Volcanic Eruption Rates and Relative Volumes for Magma Types in Composite Cones of Volcanic Arcs

Volcanic arcs and their composite cones contain a full spectrum of magma compositions—from basalt to rhyolite. Temporal variations in these compositions provide clues about the depth and size of the intrusive rocks that were their thermal sources. Compositional variations are affected by the rate of plate movement, the angle of plate descent, irregularities in the descending plate, crustal thickness, and the depth and residence time of the magma reservoirs.

Fig. 7.4
The segmented volcanic front of Central America, in which active volcanic fields are shown as
shaded areas. Stippled vertical bars mark the transverse breaks in the arc. The thin, parallel lines
mark depths to the inclined seismic zones (contour interval is 50 km); offshore 1000-m
contours are depths below sea level.
(Adapted from Carr et al ., 1982.)

Another factor affecting the longevity of thermal sources is the extrusion rate for individual volcanoes and volcanic chains. This factor is difficult to evaluate because of the buried eruption sequences, erosion, and widespread distribution of pyroclastic products, but a number of studies have provided enough data to allow general estimates. Nakamura (1974) and Crisp (1984) reviewed data for volcanic output on a global scale and found that subduction-zone-related volcanoes produce from 0.4 to 0.75 × 10⁶ km³ /m.y. Working with individual arcs, McBirney et al . (1974) and Sugimura and Uyeda (1973) concluded that the volume of material erupted for the Cascade Range and Japan, respectively, was ~5 km³ /m.y./km of arc. These general estimates were confirmed in a more detailed work by Sherrod and Smith (1990), who found that the extrusion rates in arc segments of the Quaternary Cascades volcanic arc range from 0.21 to 6 km³ /m.y./km of arc. Variations in the volume of material erupted from volcanoes of the Lesser Antilles and Central America may be related to both crustal thickness and rates of plate convergence; over the last 100,000 years, production rates have been 3.1 km³ /m.y./km of arc in Central America and 4 km³ /m.y./km of arc in the Lesser Antilles (Wadge, 1984). This relationship between convergence rate, crustal thickness, and magma types is shown in Table 7.2.

If an intermediate or silicic magma body—either small or large—is to rise buoyantly to crustal depths, it must be heated from below by basaltic magmas from the asthenosphere. Without this thermal boost, silicic magma chambers cool and solidify; they may never reach the upper crust (Lachenbruch et al ., 1976; Eichelberger, 1978). Fractionation and mixing of basaltic and silicic melts can produce the spectrum of magma types seen in composite cones. These compositional variations are controlled by the rate of magma supply, crustal thickness, rate of magma percolation through the crust, and extrusion to intrusion ratio (Figs. 7.2 and 7.3; Hildreth, 1981).

To calculate the number of shallow crustal magma bodies that might provide heat to geothermal systems, it is necessary to determine the relative volumes of magma types and their ages for each composite volcano and, if possible, for an entire arc. Central Cascade Range volcanism in North America began in the earliest Pleistocene with the eruption of widespread basaltic cones and flows and the construction of overlapping shield volcanoes (McBirney and White, 1982). Activity became more localized at centers from which more andesitic lavas and tephra were erupted. This activity formed the base upon which the large composite cones were constructed during the past million years. The volume measurements of

McBirney et al . (1974) indicated that most of the province consists of basaltic scoria cones and lava flows and that andesitic composite cones make up only 15% of the erupted material. Studies of Mount Jefferson indicate that the early basaltic activity produced >100 km³ , but that the cone-building stages involved only 25 km³ of andesitic magma (Fig. 7.5).

Clark (1983) found that the Three Sisters volcanic complex of the southern Cascade Range was erupted onto a broad base of basaltic lavas (57 km³ ), which included a much larger relative volume of andesite (30 km³ ) and rhyodacite-rhyolite (3.5 km³ ). Rhyodacitic domes erupted during the past 2300 years from aligned vents that cross the summit regions of the complex. These eruptions may have tapped only a small volume of a compositionally zoned, shallow magma chamber of much greater volume (Scott (1987).

By plotting volumes of erupted material vs SiO₂ compositions for the Quaternary volcanic rocks of Japan, Aramaki and Ui (1982) demonstrated that changes along the arc may be related to both plate movement and crustal thickness. Arc segments that consist of composite cones, domes, and calderas are mostly andesite-dacite-rhyolite—all of which are indicative of shallow crustal magma bodies. Most of the rhyolitic materials are associated with large calderas. The basaltic segments are made up of pre-dominantly simple cones and lava flows (Fig. 7.6).

Fig. 7.5
Relative volumes of rock types associated with
Quaternary composite cones in the Cascade
Range of North America. (a) Mount Jefferson was
constructed during four main periods of activity;
rock volumes vs SiO₂  content for each stage are
shown in the graph. (b) Three Sisters complex,
which is characterized by more siliceous rocks
than Mount Jefferson. Of the two, the Three
Sisters complex has more potential as a
geothermal resource.
(Adapted from McBirney and White, 1982.)