Igneous Textures: What Can They Tell Us?
Identify and describe the six major igneous textures.
The term texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of its mineral grains—not how it feels to touch. Texture is an important property because it reveals a great deal about the environment in which the rock formed (Figure 1). Geologists can make inferences about a rock’s origin based on careful observations of grain size and other characteristics of the rock.
Three factors influence the textures of igneous rocks:
• The rate at which molten rock cools
• The amount of silica present in the magma
• The amount of dissolved gases in the magma
Among these, the rate of cooling tends to be the dominant factor. A very large magma body located many kilometers beneath Earth’s surface remains insulated from lower surface temperatures by the surrounding rock and thus will cool very slowly over a period of perhaps tens of thousands to millions of years. Initially, relatively few crystal nuclei form. Slow cooling permits ions to migrate freely until they eventually join one of the existing crystals. Consequently, slow cooling promotes the growth of fewer but larger crystals. On the other hand, when cooling occurs rapidly for example, in a thin lava flow—the ions quickly lose their mobility and readily combine to form crystals.
This results in the development of numerous embryonic nuclei, all of which compete for the available ions. The result is a solid mass of many tiny intergrown crystals.
Types of Igneous Textures
The effect of cooling on rock textures is fairly straightforward:
Slow cooling promotes the growth of large crystals, whereas rapid cooling tends to generate small crystals.
However, a magma body may migrate to a new location or erupt at the surface before it completely solidifies. As a result, several types of igneous textures exist, including (a 5 not, phaner 5 visible). By definition, the crystals that make up aphanitic rocks are so small that individual minerals can be distinguished only with the aid of a polarizing microscope or other sophisticated techniques. Therefore, we commonly characterize fine-grained rocks as being light, intermediate, or dark in color. Using this system of grouping, light-colored aphanitic rocks are those containing primarily light-colored nonferromagnesian silicate minerals.
Phaneritic (Coarse-Grained) Texture When large masses of magma slowly crystallize at great depth, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic.
Coarse-grained rocks consist of a mass of intergrown crystals that are roughly equal in size and large enough to distinguish the individual minerals without the aid of a microscope. Geologists often use a small magnifying lens to aid in identifying minerals in a phaneritic rock.
Porphyritic Texture A large mass of magma may require thousands or even millions of years to solidify.
Because different minerals crystallize under different environmental conditions (temperatures 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 moves to a different environment—for example, by erupting at the surface—the remaining liquid portion of the lava cools more quickly. The resulting rock, which has large crystals embedded in a matrix of smaller crystals, is said to have a porphyritic texture (Figure 2). The large crystals in porphyritic rocks are referred to as phenocrysts (pheno 5 show, cryst 5 crystal), whereas the matrix of smaller crystals is called groundmass. A rock with a porphyritic texture is termed a porphyry.
Vesicular Texture Common features of many extrusive rocks are the voids left by gas bubbles that escape as lava solidifies. 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 3).
Another common vesicular rock, called pumice, forms when silica-rich lava is ejected during an explosive eruption.
Glassy Texture During some volcanic eruptions, molten rock is ejected into the atmosphere, where it is quenched (very quickly cooled) to become a solid. Rapid cooling of this type may generate rocks having a glassy texture. 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.
Because of its excellent conchoidal fracture and ability to hold a sharp, hard edge, obsidian was a prized material from which Native Americans chipped arrowheads and cutting tools (Figure 4).
Obsidian flows, typically a few hundred feet thick, provide evidence that rapid cooling is not the only mechanism that produces a glassy texture. Magmas with high silica content tend to form long, chainlike structures (polymers) before crystallization is complete.
These structures, in turn, impede ionic transport and increase the magma’s viscosity. (Viscosity is a measure of a fluid’s resistance to flow.) So, granitic magma, which is rich in silica, may be extruded as an extremely viscous mass that eventually solidifies to form obsidian.
By contrast, basaltic magma, which is low in silica, forms very fluid lavas that, upon cooling, usually generate fine-grained crystalline rocks. However, when a basaltic lava flow enters the ocean, its surface is quenched rapidly enough to form a thin, glassy skin.
Pyroclastic (Fragmental) Texture Another group of igneous rocks is formed from the consolidation of individual rock fragments ejected during explosive volcanic eruptions. The ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of a vent during an eruption (Figure 5). Igneous rocks composed of these rock fragments are said to have a pyroclastic texture, or fragmental texture.
A common type of pyroclastic rock, called welded tuff, is composed of fine fragments of glass that remained hot enough to fuse together. Other pyroclastic rocks are composed of fragments that solidified before impact and became cemented together at some later time. Because pyroclastic rocks are made of individual particles or fragments rather than interlocking crystals, their textures often resemble those exhibited by sedimentary rocks rather than those associated with igneous rocks.
Pegmatitic Texture Under special conditions, exceptionally coarse-grained igneous rocks, called pegmatites, may form. Rocks of this type, in which most of the crystals are larger than 1 centimeter in diameter, are described as having a pegmatitic texture (Figure 6). Most pegmatites occur as small masses or thin veins within or around the margins of large intrusive igneous bodies.
Pegmatites form late in the crystallization of a magma, when water and other materials, such as carbon dioxide, chlorine, and fluorine, make up an unusually high percentage of the melt. Because ion migration is enhanced in these fluid-rich environments, the crystals that form are abnormally large.
Thus, the large crystals in pegmatites do not result from inordinately long cooling histories; rather, they are the consequence of a fluid-rich environment that enhances crystallization.
The composition of most pegmatites is similar to that of granite. Thus, pegmatites contain large crystals of quartz, feldspar, and muscovite. However, some contain significant quantities of relatively rare and hence valuable elements—gold, tungsten, beryllium, and the rare earth elements that are used in modern high-technology devices, including cell phones and hybrid auto.