Andesitic rocks

Andesite is an intermediate extrusive igneous rock. Active volcanoes on the earth erupt andesite more than any other rock type. Andesites are primarily associated with subduction zones along convergent tectonic plate boundaries.

Interest to Geologists

Andesite takes its name from lavas in the Andes mountains of South America. To most geologists, andesites are light gray porphyritic volcanic rocks containing phenocrysts of plagioclase, very little quartz, and no sanidine or feldspathoid. Despite their lackluster appearance, andesites are of great interest to geologists for several reasons. First, active volcanoes on the earth erupt andesite more than any other rock type; andesite is the main rock type at 61 percent of the world’s active volcanoes. Second, andesites have a distinctive tectonic setting. They are primarily associated with convergent plate boundaries and occur elsewhere only in limited amounts. Of the active volcanoes that occur within 500 kilometers of a subduction zone, 78 percent include andesite; only three active volcanoes not near a convergent plate boundary erupt it. Third, andesites have bulk compositions similar to estimates of the composition of continental crust. This similarity, in association with the tectonic setting of andesites, suggests that they may play an important role in the development of terrestrial crust. Fourth, the development and movement of andesitic magma seem to be closely related to the formation of many ore deposits, including such economically important ores as molybdenum and porphyry copper deposits.

Viscosity

Rocks are classified chemically according to how much silica they contain; rocks rich in silica (more than 64 percent), such as rhyolite, are called silicic. They consist mostly of quartz and feldspars, with minor amounts of mica and amphibole. Rocks low in silica (less than 54 percent), with no free quartz but high in feldspar, pyroxene, olivine, and oxides, are called basic. Basic rocks, free of quartz, tend to be dark, while silicic rocks are lighter and contain only isolated flecks of dark minerals. Basalts are examples of basic volcanic rocks. Andesites, having a silica content of about 60 percent, are volcanic rocks termed intermediate. Andesite’s plutonic equivalent is diorite. There is no cut-and-dried difference between basalt and andesite, or between andesite and rhyolite. Instead, there is a broad transitional group of rocks that carry names such as “basaltic andesite” or “andesitic rhyolite.”

Nevertheless, some generalizations can be applied to the lavas and magmas that form these rocks. One generalization has to do with viscosity. Andesite lavas are more viscous than basalt lavas and less viscous than rhyolite lavas. This difference is primarily an effect of the lava’s composition, and to a certain extent it is a result of the high portion of phenocrysts present in the more viscous lavas.

Different minerals crystallize at different temperatures. As a basalt magma cools, a sequence of minerals appears. The first mineral to crystallize is usually olivine, which continues to crystallize as the magma cools until a temperature is reached at which a second mineral, pyroxene, begins to crystallize. As the temperature continues to drop, these two continue to crystallize. Cooling continues until a temperature is reached when a third mineral, feldspar, crystallizes. This chain of cooling and mineral crystallization is known as Bowen’s reaction principle. Often, olivine and pyroxene crystallize out early in the process, so they may be present in the final rock as large crystals up to a centimeter across. These crystals are called phenocrysts. The size of these phenocrysts is in direct contrast to the fine-grained crystals of the groundmass. Igneous rocks with phenocrysts in a fine-grained groundmass are known as porphyries. Most volcanic rocks contain some phenocrysts. The groundmass crystals form when the lava cools upon reaching the surface. If the lava has a low viscosity, reaches the surface, spreads out, and cools quickly, individual crystals do not have enough time to grow. The overall rock remains fine-grained. The phenocrysts crystallized out much earlier, while the magma was still underground. There, they had plenty of time to grow and were then carried to the surface with the magma during eruption.

