Plutonic rocks are named after Pluto, the Roman god of wealth and the underworld, and for good reason.
They are found under the earth and often contain precious metals, making them an underworldly geologic phenomenon containing riches.
If you are looking to learn more about plutonic rocks, you’ve come to the right place; keep reading to find out all about these fascinating rocks.
Plutonic Rocks Examples (Characteristics and Types)
What are Plutonic Rocks?
Plutonic rocks are a type of igneous rock, which is one of the most basic and common kinds of rocks on Earth.
They are formed when hot, molten magma from volcanoes crystalizes and solidifies over thousands of years.
Most igneous rocks cool and harden above the earth’s crust.
However, plutonic rocks are different from other igneous rocks in that they have formed from magma that has not risen above the earth’s crust and instead harden deep under the earth.
Magma often carries precious metals, gold and silver being two of the most sought-after. Iron, copper, platinum, and chromium are also commonly present in magma.
All plutonic rocks have course grains of all the different materials present in the rocks. Also, to clear up a point of common confusion, plutonic rocks and plutonium have nothing to do with each other, despite the similarities in their names.
Types of Plutonic Rocks
Since plutonic rocks are some of the most common rocks on our planet, there are many rocks and stones that are classified under that category.
Here are the four major types of plutonic rocks and important information about each one.
This is a darkly-colored, coarsely-grained plutonic rock.
It is usually dark green to black in color with dapplings of lighter grains.
It is the most abundant rock in the deep parts of oceanic crust.
Plagioclase feldspar and pyroxenes, including augite, are the two dominant minerals in Gabbro.
Sometimes olivine is present, which can give it its green color.
When polished, gabbro is a great material for countertops, floor tiles, headstones, and other stone products due to its shine and durability.
There are different subtypes of gabbro. They include:
- Normal Gabbro: this is composed of almost all plagioclase and clinopyroxene. Less than 5% is composed of the other inclusions, including olivine and hornblende.
- Norite: orthopyroxene and plagioclase comprise the chemical makeup of norite, with less than five percent of the other minerals included.
- Troctolite: this type is made up almost completely of plagioclase and olivine, made up of less than 5% of the other minerals.
- Hornblende gabbro: as its name suggests, hornblende, along with plagioclase, is almost the entire chemical composition, with less than 5% being other minerals.
Diorite is a rock with a characteristic salt and pepper look, due to equal amounts of coarse black and white grains.
Its composition is mostly of sodium-rich plagioclase and smaller amounts of other minerals, such as biotite, pyroxenes, and hornblende.
It is very commonly polished and used for countertops due to its black and white granite-like appearance.
Historically, it was used for structural purposes by the Incans and Mayans, along with ancient to modern Middle Eastern civilizations.
Europeans would often use it for cobblestones, which can still be found today.
Diorite is a relatively rare species of igneous rock.
Its composition is very specific.
If it has a higher occurrence of olivine or augite with more abundant levels of iron, it transitions to gabbro; it can also easily merge into granite, if the chemical composition is slightly altered.
It is so chemically close to both of those stones that it is often mistaken for one or the other. It is an incredibly hard rock–so hard, in fact, that ancient civilizations used it to shape and work granite.
However, its hardness and durability are what helps it take a high polish so well and makes it great for carving.
Although the previous two stones have granite-like speckling, they are not true granite.
Granite is a lightly-colored stone, mostly composed of quartz and feldspar, with inclusions of mica.
The feldspar gives granite white, pink, red, or gray speckling, while quartz gives it its sparkle.
Granite is the most widely-known plutonic rock due to its popularity for countertops and other structural appliances.
The geologic occurrence of granite is different from the commercial application of the word.
Therefore, common types of granite in countertops, such as black pearl granite, imperial red granite, and white galaxy granite are not necessarily under a granite classification, according to a petrologist.
In fact, many of those could go under a diorite classification, or any other number of plutonic rocks.
Instead, granite is classified by the occurrences of quartz, feldspar, and mica in its composition.
Pegmatite contains the most coarse grains of any other plutonic rock.
It is composed almost completely of very large crystals, of quartz, feldspar, and/or mica, and much smaller amounts of other minerals.
Their composition is most similar to granite, but the size of the crystal grains are much different.
Pegmatite is very widely known as a source for other precious metals and crystals.
Lithium ore called spodumene and beryllium ore called beryl are highly prized when found in pegmatite stones.
Very high-quality gemstones are also found in pegmatite.
Some of the highest grade deposits of aquamarine, garnet, and tourmaline have been discovered in samples of pegmatite.
Some of the world’s most expensive crystals have also been found in pegmatite, such as zircon, emerald, and topaz.
Pegmatite is classified in three groups. Here is a list of each group:
- Peralkaline rocks: these are recognized by their scarcity of aluminum, but subsequently high levels of both potassium and sodium. Also commonly found in peralkaline are riebeckite and aegirine. They are typically found around volcanic activity of continental rifts and oceanic hotspots.
- Metaluminous rocks: these rocks are classified by their molar proportions. They contain less aluminum oxide than the combination of potassium oxide, sodium oxide, and calcium oxide.
- Peraluminous rocks: these rocks are different from the previous two in that they have higher molar proportions of aluminum than the combination of potassium oxide, sodium oxide, and calcium oxide. They are associated with the formation of tungsten, tin, and silver. Many deposits of these metals in the Bolivian tin belt can be attributed to peraluminous rocks.
Plutonic Rocks are Some of the Most Important Rocks to Mankind
Plutonic rocks are not only abundant and ancient, but they are incredibly useful. They are present in our everyday lives, whether we are aware of it or not.
They may not be the prettiest stones when found in the raw, but they have been used for many different important applications since the beginning of civilization.
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