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Many people know that igneous rock forms when magma cools, but because they are not around us daily, many people don’t know their characteristics and their different types or uses. If you’re a rockhound, the igneous rock should be dear to your heart because of all the minerals it can bring along.
The formation of igneous rocks can take two routes: fast or slow. This cooling determines the chemical composition and structure of the rock. Crystals can form in the mass if cooling happens very slowly, allowing the natural geometrical shapes of the molecules to form. Glassy texture, sharp edges, or bubbles often characterize igneous rocks.
Knowing the characteristics and the differences between the types of igneous rocks and being able to spot these differences will change your rock-hunting game and allow you to find crystal-rich areas with much less effort.
If you want to check out the best books about rock and mineral identification, you can find them here (Amazon link).
Igneous Rocks are Formed when Molten Magma Cools
This is the most basic fact about igneous rocks, but it’s elementary in understanding their behavior.
Molten rock approaches Earth’s crust from the mantle because it is less dense than the material around it, much like how warm air rises in a house. Once it gets to the top, it can escape to the surface in a few different ways.
Whichever way it solidifies, it goes from around 625-1200 degrees Celsius to around 25 degrees Celsius. Besides the temperature change, the pressure change between molten rock cavities beneath the earth and the open atmosphere is extreme.
The degree to which this environmental change affects the magma determines what igneous rock is formed.
Intrusive and Extrusive: The Classification of Igneous Rocks
On its path to the crust, magma can take two different paths.
Extrusive Igneous Rocks
First, if the geology is correct, it can remain liquid until very near the top and build up very high pressures until it explodes through a volcano. This material is called extrusive igneous rock when it cools down because of how quickly it extrudes from underground.
Extrusive igneous rock is what most people think of when they think of igneous rock – the black, glassy rock that lays around the volcanic area and forms unnatural edges and corners.
This is one type of extrusive igneous rock, but basalt, pumice, and many others also belong to this category.
Pumice is the lightest rock which we know of on Earth. It is so light because the magma that formed it had so many air bubbles that most of the rocks are not rocks, just pockets of air in a now-solid host.
Basalt is an extrusive igneous rock that forms so that there is very little force keeping it stuck together, and it can be cut using just a shovel, like dirt.
Intrusive Igneous Rocks
The other option the magma has on its way to the surface is through the back roads rather than on the highway. The back roads of Earth’s crust take the form of pockets high enough to solidify the rock from a lack of heat but are not in regions of high pressure and volcanic activity. Magma in these pockets slowly cools and solidifies.
In contrast to extrusive rock, the elements of this kind of igneous rock take a lot of time to take whatever shape they want. Typically, this produces crystalline structures so pure that they can be seen with the naked eye.
Therefore, rather than being a homogenous material with particles so small that it takes the appearance of glass, this rock ends up with a heterogenous structure, which, if the right conditions are present, can lead to the formation of gemstones.
TIP: Igneous rocks are an integral part of all rocks that make up our planet; that is why there are a lot of interesting facts to be shared. Check them out in the article below:
11 Cool & Interesting Facts About Igneous Rocks (Must Read)
Chemical Composition
There are also ways to divide the igneous rock by the elements they contain. The most general classification breaks igneous rocks into whether they contain felsic or mafic minerals.
Felsic Igneous Rocks
Felsic igneous rocks are primarily made of feldspar and silica. Those minerals are lighter in color and melt earlier than their counterparts. Felsic igneous rocks are then divided by texture.
Granite, pegmatite, and rhyolite are common rock structures that fit into the category of felsic igneous rocks, each with its texture requirements. Pumice and obsidian also fall into this classification.
Silica, silicon oxide, or SiO2, comprise many magma forming felsic rocks. It can take up anywhere from 45% to 75% of the composition.
Mafic Igneous Rocks
The second classification based on chemical composition can be considered the more metallic of the two. Magnesium and iron are the minerals that dominate this composition, and they melt at much higher temperatures. Basalt and gabbro fall into this category.
