Orogenic Andesites and Plate Tectonics

Currently, active volcanoes on Earth erupt andesite more than any other rock type. These andesite sources are frequently stunning in beauty yet are sometimes lethal in effect. Although their eruptions cause more damage to property and environment than to persons, some people, including volcanologists, died violently due to eruptions of andesite while this book was being written in academic safety.

The Earth’s surface has four plate tectonic environments: divergent, convergent, and transform fault plate boundaries, and intra-plate locations. Volcanism is concentrated at plate boundaries or in linear belts or local magma floods, usually of basalt, in intra-plate locations. Andesites occur in all four environments. However, orogenic andesites are associated primarily with convergent plate boundaries, and this association is the significance of the “andesite line” frequently drawn around the Pacific Ocean. This line is a boundary seaward of which no orogenic andesites occur and is attributed to Marshall (1912) and Born (1933). Originally the andesite line was thought coincident with the boundary of the Pacific basin, but it is, instead, the western and northern boundary of the Pacific plate and the eastern boundary of the Juan de Fuca, Cocos, and Nazca plates. Sugisaki (1972) extended its meaning to include all plate boundaries converging at > 2.5 cm/yr.

Active volcanoes occur at topographic highs approximately 60 to 500 km from convergent plate boundaries which usually are associated with ocean trenches. Thus, variations in elevation of up to 13 km occur over short distances, resulting in some of the Earth’s greatest vertical relief. The high elevations of both the volcanic arcs and the nonvolcanic ridges or coast ranges in the forearcs of island arcs and continental margins, respectively, may reflect volumetric expansion accompanying hydration of the mantle above zones of dehydration of subducted lithosphere (Fyfe and McBirney 1975). Alternatively, displacement of mantle by lower density andesite beneath volcanic arcs may cause and sustain the uplift there (Gough 1973).

Studies of the tectonic significance, petrography, and chemical composition of andesites have been more common than detailed investigations of the physical properties of andesite magma, the volcanologic characteristics of its eruption, or the geologic history of its eruptive sites. Indeed, these features sometimes are regarded as of little significance for studies of andesite genesis, as opposed to near-surface phenomenology. However, as will be shown in this chapter, the volcano-logic and geologic contexts of andesite seem to favor its origin by low-pressure differentiation of basalt, and the physical properties of andesite magma probably control these differentiation processes.

Bulk chemical analyses of andesites are used primarily in two ways to evaluate andesite origins. First, average elemental and isotopic compositions can be used to test genetic hypotheses and to compare volcanic rocks from different areas or ages (between-suite comparisons). Second, variation in the concentration of elements or ratio of isotopes within suites must also be consistent with the differentiation mechanisms proposed (within-suite variations).

As noted in Section 4.2, andesites are usually porphyritic in texture, thereby providing a rich mineralogic record of their history, as well as sometimes providing Earth’s finest examples of anorthite and of coexisting pigeonite and hypersthene. However, most studies of andesite report only the identity or, at most, the proportion of phenocrysts present. Indeed, the isotopic composition of Sr is known for more andesites than is the composition of their conspicuous pyroxene phenocrysts! The data summarized below broadly constrain the physical conditions of phenocryst crystallization but are insufficient to make possible a systematic investigation of relationships between mineralogy and andesite type, eruption history, or the tectonic environment of eruption. In addition, Ewart (1976a, 1979, 1980) has summarized relationships between modal proportions of phenocrysts, bulk compositions, and crustal environments.

Comparing orogenic andesites within and between volcanic arcs or over time requires (1) choosing a reference point such as a specified wt.% SiO₂ or FeO*/MgO ratio to normalize effects of within-suite differentiation, (2) averaging or generalizing data for individual volcanoes or entire arcs, and (3) comparing data obtained from different laboratories or methods. All introduce error. I have tried to minimize these errors by summarizing analyses of Quaternary rocks by volcano (Appendix) and then averaging these by arc segment (Table 7.1). Some consistent patterns emerge from the comparisons and provide additional constraints on genetic theories.

Derivation of orogenic andesites by partial fusion of ocean crust subducted beneath volcanic arcs was first suggested by Coats (1962). Today the idea commonly is asserted in introductory geology texts and assumed by many, including some of those constructing geophysical models of convergent plate boundaries. The rapid acceptance of this oversimplified belief was due in part to the coincidence of four events in the mid-1960’s: (1) the 1968 “Andesite Conference” in Oregon, USA, which emphasized evidence that orogenic andesites originated neither by fractional crystallization nor crustal contamination of basalt; (2) discovery that clinopyroxene and garnet are co-liquidus phases of anhydrous basic and acid andesite at high pressure so that these magmas could be partial melts of basaltic eclogite (Green and Ringwood 1968a); (3) unambiguous demonstration that lithosphere, including ocean crust, is subducted at convergent plate boundaries, thereby providing a constantly replenished supply of basaltic eclogite beneath volcanic arcs (Isacks et al. 1968); and (4) recognition of correlation between magma composition and depth to the dipping seismic zone (Dickinson 1968). Because derivation of orogenic andesites from subducted crust would be their most dramatic link with plate tectonics, this genetic hypothesis is examined first. It is a rather feline hypothesis, having many lives despite repeated death threats from critics.

The upper mantle is considered parental to most terrestrial magmas with basalt being the typical offspring. However, Poldevaart (1955) and subsequently others proposed that the addition of water, such as that lost from subducted ocean crust during dehydration, would cause the progeny to become, instead, orogenic andesite. This proposal, like that discussed in the preceding chapter, provides a clear link between plate tectonics and andesite genesis, yet is oversimplified at best.

Because average sialic crust is similar in composition to orogenic andesite its total fusion can yield andesite, and its assimilation into basaltic magma can change the composition of the mixture towards andesite. However, crustal involvement cannot be necessary for either the genesis or geochemical distinctiveness of orogenic andesite because the distinctive andesites predominate even when sialic crust is thin or absent, because of arguments presented in Chapters 8 and 9 that most andesites or their parent magmas originate below the crust, and because many andesites are isotopically incompatible with crustal derivation. Qualitative geochemical arguments for crustal-level assimilation and for slab recycling are often similar because the contaminants are similar. Thus, circumstantial distinctions between the two processes which rely on geologic situations where one is inapplicable (e.g., ARC-ATL collisions) or where one is constant but the other not (e.g., along-strike variations in crustal thickness) are useful.

That orogenic andesites and their plutonic equivalents result from crystal fractionation of basalt has been a commonplace in petrology since the idea was developed by Bowen (1928). Indeed, the alternatives discussed in the three preceding chapters arose only in response to perceived inadequacies of Bowen’s proposal and especially to its lack of ready explanation for the close association between orogenic andesites and convergent plate boundaries discussed in Chapter 2. However, because each attempt to explain orogenic andesites as primary partial melts or mixtures thereof, or as crustally contaminated primary melts, has been found inadequate in most instances, we should now re-examine differentiation mechanisms carefully.

From Chapters 8 to 11 it should be clear that the name “orogenic andesite” denotes rocks sufficiently diverse that they can result from a wide variety of processes acting alone or multiply, a variety which has made andesites a problem to explain by any single mechanism. However, how often and to what extent each possible process actually operates is a different matter. My conclusion is that crystal fractionation of phenocryst phases from basalt, i.e., usually of plagioclase + orthopyroxene/olivine + augite + magnetite (POAM), is by far the most common and extensive process, supplemented to an unknown extent by magma mixing, selective interaction with the crust, and vapor fractionation. A typical andesite event may develop as follows.