Acidic rocks are abundant and found almost everywhere on our planet.
They are a specific type of igneous rock that pushes up from the inside of the Earth’s crust, and can be both interesting and beautiful.
Let’s look at what exactly makes a rock acidic, and some examples so you can identify them easily.
Acidic Rocks Examples (Characteristics and Types)
What is an Acidic Rock?
Acidic rocks are igneous rocks. What exactly do we mean by that? Well, igneous rocks are those created through the cooling and hardening of magma.
This gives them a crystalline structure and usually makes them appear glassy.
Acidic rocks are one of four classifications of igneous rocks, and are more commonly referred to as felsic rocks.
This type of rock is made up of silica, quartz, and feldspar in specific concentrations. The concentrations of each mineral classify them as felsic, or acidic.
The term felsic comes from combining the words feldspar and silica, and an acidic rock will contain this type of mineral in amounts greater than 75% of the total rock composition.
Acidic rocks will also have a specific gravity of less than three, making them lighter than other minerals.
Acidic rocks are usually light in color, and a special classification, leucocratic, meaning “light-colored,” is given to acidic rocks that contain 90% or greater concentrations of felsic minerals.
These rocks are usually named according to their texture, and they can be found mostly on the crust of the Earth, as they are extrusive and are pushed up and out of the earth by geologic processes.
Types of Acidic Rocks
Acidic rocks are much more common than you might think and can be found in different places all over the globe.
Their unique composition makes them lighter than the rocks surrounding them, making it easy for such rocks to float to the surface.
The most common minerals found in acidic rocks are quartz, silica, and plagioclase feldspar, though muscovite, hornblende, and orthoclase are also sometimes present.
Granite is one of the most abundant rocks on our planet, and it is also the most commonly found acidic rock.
Granite averages about 72 % silica content and at least 20% quartz, as rocks with less than this amount of quartz are rarely called granite.
The rest of the composition is usually feldspar, specifically orthoclase feldspar, sometimes with muscovite or biotite, which are what give granite the dark and light flakes you commonly see.
Granites are also known as phaneritic, meaning they are coarse-grained as the silica has more time to grow crystals as it cools from magma.
This is obvious when looking at the larger speckles and spots that can be seen with the naked eye when you encounter granite during your exploration.
Granite is often found in the structure of batholiths that emerge from the core of the Earth and form massive monolithic structures.
These structures can be found on almost every continent, and this is part of what makes granite the most commonly found rock of the crust.
You should find some type of granite no matter where you are.
Granite is mostly used for aesthetics currently, and you can find it on buildings and possibly as the counter top in your kitchen.
Another type of rock is a surprisingly similar composition to granite, except it is extrusive.
It is reddish-brown to off-white in color and will be composed of smaller, finer-appearing crystals with some larger crystals sprinkled throughout.
The larger crystals will predominantly be feldspar or quartz.
The main difference in rhyolites compared to granites is the presence of sanidine instead of feldspar.
This is what allows for the appearance of the crystal to look different and what gives the rock some of its characteristic coloring.
Rhyolites are found on every continent similarly to granite, mostly towards the edges or near fissures in the Earth’s crust.
This rock may also be observed with small air pockets, or bubbles, periodically, showing the way it would cool in the crust.
This is another of the most commonly found minerals in the Earth’s crust.
It is also another that is similar to granite but has a couple of key components that distinguish the two. It contains more plagioclase feldspar than orthoclase feldspar and contains biotite, augite, and hornblende.
The darker appearance of granodiorite is attributed to the higher concentrations of plagioclase in this rock.
This rock has a very similar texture to granite and also has similar concentrations of silica and quartz.
This rock gets its name as it is considered an intermediate acidic rock and is a kind of combination of granite and diorite, which is a basic or malefic type rock.
Fun fact: the Pantheon in the heart of Rome had columns twelve and half meters tall made from one solid piece of granodiorite.
This rock is a very interesting example of an acidic rock.
Found abundantly throughout the planet and having a definitive crystalline structure, this rock can be acidic, but it can also be the opposite, basic.
They occur in all parts of the Earth and are usually relatively old in geological terms.
They are especially interesting due to the wide variety of composition that they occur in.
The acidic pegmatites are often found with other minerals interspersed throughout the crystals.
Granitic and senyitic deposits of pegmatites are found with all sorts of other minerals commercially utilized found interspersed.
Sheet mica, beryllium, and lithium deposits are common, as well as deposits of gems like mica, molybdenite, and tungsten are commonly found in pegmatites on the Earth.
Acidic Rocks Make Up Earth’s Crust
Now you know that acidic rocks are also known as felsic rocks, from the silica and feldspar contents.
There is an abundance of these rocks making up the crust of the Earth and without them the planet would be lacking some of its magnificent geological structures.
Granite is the most typical type of acidic rock, and most other types are some variation of granite with slightly different compositions of feldspar, quartz, or another mineral.
Next time you take a walk or are on a hike looking for interesting rocks and formations, remember that acidic rock will be the majority of what you are bound to find.
The crystalline structure and variable colors and patterns give acidic rock a memorable appeal and any of these rocks will add value and information to a collection.
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