Geology Cafe

Introduction to Geology

Chapter 7 - Igneous Rocks, Volcanoes and Volcanic Activity

The term "igneous" applies to natural process relating to the formation, movement, and cooling of molten materials—magma (fluid underground) and lava (molten material flowing on the surface). 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. Volcanic activity refers to igneous processes happening on or near the surface. These processes can sometime have catastrophic consequences on populations and property in areas where volcanic activity occurs.
The Earth stores vast quantities of energy in the form of kinetic heat. Portions of the Earth's interior are molten, such as the outer liquid core. However, materials in the mantle are thought to be under too much pressure to melt. However, in many places, heat flow near the Earth's surface is high enough and confining pressure is low enough for rocks to melt. This molten material may find zones of weakness, such as along fault zones, to inject, melt, or under great pressure, find means to migrate to the surface. Where it reaches the surface it produces volcanic eruptions and may flow on the surface in the form of lava flows. Some volcanic eruptions can be catastrophic enough to blow volcanic mountains apart and blanket large regions with volcanic ash and cinders, even causing changes to the global climate. Nearly a quarter of world's human population live in regions that could experience moderate to catastrophic effects of volcanic eruptions.

This chapter examines the character of igneous rocks, including varieties of intrusive or plutonic rocks (where magma crystallizes below the surface) and extrusive or volcanic rocks (where magma reaches the surface and erupts), how and where they occur, and how to recognize landforms and rock types associated with volcanic areas where gemstones may occur.
Click on thumbnail images for a larger view.
The Rock Cycle
Fig. 7-1. Igneous rocks include intrusive (plutonic) [formed underground] and extrusive (volcanic) [formed on the surface].

The word igneous is derived from the Latin word ignus, meaning “fire or fiery.” In geology, the word igneous applies to materials that have solidified from molten rock material, it also applies to the processes associated with the movement of molten material underground or erupting or the surface.

Rock in its molten state

Magma is hot, fluid or semi-molten rock material underground (within the earth's crust). Magma that reaches and erupts on the land's surface is called lava. Lava is molten rock flowing on the land's surface.

Rocks formed from molten material

Rocks formed from magma cooling and crystallizing underground 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.

Rocks formed from cooling lava on or very near the surface are called extrusive igneous rocks. When lava cools rapidly it crystallizes quickly, preventing visible crystals from forming. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash, cinders, etc.

refers magma moving, cooling, and crystallizing underground. A pluton is a body of igneous rock formed underground.
Plutonic rock
is a rock formed at considerable depth by crystallization of magma and/or by chemical alteration. It is usually medium- to coarse-grained with a granitic (phaneritic) texture.

Volcanism is any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam, including volcanoes, geysers, and fumaroles. Volcanic rock is any rock formed by volcanism.
What causes to rocks melt?

The heat released by volcanoes and volcanic activity comes from heat convection from the Earth's core and mantle.

geothermal gradient—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 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.

melting—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).

degassing—the process of separation of volatile gases (H2O, CO2, SO2, etc.) and steam from molten material (magma or lava).

Magma of granitic composition can have as much at 5 percent water dissolved in it. The release and expansion of gases is the driving force in volcanic explosions and eruptions (Figures 7-3 and 7-4).

Where do volcanic gases come from? Some of it is new (from the mantle such as at divergent zone) and some of it is recycled from older crust sinking into a subduction zone: Studies of carbon isotopes (C13/C12) show that carbon emitted from Cascades volcanoes has essentially the same isotopic signature as carbon in sediments deposited in the ocean basin trenches offshore of Oregon and Washington. Figure 7-5 shows the composition of volcanic gases from different geologic settings.
Convection heat flow in through the mantle Volcanic gases
Fig. 7-2. Heat convection through the mantle is the source of heat driving volcanic activity. Current thought is that most of the heat in the Earth is remnant for its period of formation, but decay of radioactive matter and tidal forces may contribute heat as well. Fig. 7-3. Why do volcanoes erupt? Because magma contains large amount of dissolved gases! When magma containing dissolved water and gases is released in a volcanic eruption it expands hundreds of times in volume creating ash-filled clouds.
Volcanic gases Volcanic gases
Fig. 7-4. Gases from volcanic eruptions Fig. 7-5. Volcanic gases from different plate-tectonic settings

How hot is magma/lava?

Lava/Rock composition (place) Geologic setting Temperature (C)
sample (estimate range)
Temperature (F)
sample (estimate range)
Type of volcanic eruptions
(Kilauea, Hawaii):
hotspot - shield volcano 1170 (1000-1250)
2138 (1832 to 2282) hot, fluid eruptions with extensive lava flows, rare explosive eruptions
(Erta Ale, Ethiopia)
divergent zone - rift zone - shield volcano 1130 2066 hot, fluid eruptions with extensive lava flows, rare explosive eruptions
(Mount Pelee, Martinique)
convergent zone - arc - composite cone 1075 1967 explosive eruptions and pyroclastic flows
(Lascar volcano, Chile)
convergent zone - arc - composite cone 1069 (950-1200) 1956 (1742 to 2192) both explosive eruptions and lava flows
(Mototombo, Nicaragua)
convergent zone- arc - composite cone (800 to 1100) (1472 to 2012) both explosive eruptions and lava flows, lava domes, pyroclastic flows
(Mount St. Helens, Washington)
convergent zone- arc - composite cone (750 to 850) (1382 to 1562) both explosive eruptions and lava flows, lava domes, pyroclastic flows
Rhyolite (Yellowstone National Park) continental hotspot (700-900) (1292 to 1652) very large, catastrophic explosive eruptions, regional ash fall coverage

General Characteristics of Intrusive and Extrusive 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. Rapidly cooling magma (or lava) near or on the surface, crystallizes quickly, preventing visible crystals from forming. Both rocks shown below have the same mineral (and chemical) composition, but different texture due to the rate of cooling of the molten material.
intrusive rock texture of granite Figure 7-6. Phaneritic texture is a term usually used to refer to igneous rock crystal grain size and texture. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic (which is too small to see with the naked eye). Rocks with phaneritic texture are usually intrusive igneous rocks.
extrusive rock texture: rhyolite
Fig. 7-7. Aphanitic texture applies to dense, homogeneous rock with constituents so fine grained that they cannot be seen by the naked eye.Rock's with aphanitic texture are usually extrusive igneous rocks. This example is rhyolite.
Intrusive igneous rock. This example is granite. extrusive igneous rock.This example is rhyolite.

General Classification of Igneous Rocks

Figure 7-8 illustrates a general classification of igneous rocks. Igneous rocks are named based on combinations of their: 1) mineral composition, 2) crystalline sizes, 3) general color, and 4) textural characteristics.

1) The mineral composition part is based on percentages of common rock-forming minerals of igneous origin and is the most accurate means of identifying an igneous rock. However, this method is difficult for fine-grained rocks without a microscope.

2) Crystal size is subdivided into 2 classes: visible crystalline grains (phaneritic) or grains too small to see with the naked eye (aphanitic)(discussed above with Figures 7-6 and 7-7). Igneous rocks with larger crystals are easiest to identify by mineral composition.

3) Color is an easy and important way to generally classify igneous rocks, especially if mineral composition and crystals are difficult to see (as with most extrusive igneous rocks). The terms felsic, intermediate, and mafic are general terms used to describe and classify fine-grained igneous rocks (Figure 7-9; also see discussion below).

4) Textural characteristics are used to characterize some kinds of igneous rocks, and mostly applies to igneous rocks that form under special conditions. A textural name often relates to how a rock forms. For example, pumice is a volcanic rock that has so many gas bubbles in it that it can float! Obsidian, a rock that is sometimes used in jewelry, has a glass-like texture with a conchoidal fracture.
types of igneous rocks
Fig. 7-8. Classification of
Igneous Rocks
volcanic rocks
Figure 7-9. Color of volcanic rocks. Light colored volcanic rocks are described as felsic; dark colored are mafic, and color shades in the middle are considered intermediate.

How do different igneous rocks form from one original supply of magma?

Igneous rocks form as molten material cools and crystallizes into rock. As the molten material cools, chemical compounds in the melt crystallize into minerals at different temperatures, with "high temperature" minerals crystallizing first. These high temperature minerals are denser than the molten material and tend to settle out in the bottom of a magma chamber (pluton). As the melt continues to cool, the composition of the melt changes as more crystals form and settle out. Finally the melt completely cools with the composition of the rock enriched in low temperature minerals. This process is called magmatic differentiation.

A simple comparison is what happens when seawater freezes. The ice that forms directly from seawater is nearly pure water in composition. As sea ice forms, the remaining seawater becomes enriched in dissolved salts, lowering the freezing temperature of the remaining seawater (concentrated as brine). As seawater freezes and crystalline ice forms, and liquid brine (concentrated salt water) and air are trapped in tiny pore spaces within a matrix of pure ice crystals. With further cooling, solid salt crystals subsequently precipitate in pockets of brine within the ice. The net volume of the ice, volume of brine, and chemical composition of the solid salts are temperature-dependent.
volcanic rocks Magmatic differentiation involves processes by which chemically different igneous rocks, such as basalt and granite, can form from the same initial magma (Figure 7-10). High-temperature minerals can crystallize and settle out, causing the remaining molten material to be concentrated with component 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, mica, and potassium- and sodium- feldspars). Gases and fluids including water, carbon dioxide, nitrogen and other compounds are also dissolved in magma and will be concentrated in the remaining lava before being expelled as the last traces of magma cools into rock.
Fig. 7-10. Different rocks from from one magma by
magmatic differentiation.

High temperature vs. low temperature minerals (and the rocks they form)

Felsic minerals melt at lower temperatures than mafic minerals.. This was demonstrated by the work of "19th century petrologist" Norman Bowen, who showed 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 as it cools, allowing different minerals to form as the melt proceeds cooling. High-temperature minerals like olivine and Ca-rich feldspar cool first, minerals like quartz, K-rich feldspar, and biotite crystallize last (Figure 7-11). In addition, fluids, such as gases and water, are concentrated in the remnants of a melt. This pattern of mineral (and rock) formation is called the Bowen's Reaction Series (Figures 7-12 to 7-14).
volcanic rocks Bowens Reaction Series Bowen's Reaction Series Bowen's reaction series
Fig. 7-11. Low- and high-temperature "common" igneous minerals. Fig. 7-12. Bowen's Reaction Series illustrates the order that minerals form in a cooling melt. Fig. 7-13. Common igneous rock-forming minerals in Bowen's Reaction Series. Fig. 7-14. Igneous rocks formed through Bowen's Reaction Series.

What is the significance of the composition of igneous rocks?

felsic—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)

mafic—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). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt and gabbro.

mafic rocks vs. felsic rocks
—mafic rocks (rich in iron and magnesium) are denser than felsic rocks (rich in silica and aluminum).

ultramafic—a rock composed chiefly of mafic minerals (rich in iron and magnesium, and less than about 45 percent silica, such minerals as olivine, augite, or hypersthene. Peridotite, pyroxenite and serpentinite are rocks with ultramafic rocks. The mantle is ultramafic in composition and has a density of about 3.5 grams or cubic centimeter (g/cm3).

oceanic crust vs. continental crustoceanic crust is dominated by mafic and ultramafic intrusive igneous rocks whereas continental rocks are dominated by granitic (felsic) intrusive igneous rocks. The difference in density has an impact on isostacy of crust floating on the semi-fluid upper mantle (asthenosphere), with continental crust (about 2.7g/cm3) rising or floating above oceanic crust (about 3.5 g/cm3).

Isostacy and the density of crustal rocks
Fig. 7-15. Isostacy and the role of density of oceanic crust and continental crust. Both types of crust "float" on the asthenosphere, but because continental rocks are less dense, they rise above the ocean crust.
Refining minerals in the crust

The Mafic/Felsic Refining Process

Through geologic time, as new ocean crust is forming along spreading centers, older ocean crust is sinking back into the mantle along subduction zones. As the old, denser, "mafic rich" rock sinks, it heats up. First low-temperature minerals begin to melt, assisted by fluids trapped in the rock (including water, carbon dioxide, etc). This new molten material is enriched in felsic minerals. As the process proceeds, the remaining mafic minerals are concentrated in the material sinking back into the mantle, whereas the lighter felsic material rises toward the surface forming plutons that may or may not erupt on the surface as volcanic eruptions. Over time, the continents grow by the accretion of felsic-rich rocks (Figure 7-16).
Fig. 7-16. Refining minerals into the crust through subduction, plutonism and volcanism. Over time, felsic materials accumulate in continental crust.

How do granitic (felsic), andesitic (intermediate), and basaltic (mafic) rocks form, and where do they occur?

Granite Mountains, Mojave National Preserve Granitic Rocks (mostly felsic minerals)(continental land masses)

granite—a common, coarse-grained (crystalline), light-colored, hard plutonic (intrusive igneous) rock consisting chiefly of quartz, orthoclase or microcline (feldspars), and mica. Granite is found in plutonic rocks that have been exposed by erosion. In North America, granite is abundant in the core of mountain ranges exposed throughout the Rocky Mountain region and the Canadian Shield.

rhyolite—a pale fine-grained volcanic (extrusive igneous) rock of granitic composition. Rhyolite is common in continental volcanic regions with notable deposits around Yellowstone and volcanic centers throughout the Great Basin region extending from Nevada to New Mexico.

