Igneous Rocks

 

Introduction
Igneous rocks are those that form from the cooling and crystallization of hot silicate liquids. The silicate liquids themselves are the product of partial or complete melting of the crust or mantle.  Igneous rocks play an extremely important role in the evolution of the Earth;  they constitute the bulk of the oceanic crust and comprise significant portions of the continental crust as well.  Their formation is testimony to the high temperatures that exist to this day in the Earth's interior. The terminology of these liquids is as follows:

magma:    The term used to describe the liquid when it is within the Earth's crust or mantle.

lava:          The term used to describe the liquid when it has reached the surface of the Earth.
 

Melting and Crystallization Processes
The fact that volcanoes exist and have erupted lava both in the geological past and at the present leads to the conclusion that temperatures within the Earth are sufficiently high for at least partial melting to occur. Geophysical evidence tells us that with the exception of the outer core, the interior is nowhere completely melted. Consequently, magmas generated within the mantle must be the result of partial melting.

The diagram above illustrates an important point for both the melting of mantle rocks and the crystallization of high temperature silicate liquids (magma or lava).
 

Melting, or its reverse, crystallization, occurs over a range of temperatures with different minerals melting (or crystallizing) at different temperatures.  In terms of melting, a fundamental consequence of this behavior is that the composition of magma generated depends on 1) what minerals are present in the rock being melted and 2) the degree to which it is melted.

A better understanding of the processes governing the melting and crystallization behavior of igneous rocks can be gained by learning how to understand simple phase diagrams.  An excellent web-based treatment of simple melting phase diagrams has been developed by Lynn S. Fichter
 (fichtels@jmu.edu)  of the  Department of Geology and  Environmental Science at  James Madison University in  Harrisonburg, Virginia.  The text and diagrams below are copyrighted by Dr. Fichter and are used with sincere appreciation of the work Dr. Fichter put into these educational modules.

Click on the buttons below to learn more about phase diagrams.
 

Binary Eutectic Phase Diagram

 

Solid Soultion Phase Diagram

Important points to learn from the phase diagrams presented above are the following:
 

1.    Melting and crystallization are reverse processes. The paths followed are opposite for increasing versus decreasing temperature.

2.    The paths followed with increasing or decreasing temperature are controlled by the bulk composition of the rock being melted or the liquid being crystallized.

3.    For multi-component magmas or rocks crystallization (or melting) produces changes in the composition of the liquid as crystallization (or melting) proceeds.  Therefore, the composition of magmas will be determined in part by how much crystallization has occurred, or during melting, how much melting has occurred.

4.    For minerals which exhibit no solid solution (because their crystal structures are different) melting or crystallization proceeds in discrete or discontinuous steps. On crystallization, a mineral will suddently appear and with further cooing others inerals will apear at definite temperatures.

5.    For minerals which exhibit solid solution (e.g., plagioclase feldspar), crystallization (or melting) proceeds in a smooth or continuous manner with the composition of the mineral changing along with that of the liquid.

Bowen's Reaction Series
The differences in behavior between minerals exhibiting solid solution from those that do not were first recognized in the early 1900's by the developer of the discipline known as igneous petrology, N.L. Bowen.  Bowen proposed a simple explanation for the crystallization of magma in terms of common minerals that exhibit either continuous or discontinuous crystallization behavior.  This simple scheme is now know as Bowen's Reaction Series.

 

The sequence of minerals on the discontinuous side of the series appear abruptly during crystallization upon falling temperature, or begin melting abruptly on increasing temperature.  This behavior corresponds to crystallization or melting a distinct points such as eutectic or peritectic points in the simple phase diagrams shown above.  In contrast, the continuous series illustrates the consequences of crystallization or melting of minerals exhibiting solid solution such as plagioclase.  Note the following important features:
 

6.    Minerals that crystallize (or melt) at high temperatures are rich in iron and magnesium and poor in silicon.  Igneous rocks crystallizing these minerals will be mafic in composition (basalt and gabbro).

7.    The minerals crystallizing at the lowest temperatures are rich in silicon and aluminum, and rocks consisting of such minerals are called felsic.

Crystalization Simulation

Here is a real cool and informative simulation that allows one to visualize the crystallization of a variety of igneous rock compositions based on Bowen's Reaction series. The software allows you to control the composition (mafic, intermediate or sialic) and the rate of cooling.  Give it a try.

