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Many people know that igneous rock forms when magma cools, but because they are not around us every day, many people don’t know their characteristics and many different types or uses. If you’re a rockhound, 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. Igneous rocks are often characterized by glassy texture, sharp edges, or bubbles.
Knowing the characteristics and the difference 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.
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Igneous Rocks are Formed when Molten Magma Cools
This is the most basic fact about igneous rocks but it’s elementary in the understanding of how they behave.
Molten rock approaches Earth’s crust from the mantel 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 ends up solidifying, it goes from around 625-1200 degrees Celsius to around 25 degrees Celsius. Besides the change in temperatures, the pressure change between molten rock cavities beneath the earth and open atmosphere is extreme.
The degree to which this change in environment affects the magma determines what kind of 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 right, it can remain liquid until very near the top and build up very high pressures until it explodes through a volcano. When it cools down, this material is called extrusive igneous rock, for 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 which lays around volcanic area and forms unnatural edges and corners.
This is one type of extrusive igneous rock, but basalt and pumice also belong to this category, along with many others.
Pumice is the lightest rock which we know of on Earth. It is so light because the magma which formed it had so many air bubbles that most of the rock is actually not rock at all, just pockets of air in a now-solid host.
Basalt is extrusive igneous rock which forms in such a way that there is very little force keeping it stuck together, and 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 which are 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 have lots of time to take whatever shape they want. Typically, this produces crystalline structures which are so pure 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.
There are also ways to divide up igneous rock by the elements which they contain. The most general classification breaks igneous rocks into whether it contains felsic minerals 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 which fit into the category of felsic igneous rocks which each have their own texture requirements. Pumice and obsidian also fall into this classification.
Silica, or silicon oxide, or SiO2 makes up a large percent of the magma which forms 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 thought of as the more metallic of the two. Magnesium and iron are the minerals which dominate this composition, and they melt at much higher temperatures. Basalt and gabbro fall into this category.
While the categorizations of felsic and mafic do a good job in characterizing 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 exists sometimes with some mafic and some 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 which 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.
The Many Uses of Igneous Rock
Now that we have learned that granite, rhyolite, and other common types of rock are igneous, we can think about all the times we see igneous rocks in day-to-day 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 a component of asphalt, pavers, or curbing.
Peridotite is a very important type of rock. Among the types of rock which classify as peridotites is kimberlite, which you may have heard of as the rock which houses diamonds.
TIP: Gabbro is one of the most common rock on the Earth and it is located mainly at the bottom of the oceans. Oceans are full of amazing and interesting rocks, minerals and even more! Check out which gemstones come from ocean in the article bellow:
Structures where Igneous Rock is Found
Because extrusive igneous rock forms during volcanic activity, you will usually find it 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 the finding of minerals.
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 earth’s composition.
This is just what it sounds like, a long vertical cavity which has previously filled with magma and allowed it to cool, causing intrusive rock. Because it is so long, the magma which is present in these is typically from far below the surface.
Dikes and Sills
These are structures which house igneous rock and are obedient to the layers of the earth’s metamorphic and sedimentary rock.
If magma can enter this structure, then it forms a flat sheet. If this sheet does not cross over any of the existing layers, it is called a sill, otherwise it is called a dike.
These are cracks in the earth’s crust which allow magma to come up from the mantle and cool relatively close to the surface.
You will hear this word referred to geology a lot. It just 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 that 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 intrusive rock.
First, find the type of rock which typically houses the mineral you are searching for. Then, look around online for the types of structures which typically exhibit this type of igneous rock.
Next, find some geological information about your region – typically these structures will appear on a map or in a geological explanation.
Lastly, when you go out searching for rocks, be aware of the geology around you. If you notice a stark change in the properties we’ve 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 down the search to a much smaller area, letting you find much more and higher quality rocks and minerals!
TIP: There is a lot of locations in the USA where you can find igneous rocks. Check out the rockhounding guides for each state in the United States with the best locations and most common rocks and minerals youcan find there.