Rhyolite exposed in Grand Canyon of the Yellowstone
Fig. 7-17. Granitic rocks exposed in the Granite Mountains, Mojave National Preserve, CA Fig. 7-18. Rhyolite exposed in the Grand Canyon of the Yellowstone, WY

Granodiorite exposed in Yosemite NPFig. 7-19. Granodiorite exposed in the Sierra Nevada (Yosemite NP) Dacitic Rocks (continental margin volcanic arcs)

—a coarse-grained (crystalline) plutonic igneous containing quartz and plagioclase, intermediate between granite and diorite in composition. The rocks exposed in the Sierra Nevada Range (including Yosemite National Park) are mostly granodiorite.

—an extrusive igneous (volcanic) rock with an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. Lassen Volcano in northern California and many volcanoes in the Cascade Range are dacitic in composition

Chaos Crags, Lassen Volcano Fig. 7-20. Chaos Crags on Lassen Volcano consists of dacite.

Origin of Diorite
Fig. 7-21. Origin of diorite (a plutonic rock).
If magma of the same composition erupts on the surface it will form andesite.
Andesitic Rocks (oceanic Island Arc volcanics, subduction zones)

—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. Diorite is found in mountain ranges throughout the Pacific Northwest

andesite—A fine-grained, brown or grayish volcanic rock that is intermediate in composition between rhyolite and basalt, dominantly composed of plagioclase feldspar. Andesite is the most abundant rock found in the volcanic rocks of the Cascade Range extending from northern California into British Columbia. Andesite is the common rock found in volcanic arc (island chains) throughout the Pacific "Ring of Fire."

Eruption of Mount St. Helens, May 18, 1980
Fig. 7-22.
Mount St. Helens has an andesite composition. This image shows the massive eruption of May 18, 1980. The volcano continues to erupt intermittently.

Black Canyon of the Gunnison
Fig. 7-23.
Gabbro cliffs in the Black Canyon of the Gunnison, Colorado
Basaltic Rocks (rift zones, hot spots, spreading centers)

—dark-colored, crystalline intrusive igneous rock composed principally of calcic-plagioclase minerals (labradorite or bytonite) and augite, and with or without olivine and orthopyroxene. It is the approximate intrusive equivalent of basalt.
Gabbro is associated with silvers (terranes) of ancient oceanic crustal rocks that are preserved in within continental rocks, locally in California and elsewhere.

— 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. Basalt is associated with areas associated with crustal extension, such as in the Great Basin Region. All volcanic rocks on Hawaii are basalt in composition.

Basalt lava flows in Hawaii Volcanos National Park
Fig. 7-24. Basalt lava flows in Hawaii Volcanoes National Park

Fig. 7-25.
Peridotite from Hawaii. The green mineral is olivine (gem mineral is called peridote)
Ultramafic Intrusive Igneous Rocks (Mantle Rocks)

peridotite—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.

Rocks of ultramafic composition are thought to be very similar to rocks found in the earth's upper mantle. Material of ultramafic composition is carried to the surface in some magmas of deep origin.
Pyroxenite from Monterey County, CA
Fig. 7-26. Pyroxenite from Arroyo Seco Canyon, Monterey Co., CA. The black mineral is pyroxene.

Volcanic rocks with unusual textures

Materials ejected from volcanic eruptions have some unique characteristics. Some are of interest to the gem community, not that they are gems or considered precious, but they can be cut or shaped into interesting variety of uses for jewelry and art. Visitors to Hawaii are introduced to the terms pahoehoe (pronounced "pā-hoi'hoi') and aa (ä’ä) (Figures 6-27 and 6-28). Pahoehoe has a ropey fluid texture formed when hot basaltic lava cools quickly. Aa is lava rock with a rough, blocky surface when a lava flow continues to move slowly as it cools, and congealed rock breaks into rough pieces. People who walk on it barefoot frequently yell "Ah! Ah!" (Old Hawaiian joke).

Vesicular lava rock is any igneous rock that has gas bubbles trapped in a fine-grained volcanic rock. Scoria is volcanic rock with a light, frothy consistency due to the high volume of gas bubbles trapped in the rock as it cools as lava is ejected from a volcano (Figure 7-29). If the rock is so frothy from trapped gas inside that it will float it s called pumice. Huge mats of pumice have been observed floating on the ocean after massive volcanic eruptions.

Tuff is a volcanic rock that contains an abundance of visible fragments of volcanic rock that have been crushed or welded together by the heat released during an explosive volcanic eruption (Figure 7-30).

Flow banded lava rock is a volcanic rock that has a layered appearance due to flowing or stretching (like taffy candy) that formed as the lava was still flowing as it cooled (Figure 7-31).

Obsidian is a dark, glasslike volcanic rock formed by the rapid solidification of lava without crystallization (natural glass)(Figure 7-32). Obsidian breaks with a conchoidal fracture like glass. Bubbles in volcanic rocks can fill with minerals, including gem minerals. Snowflake obsidian is very attractive when tumbled or polished. The snowflakes in the obsidian are crystals (phenocrysts) of feldspar.
Pahoehoe lava Aa lava, Hawaii
Fig. 7-27. Pahoehoe lava has a ropy texture (Hawaii). Figure 7-28. A'a lava has a rough, blocky texture (Hawaii)
scoria volcanic tuff
Fig. 7-29. Scoria or pumice Fig. 7-30. Volcanic tuff
Andesite Obsidian
Fig. 7-31. Flow-banded lava rock display lines where the partly molten lava stretched like pulling taffy. Fig. 7-32. Obsidian (natural glass) is usually black but can occur in a variety of colors (from Glass Mountain, CA)

Special Igneous Rock Features

Xenolith in granite xenolith—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. Xenoliths stand out in appearance from the surrounding magma that cooled into stone around them.
Fig. 7-33. Dark xenolith in granite in Joshua Tree National Park. Xenoliths tend to survive sinking into a pluton (magma chamber) if it is composed mostly of high-temperature mafic minerals.
phenocryst porphyry—A hard igneous rock containing visible crystals, usually of feldspar, in a fine-grained (microcrystalline), typically dark gray, reddish, or purplish groundmass.

phenocryst—a large or conspicuous crystal in a porphyritic 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 matrix.
Fig. 7-34. Feldspar phenocrysts in andesite porphyry Pink feldspar (K-spar) phenocrysts in andesite porphyry.
Pegmatite pegmatite—a coarsely crystalline granite or other igneous rock with crystals several centimeters in length, and sometimes containing rare minerals rich in rare elements such as uranium, tungsten, beryllium and tantalum. Fluids (water, CO2, etc.) dissolved in the magma in the late stages of cooling allow larger crystals to form in pockets within a larger plutonic body.
Fig. 7-35. Pegmatite vein in granite. Many important gem minerals and strategic metal-bearing minerals occur in pegmatites (see discussion below with "Igneous Gems").

Volcanoes and Volcanic Features

Volcanism is any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam and gases, including features including volcanoes, geysers, and fumaroles.