Andesite Flows

Basalts, as a result of their low viscosity, tend to produce thin lava flows that readily spread over large areas. They rarely exceed 30 meters in thickness. Andesite flows, by contrast, are massive and may be as much as 55 meters thick. The largest single andesite flow described, which is in northern Chile, has an approximate volume of 24 cubic kilometers. Because of their low viscosity, basalt flows can advance at considerable rates; speeds up to 8 kilometers per hour have been measured. Andesite flows often move only a matter of tens of meters over several hours. As a result of their higher viscosity, andesite flows show none of the surface features of more “liquid” lavas. Flow features such as wave forms, swirls, or the ropy textures often associated with basalt flows never occur in andesite flows. Andesite flows tend to be blocky with large, angular, smooth-sided chunks of solid lava. The flow tends to behave as a plastic rather than a liquid. An outer, chilled surface develops on the slow-moving flow, with the interior still molten. Plasticity within the flow increases toward the still-molten inner portion. As the flow slowly shifts, moves, and cools, the hard, brittle outer layer breaks into the large angular blocks characteristic of andesite lava flows. The flow slowly moves, with the blocks colliding and overriding one another to form piles of angular andesite blocks.

The viscous nature of andesite lavas is responsible for many classic volcanic features. Most notable is the symmetrical cone shape of the stratovolcanoes of the circum-Pacific region. Short, viscous andesitic flows pouring down the flanks of these volcanoes are alternately covered by pyroclastic material and work to build the steep central cone characteristic of composite stratovolcanoes. When an andesite’s silica content rises to a point that it approaches rhyolite composition, it is termed a dacite. Dacite is often so viscous that it cannot flow and blocks the vent of the volcano. This dacite plug is called a lava dome. Such a dome can be seen in pictures of Mount St. Helens. If the plugged volcano becomes active again, the lava dome does not allow for a release of accumulating pressure and explosive gases. Pressure builds until the volcano finally erupts with great force and violence.

Andesite Line

The differences between basalts and andesites reflect their differences in composition, which is a function of the environment in which their source lavas occur. Basalts are typically formed at midocean ridges and build up oceanic crust. The generation of andesite magma is characteristic of destructive plate margins. Here, oceanic plates are being subducted below continental plates. Destructive plate boundaries tend to produce a greater variety of lavas than spreading zones (ocean ridges). The close association of andesite with convergent plate boundaries is the significance of the “Andesite Line” often drawn around the Pacific Ocean basin. This fairly well-defined line separates two major petrographic regions. Inside this line and inside the main ocean basin, no andesites occur. All active volcanoes inside the line erupt basaltic magma, and all volcanic rocks associated with dormant volcanoes within this region are basaltic. Outside the line, andesite is common. The Andesite Line parallels the western and northern boundary of the Pacific plate and the eastern boundary of the Juan de Fuca, Cocos, and Nazca plates. The Andesite Line parallels the major island arc systems, the subducting edges of the tectonic plates listed above, and a chain of prominent and infamous stratovolcanoes known as the Ring of Fire. These stratovolcanoes are exemplified by the following: Mount Rainier and other volcanoes of the western United States, El Chichón of Mexico, San Pedro of Chile, Mounts Egmont and Taupo of New Zealand, Krakatau and Tambora of the Indonesian Arc, Fujiyama of Japan, Bezymianny of Kamchatka, and the Valley of Ten Thousand Smokes in Alaska.

Benioff Zone

The Benioff zone is a plane of seismic activity dipping at an angle of about 45 degrees below a continent, marking the path of a subducting oceanic plate at the convergent plate margin. At the Benioff zone, the subducting plate heats up and gives off water, which lowers the melting point of the overlying mantle and causes it to melt. As magma is formed at the Benioff zone and begins its slow rise toward the surface, it passes through and comes in contact with regions of the mantle and continental crust. During its rise, the magma also comes in contact with circulating meteoric water (water originating on the surface), and new fluids derived from magma are formed. Because andesitic magma rises through a variety of host rocks, the variety of minerals that enrich the new fluids is increased. The rising magma also stresses the surrounding host rocks, causing them to fracture. These fractures and similar open spaces are filled with mineral-rich water and fluids derived from magma. As the fluids cool, mineral ores precipitate from the solutions and fill the open spaces. These filled spaces often become mineral-rich dikes and sills.

