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Chapter 5 - Igneous Rocks and Processes

Introduction to Igneous Rocks

Rocks formed from cooling melted (molten) earth materials are called igneous rocks (derived from the Latin word ignis meaning fire). A unique variety of gems occur in or in association with igneous rocks. There are a multitude of different physical and chemical conditions that lead to the formation of specific gems. Understanding these conditions are important to understanding where they form and where they can be found today. Some gems formed in these rocks crystallize as the molten rock cools. Others form in association with hot fluids (water, carbon dioxide, and other gases) that separate and escape from cooling magma. Others form in open spaces (vugs) within igneous rocks long after the rocks have cooled. Igneous rocks are classified by their unique properties and characteristics which are related to composition of their host melt and the environmental setting where they form, underground or on the surface.

The molten material is called magma below the surface and lava when it flows on the surface. The igneous rocks are primary and formed the crust as the earth cooled starting several billion years ago. Igneous rocks are still being formed today both above and below ground. Igneous rocks exist on all continents and within the ocean basins (Figure 5-1). Their occurrence is most obvious in regions where active volcanoes erupt frequently, but large areas around the world are underlain by ancient igneous rocks, both plutonic (formed underground) and volcanic (formed on the surface)(see Figure 5-2).

Plutonic rocks formed at considerable depth by crystallization of magma and/or by chemical alteration. (The word plutonic is derived from Pluto, the ruler of the Underworld in classic mythology). Plutonic rocks form by plutonism—magma moving, cooling, and crystallizing underground. Rocks formed in association with plutonism are called intrusive igneous rocks. Intrusive igneous rocks form in naturally insulated settings (rock is a poor conductor of heat) so that minerals crystallize slowly, forming large, visible crystals (usually medium- to coarse-grained with a "granitic" texture. Plutons are also called intrusions in that magma intrudes into other pre-existing rocks. Intrusions range is size from small bodies filling fractures to great bodies of igneous rocks or magma underlying whole mountain ranges emplaced in stages over long periods of time.

Volcanic rocks are any rock formed by volcanism. (Vulcan was the god of fire in Roman mythology.) Volcanism is any of various processes and phenomena associated with the surface discharge of molten rock or hot water, steam, and gases this include volcanoes, geysers, hot springs, and fumaroles. Volcanism produces extrusive igneous rocks—rocks formed from rapidly cooling magma (or lava), on or near the surface that crystallizes quickly. Rapid cool preventing visible crystals from forming. Extrusive igneous rocks include lava flows and pyroclastic material such as volcanic ash.
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Map of the world showing the location of volcanoes
Fig. 5-1. Map of geologic provinces of the world. Igneous rocks occur in association with active volcanic areas (shown in red). More ancient igneous rocks are preserved and exposed in shield regions and mountain belts around the world, both on land and within the ocean basins.
features associated with igneous plutons and surrounding host rock
Fig. 5-2. Formation of igneous rock by magmatic intrusions. Features underground are plutonic, above ground are volcanic.
Why do rocks melt?

In most regions, the average temperature increases an average of 20 to 30 degrees C per kilometer with increasing depth in the upper crust. This temperature change, called the geothermal gradient, varies with depth and location, depending on plate-tectonic settings. Regions experiencing plutonism and volcanism are locations where hot material may be located closer to the surface. The interior of the earth is very hot with rocks in the outer core in a molten state. Much of this heat is left over from the time the earth formed in the stellar nebula almost 5 billion years ago. More heat is generated by the decay of radioactive elements, and possibly tidal forces between Earth, moon, sun, and other planets. This heat slowly escapes toward the surface by convection currents in the mantle (Figure 5-3). Rocks will melt (generating magma) if heat flow increases in an area to the point that minerals reach their melting points. Other factors that cause melting include the introduction of volatiles (water and gases) into rocks under pressure, or if there is a decrease in pressure confining hot rocks (such as the release of pressure caused by a great earthquake). Once the heat is released, the melt crystallizes, forming igneous rocks.
Heat convection from the mantle is the source of energy that melts rock near the surface.
Fig. 5-3. Convection in the mantle bring heat to the surface.

