Igneous Rocks

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 into two categories:

Classification: Dark rock or Light rock?

Dark Rock

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.

Light Rock

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.

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.

Classification: Large Crystals or small crystals?

Can you see individual 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 and an intrusive version, Chemically identical to each other. Basalt is the extrusive (from lava flows) version of Gabbro. Rhyolite is the extrusive version of Granite.

Porphyritic andesiteAnd 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.

Basalt: Fine grained extrusive Mafic rock

Extrusive Mafic examples

Gabbro: Coarse grained intrusive Mafic Rock

Mafic intrusive examples

Felsic Extrusive examples

Felsic intrusive examples

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