(a) Texture (coarsely versus finely crystalline rocks)

(b) Chemical Composition (silica content)

(c) Mineral Content: (proportion of lighter to darker minerals)

Texture determines whether an igneous rock is defined as Intrusive (coarsely crystalline) or Extrusive (finely crystalline).

1. Fig. 3.2: Intrusive igneous rocks form at some depth within the crust due to slow cooling of magma, resulting in formation of large crystals. Individual crystals can be seen with the naked eye. These rocks are termed texturally as coarsely crystalline or phaneritic.

2. Intrusive igneous rocks are also called plutonic rocks which is more of a genetic term.

1. Fig. 3.2: Extrusive (volcanic) igneous rocks crystallize rapidly from lava erupted onto the surface. Individual crystals usually cannot be seen by the naked eye and require a microscope. These rocks are termed finely crystalline or aphanitic and generally crystallize from lava flows.

2. Fig. 4.4: A porphyry is predominantly a finely crystalline (aphanitic) rock containing some large crystals called phenocrysts. Phenocrysts are typically plagioclase although olivine, pyroxene and amphibole can also occur as large crystals.

3. The large crystals in a porphyry probably formed while the magma was still below the Earth’s surface, whereas the finely crystalline matrix formed quickly after the remaining lava with large crystals was extruded and rapidly cooled.

4. Fig. 3.2: Volcanic rocks can also be formed from broken pieces of lava, glass and mineral grains that were thrown high in the air during explosive eruptions. These Pyroclasts (Fig. 4.3) initially rain down from the sky and accumulate on the ground as layers of loose material. Eventually these pyroclasts become lithified to form tuff (Fig. 5.23) or volcanic breccia (Fig. 5.8).

5. Fig. 4.3: Volcanic glass forms from rapid quenching of lava which occurs so quickly that no crystals have time to form. One common volcanic glass is pumice that appears as a frothy mass riddled with holes and formed by the quenching of melts rich in gas bubbles. Melts without any gases are quenched to form a dense, black rock called obsidian.

(a) Coarsely Crystalline (phaneritic)

(b) Finely Crystalline (aphanitic)

(c) Porphyritic

(d) Pyroclastic

(e) Glass

Silica content can be used to classify igneous rocks. Most igneous rocks fall within the range of 40 to 70 % SiO2 by weight.

1. Table 4.1: Classification of igneous rocks also takes into account the proportions of various minerals.

(a) Mafic Minerals: Dark minerals consisting of olivine, pyroxene, amphiboles and biotite mica. These silicates are rich in Fe, Mg and Ca.

(b) Felsic Minerals: Light-colored minerals consisting of quartz, K-feldspar, plagioclase and muscovite mica. These silicates are rich in Si, Na and K.

2. Fig. 4.6 Classification Diagram:

(a) Felsic Rocks: Granite and Rhyolite

(b) Intermediate Rocks: Andesite and Diorite.

(c) Mafic Rocks: Basalt and Gabbro

(d) Ultramafic Rocks: Peridotite.

1. Rocks are composed of a mixture of minerals. Not all minerals melt at the same temperature. For instance, a peridotite is an ultramafic rock composed of olivine and pyroxene. If we were to gradually raise the temperature of the rock, we’d eventually reach a temperature in which the pyroxenes began to melt, whereas the olivines would remain solid. We would have to raise the rock temperature higher in order to melt the olivine.

2. Overhead of Partial Melting: The phenomenon where certain minerals in a rock melt while others remain solid is called partial melting. The resulting liquid is termed a partial melt.

3. Figure 4.11: The sequence or order in which minerals melt as the temperature of a rock is slowly increased can be predicted using Bowen’s reaction series. Quartz, muscovite and K-feldspars melt at much lower temperatures whereas Olivine and Ca-rich feldspars are the last minerals to melt.

4. The partial melt formed through the melting of certain minerals takes on the composition of the melted minerals. For example, if quartz and K-feldspar preferentially melted, the resulting liquid would have a chemical composition similar to quartz + K-feldspar. The minerals remaining behind would define the composition of the residual solid.

5. In actuality, no rock would contain all the minerals shown in Bowen’s reaction series.

(a) Ultramafic rocks: olivine + pyroxene.

(b) Mafic rocks: olivine, pyroxene, Ca-feldspar + amphiboles

(c) Intermediate rocks: amphibole, biotite, Na-rich feldspar + quartz

(d) Felsic rocks: quartz, K-feldspar, muscovite + biotite

1. Figs. 4.8: Usually melting of rocks occurs within the lower crust or upper mantle depending on the tectonic environment. Mantle plumes arise from melting of rocks deep within the mantle. The composition of the rocks melted, and thus the composition of the magma produced, depends on the tectonic environment.