Field Study

The study and interpretation of andesitic rocks are accomplished in three basic ways: fieldwork, in which researchers travel to locations to assess and interpret a specific region of the earth that is known or suspected of being andesitic terrain; petrological studies, which use all available methods of study to ascertain the history, origin, conditions, alterations, and weathering of the rocks collected during fieldwork; and mineralogical studies, which through intensive laboratory investigations identify the specific mineral characteristic of a sample.

If a scientist suspects that a region of the earth is andesitic terrain, the first step is to read the information already available about the region. If travel to that region for fieldwork is considered useful, the researcher will most likely collect a large number of samples. The location of each sample is carefully recorded on a map and additional data describing the geologic setting of the sample are recorded. These data include thickness, areal extent, weathering, and strike and dip of the location. Photographs are also commonly used to record the surrounding environment of the sample location. Samples are usually given preliminary study at the researcher’s field camp.

Petrological and Mineralogical Studies

When the fieldwork is over, the researcher returns with the samples to a laboratory setting, where petrological and mineralogical investigations begin. A petrological study of an andesitic sample will include both petrographic and petrogenetic analyses. The petrographic analyses will describe the sample and attempt to place it within the standard systematic classification of igneous rocks. Identification of the sample is accomplished by means of examining a thinly sliced and polished portion of the sample under a petrographic microscope—a process known as thin-section analysis. The information obtained by microscopic examination gives a breakdown of the type and amount of mineral composition within the sample. More advanced analysis techniques, such as X-ray fluorescence and electron microprobe, are often used to obtain very precise chemical compositions of samples. Knowing the conditions under which these minerals form allows the researcher to make a petrogenetic assessment.

Petrogenesis deals with the origin and formation of rocks. If a mineral is known to form only at certain depths, temperatures, or pressures, its presence within a sample allows the researcher to draw some specific conclusions concerning the rock’s formation and history. Mineralogical studies of field-gathered samples aid in the petrogenetic portion of the analysis. Mineralogy involves the study of how a mineral forms—its physical properties, chemical composition, and occurrence. Minerals can exist in a stable form only within a narrow range of pressure and temperature. Experimental confirmation of this range enables the researcher to make a correlation between the occurrence of a mineral in a rock and the conditions under which the rock was formed. In this sense, mineralogical and petrological analyses complement each other and work to formulate a concise history of a given sample.

When data gathered from the field and laboratory are combined, the geological history of a given field area can begin to be interpreted. When areas of similar igneous rock types, ages, and mineral compositions are plotted on a map, they form a petrographic province. A petrographic province indicates an area of similar rocks that formed during the same period of igneous activity. On a global scale, the Andesite Line marks the boundary between two great provinces: the basaltic oceanic crust and the andesitic continental crust. Both crustal forms have distinctly different geological histories and mineral compositions.

Principal Terms

Bowen’s reaction principle: a principle by which a series of minerals forming early in a melt react with the remaining melt to yield a new mineral in an established sequence

extrusive rock: igneous rock that has been erupted onto the surface of the earth

groundmass: the fine-grained material between phenocrysts of a porphyritic igneous rock

intermediate rock: an igneous rock that is transitional between a basic and a silicic rock, having a silica content between 54 and 64 percent

phenocryst: a large conspicuous crystal in a porphyritic rock

plutonic rock: igneous rock formed at a great depth within the earth

porphyry: an igneous rock in which phenocrysts are set in a finer-grained groundmass

stratovolcano: a volcano composed of alternating layers of lava flow and ash; also called a composite volcano

subduction zone: a convergent plate boundary

viscosity: a substance’s ability to flow; the lower the viscosity, the greater the ability to flow

Bibliography

Bowen, N. L. The Evolution of Igneous Rocks. Mineola, N.Y.: Dover, 1956. An unmatched source and reference book on igneous rocks, written by the father of modern petrology. The basis of Bowen’s reaction principle. Written for graduate students and professional scientists. A classic but difficult work.

