melting but also that necessary to continue raising the temperature of the partially molten assembly to melt the more stable components. As a result, total melting seldom, if ever, occurs within the Earth. Instead, magma is produced as interconnected tubules, or as thin skins of liquid along grain boundaries. Even at high degrees of melting, the mixture of solid and liquid rock probably never exceeds a crystal-dominated mush.
The relatively low density of the liquid rock results in separation, with the liquid rising toward the surface. The magma may ascend along grain boundaries like liquid flowing through a porous solid. At the confining pressures within the Earth, a liquid cannot move unless replaced by some other material; gaps do not remain within rocks of the interior. Liquid must be kneaded, or squeezed, out of the solid component of magma-producing rock. This occurs through recrystallization that fills gaps left behind as the liquid travels upward along grain boundaries. This process allows the liquid to coalesce into accumulations of increasing size. At some stage the pressure from accumulating magma forms veins and cracks in the overlying rock. The flow of magma through rock shares many similarities with the movement of other fluids, such as groundwater and petroleum. A better understanding of the physical processes of fluid flow through the Earth could provide answers to a wide variety of problems in the earth sciences.
Extensive geophysical observations of Hawaiian eruptions carried out by the Volcano Observatory of the U.S. Geological Survey have produced tremendous improvements in our understanding of magma transport and storage (Figure 2.6). This work is a major advance in volcanology because it has established a predictive framework for eruptive activity in a volcanic system such as Hawaii's. The nearly continuous volcanism occurring in Hawaii offers a rare opportunity to study geological processes operating on human time scales.
Hawaii represents only one of the many types of volcanic systems active on Earth. Many evolutionary steps in Hawaiian volcanism are similar to those that occur in other settings, but the distinct end products erupted at the surface testify to different evolutionary paths for magmas produced in different tectonic settings. Continental volcanism almost invariably involves longer storage times in larger intermediate-depth magma chambers, resulting in the production of more chemically mature volcanics.
Once a magma has been produced within the Earth's interior, the likelihood that it will erupt on the surface depends strongly on its chemical composition. Silica-rich magmas are more viscous than those low in silica. In other words, the lower the silica content, the more easily the magma flows. Consequently, basalt—the common silica-poor lava—erupts in great abundance on the surface to form the ocean floor, islands like Hawaii and Iceland, and large flood basalt provinces on the continents, such as those of the Columbia River Plateau and the Deccan Traps of India. Basaltic magmas also tend to have relatively low concentrations of volatiles, which, along with their low viscosities, generally allow basalts to be erupted quiescently. As a result, basaltic eruptions rarely cause loss of life because the flow paths are easily predicted and the flow velocities generally are slow enough to allow effective evacuations. Destruction of immovable objects, however, can be significant because relatively low viscosities allow even small-volume flows to cover substantial areas. The largest basaltic provinces can be devastating in this respect. For example, the Columbia River basalts cover an area of about 200,000 km2, an area roughly equal to Virginia, Maryland, West Virginia, and Delaware combined.
As the silica content of the magma increases, eruption becomes less likely. This is because the high viscosity of silica-rich magma makes it susceptible to heat loss and crystallization during its slow ascent through the crust. Consequently, high-silica subsurface intrusions, which cool to become granite, are abundant, while low-silica intrusions are rarer. However, granitic magmas that do reach the surface erupt violently and may cause widespread destruction.
Eruptions of silica-rich magmas devastate because they are driven by the explosive exsolution of volatiles, particularly water. Even small eruptions, such as that at Mount St. Helens, can be accompanied by tremendous explosions that destroy life and landscape over a wide area. The 1883 eruption of Rakata, on the Indonesian island of Krakatoa, was a moderate-sized event that exploded with the energy equivalent of 20,000 Hiroshima-sized atomic bombs. The explosion ejected approximately 80 km3 of rock into the atmosphere. Some of the finer-grained dust, ash, and vapor circled the globe and stayed in the atmosphere for several years. The explosion left a crater 6 km in diameter and induced tsunamis that killed more than 36,000 people.