A volcanic cone is a hill or mountain formed by the accumulation of volcanic material around a vent where magma reaches the surface (Figure 7-36). The size and shape of a volcanic cone depends on the volume of material ejected over time, the composition and temperature of the material (lava, rock and gases) vented from the volcano, and the nature of the eruption (explosive or otherwise). There are a variety of different kinds of volcanoes.

As discussed above, different plate-tectonic settings produce different kinds of volcanoes (Figure 7-37). Most volcanoes of the world are associated with plate boundaries, either divergent boundaries (spreading centers) or convergent boundaries (volcanic arcs associated with subduction zones).

Volcanic eruptions release a variety of material including lava, tephra (ash, cinders and rock fragments), gases, and water. Volcanic eruptions can be gentle venting of fluid lava to catastrophic explosions that blow volcanoes apart and scatter ash and material over large areas (Figures 6-38 and 6-39).

In general, very hot magmas produce more gentle eruptions that vent gases continuously where they erupt and pour lava on the surface that can flow under the influence of gravity over long distances (Figures 7-40 and 7-41). Hot magmas occur in association with spreading centers and hotspots. These hot magmas are mafic to ultramafic in composition.

Cooler magmas associated with subduction zones tend to be rich in dissolved water and gases. As this magma approaches the surface, the water and gas trapped in the cooling magma escapes producing great pressure that expands explosively causing tremendous eruptions of clouds of steam and ash. Most of the material falls to the earth in the vicinity of the eruption piling up to build up tall volcanic cones. These eruptions can occur suddenly and be tremendous, blowing entire mountain-size volcanoes apart, only to be rebuilt by later eruptions. Explosive volcanoes tend to be felsic to intermediate in composition.

Features Associated with Volcanoes and Volcanism

Lava flows are lava flowing on the surface under the influence of gravity. The term lava flow also applies to a deposit of volcanic rock formed from lava flowing and cooling on the land's surface (Figures 7-40 to 7-42).

A cinder cone is a cone-shaped hill formed around a volcanic vent by fragments of lava (blocks, cinder, ash) blown out during eruptions (Figure 7-42 and 7-43). Cinder cones and lava flows can form from a vent or series of vents associated with an eruption.

A shield volcano is broad, domed volcano with gently sloping sides, characteristic of the eruption of fluid, basaltic lava. The large volcanoes on Hawaii are shield cones (Figure 7-44). Shield cones are formed from very hot lavas.

A composite cone (also called stratovolcano) is a typically tall and large, steep volcanic cone built up of many layers of both lava and pyroclastics (tephra, pumice, and volcanic ash), often created by a series of cyclic eruptions in which pyroclastics are created by explosive eruptions until the vent is open, then lava flows occur (Figure 7-45). Most large continental volcanic cones are this type.

A dome volcano is a volcano composed of lava domes; a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Lava domes can vary from basalt to rhyolite in composition although most preserved domes tend to have high silica content (Figure 7-46).

A fissure eruption is a volcanic eruptions along rift fault zones that can flood large area with basalt flows (Figure 7-47). In prehistoric times, great fissure eruptions have occurred. Flood basalts are the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Another older name is trap basalt.

A crater is a large bowl-shaped vent or collapsed top of a volcano created by explosive eruptions (Figures 7-48 and 7-50). A crater may also be a large bowl-shaped hole created by an meteor or asteroid impact and explosion.

A caldera is a very large volcanic crater, typically one formed by a major eruption (explosion) or the inward collapse of a volcanic cone following an eruption (Figures 7-49 and 7-51).

When molten rock move through the earth, it melts or forces its way through other existing rocks. As the magma cools underground it forms bodies of igneous rocks which are recognizable when exposed by erosion (or mining). Although active volcanoes may have rich mineral deposits associated with them, they are not places that any safety-conscious person might want to dig a mine! On the other hand, ancient igneous rocks (plutonic and volcanic) have recognizable landforms that can be sites for mineral exploration. Over time, erosion strips away materials on the surface, striping away exposing volcanic and plutonic features that were once deeply buried.

Plutonic Features

A pluton is a body of intrusive igneous rock (plutonic rock) that crystallized from magma slowly cooling below the surface of the Earth. Types of plutons include batholiths, dikes, sills, laccoliths, stocks, and other igneous bodies. A plutonic rock is a rock formed at considerable depth by crystallization of magma and/or by chemical alteration. It is usually medium- to coarse-grained with a granitic (phaneritic) texture. Figure 7-52 shows a pluton exposed by erosion. Figure 7-53 illustrates features associated with modern volcanic and plutonic features and how these ancient volcanic features appear on the landscape after exposed to erosion.

Large Scale Plutonic Features

These features are typically extensive, measurable or mapable in the range of tens to hundreds of miles in size.

A batholith is a great mass of igneous rock, extending to great depths, formed from extensive magmatic intrusions (plutons) over a long period of time and throughout a region, typically associated with volcanic arcs.
For example, the core of the Sierra Nevada Range in California is the exposed remnant of a great batholith (Figure 7-53). Figure 7-54 is a map of California showing the extent of the Sierra Nevada Batholith, the Peninsula Range Batholith, and other large igneous plutonic bodies in California. The Sierra Nevada Range in California is a great batholith exposed by erosion; It is nearly 400 miles long and up to 70 miles wide.. In Cordilleran volcanic arc formed during the Mesozoic Era (~200 to 80 million years ago). Only traces of the volcanic chain that probably existed above the batholith. The volcanic overburden has long since worn away by erosion, exposing the plutons that existed deep beneath the volcanoes.

A volcanic arc is a generally curved or linear belt of volcanoes above a subduction zone, including the volcanic and plutonic rocks formed there. The modern Cascade Mountains and the Aleutian Island of Alaska are examples of volcanic arc. Volcanic arcs exist in locations where batholiths are actively forming deep below. The intrusive igneous rock exposed throughout the Sierra Nevada Range are part of a batholith region that existed along the West Coast of North America during the Mesozoic era. The volcanoes have long since eroded away, exposing the batholith region underneath (Figure 7-54). Figure 7-55 shows a volcanic arc region associated with the Antilles Island Chain in the Caribbean Sea region.

The most extensive regions on earth where volcanism is occurring, but is not seen is along the submarine ocean ridges associated with plate-tectonism spreading centers. Iceland is an exposed portion of the Mid-Altlantic Ridge, and has at least 17 volcanoes showing active or recent activity. Iceland's volcano's produce very hot, basaltic lava eruptions (Figure 7-56).

A hotspot is a place in the upper mantle of the Earth at which extremely hot magma from the lower mantle upwells to melt through the crust (oceanic or continental) usually in the interior of a tectonic plate to form volcanic features on the surface. Examples include the Hawaii Hotspot (Figures 7-57 and 7-68) and Yellowstone Hotspot (Figure 7-59 and Figure 7-60). Map of showing the extent of the caldera in Yellowstone National Park, Wyoming. A massive eruption about 2 million years ago ejected rhyolitic material followed by basalt flows filling in its giant caldera. Valles Volcanic field is also another hotspot north of Albuquerque, New Mexico (see Figure 7-51). Hotspot can be locations of very intense volcanism!