Another important difference between the minerals high on Bowen's reaction series and those that crystallize at low temperatures is the degree of polymerization exhibited in the crystal structures f the minerals. The silicate minerals on Bowen's reaction series are representative of the major groups of silicate crystal structures.  As shown in the following table, the different silicate mineral groups are distinguished by the ways in which the silicon-oxygen tetrahedra are linked together by sharing of corner oxygen atoms.

Notice that as minerals crystallize down the discontinuous branch of Bowen's reaction series, the minerals become increasingly more complex in terms of the structure of the silicate framework.   Conversely, during melting the more interconnected silicate structures melt before at lower temperatures than those with more open frameworks and lower degrees of polymerization.

Structures of Silicate Liquids
Without going into great detail, you should know that silicate liquids actually have considerable structure to them.  This is largely due to the very strong bonds between Si and O in the SiO₄ tetrahedron.  As illustrated in the figure below, the strength of these bonds keeps the SiO₄ tetrahedra intact even in the liquid state.  Furthermore, the ways in which the SiO₄ tetrahedra are linked together (polymerization) are also largely preserved in magmatic liquids.

Note that liquid silica does not have a structure greatly different thatn that of crystalline silica.  The framework of SiO₄ tetrahedra is distorted and more open, but it is still present, and obviously must give the liquid considerable strength.  Similarly with diopside (a pyroxene), the single chain structure can be seen in the liquid state.

What this means is that silicate magma has considerable more strength (structure) that liquids such as water.  This structure manifests itself in terms of viscosity.  Furthermore, given the increase in polymerization of minerals, and hence liquids, going down Bowen's Reaction Series, we can see that the viscosity of the liquids must also increase.  Because viscosity is also temperature dependent (increasing with decreasing temperature), it is apparent that high temperature magma in equilibrium with minerals such as olivine, pyroxene and Ca-rich plagioclase (i.e, basalt) will have low viscosity whereas low temperature magmas crystallizing quartz and alkali feldspar (i.e, rhyolite) will have very high viscosity.  This fact explains why 1) basalts are common and gabbros are rare  -- mafic magmas are sufficiently fluid (low viscosity) to make it to the surface; and 2) granites are common and rhyolites are rare -- felsic magmas are too sticky, and rarely make it to the surface.

Volatile Content
One final feature of great importance to volcanism, is the fact that minerals high on Bowen's Discontinuous Reaction Series (e.g., olivine and pyroxene) are anhydrous, whereas those toward the bottom may be hydrous (e.g., muscovite and biotite).  An important difference in magmas is their volatile content (chiefly the amount of H₂O).  Mafic magmas are low in volatiles and felsic magmas contain significant amounts of volatiles.

Putting this all together, Bowen's Reaction Series tells us the following:
 

• High temperature magmas are low in SiO₂, i.e., mafic
• Magmas low in SiO₂ are hot and have low viscosities, hence are very fluid (e.g., Hawaiian basalts)
• Magmas low in SiO₂ have low volatile contents.  Consequently their eruptions are not explosive.

In contrast

• Low temperature magmas are relatively high in SiO₂, i.e., felsic
• Magmas high in SiO₂ are relatively cooler and have high viscosities, hence are very sticky (e.g., rhyolite or obsidian)
• Magmas highin SiO₂ have relatively high volatile contents.  Consequently their eruptions are  explosive.

Classification of Igneous Rocks
The Classification of igneous rocks is based on a combination of chemical composition and texture.

 

Chemical composition of is controlled by:
1) the composition of the parent material (e.g., mantle peridotite vs continental crust).
2) the degree to which the parent material is melted.
3) modification of the composition through crystallization and differentiation.

Compositional ranges of some important igneous rocks are shown in the following table:
 

	Peridotite	Basalt	Granite
Element	(Mantle)	(Oceanic Crust	(Continental Crust)
Si	20.3	24.7	34.3
O	43.1	43.1	48.5
Mg	22.3	5.2	<1
Fe	9.4	8.3	2.0
Ca	2.1	7.6	<1
Al	1.1	8.2	8.0
K	<1	<1	4.5
Na	<1	1.7	2.4
	Ultramafic	Mafic	Sialic

 Note that as one goes from peridotite to granite the amount of Si and Al increases whereas the amount of Fe and Mg decreases.  It is therefore useful as a first approximation to use broad compositional terms such as ultramafic, mafic and sialic to describe igneous compositions.  This eliminates the need to memorize chemical numbers, and can easily be recognized based on the color of the rocks.