Burns, Dale H. and Silva, Shanaka L. "Andesites and Evolution of the Continental Crust: Perspectives from the Central Volcanic Zone of the Andes." Frontiers, 13 Jan. 2023, www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.961130/full. Accessed 25 July 2024.

Carmichael, I. S. E., F. J. Turner, and J. Verhoogen. Igneous Petrology. New York: McGraw-Hill, 1974. A college-level textbook on the formation and development of igneous rocks. Very detailed and complete.

Cocker, Kate. "A History of Andesite Production via Magma Mixing and Mingling Revealed Microscopically at Ngauruhoe Volcano." Geochemistry, Geophysics, Geosystems, 25 Sept. 2022, agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GC010589. Accessed 25 July 2024.

Faure, Gunter. Origin of Igneous Rocks: The Isotopic Evidence. New York: Springer-Verlag, 2010. Discusses chemical properties of igneous rocks, and isotopes within these rocks formations. Specific locations of igneous rock formations and the origins of these rocks are provided. There are many diagrams and drawings along with the overview of isotope geochemistry in the first chapter to make this accessible to undergraduate students as well as professionals.

Gill, J. B. Orogenic Andesite and Plate Tectonics. New York: Springer-Verlag, 1981. A well-documented summary of the entire field of andesite genesis. Written for graduate students and professional earth scientists.

Klein, Cornelis, and Cornelius S. Hurlbut, Jr. Manual of Mineralogy. 23d ed. New York: John Wiley & Sons, 2008. An introductory college-level text on mineralogy. Discusses the physical and chemical properties of minerals and describes the most common minerals and their varieties, including gem mineral varieties. Contains more than 500 mineral name entries in the mineral index and describes in detail about 150 mineral species.

Perchuk, L. L., ed. Progress in Metamorphic and Magmatic Petrology. New York: Cambridge University Press, 1991. Although intended for the advanced reader, several of the essays in this multiauthored volume will serve to familiarize new students with the study of igneous rocks. In addition, the bibliography will lead the reader to other useful material.

Ross, Pierre-Simon, et al. “Basaltic to Andesitic Volcaniclastic Rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, Geochemistry, and Geochronology.” Canadian Journal of Earth Sciences. 48 (2011): 757-777. Discusses the rock formations in the Archean Blake River Group. Physical characteristics, age relationships, and geochemistry data are measured and interpreted to determine the origins of these rock formations. The authors hypothesize the volcanic processes that resulted in these formations. Best suited for graduate students or researchers studying volcanism, petrology, or mineralogy.

Science Mate: Plate Tectonic Cycle. Fremont, Calif.: Math/Science Nucleus, 1990. Part of the Integrating Science, Math, and Technology series of manuals intended to help teachers explain basic scientific concepts to elementary students. Provides a basic understanding of plate tectonics, earthquakes, volcanoes, and general geology for the student with no background in the Earth sciences.

Sutherland, Lin. The Volcanic Earth: Volcanoes and Plate Tectonics, Past, Present, and Future. Sydney, Australia: University of New South Wales Press, 1995. Provides an easily understood overview of volcanic and tectonic processes, including the role of igneous rocks. Includes color maps and illustrations, as well as a bibliography.

Tamura, Y., et al. “Andesites and Dactites from Daisen Volcano, Japan: Partial-to-Total Remelting of an Andesite Magma Body.” Journal of Petrology 44 (2003): 2243-2260. Discusses the genesis of andesitic and dacitic rocks from the Daisen Volcano. Lava characteristics and the chemical and physical characteristics of local rocks are compared to determine the stages of the rock forming process.

Williams, H., and A. R. McBirney. Volcanology. San Francisco: Freeman, Cooper, 1979. A classic textbook on volcanoes and volcanology. Well illustrated and very descriptive. Written for the undergraduate or graduate student.

Windley, B. F. The Evolving Continents. 3d ed. New York: John Wiley & Sons, 1995. An excellent reference book on plate tectonics and tectonic processes. Written for the college-level reader.