A volcanic field is an area of the Earth's crust that is prone to localized volcanic activity. They may only contain a few volcanoes or they may contain many tens to hundreds of volcanoes, vents and lava flows. Volcanic fields may have hundreds of vents forming volcanoes and lava flows intermittently over long periods of time (field may be active for millions of years). An example is the San Francisco Volcanic Field in northern Arizona (Figure 7-61 and 7-62). Volcanic fields may be associated with plate boundary regions (such parts as an island arc system along subduction zones) or associated with hotspots, or zones of continental rifting.

Intermediate- to small-size igneous features

These are features associated with ancient plutonic activity (and now exposed by erosion of the landscape). Individual plutons commonly have a dome-like shape when exposed by erosion (see Figure 7-53).

A stock is an igneous intrusion having a surface exposure of less than 40 square miles, differing from batholiths only in being smaller (examples: Figures 6-63 and 6-64). Circular or elliptical stocks may have been vents feeding former volcanoes.

A laccolith is a lens-shaped mass of igneous rock that has been intruded between rock layers creating a dome-shape chamber filled with igneous rock (Figure 7-65).

A dike in a vertical or near vertical wall of igneous rock formed where magma squeezed into a fault zone before crystallizing. Dike form in volcanic regions, and often appear as dark castle wall-like features on landscapes where the host rock surrounding the intrusion have eroded away . A sill is a tabular, typically more horizontal than vertical, sheet of intrusive igneous rock that has intruded between layers of older rocks. Dikes and sills can form simultaneously of at different stages in igneous activity in an area (Figure 7-66). Dikes and sills can be small to massive is in size (Figures 7-67 and 7-68).

Volcanic ash beds in John Day Fossil Beds National Monument Ash Beds are layered deposits of materials ejected from volcanic eruptions. Successive volcanic eruptions in a region can accumulate in basins. Individual beds can sometimes be traced over long distances across a region and can range in thickness from very thin to massive, hundred of feet thick.
Fig. 7-69. Massive ash beds exposed in John Day Fossil Beds National Monument, Oregon.
Columnar jointing in Devils Postpile National Monument, California Columnar Jointing forms where magma or lava pooled at or near the surface shrinks as it cools. This shrinkage causes a great amount of stress within the cooling body of material, causing it to fracture in polygonal shaped columns perpendicular to the stress.
Fig. 7-70. Columnar joints exposed in an old lava flow in Devils Postpile National Monument, California
Flood basalts, John Day, Oregon Lava flows are typically interbedded with volcanic ash fall deposits. Canyons carved by erosion expose stacked layers of lava and ash.
Fig. 7-71. Flood basalt layers exposed in John Day, Oregon. Formed from hot, fluid basaltic lava spreading over a large region associated with the Columbia River Basalts.

Features formed from flowing lava

a'a—a lava rock with a blocky, frothy, very rough surface texture formed as a cooling crust on a fluid lava flow (Figure 7-72).

pa'hoe'hoe—textural description basaltic lava forming smooth undulating or ropy masses on the surface of a rapidly cooling lava flow (Figure 7-73).

lava tube—a natural tunnel within a solidified lava flow, formerly occupied by flowing molten lava. (Figures 7-74 and 7-75)

Aa lava, Hawaii Pahoehoe lava
Fig. 7-72. A'a lava flow, Hawaii Fig. 7-73. Pa'hoe'hoe lava, Hawaii
Lava tube, El Malpais National Monument, New Mexico Lava tube, Valentine Cave, Lavabeds National Monument, California
Fig. 7-74. Lava tube entrance
El Malpais NM, New Mexico
Fig. 7-75. Lava tube
Lavabeds NM, California
Types of volcanoes
Fig. 7-36. When magma reaches the surface it becomes lava. Fig. 7-37. Types of volcanoes are based on shape of their cones.
Pu'u'o'o Volcano erupting on Hawaii volcanic
Fig. 7-38. Gas and lava venting at Pu'u'o'o Volcano on Hawaii. Hawaii volcanoes produce some of the hottest lava on Earth. Fig. 7-39. Explosive volcanic eruptions like this one in the Aleutian volcanic chain are associated with relatively "cool" and "wet" magmas.
Lava flowing in Hawaii Volcanoes National Park cone and lava flow of SP Crater, Arizona
Fig. 7-40. Hot flowing lava on Hawaii. The lava behaves predictably enough for visitors to approach close enough to view basalt rocks and landforms forming. Fig. 7-41. Airliner view of SP Crater, a cinder cone and lava flow in NW Arizona. The cinder cone formed around a vent, lava flowed from the base of the cone.
Pu'u'o'o volcano Cinder Cone in the Mojave National Preserve, California
Fig. 7-42. Fountaining lava produces a cinder cone near the vent of a volcano. Lava flows spread away and down hill from the vent. Pu'u'o'o volcano on Hawaii. Fig. 7-43. Cinder Cone
Mojave National Reserve, CA
Formed from gaseous lava eruptions producing a cone composed of ash and cinders (tephra).
Halualai volcano is a shield cone on Hawaii Mt. Shasta
Fig. 7-44. Shield Cone
Halualai Volcano, Hawaii
Gentle slope formed from numerous hot fluid basaltic lava flows over time.
Fig. 7-45. Mt. Shasta is a large and complex stratovolcano located at the southern end of the Cascade range in northern California.
Lava dome in Mount St. Helens crater Fissure eruption
Figure 7-46. A lava dome forming in the crater of Mount St. Helens in Washington.
Figure 7-47. A fissure eruption along a rift zone on Mauna Loa on Hawaii.
Halemaumau Crater on top of Kilauea Volcano on Hawaii Crater Lake in Oregon is a small caldera relative to the massive scale of the one in Yellowstone.
Fig. 7-48. Halemaumau Crater on top of Kilauea Volcano on Hawaii. The crater is one of the vents on the volcano, sometime filling with a lava lake, rising and sinking with different phases of the erupt. Fig. 7-49. Crater Lake in Oregon is a small caldera about 2 miles in diameter. It formed from the eruption of Mt. Mazama about 9000 years ago. It is now hosts the deepest lake in North America.
Mount Rainier, Washington Valles Caldera
Fig. 7-50. View of a crater on top of Mount Rainier, Washington Fig. 7-51. Satellite view of Valles Caldera, New Mexico
Ryans Peak pluton, Joshua Tree National Park, California Modern and ancient volcanoes Fig. 7-53. Igneous features associated with modern and ancient igneous rocks
Fig. 7-52. Granite pluton
Ryans Peak in Joshua Tree National Park, California
Salinian basement
Yosemite plutons
Fig. 7-53. Sierra Batholith includes plutonic rocks exposed in Yosemite National Park, California Fig. 7-54. Batholith regions in California include the Sierra Nevada, Mojave region, and Peninsular Ranges
Caribbean  volcanic arc Volcanism at a mid-ocean ridge exposed in Iceland.
Fig. 7-55. Volcanic arc in the Caribbean region. A magma chamber at depth feeds molten material to volcanoes on the surface. Fig. 7-56. Iceland is an exposed portion of the Mid Altantic Ridge where spreading center volcanism is taking place.
Hawaii map Hawaiian Hotspot Fig. 7-58. Hawaii and the Emperor Seamount Chain.
Fig. 7-57. Hawaii is a volcanic center over an oceanic hotspot. Historic lava flows are shown in red.
The Hawaiian Hotspot is a deep-seated igneous plume rising beneath the overriding Pacific Plate. The hotspot has been active for over 80 million years. This image also shows the chains of volcanoes (island arcs) of the Aleutian Island, Japan, Philippines, and Indonesia.
Yellowstone_hotspot Yellowstone caldera
Fig. 7-59. The Yellowstone Hotspot is a deep-seated igneous plume that has migrated beneath the North American Plate as it has moved westward. Fig. 7-60. Map showing extent of the Yellowstone's "supervolcano" caldera. About 2 million years ago, massive eruptions of rhyolitic material were followed by basalt flows filling in crater.
San Francisco volcanic field Humphreys Peak near Flagstaff Arizona
Fig. 7-61. San Francisco Volcanic Field covers 1,800 square miles in northern Arizona. Eruptions from about 600 vents (forming volcanoes) occurred in the last 7 million years. Fig. 7-62. Humphreys Peak rises above Flagstaff, Arizona. The Mountain Peaks are all part of the San Francisco Volcanic Field, a region that is still considered volcanically active.
Devils tower in Wyoming is a stock Shiprock, a stock with radiating dikes
Fig. 7-63. Devils Tower in Wyoming is an eroded remnant of a stock Fig. 7-64. Shiprock in northwest New Mexico is a stock with radiating dikes.
Bear Butte, South Dakota dikes and sills
Fig. 7-65. Bear Butte in South Dakota is a classic example of a laccolith. Fig. 7-66. dikes (vertical) and sills (horizontal) in Black Canyon, Arizona
Palisades Sill along the Hudson River, New Jersey Palisades sill in the Grand Canyon
Fig. 7-67. The Palisades along the Hudson River in New Jersey (across the river from Manhattan) is a massive sill nearly 50 miles long. Fig. 7-68. Another great sill (also called the Palisades Sill) is exposed along the Colorado River in the Grand Canyon.

