Prof. Dr. Sobhi Nasir>
Department of Earth Science, P.O.Box 36, 123 Al-Khod, Sultan Qaboos university, Sultanate of Oman
Textures of Igneous Rocks
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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.
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
2) the degree to which it is melted.
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:
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).
The minerals crystallizing at the lowest temperatures are rich in silicon and aluminum, and rocks consisting of such minerals are called felsic.
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.
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.
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:
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 magma. Melting of the crust, however, is not completely straightforward. temperatures required for melting under dry conditions exceed those attained in normal continental crust. Note that eunder 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.
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 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.
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. 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.