Common Minerals In Igneous Rocks

General composition of the Earth's crust: Igneous rocks are mainly composed of silicon and oxygen, the two dominant elements in the Earth's crust and mantle. These two elements bond together making the back bone of a group of minerals called the silicates. Along with silicon and oxygen (these two element make up 76% of the crust), aluminum, iron, calcium, sodium, magnesium, and calcium make up about 99% of the earth’s crust by weight (see Figure 5-4). So in a melt, these elements usually predominate and make up most of the rock forming minerals (a dozen or so common minerals—see Figures 5-5 to 5-9).

Common igneous minerals combine to form igneous rocks.
Composition of the Earth's crust
Fig. 5-4. Elemental composition of the Earth's crust; oxygen and silicon are most abundant elements.
Common Igneous Rock-Forming Minerals Are Silicates
Ultramafic silicate minerals, olivine and pyroxene Mafic minerals, amphibole and biotite mica Felsic minerals, quartz and muscovite Feldspars, plagioclase Feldspar minerals: orthoclase, microcline
Fig. 5-5. Olivine and Pyroxene Fig. 5-6. Amphibole and Biotite (Mica) Fig. 5-7. Quartz and Muscovite (Mica) Fig. 5-8. Plagioclase Feldspars Fig. 5-9. Orthoclase Feldspar

Classification of Igneous Rocks

For geologist to have conversations about rocks special vocabulary and descriptive terminology has been developed. Igneous rocks are named based on combinations of their mineral composition, crystalline sizes, and other textural characteristics. Most important is their mineral composition. Figure 5-10 is a simplified classification of igneous rocks that show the name of igneous rocks based on their mineral composition, based on their content of the common rock forming minerals (illustrated above). The general classes of igneous rocks are named by the percentage of dominant minerals they contain. They are also named based on whether they are intrusive or extrusive where they formed.

For instance, based on the color chart in Figure 5-10, for a rock to be called "granite" or a "rhyolite" it must contain in the general range of about 20% to 75% orthoclase (potassium feldspar), 10% to 40% quartz, 5% to 20% plagioclase (sodium- and/or calcium- feldspar), and traces of muscovite, biotite, and amphibole.

Another example, for an extrusive igneous rock to be called a "basalt" it should have little or no quartz or orthoclase (potassium feldspar), but it can have a wide range of plagioclase, ranging from about 75% to 0%. Most basalts contain a significant amount of pyroxene, but may or may not contain any olivine!

The intrusive igneous rock called "dunite" is composed of a mostly of olivine, but may have some pyroxene as well.
types of igneous rocks
Fig. 5-10. Classification of
Igneous Rocks

Note that this is a simplified version; there are additional names for igneous rocks that are defined with more specific ranges in mineral composition and texture characteristics.
 
Naming igneous rocks based on mineral composition alone is not easy!

Although the primary means of naming an igneous rock is to determine the mineral composition, this is only possible if the mineral grains are actually large enough to see!

Coarse-grained igneous rocks have interlocking crystals visible to the unaided eye are described as having a phaneritic texture (see the example granite in Figure 5-11). Conversely, dense, homogeneous igneous rock with mineral constituents so fine grained that they cannot be seen by the naked eye are described as having an aphanitic texture (see the example in Figure 5-12).

In general, intrusive igneous rocks have a phaneritic texture. Intrusive igneous rocks form in naturally insulated settings (rock is a poor conductor of heat) so that deep underground minerals tend to crystallize slowly, forming large, visible crystals.

Likewise, extrusive igneous rocks tend to have an aphanitic texture. Extrusive igneous rocks form from rapidly cooling magma, on or near the surface. The minerals crystallize quickly, preventing visible crystals from forming.
Granite is an intrusive igneous rock with a phaneritic texture extrusive rock texture: rhyolite
Fig. 5-11. Phaneritic texture of the intrusive igneous rock: granite Fig. 5-12. Aphanitic texture of the extrusive igneous rock: rhyolite
Both rocks shown above have the same mineral (and chemical) composition, but the different textures are due to the rate of cooling of the molten material. The granite cooled slowly underground, whereas the rhyolite cooled quickly on the earth surface, such as in a volcano or lava flow.