Volcanic Hazards

Extremely hot lava tends to release its gases quickly where it erupts, and the extreme heat allows lava to flow long distances, filling in valleys before spreading over the surface. Conversely, lava that is near the point of cooling typically contains a higher percentage of dissolved gases trapped in the molten material. As a result, "cooler" volcanoes tend to release their gases as explosive volcanic eruptions when the molten material reaches the surface.
Volcanic features associated with explosive volcanic eruptions

tephra—rock fragments and particles ejected by a volcanic eruptions; types include ash, lapili (pea-sized fragments), cinders (pebble-sized), blocks (cobble- to boulder-sized), and bombs (drip-shaped blobs of lava ejected that remain liquid on the inside with an outer crust when they land).

pyroclastic flow
—a dense cloud of very hot ash, lava fragments, and gases ejected explosively from a volcano and typically flowing downslope at great speed and with destructive force.

volcanic tuff—a term describing rocks composed of volcanic ejecta, such as broken pieces of volcanic glass, phenocrysts, rock fragments, etc.

—A landslide or mudflow of volcanic materials on the flanks of a volcano. Also the name of the deposit created by a landslide or mudflow on a volcanic landscape.
Parts of a volcano
Fig. 7-76. Parts of a volcano illustrating select features
Pyroclastic flow
Fig. 7-77. Pyroclastic flow descending the Mayon Volcano, Philippines
Cascade eruptions in last 4,000 years
Fig. 7-78. Cascade Range volcanic eruptions of the past 4,000 years
Map of volcanic rocks
Fig. 7-79. Igneous provinces of North America: plutonic and volcanic rocks and volcanic features are found throughout North America. All "recent" activity is in the western US.
Columbia River Basalt region
Fig. 7-80. Columbia River Basalts - a region covered by extensive lava flows from great fissure eruptions.
California Volcanic Rocks
Fig. 7-81
. California's volcanic areas. Many volcanic areas are now within state and national parks.
tephra fall
Fig. 7-82. Major tephra falls map of the Western United States - large regions are blanketed by large eruptions.

Volcanic Hazards

Bad days when you might want to say "I hate when that happens."

Below are some highlights of some of world's worst volcanic disasters. Massive volcanic eruptions are destroyers of civilizations. Volcanic eruptions produce earthquakes, massive hot ash clouds (pyroclastic flows), generate tsunamis, and can even change the climate lasting for decades, and much more (Figure 7-83). Figures 7-84 to 7-90 highlight some the biggest to impact humanity in historic times, and there were probably many much worse in prehistoric times. Some of the most notorious in history include the eruption of Tambora in 1816, Krakatoa Volcano in 1881, and the Mt. Pelee in 1902. Great ones in ancient history including the eruption of Mt. Vesuvius in Italy, and perhaps the greatest, the great eruption on Thera (what's left is now Santorini) that is considered to have wiped out the Minoan Civilization and reeked havoc on ancient Egypt and other cultures in the Mediterranean region.
Volcano Hazards Volcanic disasters tamboraMt. Krakatoa eruption
Fig. 7-83. Volcano hazards Fig. 7-84. Worst Volcanic Disasters Fig. 7-85. Mt. Tambora Volcano Fig. 7-86. Krakatoa Volcano
1902 Erupt of Mt. Peleee Santorini Volcano Thera Volcano Volcanic bad day on Mt. Vesuvius
Fig. 7-87. Mt. Pelee is a very active volcano in the Caribbean island chain. Fig. 7-88. Santorini is what is left of the Thera volcano, blown apart in 1650 BC. Fig. 7-89. Santorini (Thera) Volcano destroyed the Minoan Empire. Fig. 7-90. Mt. Vesuvius buried the cities of Pompeii and Herculaneum in 79 AD.

Hydrothermal Features and Deposits

Igneous activity releases fluids (water and gases). Heat from hot material also heats the groundwater. As fluids move, they can dissolve and precipitate minerals as the physical and chemical conditions change. Where water (steam) and gases vent at the surface they form fumaroles, hotsprings and geysers.

A fumerole is an opening or vent in or near a volcano, through which hot sulfurous gases, steam, and other gases emerge (Figure 7-91).