1. Figs 4.12: After partial melt is formed, the liquid starts migrating through the surrounding rocks towards the surface via pore spaces, boundaries between crystals, and possibly fracture systems. The melts eventually pool in large cavities to form magma chambers.

2. The magma body slowly begins to cool until eventually the first minerals begin to form.

1. Fig. 4.11: Bowen’s reaction series is divided into two branches

(a) Discontinuous Reaction Series: Different minerals crystallize at different temperatures. An earlier formed mineral may react with the magma and be converted to the next mineral in the series (e.g. olivine to pyroxene).

(b) Continuous Reaction Series: Crystals initially formed become compositionally modified through continued reaction with the evolving melt as temperatures drop. Early Ca-rich plagioclase transform to more Na-rich plagioclase through reaction with an increasingly silicic melt as the temperature drops.

2. Both reaction series occur simultaneously in the crystallizing magma chamber.

3. As early-formed crystals remove certain elements from the melt (e.g. Mg, Fe by olivine and pyroxene), the remaining melt becomes enriched in elements left behind (e.g. Si, Na, K, Al). As a result, minerals crystallizing later from the compositionally modified melt will be composed of those elements left behind in the liquid.

4. This process by which an initial (parent) magma of one composition can produce rocks of a variety of compositions via crystallization is called magmatic differentiation. Early formed mafic rocks riched in Mg and Fe give way to intermediate rocks containing less Mg and Fe, but having more Na, Al and Si. Finally at late stages of differentiation, felsic rocks are primarily formed from elements left within the remaining liquid, Si, K, Na, and Al.

2. Some igneous rocks show compositionally zoned crystals where the rims have a more evolved composition relative to the cores. Plagioclase can have Ca-rich cores and Na-rich rims. Olivine may be surrounded by a rim of pyroxene. In both cases, only partial reaction with the melt has taken place.

3. Figure 4.12: Other igneous rocks, however, are only composed of early formed minerals like olivine and Ca-rich plagioclase and show no evidence of reaction with an evolved melt. Somehow, these early formed crystals became separated or isolated from the remaining melt before any reaction could occur.

4. Fractional crystallization is the term used for this separation and removal of successive fractions of crystals formed from a cooling magma. In fractional crystallization, the first crystals of plagioclase or olivine may be removed from further reaction with the magma.

5. This may occur when these early formed crystals are buried by other crystals and thus are no longer in contact with the magma. Alternatively, the remaining liquid may somehow escape the magma chamber, leaving behind only the early-formed crystals.

1. Figure 4.12: Geologists now recognize that magmatic differentiation is far more complex than previously outlined. In addition to simple crystallization, magmas can be compositionally modified through:

(a) Mixing between two magma chambers of different composition to form a mixed or hybrid magma.

(b) Separation of different magma compositions that are immiscible (oil and water).

(c) Partial melting of rocks of different compositions, followed by mixing of melts.

(d) Turbulent mixing of magmas of differing temperatures within a single chamber.

(e) Assimilation of country rock.

(a) The composition of the original rock melted to form the magma.

(b) Crystallization of minerals changes the composition of the remaining liquid.

(c) Composition of any wallrock digested by the magma

(d) Mixing with magma of different composition.

(e) Isolation of newly-formed crystals from further reaction with the magma (fractional versus equilibrium crystallization).

1. Plutons are large igneous bodies that form at depth within the crust due to crystallization of magma. Plutons can range in size from 1 km3 to hundreds of cubic kilometers.

2. Figure 4.14: Magma can force its way towards the surface in three ways:

(a) Wedging open the overlying rock: Overlying rock may explode during this process.

(b) Breaking off large blocks of rock in a process called stoping. Blocks that are broken off sink into the magma, melt, and blend in with the liquid.

(c) Melting surrounding rock: The melted rock mixes with the magma, slightly changing its composition.

1. Figure 4.13: Intrusive igneous bodies have different terms depending on the size of the body, it’s overall geometry and relationship with surrounding rocks.

2. Batholiths are the largest plutons and consist of massive irregular igneous rock covering an area over 100 km2.

3. Intrusions covering less than 100 km2 are called stocks.

4. Batholiths and stocks are irregular masses of igneous rock referred to as discordant intrusions since they cut across layers of country rock.

5. Offshoots of batholiths and stocks are termed either dikes or sills and represent major routes of magma transport within the crust. Dikes are tabular igneous bodies that are discordant with the country rock since they cut across layers of bedding.

6. Sills are concordant tabular bodies in that sills form by injection of magma between parallel layers of country rock.

7. Veins are deposits of coarse minerals found within a rock fracture and can range from a few millimeters to several meters across. Veins can extend lengthwise from several meters to several kilometers long. Pegmatites (Fig. 4.17) are a special type of vein deposit consisting of coarse-grained granite that cuts across much finer-grained country rock.