Using color to name fine-grained volcanic rocks

For simplicity, aphanitic igneous rocks (generally meaning volcanic rocks) are roughly classified by color into three rock types. Rhyolite is "light colored." Basalt is "dark colored." And, andesite is "intermediate" grays or mid-tone colors (Figure 5-13). Volcanoes in different regions produce large quantities of on kind of volcanic rock (but not always). The composition of magmas associated with volcanic areas can change over time. As magma cools, minerals that crystallize at higher temperatures settle out and turn to stone. Igneous rocks enriched in "high temperature minerals" tend to be dark in color whereas the rocks enriched in "low temperature minerals" tend to be light in color.
volcanic rocks
Fig. 5-13. Color of volcanic rocks.

Four General Classes of Rocks: Felsic, Intermediate, Mafic, and Ultramafic

Minerals, rocks and the magmas they form from are also grouped by general compositions: felsic, intermediate, mafic, and ultramafic.
These four classes are useful for describing geologic processes, mineral and rock composition on many scales. These terms are also important for describing geologic settings of igneous rocks (including the gem deposits they contain). See Figures 5-10 where these terms are related to rock and mineral compositions.
Felsic Igneous Rocks

Felsic applies to minerals of silica and aluminum-rich composition, and the rocks that form from them. a term used to describe molten material (magma), minerals, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminum, sodium, and potassium. The term combines part of the words "FELdspar" and "SILica". Most felsic minerals are light in color and have a density less than 3 grams or cubic centimeter (g/cm3) . Felsic minerals include quartz, feldspars (orthoclase and Na-plagioclase), and muscovite. Felsic rocks are most abundant in continental settings.

granite—a common, coarse-grained (crystalline), light-colored, hard plutonic (intrusive igneous) rock consisting chiefly of quartz, orthoclase or microcline (feldspars), and mica.

rhyolite—a pale fine-grained volcanic (extrusive igneous) rock of granitic composition.
Granite Mountains, Mojave National Preserve Rhyolite exposed in Grand Canyon of the Yellowstone
Fig. 5-14. Massive granite outcrops in the City of Rocks National Preserve, Idaho. These dome-shaped features represent numerous individual plutons formed in association the Cassia Batholith in southern Idaho. Fig. 5-15. Rhyolite exposed in the Grand Canyon of the Yellowstone in Yellowstone National Park. These rocks formed from massive volcanic eruptions that created the Yellowstone landscape.
Intermediate Igneous Rocks

intermediate class applies to rocks that are intermediate in composition between "felsic" (see above) and "mafic" (see below). Rocks of intermediate composition are most abundant in belts of volcanoes (volcanic arcs) associated with subduction zones, past and present.

diorite—a crystalline intrusive igneous rock intermediate in composition between granite and gabbro, consisting essentially of plagioclase and hornblende or other mafic minerals; having a "salt and pepper"-like appearance. Granodiorite is intermediate between granite and diorite.

andesite—A fine-grained, brown or grayish volcanic rock that is intermediate in composition between rhyolite and basalt, dominantly composed of plagioclase feldspar.
Diorite Eruption of Mount St. Helens, May 18, 1980
Fig. 5-16. Diorite has a "salt and pepper" appearance due to relatively even mix of felsic and mafic minerals. Fig. 5-17. Much of the Cascade Range, a volcanic arc extending from California to Canada (including Mount St. Helens shown here) has andesitic composition.
Mafic Igneous Rocks

Mafic is an adjective describing a molten material (magma), mineral, or rock which are enriched in magnesium and iron; the term combines part of the words "MAGnesium" and "FErric" (ferric iron are compounds with the Fe+3 ion). Most mafic minerals are dark in color and the relative density is greater than 3 grams or cubic centimeter (g/cm3). Mafic rocks occur in many geologic setting where hot magma from the mantle and lower crust migrate toward the surface, forming rocks in ocean crust and continental rift settings.

gabbro—dark-colored, crystalline intrusive igneous rock composed principally of calcic-plagioclase minerals (labradorite or bytonite) and mafic minerals: pyroxenes, amphiboles, and with or without olivine and biotite. It is the approximate intrusive equivalent of basalt.