A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust, and usually defined as spring water warmer than the human body (Figure 7-92). If they are cooler than body temperature but warmer than average air temperature it is called a warm spring.

A geyser is a hot spring in which water intermittently boils and erupts, sending a tall column of water and steam into the air (Figure 7-93).

Hydrothermal vents occur on the seafloor and are best known from locations on or near mid-ocean ridges. Under the great pressures at the bottom of the ocean water will not turn to steam. Fluids venting from the seafloor have been measured at temperatures hot enough to melt glass! These fluids are rich in a variety of metals and rare elements that precipitate around the vents called "black smokers." (Figure 7-94). Ancient black smokers and related deposits are associated with many ore deposit now found on continents.

Hydrothermal veins are fractures in rock that have been filled with minerals (most commonly quartz and/or calcite) precipitated from groundwater or hot fluids of magmatic origin (Figure 7-95). Hydrothermal veins are transitional to cooler setting that those associated with pegmatite. Open pocket can also be filled with gem minerals and metals ores including gold, silver, and copper. Most of the world's copper come from large copper porphyry deposits (Figure 7-96). These copper-rich deposits contain precious stone and minerals including azurite, chrysocolla, malachite, turquoise, and others. Cinnabar (mercury ore) also forms in hydrothermal deposits. Hydrothermal deposits are also associated with sedimentary and metamorphic environments.

Some gems form after beds of ash and lava settle because the ash is soluble in the hot fluids that may rise from hydrothermal (hot water) vents emanating from below. Hot gases also rise and can carry mineral forming elements. Because the hot water is filled with dissolved minerals, on cooling a precipitation of crystals into spaces in the rock may occur as the hot gases and water rise and cool. Some opals and nodules such as thunder eggs form in this way (Figures 6-97 and 6-98). Geodes are small crystal-filled cavities, usually with hard, typically with spherical or ovoid outer surfaces. Although geodes occur in igneous deposit, most are of sedimentary origin. Any open gas pocket can become a void filled with crystals if conditions are right.
Fumerole, Hawaii Hotspring, Yellowstone
Fig. 7-91. Fumerole with sulfur deposits, Hawaii Fig. 7-92. Boiling hot spring, Yellowstone, Wyoming
Geyser, Yellowstone black smoker deposits on the seafloor
Fig. 7-93. Grand Geyser erupting in Yellowstone National Park, Wyoming Fig. 7-94. Hydrothermal veins in North Cascades National Park, Washington
Quartz veins Copper porphyry
Fig. 7-95. Quartz veins in North Cascades NP, WA Fig. 7-96. Copper porphyry from Arizona
Opal thunder egg
Fig. 7-97. Precious opal is one variety of opal. Fig. 7-98. Thunder egg (with banded agate).

Earth is not the only place where volcanism is known to occur. Volcanism occurs on other planets and moons(NASA images).

Moon Volcano on Venus Olympus Mons on Mars Volcano erupting on Jupiter's moon IO
Fig. 7-99. Dark areas on the moon are great lava-filled basins that formed early in its history. Fig. 7-100. With surface temperatures around 800 degrees, the surface of Venus is a volcanic planet. Fig. 7-101. Olympus Mons on Mars is the largest volcano in the solar system. Fig. 7-102. Jupiter's moon Io is covered with volcanoes, many are currently active.

Gems of Igneous Origin

Gems are minerals that can form under limited physical and chemical environments. In some cases, these may be only igneous settings. However, some minerals, quartz as an example, can form in a variety of settings, igneous, sedimentary, and metamorphic. Is a discussion of two (of many) settings of how gems form and how they occur. Below is a chart showing selected gems formed in igneous settings.

The Igneous Origin of Diamonds

Diamonds are a rare occurrence on the surface of the planet because it takes extremely hot and high pressure conditions to create them. Physical and chemical conditions where diamonds form only exist in the mantle, nearly 70 miles down or more. In that environment in the upper mantle, diamonds may be a common mineral! It takes incredible events, nothing that has ever been witnessed in historic times, to bring diamonds to the surface. Diamond deposits around the world (that have any economic significance) are associated with volcanic features called diatremes (Figure 7-103). A diatreme is a long, vertical pipe formed when gas-filled magma forces its way through the crust to explosively erupt at the surface. Kimberlite a special kind of intrusive igneous rock associated with some diatremes that sometimes contain diamonds, typical coarse grained an bluish in color. Diamonds are xenoliths carried up from deep sources in the mantle, and often occur in association with other gem minerals including garnet, spinel and diopside. Most "economically significant" diamond deposits occur in ancient rocks (Precambrian age), but have been discovered on all continents. Because diamonds are so hard, they survive torturously-long histories, recycled through sedimentary and metamorphic environments without being destroyed. As a result they have been found almost everywhere as very rare, isolated discoveries. Diamonds of microscopic size have been discovered in meteorites and asteroid impact sites, and some metamorphic rocks. They are most extensively mined from kimberlite pipes or from alluvial gravels derived downstream from diamond source areas. It should be noted that most diamonds are not of gem quality, but those that are not are used for industrial purposes.
Fig. 7-103. Diamond-bearing kimberlite pipes are diatremes that originate in the mantle.


Pegmatite is a coarsely-grained crystalline igneous rocks with interlocking crystals typically several centimeters in length (or larger, including the world’s largest crystals, some larger than 10 meters in length). many exotic and important minerals, including many gemstones, are found in them (Figures 7-104 to 7-106).

Mineralogy: Most pegmatites are granitic in composition, having granite’s constituent minerals quartz, feldspar (Na-plagioclase and orthoclase) and mica, commonly muscovite. Less common are gabbroic pegmatites—a gabbro with very large crystal grains of amphiboles, biotite, and some pyroxenes.

Occurrence: Pegmatite typical form as masses in igneous dikes and veins. They are most common along the margins of large igneous intrusions, especially in regions associated with batholiths. Some pegmatites apparently form from very fluid remnants of cooling magma with incompatible elements that are driven off of the main cooling plutonic rock body. An aureole zone is where host rocks show the effects of being nearby cooling large intrusions. Fracture formed in wall rocks surrounding a nearby intrusion can be flooded with late-stage magma and high-pressure fluids derived from magma that are rich in volatile components including water, CO2, and other gases. The volatile components both lower the temperature that mineral can form, but also enhance diffusivity (the ability for elements to move around within a fluid to interact with crystal nucleation sites). This diffusivity allows large and rapid crystal growth—allowing minerals to separate into pockets, some with rare and unusual compositions. These late stage fluids can be enriched in the elements necessary for important gems. Not all pegmatites form the same way.

Pegmatites frequently occur in association with aplite dikes and veins (Figure 7-108 to 7-109). Aplite is a light-colored granitic rock composed of quartz and feldspar with sugary texture. Pegmatites occur in pod-shaped mass, usually as small pods to long linear zones, in some rare cases hundreds of feet in size and thickness. Some pegmatites may contain open pockets (called vugs) that are typically surrounded by well-formed crystal masses radiating into the open voids.

Some pegmatites are best described as metamorphic in origin, formed as rocks rich in fluids only begin to melt and separate from host rocks into isolated pockets and veins (discussed with metamorphic rocks).

Economic Value: Gem minerals found in pegmatites include: apatite, beryls (including emerald), cassiterite, corundum (sapphires), feldspars (including aquamarine and perthite), fluorite, garnet, lepidolite quartz varieties (crystal, rose, and smoky), spodumene, topaz, and tourmaline. Pegmatites sometimes containing rare minerals rich in uncommon elements, including important rare-earth elements. Pegmatite ore around the world have been mined for such economically strategic elements, primarily for beryllium and lithium, but also aluminum, bismuth, boron, cesium, molybdenum, niobium, potassium, tantalum, thorium, tin, tungsten, and uranium.

pegmatite schorl
Fig. 7-104. Pegmatite dike in granite displaying large crystal of amphibole and feldspars (orthoclase and plagioclase). Fig. 7-105. Pegmatite with schorl (black tourmaline) in quartz and orthoclase feldspar from Black Hills, South Dakota.
Tourmaline-bearing pegmatite from San Diego County, California
Fig. 7-106
. Pegmatite with watermelon tourmaline. minerals: pink (elbaite) and green (liddicoatite) from from San Diego County, California
Pegmatite vein exposede in a boulder in Death Valley, California
Fig. 7-108.
A boulder of with aplite and pegmatite veins in Death Valley National Park, California.
Gabbro pegmatite from Aromas Quarry, California
Fig. 7-107.
Gabbro pegmatite (crystals of amphibole and plagioclase) from Aromas Quarry, Monterey County, CA
Pegmatite dikes at Kehoe Beach, Point Reyes National Seashore, CaliforniaFig. 7-109. Pegmatite and aplite dikes and veins in granitic rocks on Kehoe Beach, Point Reyes National Seashore, California

Many Gems of Igneous Origin Are Silicates

A silicate is a mineral that contains silica (SiO2) within its crystal structure. Silica content refers to the total content of (SiO2) in a rock or mineral. For instance, a rock composed entirely of albite (a feldspar with the chemical formula NaAlSi3O8 has a silica content of 77%. Pure quartz (mineral) has 100% silica (Figure 7-110). Although silicate minerals are abundant and important not all that many gems are silicates. Gem minerals that are silicates include varieties of beryls (including emerald), feldspars, olivine (including peridote), quartz, garnets, topaz, tourmalines, and others. A lot of gems, even in of silicate family, have exotic elements. These exotic elements, such as beryllium, boron, lithium, chromium, vanadium, and zirconium need to be concentrated by changing physical and chemical conditions associated with the molten material and fluids that form them. So gem formation often requires unusual geologic circumstances. Sometimes water-rich hot fluids generated from the cooling molten material and rock separate from the main magma and bring with them the exotic elements that do not easily fit into the common silicate mineral crystal structures (that includes ferromagnesian silicates, quartz, feldspar, mica, etc.).

Quartz is one of the most abundant minerals in the crust. It is the most abundant "gem" so all varieties are considered "semiprecious."

Quartz has a variety of forms that are used a gems or precious stones. Pure, clear quartz is called "rock crystal" (Figure 7-110). The combination of trace impurities and exposure to radiation give other varieties of quartz their color. Smoky quartz gets its dark color from silicon atoms set free in the crystalline structure of quartz (Figure 7-111). Amethyst, citrine, and rose quartz have traces of iron and other metals in the crustal structure and are also discolored from natural radiation exposure in the environments where they form (Figures 7-112 and 7-113). Heating colored varieties of quartz can undue the effects of radiation exposure and alter the color of the mineral. Most commercially sold citrine gems are actual heat-treated amethyst or smoky quartz.

Quartz has different crystal structures. The most common, called α-quartz, is stable in the pressures and temperatures of the surface environment. At at 573 °Celsius it undergoes a reversible change in crystal structure to form β-quartz. Over time, if conditions are right, β-quartz converts to α-quartz. Silica (Si02) has a polymorph (an alternative crystal structure) called moganite. Quartz and moganite combine in microcrystalline size to form the mineral chalcedony (Figure 7-1114). Different varieties of quartz combine to form different kinds of stones that show contrasting bands or patches of color. These include agate, sard, onyx, carnelian, heliotrope, and jasper. However, most of these forms also occur in sedimentary or metamorphic settings, not entirely igneous. For instance, fossil wood is frequently preserved as brightly colored varieties of quartz and is abundant in deposits rich in volcanic ash (Figure 7-115). Chemical interactions between the volcanic ash, groundwater, and wood buried in the volcanic ash replace the organic tissue of the wood with silica (in various forms of silica and other minerals).
Quartz varieties with different trace impurities
quartz crystal smoky quartz
Fig. 7-110. Rock crystal is pure quartz. Quartz is one form of silica (SiO2) Fig. 7-111. Smoky quartz is dark from radiation exposure
Amethyst rose quartz
Fig. 7-112. Amethyst gets it color from both traces of iron and radiation exposure over time. Fig. 7-113. Rose quartz gets its color from traces of titanium, iron, or manganese.
chalcedony Fossil wood as onyx
Fig. 7-114. Chalcedony is a variety of microcrystalline quartz with a botryoidal appearance. Fig. 7-115. Fossil wood can be wood replaced by silica and other minerals derived from volcanic ash.

Selected Gem Minerals From Igneous Environments

Xenoliths (X)
Phenocrysts (F)
Pegmatites (P)
Crystalline Plutonic (C)
volcanic (V)
hydrothermal (H)
other geologic settings (O)
apatite P O   corundum X O     peridote X       spinel X P C  
augite X F C - ruby X O     prehnite V O     topaz P O    
beryls P O   - sapphire X O     quartz varieties P V H O tourmalines P O    
- aquamarine P O   diamond X       - agate V H O   - achroite P O    
- emerald P O   feldspars F P C O - amethyst P V H O - elbaite P O    
- morganite P O   - amazonite F P C O - ametrine P V H O - liddicoatite P O    
chrysoberyl P     - labradorite F P C O - bloodstone V O     - rubellite P O    
cinnabar H     - moonstone X P O   - chalcedony V O     - schorl P O    
copper minerals H O   fluorite P H O   - citrine P V H O zircon

X C O  
*azurite H O   garnets X P O   - crystal P V H O          
*chrysocolla H O   spodumene
P       - rose P                
*malachite H O   - kunzite P       - smoky quartz P O              
*turquoise H     opals H O     snowflake obsidian V                
Links go to Smithsonian Institution's "Gem Gallery" website

Quiz questions