Textures vary widely in igneous rocks.  In terms of classifying igneous rocks, the most important texture is the grain size.  The grain size of an igneous rock is primarily controlled by the rate at which the magma or lava cools and crystallizes.  Magmas that do not rise to the surface but cool at depth are known as intrusive.  Because they are surrounded by heat-insulating rock, they crystallize slowly, and consequently have the opportunity to grow large crystals.  At the other extreme, liquids that are quenched (nearly instantaneous cooling) have no time or opportunity to grow minerals at all, and thus freeze into a glass.  Lavas that erupt on the surface are known as extrusive.  These lavas cool relatively quickly, but do crystallize minerals.  These, however, are often too small to be seen with the naked eye.

By combining composition and grain size, we arrive at the following classification for the igneous rocks:
 

Composition	Coarse Grained Intrusive	Fine Grained Extrusive
Sialic	granite	rhyolite
Intermediate	diorite	andesite
Mafic	gabbro	basalt
Ultramafic	peridotite

Additional characteristics of these rocks are as follows:
 

	Basalt/Gabbro	Andesite/Diorite	Granite/Rhyolite
Minerals	olivine	pyroxene	muscovite & biotite
	pyroxene	amphibole	quartz
	Ca-rich plagioclase	intermediate plagioclase	Na-rich plagioclase
Color Index	50-60	40	<30
Eruption Temperature	1100 - 1200	1000 - 1100	800 - 1000
Wt. % SiO2	45 - 53	53 - 60	65 - 75
Volatile content	low	intermediate	high

Color index is simply the volume percentage of dark-colored (mafic) minerals.  Basalts, for example typically contain approximately 60 volume % olivine and pyroxene and 40% plagioclase.

Temperatures and compositions are shown, not because you should memorize them, but in order to demonstrate primary controls on a magma's or lava's viscosity.  For all silicate liquids, viscosity is primarily controlled by temperature and SiO₂ .  The higher the temperature, the lower the viscosity, and the more fluid the magma is.  In contrast, the higher the SiO₂-content, the higher the viscosity.  This is a result of the fact that liquids with higher SiO₂-contents will be more polymerized and consequently more "sticky".  Note that these two parameters combine to make mafic magmas/lavas very fluid, and sialic magmas/lavas very viscous.  Consequently, mafic magmas almost always are capable of rising through the crust to erupt at the surface.  Hence, basalts are very common, but gabbros are rare. At the other extreme, sialic magmas are very viscous (sticky) and have difficulty rising through the crust to erupt at the surface.  Hence granites are very common and rhyolites more rare.

Viscosity combines with the volatile content to largely control the style and explosivity of volcanic eruptions.  Low viscosity liquids allow volatiles to separate from the liquid easily.  On the other hand, high viscosity liquids hinder the separation of a gas phase and consequently allow gas pressure to build up.  At one extreme, basaltic eruptions are typically not explosive, because of the low viscosity and low volatile contents.  Sialic eruptions, however, can be extremely explosive due to the build-up of very high gas pressures prior to the eruption.

Origin of the Three Primary Magma Types
Basalt.  We have seen previously that basaltic rocks form the bulk of the oceanic crust.  Basaltic lavas are erupted at mid-ocean ridges and subsequently move away from the ridge by sea floor spreading.  The source for the magma is partial melting of peridotite in the asthenosphere.  Upwelling convection currents in the asthenosphere bring hot, partially melted peridotite close to the surface.  Typical peridotite, if partially melted about 5% yields a liquid with the composition of basalt.

Granite.  Sialic magmas such as granite/rhyolite cannot form by melting of the mantle.  They require melting of a parent material considerably more rich in Si and Al.  Such parent material is only found in the continental crust.  Partial melting of the continental crust is therefore the source for sialic magmas.
Melting of the crust, however, is not completely straightforward.  As illustrated in the figure to the left, temperatures required for melting under dry conditions exceed those attained in normal continental crust.  Note that even under high heat-flow conditions typical of the ocean basins, temperatures would not be high enough to melt dry material until depths in excess of 100 km -- about three times the thickness of normal continental crust.  The disparity is worse for normal continental geothermal gradients.  Note, however, that the situation is drastically different if H₂O is present. H2O is a very effective fluxing agent for rock melting and when present significantly lowers the melting temperature of crustal materials. Consequently, whereas dry melting in unlikely in the continental crust, "wet" melting is very possible.

Andesite.  This rock type takes its name from the lavas erupted from volcanoes in the Andes.  The rock is not, however, geographically restricted to the Andes, but is characteristic of all subduction-zone-related volcanoes, including those that make up the Cascades.

