In the discussion of the rock cycle, we pointed out that igneous rocks form as magma or lava cools and crystallizes. Magma is most often generated by melting of rocks in Earth’s mantle, although some magma originates from the melting of crustal rock. Once formed, a magma body buoyantly rises toward the surface because it is less dense than the surrounding rocks.

When magma reaches the surface, it is called lava (Figure 2). Sometimes, lava is emitted as fountains, which are produced when escaping gases propel molten rock skyward. On other occasions, magma is ejected explosively from vents, producing a spectacular eruption, such as the 1980 eruption of Mount St. Helens. However, most eruptions are not violent; rather, volcanoes most often emit quiet outpourings of lava.

Molten rock may solidify at depth, or it may solidify at Earth’s surface. When molten rock solidifies at the surface, the resulting igneous rocks are classified as extrusive rocks, or volcanic rocks (after the Roman fire god, Vulcan). Extrusive igneous rocks are abundant in western portions of the Americas, including the volcanic cones of the Cascade Range and the extensive lava flows of the Columbia Plateau. In addition, many oceanic islands, including the Hawaiian Islands, are composed almost entirely of volcanic igneous rocks.

Most magma loses its mobility before reaching Earth’s surface and eventually solidifies deep below the surface. When magma solidifies at depth, it forms igneous rocks known as intrusive rocks, or plutonic rocks (after Pluto, the god of the underworld in classical mythology). Intrusive igneous rocks remain at depth unless portions of the crust are uplifted and the overlying rocks are stripped away by erosion. Exposures of intrusive igneous rocks occur in many places, including Mount Washington, New Hampshire; Stone Mountain, Georgia; Yosemite National Park, California; and Mount Rushmore in the Black Hills of South Dakota (Figure 3).

From Magma to Crystalline Rock

Magma is molten rock (or melt) composed of ions of the elements that move about freely, mainly as the silicon and oxygen found in silicate minerals. Magma also contains gases, particularly water vapor and carbon dioxide, that are confined within the magma body by the weight (pressure) of the overlying rocks, and it may contain some solids (mineral crystals). As magma cools, the once-mobile ions begin to arrange themselves into orderly patterns—a process called crystallization. As cooling continues, numerous small crystals develop, and ions are systematically added to these centers of crystal growth. When the crystals grow large enough for their edges to meet, their growth ceases. Eventually, all the liquid melt is transformed into a solid mass of interlocking crystals.

The rate at which magma cools strongly influences crystal size. If magma cools very slowly, ions can migrate over great distances. Consequently, slow cooling of magma results in the formation of fewer, larger crystals. If magma cooling occurs rapidly, the ions lose their motion and quickly combine. This results in a large number of tiny crystals that all compete for the available ions. Therefore, rapid cooling of magma results in the formation of a solid mass of small intergrown crystals.

If the molten material is quenched or cooled almost instantly, there is insufficient time for the ions to arrange themselves into a crystalline network. Solids produced in this manner consist of ions distributed somewhat randomly. Such rocks are called glass and are quite similar to ordinary manufactured glass. “Instant” quenching often occurs during violent volcanic eruptions that produce tiny shards of glass called volcanic ash.

In addition to the rate of cooling, the composition of a magma and the amount of dissolved gases influence crystallization. Because magmas differ in each of these aspects, the physical appearance and mineral composition of igneous rocks vary widely.

Igneous Compositions

Igneous rocks are composed mainly of silicate minerals. Chemical analysis shows that silicon and oxygen—usually expressed as the silica (SiO2) content of a magma—are by far the most abundant constituents of igneous rocks. These two elements, plus ions of aluminum (Al), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), and iron (Fe), make up roughly  percent by weight of most magmas. Magma also contains small amounts of many other elements, including titanium and manganese, and trace amounts of much rarer elements, such as gold, silver, and uranium.

As magma cools and solidifies, these elements combine to form two major groups of silicate minerals: dark silicates and light silicates. The dark silicates are rich in iron and/or magnesium and are comparatively low in silica (SiO2). Olivine, pyroxene, amphibole, and biotite mica are the common dark silicate minerals of Earth’s crust. By contrast, the light silicates contain greater amounts of potassium, sodium, and calcium and are richer in silica than dark silicates. Light silicate minerals include quartz, muscovite mica, and the most abundant mineral group, the feldspars. Feldspars make up at least 40 percent of most igneous rocks. Thus, in addition to feldspar, igneous rocks contain some combination of the other light silicates and/or dark silicates.

