together of island arcs. Island arc volcanism is accompanied by igneous intrusions. If these arcs swept together, the underlying intrusions could eventually form the huge batholiths that form the roots of cordilleran mountain ranges characterizing ocean-continent convergent plate boundaries. This is the process going on today in Southeast Asia, especially the Philippines and Indonesia.

Detailed study of oceanic island arcs, and especially the Central Aleutians, has cast doubt on this simple picture. The average composition of the arcs differs significantly from that of the continents. For example, silica contents average 50 percent rather than the 60 percent typical of continental crust. A likely explanation appears to be that the upper parts of the oceanic arcs, which are richer in silica, are concentrated in the continents during the process of continental assembly. If this suggestion proves valid, it has far-reaching implications for the evolution of both crust and mantle. Earth scientists have generally considered that the bulk of the buoyant material of island arcs and continents remains at the Earth's surface once it has formed. It is a new and challenging concept that some fraction of this material is being returned to the mantle.

Volcanism along convergent margins is a perplexing subject of debate. The problem is compounded by lack of theoretical and observational information on the temperature and flow structure in the mantle beneath convergent margins. The premise is that the subduction process injects a cold surface plate into the mantle. Intuitively, this should cool the mantle that surrounds the subducting plate, but somehow the subducting plate instigates volcanism.

Two distinctive features of volcanism in the environment are the involvement of volatile components and the eruption of andesite in addition to basalt. Melting seems to be aided by the transport to depths greater than 90 km of water and carbon dioxide held within the subducting plate. Water-and carbon-dioxide-rich fluids may be released from the subducting plate as it becomes heated. These fluids then rise into the overlying mantle to act as fluxes that trigger melting. Trace element and isotopic analyses of arc lavas have shown that the sediment coating of the subducted oceanic crust indeed is transported to the sources of the volcanoes to play a role in initiating magma genesis.

Experimental studies of magma source materials at appropriate temperatures, pressures, and varying fluid contents have outlined possible melting conditions under a variety of thermal conditions. Geochemical studies of the volcanic products have described their variability in volcanic products and have identified the physical processes that contribute to determining magma composition. But these studies have not identified a unique origin or a differentiation path for arc magmas.

The current initiatives to establish a global network of broad-dynamic-range seismometers and abundant portable seismometers will produce detailed seismic images of the mantle beneath arcs. Some work already has been done on this topic, particularly in Japan, which has provided new information on the temperature distribution beneath arcs. Additional information on the character of mantle flow beneath arcs may derive from studies of anisotropic seismic wave propagation in these areas. Seismic waves passing through olivine, the dominant mineral in the upper mantle, travel at different speeds, depending on their orientation with respect to the crystal growth axes of the olivine. The crystals can be preferentially oriented by the stresses associated with mantle flow. This orientation causes detectably different transit times for seismic waves passing parallel and perpendicular to the flow direction. Because of this anisotropic behavior of olivine, seismic data may possibly be used to determine the direction of mantle flow beneath arcs. Detailed seismic studies of the subarc mantle, especially with the improved resolution expected from the next generation of digital seismic sensors, may provide much clearer images of these inaccessible regions.

Volcanism at convergent margins may be unique to the Earth, and understanding the process is particularly important for several reasons. Arc volcanism may be the primary means by which the continents are formed. If that is true, the continents ultimately owe their origins to subduction and the volcanism it instigates. Subduction returns surface materials to the mantle, which keeps the interior fertile for continued volcanism. If it were not for subduction, continued crustal formation would have removed most of the easily meltable components from the mantle, leaving a residue immune to melting and hence incapable of driving the plate tectonic cycle.

Arc magmas often are very rich in dissolved volatile components that interact with hydrothermal circulations, resulting in economic concentrations of certain elements, particularly copper, but also gold, silver, and molybdenum. This same characteristic makes arc magmas likely to result in dangerously explosive eruptions when they near the surface—explosions driven by the violent boiling of the gases that were dissolved in the magmas at greater depths. Arc volcanism, consequently, rep-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement