges are basaltic as well.
This is because basalt is the most common extrusive, igneous rock.
When volcanoes erupt, they typically produce basalt rock.
Therefore, any volcanic activity that happens beneath the ocean lines the seafloor with rapidly cooling volcanic basalt, and when volcanoes erupt on land they grow into mountains of basalt.
The Hawaiian islands were created by a volcanic explosion 300,000 to 600,000 years ago, creating basalt mountains that would be higher than Mount Everest if they were compared from the base.
To this day, there are active basalt volcanoes in Hawaii that pour magma into the ocean and cool it rapidly into basaltic rock.
Basalt is a very hard black or very dark gray rock with a fine grain.
It is composed primarily of plagioclase and pyroxene minerals.
Although it most commonly forms as extrusive magma, it can also form intrusively in a thin sill or igneous dike.
You can find basalt almost anywhere, but it is most common near oceans and areas that were previously submerged, and around volcanoes.
Related to basalt, Gabbro is essentially made of the same substrate materials (plagioclase feldspar and pyroxenes like augite) but forms in larger grains.
Gabbro is dark green to black and is sometimes marbled with lighter spots from contaminant minerals, giving it an inconsistent coloration.
The difference between basalt and gabbro is in the grain size, which is a direct result of how the rock is formed.
Basalt is an extrusive igneous rock that erupts from volcanoes or fissures in the earth’s crust.
This leads to a rapid cooling process as the liquid magma is exposed to water or air.
Because the cooling process is so rapid, the grains become very fine.
Gabbro is an intrusive igneous rock, meaning that it is formed inside the earth rather than as a result of volcanic activity.
While basalt cools quickly, gabbro cools more gradually inside the earth and forms larger grains.
Basalt is extremely common because it is located at the earth’s surface or on the seafloor where volcanic magma erupts.
It underlies the world’s oceans. However, beneath the layer of rapidly cooled basalt where the magma surfaced, the same substrates continue to cool more slowly, forming gabbro.
Somewhere between gabbro and basalt in grain size, Diabase is a medium grain intrusive igneous rock that is also sometimes called dolerite.
Diabase appears dark gray or black with a medium grain and is extremely hard. It is sometimes crushed and used for paving or construction.
It is also used for tiles.
It can even be used for building monuments or sculptures, and is sometimes called “black granite”.
Diabase is an intrusive rock that does not form in volcanic eruptions. Instead, it develops in shallow areas or sills in the earth.
Although it is compositionally similar to volcanic basaltic rock, Diabase includes bits of glass and is coarser-grained.
If the grains in the diabase get large enough, it can be considered gabbro.
The characteristic diabisic look is a morphological difference caused by chemistry.
Diabase is composed of plagioclases and pyroxenes, like basalt and gabbro.
However, in diabase, the malformed plagioclase crystals mold to or wrap around the pyroxene crystals to create a different texture.
Phonolite is an extrusive igneous rock that forms during volcanic eruptions.
It forms in thin, hard plates that actually resonate when struck with a hammer, hence the name which derives from “phonos”, Greek for “voice” or “sound”.
Phonolite refers to extrusive igneous rocks that contain a high proportion of nepheline and alkali feldspar.
The feldspar is usually sanidine or anorthoclase and makes up the majority of the ground mass of the rock.
Pyroxene phenocrysts form as green needles that appear interstitially in the rock.
Nepheline appears as smaller interstitial crystals or microphenocrysts.
Most of the phonolite on earth now dates back to the Cenozoic era.
There are deposits of phonolite in many places: the Eiffel plateau in France, Naples in Italy, Cripple Creek in Colorado in the United States, and Olbruck, Germany.
You can also find phonolite trachytes on volcanic islands like St. Helena.
Reviewing Basic Rocks
What makes a rock “basic” is not its Ph, but its level of silica content – 45-55% silica makes a rock basic.
Basalt, gabbro, diabase, and phonolite are all igneous rocks, produced in volcanic conditions.
Basalt and phonolite are formed in actual eruptions, while gabbro and diabase with their larger grain sizes are formed by cooling more slowly beneath the earth’s surface.
Basic rocks all contain plagioclases and pyroxenes, which crystallize differently depending on the geological conditions.
If you are interested in finding basic rocks, the best place to look is near the ocean or a volcano where they form.
If you can’t make it there, you can find the basic rocks described here used in construction.
These dark, hard, tough stones are common and useful as crushed rock.
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