What Rocks Are Igneous? | Igneous Rocks are Formed | Types of Igneous Rock
Today’s article will be taking you on a journey surrounding a pretty hot topic: igneous rocks. There are three types of rock, but we’ll mainly be focusing on what rocks are igneous and what makes a rock igneous before looking at how they’re used in a modern world.
To begin with, we’ll need to quickly explain the three types of rock: there’s igneous rock, sedimentary rock and metamorphic rock.
Igneous rock is a very unique rock as it is only formed using volcanic movement whereby the cooling down of molten lava determines the rock that is formed. Not only this, but it can happen both at the Earth’s surface and underground in the magma chambers.
Sedimentary rock, on the other hand, is formed through time when layers of minerals from the air and sea mound on top of each other. And, metamorphic rock is created under the Earth’s surface with heat, pressure and chemical exposure.
But, we digress. Moving back to the interesting parts of igneous rocks and the awe of volcanic movement:
- What is Igneous Rock and How is it Formed?
As mentioned, igneous rock is a by-product from volcanic magma. When the molten lava begins to cool it can create a rock. Depending on the duration of the time taken to cool and solidify, it will determine whether or not a rock will have crystals.
Rocks such as Granite are cooled near the Earth’s mantle, where it is still incredibly hot, but slightly less hot than the magma, itself. During this slow process, crystals are able to form in the rock, giving granite its particularly speckled composition.
Whereas, when igneous rocks are formed closer to the Earth’s surface, you’ll find heavy types of igneous rock with very small crystals such as obsidian. This difference in temperature and cooling has given way to two identifiable types of igneous rock: intrusive and extrusive. Meaning one is formed on the surface of the Earth while the other is created near the Earth’s core.
As magma can vary across our planet, it means that the result of the igneous rock can vary, too. Not only this, but even the cooling process from which igneous rocks are formed can create multiple types of igneous rocks from the exact same magma.
So, when we ask ourselves; “what rocks are igneous?” the simple answer is that it must have been formed from volcanic magma inside the Earth and cooled. As well as this, when geologists are examining an igneous rock, they’ll have the basic chemical composition building blocks to look out for. Igneous rocks are mainly formed with silicate materials (silicone and oxygen).
- Intrusive Igneous Rock
Simply put, intrusive igneous rock is formed inside the Earth’s crust. It occurs when molten magma comes into contact with pockets of gas that is at a lower temperature than the magma, itself.
Also called plutonic rock, intrusive igneous rocks are most commonly found within subterranean layers on the planet such as in the sea or underground. However, they can often make their way to the surface with tectonic activity such as earthquakes.
As intrusive rocks are cooled at a temperature still incredibly high, it means that the plutonic rock is forming at a very slow rate. This allows for large crystals to form over time. The biggest and most commonly recognised example of this is granite. The large white deposits in granite are crystals that have formed over time.
This process will only occur with intrusive rocks, meaning that when asking what rocks are igneous, you should look for the crystal structures than have formed. Examples of intrusive types of igneous ricks are; granite, gabbro, pegmatite, peridotite and diorite.
- Extrusive Igneous Rock
While it may seem obvious, extrusive igneous rock is formed on the Earth’s surface, directly opposing intrusive types of igneous rock.
Molten rock – or, magma – floating close to the Earth’s core in chambers, is swiftly brought to the Earth’s surface with tectonic movement such as volcanic eruptions. While there are several reasons for the molten rock to be brought to the surface, there are three main reasons.
The first would be due to pressure released from the gases in the magma chamber. This pressure causes an abrupt discharge through the conduit/vent. The second reason being the buoyancy of the magma and the third being the addition of new molten rock increasing the chamber’s pressure.
When the molten lava exits the throat of the volcano, lava will flow down the volcano while volcanic ash will also fill the sky. When thinking about what rocks are igneous, we need to think about the smallest of rocks as well as the large rocks that will form from magma. Volcanic ash, such as that from Mount Vesuvius that covered Pompeii, can soar into the atmosphere, harden and fall at speeds over 100km/hour.
With cooling speeds such as that, or even when flowing down the side of the volcano, extrusive igneous rock doesn’t have the same crystallisation opportunities. Meaning examples of extrusive igneous rock such as obsidian, basalt, rhyolite, andesite, scoria, dacite, tuff and pumice, all have minimal crystallisation.
- The Uses of Types of Igneous Rock
Now we know what rocks are igneous and where they come from, we can have a look at the modern applications.
As most igneous rocks are dense and durable, many have been used for their construction benefits dating back to ancient civilisations. Granite, being the main example in this respect, is still commonly used as a construction tool and for statues. Then there’s the choices to use granite as a stylish and modern kitchen countertop.
Dorite, on the other hand is extensively a part of works of art including vases. Peridotite is then farmed to make peridot, a stunning green gem used within jewellery. And, then a final example would be pumice. A highly porous extrusive igneous rock, it is commonly used as an abrasive on skin, in erasers and in washing machines to make stone-washed jeans. So, next time you decide to scrape some unwanted skin from the bottom of your foot, just know that it once flowed out of a volcano.
- There are over 700 types of igneous rock.
- Granite is a common rock which is made of 25% quartz and often used in construction because of its strength.
- The upper section of our Earth’s crust is actually made of 95% igneous rock, the other 25% is made with the other two types of rock.
- Magma is made of the atoms and molecules resulting from molten minerals.
- There is a rock called Tuff that is formed only with volcanic ash.
- On rare occasions, fossils can be found where molten rock has hardened around a object (such as a tree) before it has burned away causing an imprinted chamber.
- Many igneous rocks are formed with holes inside due to trapped gas in the lava.
- The more silica that is in an igneous rock, the paler it will be in colour.