Some current aspects of volcanology

Summary The remarkable progress of volcanology in recent years has been a natural consequence of the theoretical and technological advances that now permit quantitative interpretations of the conditions governing the generation and eruption of magmas. The result has been a better insight into the interior of the earth and moon. Much recent work has been directed toward the study of oceanic volcanism. The distribution, rates of eruption, and compositional variations of lavas with respect to large-scale features of the ocean floor seem to reflect major tectonic processes. A greatly improved understanding of the mechanisms of eruption has been gained through precise measurements of the deformation of the Hawaiian volcano Kilauea during the course of successive eruptive episodes. Magma rises from the upper mantle to fill a shallow reservoir of dikes and sills a kilometer or two beneath the summit. It resides at this level, precipitating mafic minerals and accumulating water vapor, until its density is reduced and it can rise and vesiculate. Under certain conditions, magma has been seen to drain back down its vent. It is estimated that about 50 · 10 6 m 3 of new magma rises from the mantle under Hawaii each year, but only part of this volume is erupted at the surface as lavas. The depths at which oceanic magmas are generated and acquire their chemical and mineralogical features are still difficult to estimate. Much has been learned from ultramafic inclusions and from experimental studies of phase equilibria, but many thermodynamic aspects of the generation and rise of magma need to be evaluated. Large numbers of volcanoes are associated with island arcs and the margins of continents next to trenches and active seismic zones. Volcanic activity in these places is episodic and may shift abruptly from one axis to another. The direction of migration is usually seaward. Lavas tend to be more alkaline at increasing distances from the trench, and there seems to be a relation between potassium-sodium ratios and the thickness of the continental crust through which the magma has risen. Most workers have abandoned the theory that andesitic lavas result from wholesale assimilation of sialic crustal material in basaltic magma, but there is still disagreement on the question of whether andesites are produced directly by partial melting at high water pressures or indirectly by differentiation of a more basic magma. It has been proposed that plates of oceanic crust are being thrust under the island arcs and continental margins, but there is an apparent problem in reconciling the cooling effect of such underthrusting with the prolonged volcanism and high heat flow observed at the surface. The greatest volumes of continental volcanic rocks are found in flood basalts and siliceous ignimbrites. Recent field studies have shown that many of the ignimbrite sheets are related to gigantic calderas. Although it has generally been accepted that the voluminous siliceous magmas were produced by anatectic melting of the continental crust, this view has recently been questioned. Even though no large ignimbrite eruption has been witnessed in historic times, the basic mechanism by which the magma is discharged and transported over great distances seems to have been adequately explained. Nevertheless, much remains to be learned about the behavior of erupting magmas and especially those that produce pyroclastic rocks. The scarcity of data on the physical properties of magmatic liquids and gases has been a serious obstacle. Techniques of predicting volcanic eruptions are still based almost entirely on empirical data rather than a true understanding of volcanic processes. Nevertheless, remarkable progress has been made in anticipating eruptions in certain regions of active volcanism, such as Japan and Hawaii.