basalt
— A dark-colored igneous rock, commonly extrusive (from volcanic eruptions) and composed primarily of the minerals of calcic plagioclase and pyroxene, and sometimes olivine. Basalt is the fine-grained equivalent of gabbro. However, (see below).
Black Canyon of the Gunnison Sunset crater, Arizona
Fig. 5-18. Gabbro cliffs in the Black Canyon of the Gunnison, Colorado Fig. 5-19. Young basalt lava flows at Sunset Crater Volcano near Flagstaff, Arizona
lava flows on the Columbia River Plateau, Washington Fig. 5-20. A stacked series of basalt lava flows exposed in central Washington. Massive volcanic erupts produced lava flows that covered large regions in Washington, Oregon and Idaho in Miocene time.
Ultramafic Igneous Rocks

The term ultramafic applies to a rock composed chiefly of mafic minerals (rich in iron and magnesium), and less than about 45 percent silica, such as the minerals olivine and pyroxene. Some basalt rocks, such as those on Hawaii, have an ultramafic composition. The Earth's mantle has an ultramafic composition.

dunite—a dense, coarse-grained plutonic rock containing a large amount of olivine, considered to be the main constituent of the earth's mantle.

pyroxenite—a dark gray or greenish, granular intrusive igneous rock consisting chiefly of pyroxenes and olivine; a dominant rock type found in intrusive igneous rocks associated with oceanic crust.
Pyroxenite from Monterey County, CA dunite from Hawaii
Fig. 5-21. Pyroxenite (ultramafic rock consisting of pyroxene and olivine) from Monterey County, California Fig. 5-22. Dunite from Hawaii, this variety called peridotite because of green color of the olivine crystals

Minerals Crystallize At Different Temperatures

The role of temperature in the formation of rocks is important to many aspects to geology, ranging from small scale aspects of gem crystal formation to large-scale dynamic processes associated to plate tectonics. Minerals melt and form at different temperatures. Bowen's Reaction Series (named after Canadian petrologist, Norman L. Bowen, 1997-1956) explains how different common igneous minerals crystallize from molten material at different temperatures (Figure 5-23). As crystals form the chemistry of the remaining melt changes in composition. Magmatic differentiation involves processes by which chemically different igneous rocks, such as basalt and granite, can form from the same initial magma (Figure 5-24).

When a very hot magma cools high-temperature minerals like olivine and Ca-rich feldspar crystallize first. The mineral crystals that form are denser than the magma and can settle out of the melt. Over time, as a magma body continues to cool, it causes the remaining molten material to be concentrated with components that may later form rock enriched in low temperature minerals (such as granite). The last rocks to crystallize in a magmatic intrusion will be enriched in low temperature minerals (quartz, micas, and potassium- and sodium- feldspars).

High-Temperature vs. Low-Temperature Minerals (and the Magmas and Associated Rocks They Form)

Felsic minerals (and rocks) melt at lower temperatures than mafic minerals (and rocks). Bowen's Reaction Series demonstrates that as a silicate-rich melt cools, minerals that form at higher temperatures will crystallize first. As these minerals crystallize, the chemistry of the remaining melt will change becoming more felsic in composition as it cools. Fluids, such as gases and water, are concentrated in the remnants of a melt. It is in these late phase melts where many precious gems can form.

The Role of Magmatic Differentiation in Plate Tectonics

Likewise, as rocks heat up, low temperature minerals melt first. This happens where oceanic crust descends into subduction zones (Figure 5-25). Once enough material melts, it can separate and migrate, forming magma bodies. Because molten material is less dense than rock, gravity will allow it to migrate toward the surface. Magma bodies associated with spreading centers along mid-ocean ridges and hotspots like Hawaii are very hot (in the range of 1200° Celsius). At these high temperatures even ultramafic minerals including olivine and pyroxene will remain molten. Because these magmas are derived from the mantle, the rocks they form have a mafic or ultramafic composition. These rocks become ocean crust.