Granitic (Felsic) versus Basaltic (Mafic) Compositions

Despite their great compositional diversity, igneous rocks (and the magmas from which they form) can be divided into broad groups according to their proportions of light and dark silicate minerals (Figure 4). Near one end of the continuum are rocks composed almost entirely of light-colored silicates—the minerals quartz and potassium feldspar. Igneous rocks in which these are the dominant minerals have a granitic composition. Geologists refer to granitic rocks as being felsic, a term derived from feldspar and silica (quartz). In addition to quartz and feldspar, most granitic rocks contain about 10 percent dark silicate minerals, usually biotite mica and amphibole. Rocks of granitic composition include granite and rhyolite. They are major constituents of the continental crust.

Rocks that contain at least 45 percent dark silicate minerals and calcium-rich plagioclase feldspar (but no quartz) are said to have a basaltic composition (refer to Figure 4). Basaltic rocks contain a high percentage of dark silicate minerals, so geologists refer to them as mafic (from magnesium and ferrum, the Latin word for iron). Because of their high iron content, mafic rocks are typically darker and denser than granitic rocks. Rocks of basaltic composition include basalt and gabbro. They make up the ocean floor as well as many of the volcanic islands located within the ocean basins.

Other Compositional Groups

As you can see in Figure 4, rocks with a composition between granitic and basaltic rocks are said to have an andesitic composition, or intermediate composition, after the common volcanic rock andesite. Intermediate rocks contain at least 25 percent dark silicate minerals, mainly amphibole, pyroxene, and biotite mica, with the other dominant mineral being plagioclase feldspar. This important category of igneous rocks is associated with volcanic activity that is typically confined to the seaward margins of the continents and on volcanic island arcs, such as the Aleutian Islands of Alaska.

At the far end of the compositional spectrum are the ultramafic igneous rocks, composed almost entirely of dense ferromagnesian minerals (refer to Figure 4). Although ultramafic rocks are rare at Earth’s surface, the rock peridotite, which is composed mostly of the minerals olivine and pyroxene, is the main constituent of the upper mantle.

What Can Igneous Textures Tell Us?

To describe the size, shape, and arrangement of the mineral grains that make up a rock, geologists use the word texture. Texture is an important property because it allows geologists to make inferences about a rock’s origin, based on careful observations of crystal size and other characteristics (Figure 5). Rapid cooling produces small crystals, whereas very slow cooling produces much larger crystals. As you might expect, the rate of cooling is slow in magma chambers that lie deep within the crust, whereas a thin layer of lava extruded upon Earth’s surface may chill to form solid rock in a matter of hours. Small molten blobs ejected from a volcano during a violent eruption can solidify in mid-air.

Fine-Grained Texture

Igneous rocks that form at Earth’s surface or as small intrusive masses within the upper crust, where cooling is relatively rapid, exhibit a fine-grained texture (refer to Figure 5F). By definition, the crystals that make up fine-grained igneous rocks are so small that individual minerals can be distinguished only with the aid of a microscope or other sophisticated techniques. Therefore, we commonly characterize fine-grained rocks as being light, intermediate, or dark in color.

Coarse-Grained Texture

When large masses of magma slowly crystallize at great depth, they form igneous rocks that exhibit a coarse-grained texture. Coarse-grained rocks consist of a mass of intergrown crystals that are roughly equal in size and large enough so that the individual minerals can be identified without the aid of a microscope (refer to Figure 5C). Geologists often use a small magnifying lens to aid in identifying minerals in coarse-grained igneous rocks.

Porphyritic Texture

A large mass of magma may require thousands or even millions of years to solidify. Because different minerals crystallize under different conditions of temperature and pressure, it is possible for crystals of one mineral to become quite large before others even begin to form. If molten rock containing some large crystals erupts or otherwise moves to a cooler location, the remaining liquid portion of the lava will cool more quickly. The resulting rock, which has large crystals embedded in a matrix of smaller crystals, is said to have a porphyritic texture (refer to Figure 5B and Figure 6). The large crystals in porphyritic rocks are referred to as phenocrysts, whereas the matrix of smaller crystals is called groundmass.

