Igneous Rocks
Born of Fire from deep in the earth
Mining and Exploration
Geology Handbook
Dark or light?
Igneous Rocks are the most primitive of rocks – they come directly from molten rock and crystallize in varying ways to become rocks.
Since the other types of rocks (metamorphic and sedimentary) are derived from igneous rocks to start with – it is important to know the basics of where the source rock came from. This can be as easy as light vs dark. – But not always.
The Magma (molten rock before it reaches the surface) which generates the specific rocks can vary greatly in its composition and source.
To simplify things we can classify magmas (and the rocks they form) into two categories:
Light (Felsic) and Dark (Mafic)
Happening Now
Whose exploring NW Ontario?
Exploration And permitting tablesThe table provided by MNDM shows the Plan or Permit number, the project name, the claim holder, all affected claims, the project location, any activities taking place and a start and end date for the activities. Activities...
The full spectrum of rock types is classified as….
Ultramafic >> Mafic >> Intermediate >> Felsic
>>>> Increasing Silica (SiO2) >>>>>>>>
<<<< Increasing Magnesium/Iron <<<<<
Ultramafic rocks are some of the earliest rocks on earth and tend to be very old. They have great importance in mining exploration as a source rock.
The Dark Side
Mafic: Contains heavier elements – what they call a primitive magma. The heavier elements (iron, magnesium,) are more abundant and the lighter minerals (silica) are rarer.
Since these heavy elements tend to form darker minerals (olivine, pyroxene, amphibole) the corresponding rock will tend to be darker in appearance. – But not always.
As Mafic rocks are re-melted through plate tectonics, some of the heavier elements are lost as they settle deep into the earth (earths core isn’t just iron). Proceeding in geological timescales, rock are melted and solidified over and over and such over time the heavier elements are depleted until they no longer dominate the magma. Mafic rocks dominate the ocean crust.
Ocean crust is primarily mafic in nature. Darker in colour and physically it subducts under continental crust.
Importance:
As Mafic rocks contain the heavier elements, not only iron and magnesium, but the metals (gold, silver, copper, platinum…) their presence is of utmost importance when exploring for valuable minerals. Knowing a source rock is mafic in origin is the base of most mineral exploration.
The Light Side
Felsic: Contains silica as a dominant element due to the processes listed above. Common minerals become quartz, muscovite, feldspars and since these tend to be lighter (whiter) elements felsic rocks tend to be lighter in colour over their mafic cousins. – But not always.
Continental crust is primarily felsic in nature. It is lighter in colour and physically “floats” atop more mafic magmas of the oceanic crusts. Continental crust is very difficult to get to subduct/melt naturally.
Classification: Large Crystals or small crystals?
The individual crystal size of all rocks cooling from magma or lava is a direct result of how much time they have to grow. The longer a mineral has to grow, the larger the crystals will be. This means the speed at which molten rock solidifies directly determines the size of the crystals.
So for a given magma composition, there will be an extrusive (at the surface, fast cooling) and an intrusive (inside the earth slow cooling) version, Chemically identical to each other. Basalt (Lava flows on surface) is the extrusive (from lava flows) version of Gabbro which is when the magama solidifies deep in the earth. Rhyolite is the extrusive version of Granite.
And just to get confusing, some magmas may cool quickly at one point, but slowly at another (two stage cooling) which will result in some large crystals in a mostly small crystal rock. These rocks and their texture are classified as porphyritic.
A magma that has millions of years to cool deep in the earth will tend to form/grow large easily distinguishable crystals. A magma (or lava at the earth’s surface) which explodes into air or water and cools very quickly will only have very tiny crystals – so small you may need magnification to see them.
Fractional crystallization.
There are also rocks where some crystals form and sink to the bottom of the magma, then another, and another… forming layers. This is especially important to the formation of chromite deposits.
Get In Touch
CanadaMines.ca provides general information on geology, mining, and exploration in Canada and is not intended as professional advice. Users should consult qualified professionals before making decisions based on site content. While we strive for accuracy, we cannot guarantee error-free, complete, or current information and assume no responsibility for discrepancies. We are not liable for damages from using or relying on-site content, and users indemnify CanadaMines.ca and its affiliates from claims arising from their use. External links are not endorsed, and this disclaimer may change without notice. Continued use signifies acceptance of changes.
Canadamines.ca participates in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com, Amazon.ca, and affiliated sites. As an Amazon Associate, we earn from qualifying purchases through our links.