2.4: Igneous Rocks

How Igneous Rock Forms

Igneous rock forms from the cooling of magma (molten rock), either slowly at depth within the crust, or quickly at or near the surface. In general, the longer the cooling process takes (up to millions of years), the larger the crystals in the rock. Volcanic (extrusive) rock, rock that cooled at or near Earth’s surface, typically has mineral crystals that are less than 0.1 mm across (because they cooled within seconds or minutes), while intrusive (plutonic) igneous rock has crystals that are typically larger than 1 mm across.

Magma, the molten rock from which igneous rock forms, does not all cool and crystallize in an instant as soon as it reaches a certain temperature. Magma is chemically homogeneous and thus its various chemical constituents cool at different temperatures.

Order of Crystallization: Bowen's Reaction Series

The minerals that make up igneous rocks crystallize at a range of different temperatures. This explains why a cooling magma can have some solid crystals within it and yet remain predominantly liquid. The sequence in which minerals crystallize from a magma is known as Bowen's reaction series (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Bowen's reaction series describes the order in which minerals crystallize from a melt (magma or lava). Read a detailed description. This work by Steven Earle is licensed under CC BY.

Bowen’s reaction series has two pathways for minerals to form as magma cools: the discontinuous series (left side of Figure \(\PageIndex{1}\)) and the continuous series (right side of Figure \(\PageIndex{1}\)).

Discontinuous Series

On the discontinuous series, olivine normally crystallizes first, between 1200° and 1300°C. As the temperature drops, and assuming that some silica remains in the magma, the olivine crystals will react (combine) with some of the silica in the magma to form pyroxene. As long as there is silica remaining and the rate of cooling is slow, this process continues down the discontinuous branch: olivine to pyroxene, pyroxene to amphibole, and (under the right conditions) amphibole to biotite.

Mg₂SiO₄ + SiO₂ → 2MgSiO₃

olivine + silica → pyroxene

Finally, if the magma is quite silica-rich to begin with, there will still be some silica left at around 750° to 800°C, and from this last remnants of the magma, potassium feldspar, quartz, and maybe muscovite mica will form.

Continuous Series

On the continuous series, at about the point where pyroxene begins to crystallize, plagioclase feldspar also begins to crystallize. At that temperature, the plagioclase is calcium-rich (a variety of plagioclase feldspar called anorthite). As the temperature drops, and providing that there is sodium left in the magma, the plagioclase that forms is a more sodium-rich variety (albite).

Magma Composition Influences Which Minerals Form

The composition of the original magma is critical to magma crystallization because it determines how far the reaction process can continue before all of the silica is used up. The compositions of typical mafic, intermediate, and felsic magmas are shown in Figure \(\PageIndex{2}\). Note that these compositions are expressed in terms of “oxides” (e.g., Al2O3 rather than just Al). There are two reasons for this: one is that in the early analytical procedures, the results were always expressed that way, and the other is that all of these elements combine readily with oxygen to form oxides.

Figure \(\PageIndex{2}\): Chemical compositions of typical mafic, intermediate, and felsic magmas. This work by Karla Panchuk, a derivative of the original, is licensed under CC BY 4.0.

Mafic magmas have 45% to 55% SiO₂, about 25% total of FeO and MgO plus CaO, and about 5% Na₂O + K₂O. Felsic magmas, on the other hand, have much more SiO₂ (65% to 75%) and Na₂O + K₂O (around 10%) and much less FeO and MgO plus CaO (about 5%).

Table \(\PageIndex{1}\): Proportions of oxides for the main chemical components in mafic, intermediate, and felsic magma.

Oxide	Felsic Magma	Intermediate Magma	Mafic Magma
SiO2	65% to 75%	55% to 65%	45% to 55%
Al2O3	12% to 16%	14% to 18%	14% to 18%
FeO	2% to 4%	4% to 8%	8% to 12%
CaO	1% to 4%	4% to 7%	7% to 11%
MgO	0% to 3%	2% to 6%	5% to 9%
Na2O	2% to 6%	3% to 7%	1% to 3%
K2O	3% to 5%	2% to 4%	0.5% to 3%

As a mafic magma starts to cool, some of the silica combines with iron and magnesium to make olivine. As it cools further, much of the remaining silica goes into calcium-rich plagioclase, and any silica left may be used to convert some of the olivine to pyroxene. Soon after that, all of the magma is used up and no further changes take place. The minerals present will be olivine, pyroxene, and calcium-rich plagioclase. If the magma cools slowly underground, the product will be gabbro; if it cools quickly at the surface, the product will be basalt (Figure \(\PageIndex{3}\)).