Vesicular Texture

Many extrusive rocks exhibit voids that represent gas bubbles that formed as the lava solidified. These nearly spherical openings are called vesicles, and the rocks that contain them are said to have a vesicular texture. Rocks that exhibit a vesicular texture often form in the upper zone of a lava flow, where cooling occurs rapidly enough to preserve the openings produced by the expanding gas bubbles (Figure 7). Another common vesicular rock, called pumice, forms when silica-rich lava is ejected during an explosive eruption (refer to Figure 5D).

Glassy Texture

During some volcanic eruptions, molten rock is rapidly quenched and quickly becomes a solid. Rapid cooling of lava in the atmosphere, or within water, or at the edges of flows may generate rocks having a glassy texture (refer to Figure 5A). Glass results when unordered ions are “frozen in place” before they are able to unite into an orderly crystalline structure. Obsidian, a common type of natural glass, is similar in appearance to dark chunks of manufactured glass.

Pyroclastic (Fragmental) Texture

Another group of igneous rocks is formed from the consolidation of individual rock fragments ejected during explosive volcanic eruptions. The ejected rock fragments might be very fine ash, molten blobs, or large angular blocks torn from the walls of a vent during an eruption. Igneous rocks composed of these rock fragments are said to have a pyroclastic texture, or fragmental texture (refer to Figure 5E). A common type of pyroclastic rock, called welded tuff, is composed of fine fragments of glass that remained hot enough to eventually fuse together.

Common Igneous Rocks

Geologists classify igneous rocks by their texture and mineral composition. The texture of an igneous rock is mainly a result of its cooling history, whereas its mineral composition is largely a result of the chemical makeup of the magma from which it formed (Figure 8). Because igneous rocks are classified on the basis of both mineral composition and texture, some rocks have similar mineral constituents but exhibit different textures; therefore, they are given different names.

Granitic (Felsic) Rocks

Granite is a coarse-grained intrusive igneous rock that forms where large masses of silica-rich magma slowly solidify at depth. Episodes of mountain building may uplift granite and related intrusive rocks, with the processes of weathering and erosion stripping away the overlying crust. Areas where large quantities of granite are exposed at the surface include Pikes Peak in the Rockies, Mount Rushmore in the Black Hills, Stone Mountain in Georgia, and Yosemite National Park in the Sierra Nevada (Figure 9).

Granite is perhaps the best-known igneous rock, in part because of its natural beauty, which is enhanced when polished, and partly because of its abundance. Slabs of polished granite are commonly used for tombstones, monuments, and countertops.

Rhyolite is the extrusive equivalent of granite (with the same chemical composition but different texture) and likewise, is composed essentially of light-colored silicate minerals (refer to Figure 8). The silicate minerals account for the color of rhyolite, which is usually buff to pink or light gray. Rhyolite is fine-grained and frequently contains glass fragments and voids, indicating rapid cooling in a surface environment. In contrast to granite, which is widely distributed as large intrusive masses, rhyolite deposits are less common and generally less voluminous. Yellowstone Park is one well-known exception where extensive rhyolite lava flows are found (along with thick ash deposits of rhyolitic composition).

Obsidian is a common type of volcanic glass. Although dark in color, obsidian usually has a felsic composition; its color results from small amounts of metallic ions in an otherwise clear, glassy substance. Because of its excellent conchoidal fracture and ability to hold a sharp, hard edge, obsidian was a prized material from which Indigenous Americans chipped arrowheads and cutting tools (Figure 10).

Another silica-rich volcanic rock that exhibits a glassy and vesicular texture is pumice. Often found with obsidian, pumice forms when large amounts of gas escape from molten rock to generate a gray, frothy mass (Figure 11). In some samples, the vesicles are quite noticeable, whereas in others, the pumice resembles fine shards of intertwined glass. Because of the large volume of air-filled voids, most samples of pumice float in water (refer to Figure 11).