The close relationship to the process of subduction and the generation of andesititic magma implies a direct genetic link.  As a subducting slab descends to progressive deeper levels in the mantle, it heats up, is metamorphosed and it releases volatiles such as H₂O.  There currently is some debate about whether or not the subducting slab actually melts to form andesite magma, or whether volatiles given off the slab flux the overlying mantle wedge and cause it to melt.  Regardless of the details, it is clear that melting of either the slab or the overlying mantle as subduction proceeds produces andesite that rises to the surface to form chains of volcanoes inboard from the trench.  In the Cascades, these volcanoes extend from Mt. Garibaldi in British Columbia, south through Washington and Oregon and extend into northern California to Mt. Lassen.

In addition to some uncertainty as to the details of the partial melting process (slab vs overlying mantle wedge) the evolution of andesite magma is complicated by interaction with the crust as the magma migrates upward.  The figure to the left illustrates the commonly observed complexity of the crust below old (now eroded) subduction-related volcanoes.  Geologic features include complexly deformed metamorphic rocks, numerous intrusions of diorite batholiths and abundant evidence for interaction of andesitic magma with crustal rocks.
 

 Occurrence of the Three Primary Magma Types
Basalt.  In oceanic terranes, basaltic volcanism is dominant and frequent occurrence.  Eruption of submarine basalt along the mid-ocean ridges is an on-going phenomenon, but seldom observed except at places where the ridge rises above sea level (e.g., Iceland).  In addition, hot-spot activity can result in spectacular examples of basaltic volcanism on large volcanic islands such as Hawaii.  Owing to the low viscosity and low volatile content, eruptions are non-explosive and characterized by rapidly moving, long-traveled lava flows.

Although less common, significant basaltic eruptions also occur on the continents.  One of the most spectacular examples is that of the Columbia River basalts located in the Pacific Northwest.  These basaltic lava flows erupted from fissure vents near the Idaho-Oregon border between 17 and 6 Ma, and flowed mainly westward.  311 individual flows have been recognized.  The total thickness is more than 3 miles and the total volume is estimated at 42,000 cubic miles.  To give some idea of how fluid these flows were, some traveled more than 200 miles (all the way to the Pacific Ocean) and speeds have been estimated at approximately 35 mph.
 
 

Andesite.  As noted earlier, these rocks form above active subduction zones on both continental and oceanic plates.  Eruptions typically result in the construction of large, steep-sided, composite or strato volcanoes made up of alternating lava flows and pyroclastic (ash) deposits.  As documented by the 1980 eruption of Mt. Saint Helens, these volcanoes can be very explosive.

As explosive as Mt. Saint Helens seemed at the time of the 1980 eruption, it was actually a relatively small eruption compared to other Andesite volcanoes such as Mt. Pinatubo, or closer to home the devastating eruption of Mt. Mazama which occurred 6,600 years ago.  For comparison, the St. Helens eruption released between 1 and 2 cubic kilometers of material in the form of volcanic ash.  Mazama, released approximately 75 km3!  The eruption was sufficiently explosive to send ash as far north as central Alberta and as far east as central Wyoming.  The eruption so shattered the volcano that when the underlying magma chamber was drained the remains of the volcano collapsed into the void created in the chamber, thus forming Crater Lake.
 

Granite.  The high viscosity of sialic magma makes it difficult for such magmas to reach the surface.  Consequently, rocks in this compositional range are most commonly found as intrusive varieties.  Large granitic batholiths are typically found in tectonic regions undergoing mountain building and metamorphism.  Often, these intrusions result in very large batholiths that comprise a significant portion of the local continental crust.  In western North America, for example, granitic batholiths include the Sierra Nevada Batholith, the Idaho Batholith, and the British Columbia Batholith.

Rhyolite.  Although much less common than granite, rhyolite eruptions are important because they are by far the most explosive of all volcanic phenomena.  The high viscosity and the high volatile content combine to result in the build-up of enormous pressure as gasses separate from the silicate liquid.  In addition to being very explosive, these eruptions produce enormous quantities of ash.  The following table compares some explosive eruptions:
 
 
 
 
 

Eruption	Age	Volume (km3)
Mt. Saint Helens (andesite)	1980 A.D.	1-2
Mt. Mazama (andesite)	7 Ka	75
Mesa Falls (rhyolite)	1-2 Ma	280
Lava Creek (rhyolite)	0.6 Ma	1000
Huckleberry Ridge (rhyolite)	2 Ma	2500

The final three entries in this table are rhyolitic eruptions associated with the Snake River - Yellowstone hot spot.  Volcanic ash from the Huckleberry Ridge eruption is recognized as far north as Saskatchewan, south into northern Mexico, and as far east as New Orleans.  This was truly a major eruption.