Felsic magmas tend to be cooler than mafic magmas when crystallization begins (because they don’t have to be as hot to remain liquid), and so they may start out crystallizing pyroxene (not olivine) and plagioclase. As cooling continues, the various reactions on the discontinuous branch will proceed because silica is abundant, the plagioclase will become increasingly sodium-rich, and eventually potassium feldspar and quartz will form. Commonly even very felsic rocks will not have biotite or muscovite because they may not have enough aluminum or enough hydrogen to make the OH complexes that are necessary for mica minerals. Typical felsic rocks are granite and rhyolite (Figure \(\PageIndex{3}\)).

The cooling behavior of intermediate magmas lie somewhere between those of mafic and felsic magmas. Typical mafic rocks are gabbro (intrusive) and basalt (extrusive). Typical intermediate rocks are diorite and andesite. Typical felsic rocks are granite and rhyolite (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Examples of the igneous rocks that form from mafic, intermediate, and felsic magmas. This work by Steven Earle is licensed under CC BY.

Query \(\PageIndex{1}\)

Igneous Rock Compositions

Classification of igneous rock is based mainly on the rock composition and texture, which is illustrated in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): A simplified classification diagram for igneous rocks based on their mineral compositions. Access a detailed description. This work by Steven Earle is licensed under CC BY 4.0.

Four broad compositional classes of igneous rocks are shown, namely felsic, intermediate and mafic, and ultramafic. These are determined by the proportions of the dark silicate minerals (biotite, amphibole, pyroxene and olivine). Felsic rocks are light colored, often close to white, with less than 20% dark minerals. Intermediate rocks are medium-dark (typically gray) with 20 to 50% dark minerals, and mafic rocks are close to black (with more than 50% dark minerals). The three main types of intrusive igneous rocks are granite, diorite and gabbro. The equivalent volcanic (extrusive) rocks—which are fine grained—are rhyolite, andesite and basalt. Igneous rocks with close to 100% dark minerals are known as ultramafic, such as peridotite; these are rare on the Earth’s surface, but common in the mantle. While komatiite is the name given to an ultramafic, extrusive rock, these rocks are very rare and almost never occur.

Mafic igneous rocks (e.g., basalt or gabbro) are denser than felsic igneous rocks. Basalt has a specific gravity of about 3 g/cm³, while for granite the value is about 2.7 g/cm³. This small difference becomes very important in the context of plate tectonics. In comparison, the ultramafic rock of the mantle has a density of about 3.3 g/cm³.

Igneous Rock Textures

The second part of classifying igneous rocks is identifying the texture of the igneous rock. Remember, the cooling rate of the magma determines how large crystals grow. Therefore intrusive (plutonic) igneous rock and extrusive (volcanic) igneous rock are differentiated by crystal size. Rocks with crystals large enough to be easily seen with the unaided eye are plutonic and their texture is called phaneritic (from the Greek root, "phaneros" meaning visible). Rocks with crystals difficult to be seen with the unaided eye are volcanic and their texture is called aphanitic (or "not" visible). Variations occur depending upon the circumstances of the igneous rock’s formation. Common variations include:

Pegmatitic - exceptionally large crystals, usually only found in felsic and intermediate compositions (Figure \(\PageIndex{5}\)). Their basic texture will be phaneritic, not aphanitic. These rocks are therefore given the adjective "pegmatitic" before their names, such as a pegmatitic granite or a pegmatitic diorite. Or, they are referred to as either a "granite pegmatite" or a "diorite pegmatite."

Figure \(\PageIndex{5}\): Pegmatite containing lepidolite, tourmaline, and quartz from the White Elephant Mine in the Black Hills, South Dakota. This work by Jstuby via Wikimedia Commons is in the public domain.