The magmas associated with subduction zones only range in temperature of about 900° to 700° Celsius. As rocks sink into a subduction zone, felsic minerals melt first. The magmas that form become concentrated with fluids (remnants of seawater, organic compounds) that were trapped in the sinking rocks. As these magmas form, the lighter felsic material separates from the denser, high temperature minerals which concentrate and sink back into the mantle. As a result, the magma bodies that form in association with subduction zone-volcanic arc setting are enriched in materials that form rocks intermediate to felsic.

Over millions of years, subduction zones produce large quantities of igneous rock of intermediate to felsic in composition. These rocks ultimately become incorporated into continental crust. The average density of continental crust is 2.7grams/cm3 whereas oceanic crust has the average density of 3.5 grams/cm3. This explains why continents isostatically rise above ocean basin and why the oldest rocks known accumulate in continental crust (because they are too light to sink back into the mantle).
Bowens Reaction Series
Fig. 5-23. Bowen's Reaction Series explains how minerals crystallize at different temperatures as a magma body cools.High temperature minerals cool first, low temperature minerals cool last.
volcanic rocks
Fig. 5-24. Magmatic differentiation occurs as minerals of different crystallization temperatures and density separate from a cooling magma body.
Igneous settings and temperatures of lavas
Fig. 5-25. The magmas associated with the formation of new ocean crust are much hotter than magmas associated with subduction zones and volcanic arcs.
 

Igneous rock with Unique Characteristics and Origins

Gems associated with igneous rocks form in a variety of ways. Some gems (like diamonds) form deep in the crust or upper mantle and are carried upward with material migrating toward the surface. Others form in magma bodies and settle into pockets. Still others form at the tail end of the voyage of a magma body where fluids (water and gasses) trapped in the magma help crystals of unique compositions to form. Still others are associated with the mineral-rich fluids escaping from a cooling magma chamber.

A xenolith is a rock fragment foreign to the igneous mass in which it occurs. Xenoliths are commonly composed of rock derived from the sides or roof of a magma chamber. The rocks sink into the magma chamber but escape melting as the magma cools to stone. In some cases, they are materials carried up from great depths. In some cases, they most abundant in the last stages of a volcanic eruption—the last magma in to migrate to the surface is from the deepest source of the magma.

A phenocryst is a large or conspicuous crystal in a volcanic or igneous rock, distinct from a more fine-grained groundmass (mineral matrix). Phenocrysts (crystals) form in magma at depth before it reaches the surface where the magma (or lava) cools quickly for form the fine-grained groundmass. A rock with abundant phenocrysts is call a porphyry—a hard igneous rock containing visible crystals, usually of feldspar, in a fine-grained (microcrystalline), typically dark gray, reddish, or purplish groundmass (Figures 5-28 to 5-31).

Phenocrysts
and xenoliths carried up from great depths can be of value, consider peridote and diamonds. Both may exist in large quantities in the mantle but are rare on the surface and in the earth’s crust. As well, corundum (ruby and sapphire) and moonstones can be carried up with volcanic eruptions (such as alkali basalt) that flood the earth’s surface with large volumes of volcanic rock making gem-rich regions as in Thailand’s Chantbaburi-Trat area that includes the Hill of Gems (rubies) area, the Pailin ruby and sapphire gem field in Cambodia.

Peridote Mesa near San Carlos, Arizona, and Australia’s sapphire and zircon-rich Ankie and New England districts (Yellow and blue sapphires) have similar origins. The liquid magma acts like an elevator bringing these precious crystals to the surface.

peridotite Xenolith in granite
Fig. 5-26. Xenoliths of green olivine (peridote gem) in matrix of basalt, from Halualai Volcano, Hawaii Fig. 5-27. Dark xenolith of pyroxenite in granite matrix in Joshua Tree National Park, California
porphyritic texture Basalt porphyry with plagioclase phenocrysts
Fig. 5-28. Porphyry is an igneous rock consisting of phenocrysts standing out a fine-grained groundmass. Fig. 5-29. Phenocrysts of white feldspar (plagioclase) crystals in a dark andesite porphyry.
phenocryst Andesite porphyry with a phenocrysts of a variety of feldspars and quartz.
Fig. 5-30. Phenocrysts of pink feldspar (orthoclase) in andesite porphyry. Fig. 5-31. Porphyry with phenocrysts of feldspar, quartz, and topaz.

Continued with Chapter 6.
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