Andesitic (Intermediate) Rocks

Andesite is a medium-gray extrusive rock. It may be fine-grained, or it may have a porphyritic texture (refer to Figure 8), often with phenocrysts of plagioclase feldspar (pale and rectangular) or amphibole (black and elongated). Andesite is a major constituent of many of the volcanoes that are found around the Pacific Rim, including those of the Andes Mountains of South America (after which andesite is named) and the Cascade Range of North America.

Diorite, the intrusive equivalent of andesite, is a coarse-grained rock that resembles gray granite. However, it can be distinguished from granite because it contains few or no visible quartz crystals and has a higher percentage of dark silicate minerals.

Basaltic (Mafic) Rocks

The most common extrusive igneous rock is basalt, a very dark green to black, fine-grained volcanic rock composed primarily of pyroxene, olivine, and plagioclase feldspar. Many volcanic islands, such as the Hawaiian Islands and Iceland, are composed mainly of basalt (Figure 12). Furthermore, the upper layers of the oceanic crust consist of basalt. In the United States, large portions of central Oregon and Washington were the sites of extensive basaltic outpourings in the geologic past.

The coarse-grained, intrusive equivalent of basalt is gabbro (refer to Figure 8). Gabbro is not commonly exposed at the surface, but it makes up a significant percentage of oceanic crust.

How Igneous Rocks Form

Because igneous rocks exhibit a wide range of compositions, it would be logical to assume that they originate from equally diverse magmas. However, geologists have observed that a single volcano, fed by a single magma chamber, may extrude lavas exhibiting quite different compositions. Data of this type led geologists to examine the possibility that magma might change (evolve) over time and, thus, become the parent to a variety of igneous rocks. To explore this idea, a pioneering investigation into the crystallization of magma was carried out by Canadian petrologist N. L. Bowen in the early 1900s.

Bowen’s Reaction Series

In a laboratory setting, Bowen demonstrated that magma, with its complex chemistry, crystallizes over a range of temperatures, unlike liquids that consist of just a single component (such as water), which solidify at specific temperatures. As magma cools, certain minerals crystallize first at relatively high temperatures. At successively lower temperatures, other minerals begin to crystallize. This succession of minerals, shown in Figure 13, became known as Bowen’s reaction series.

Bowen discovered that the first mineral to crystallize from a body of magma is olivine, which solidifies around 1200°C (~2200°F). Further cooling results in the formation of pyroxene, as well as plagioclase feldspar. At intermediate temperatures, the minerals amphibole and biotite begin to crystallize.

During the last stage of crystallization, after most of the magma has cooled and solidified, the minerals muscovite and potassium feldspar may form (refer to Figure 13). Finally, quartz crystallizes from any remaining liquid and at temperatures around 750°C (~1400°F). Olivine and quartz are not found in the same igneous rock because quartz crystallizes at much lower temperatures than olivine.

Analysis of igneous rocks provides evidence that this crystallization model approximates what can happen in nature. In particular, we find that minerals that form in the same general range on Bowen’s reaction series are found together in the same igneous rocks. For example, notice in Figure 13 that the minerals quartz, potassium feldspar, and muscovite, located in the same region of Bowen’s diagram, are typically found together as major constituents of the igneous rock granite.

Magmatic Differentiation

So, how can a single volcano, fed from a single magma chamber, form rocks of quite different composition over its lifetime? First, as Bowen demonstrated, different minerals crystallize from magma according to a predictable pattern. As each mineral forms, it selectively removes certain elements from the melt. For example, crystallization of olivine and pyroxene selectively removes iron and magnesium, leaving the remaining melt more felsic. If some mechanism then physically separates these crystals from the remaining melt, the remaining melt crystallizes to form a different, more felsic rock type.

One such mechanism is crystal settling. If early-formed crystals are more dense (heavier) than the remaining melt, they tend to sink toward the bottom of the magma chamber, as shown in Figure 14. Consequently, the lower and upper parts of the magma chamber form rocks of differing composition. The formation of one or more secondary magmas from a single parent magma is called magmatic differentiation.

At any stage in the evolution of magma, the solid and liquid components can separate into two chemically distinct units. Furthermore, magmatic differentiation within the secondary magma can generate other chemically distinct masses of molten rock. Consequently, magmatic differentiation and separation of the solid and liquid components at various stages of crystallization can produce several chemically diverse magmas and, ultimately, a variety of igneous rocks.