While the categorizations of felsic and mafic characterize the chemical composition of igneous rocks, geologists have found it useful to form a few more classifications along the same lines.
Nature typically does not work in such rigid ways, so rock sometimes exists with some mafic and felsic minerals inside. This would be called an intermediate igneous rock, with ultramafic being the extreme of almost no silica and feldspar in the composition.
Below is a chart that visualizes all the types of igneous rocks. It’s very technical but a great way of looking at how many different types are out there and what their properties are.
TIP: Mineral properties are very specific characteristics of every separate mineral. Check out the complete guide with all minerals properties explained in the article below:
Guide: All Rock & Mineral Properties Explained by Expert
The Many Uses of Igneous Rock
Now that we have learned that granite, rhyolite, and other common rock types are igneous, we can think about all the times we see igneous rocks in daily life.
Now you can categorize any granite statue you see around as extrusive or intrusive (remember – can you visually see the crystals of pure composition inside?) and felsic or mafic. Because of its beauty, granite has been used for sculptures dating back to Roman times.
You may have also recognized pumice as popular with self-care products. It is typically used as an exfoliator because of its porosity. It’s also used as an abrasive in pencil erasers and a traction material in rubber tires.
Gabbro, an intrusive mafic rock, can be used in areas where granite is desired to be a darker color. It may even be labeled as granite, so don’t let that fool you (remember – granite has more silica and feldspar and is much lighter in color). It is also used in road construction as an asphalt, pavers, or curbing component.
Peridotite is a very important type of rock. Among the types of rock that classify as peridotites is kimberlite, which you may have heard of as the rock that houses diamonds.
TIP: Gabbro is one of the most common rocks on the Earth and is located mainly at the bottom of the oceans. Find out more about gemstones from the ocean in the article below:
Which Gemstones Come From The Ocean? Corals, Pearls, & more!
Structures where Igneous Rock is Found
Because extrusive igneous rock forms during volcanic activity, it is usually found in volcanically active areas. It can be at the surface or buried if the activity was long ago.
Intrusive rocks, on the other hand, form underground, and the areas where they form can be categorized to aid in finding minerals.
Ophiolites
Sometimes, magma is exposed to the surface through the movement of plates. In this case, the magma can come up and cool in the bottom of the ocean – this is called an ophiolite. These are mainly used by scientists studying the earth’s composition.
Pipes
This sounds like a long vertical cavity that has previously been filled with magma and allowed it to cool, causing intrusive rock. Because it is so long, the magma present in these is typically from far below the surface.
Dikes and Sills
These structures house igneous rock and are obedient to the earth’s metamorphic and sedimentary rock layers.
If magma can enter this structure, then it forms a flat sheet. This sheet is called a sill if it does not cross over any existing layers. Otherwise, it is called a dike.
Fissures
These earth’s crust cracks allow magma to come up from the mantle and cool relatively close to the surface.
Batholith
You will hear this word referred to as geology a lot. It means a large deposit or area with intrusive, mostly felsic igneous rocks.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
How Does it Affect My Rockhounding?
This is the main question of the day. You can use all you have learned about igneous rocks to help you find rocks and minerals more easily. If you want to find crystals or large chunks of minerals, you’ll be looking for an intrusive rock.
First, find the rock that typically houses the mineral you seek. Then, look online for structures that typically exhibit this type of igneous rock.
Next, find some geological information about your region – typically, these structures appear on a map or in a geological explanation.
Lastly, be aware of the geology around you when searching for rocks. If you notice a stark change in the properties discussed in this article, make a mental note of this area. Remember, the more information you have about the mineral you search for, the better.
For example, diamonds are found in ultramafic intrusive rock, usually forming kimberlite pipes. This information helps narrow the search to a much smaller area, letting you find much more and higher quality rocks and minerals!
TIP: If you are located in the United States and you love rockhounding, then we have compiled a list, maps, and guidelines about the best rockhounding sites near you! Find out more in the article below:
Rockhounding Near Me: Best Locations State-by-State (Map)