Porphyritic - the crystals are of mixed-sized, with noticeably larger crystals, or "phenocrysts", surrounded by smaller crystals, or "groundmass." These rocks are usually felsic, intermediate, or mafic, and are seldom ultramafic. They can also be either plutonic or volcanic in their basic texture. They are therefore given the adjective “porphyritic” before their names or are referred to by their compositional name followed by the term "porphyry." For example a porphyritic rock with an intermediate composition might be a diorite porphyry or a porphyritic diorite, or it might be an andesite porphyry or a porphyritic andesite. Figure \(\PageIndex{6}\) shows a sample of rock with porphyritic texture.

Figure \(\PageIndex{6}\): This is an example of a rock with porphyritic texture with a camera lens cap for scale. The white crystals are large and visible without aid whereas the darker groundmass is made of crystals too small to identify. This rock originated from the 1915 eruption of Lassen Peak (see: Cascade Range and Modoc Plateau). "Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 4" by James St. John via Flickr is licensed under CC BY.

There are three additional textural terms commonly used for igneous rock - glassy, vesicular, and pyroclastic. All of these terms are used with volcanic rock and not plutonic rock. They reflect different conditions that can occur during volcanic eruptions.

Glassy rock cooled so fast that crystals did not have time to form. Commonly it is of felsic composition and called obsidian. Note that because it does not have crystals, the commonly used color conventions for identifying whether its composition would be felsic, intermediate, or mafic don’t apply. Obsidian is almost always black, even though its composition is almost always felsic (Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{7}\): A sample of obsidian, a glassy textured rock. This sample is from the Medicine Lake Volcano (see: Cascade Range and Modoc Plateau). "Spherulitic obsidian (Medicine Lake Highlands, California, USA) 2" by James St. John via Flickr is licensed under CC BY.

When a volcano erupts and there is an abundance of gas in the lava, holes from the escaping gasses can be preserved as the crystals form. Such rocks have a porous texture and are given the adjective, vesicular, before their name, for example, vesicular basalt (Figure \(\PageIndex{8}\)). One special example of a porous rock occurs when the rock is both glassy and porous. Essentially such a rock is “rock foam” like the foamy head on a glass of root beer or beer and is given the name, pumice, from the Latin word “pumex” meaning foam.

Figure \(\PageIndex{8}\): Weathered vesicular basalt from Amboy Crater, Mojave Desert, CA. "Vesicular basalt (Upper Pleistocene, ~79 ka; lava flow from Amboy Crater, Mojave Desert, California, USA) 1" by James St. John via Flickr is licensed under CC BY.

The last of these textural terms relating to volcanic eruptions is pyroclastic or fragmental. This texture occurs when the volcano explodes. The hot explosion debris, when it falls back to Earth’s surface, compacts and cements into rock that may contain rock fragments, crystals, and glass (Figure \(\PageIndex{9}\)). Such rocks are commonly referred to as tuff and usually have either a felsic or intermediate composition. Their composition may be used as an adjective, for example, rhyolitic tuff or andesitic tuff.

Figure \(\PageIndex{9}\): Sample of tuff with a pyroclastic texture. This rock is from Hole in the Wall in the Mojave National Preserve, CA (see: Mojave and Colorado Deserts). This work by Wilson44691 via Wikimedia Commons is in the public domain.

Using Composition and Texture to Classify Igneous Rocks

Once the composition and texture of an igneous rock have been determined, rocks can be classified using Table \(\PageIndex{2}\).

Table \(\PageIndex{2}\): Classification of igneous rocks according to composition (first row) and texture (first column).

	Felsic	Intermediate	Mafic	Ultramafic
Pegmatite	Granite pegmatite	Diorite Pegmatite	Gabbro Pegmatite	Rare
Phaneritic	Granite	Diorite	Gabbro	Peridotite
Porphyritic - phanerititc	Granite Porphyry	Diorite Porphyry	Gabbro Porphyry	Rare
Porphyritic - aphanitic	Rhyolite Porphyry	Andesite Porphyry	Basalt Porphyry	Rare
Aphanitic	Rhyolite	Andesite	Basalt	Komatiite
Glassy	Obsidian	Obsidian	Obsidian	Rare
Vesicular (porous)	Pumice	Pumice/Scoria	Scoria	Rare
Pyroclastic (fragmental)	Rhyolite tuff	Andesite tuff	Rare	Rare

References

1. Earle, S. (2019). Physical Geology – 2nd Edition. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/physicalgeology2ed March 2024