Igneous Activity

Igneous Activity begins with a description of the catastrophic 1980 eruption of Mount St. Helens. A discussion of volcanism and the factors that determine the nature of volcanic eruptions (magma composition, temperature, and amount of dissolved gases) is followed by an examination of the materials that can be extruded during an eruption. The types of volcanic cones, their origins, shapes, and compositions, as well as the nature of fissure eruptions and volcanic landforms, are also presented.

An examination of intrusive igneous activity includes the classification and description of the major intrusive igneous bodies—dikes, sills, laccoliths, and batholiths. The chapter closes with a discussion of the relations between igneous activity, plate tectonics, and the origin and distribution of magma.

Learning Objectives

After reading, studying, and discussing this chapter, you should be able to:

•List the factors that determine the violence of volcanic eruptions.

•List the materials that are extruded from volcanoes.

•Describe the major features produced by volcanic activity.

•List and describe the major intrusive igneous features.

•Discuss the origin of magma.

•Describe the relation between igneous activity and plate tectonics.

Chapter Summary

•The primary factors that determine the nature of volcanic eruptions include the magma’s temperature, its composition, and the amount of dissolved gases it contains. As lava cools, it begins to congeal, and as viscosity increases, its mobility decreases. The viscosity of magma is directly related to its silica content. Granitic lava, with its high silica content, is very viscous and forms short, thick flows. Basaltic lava, with a lower silica content, is more fluid and may travel a long distance before congealing. Dissolved gases provide the force which propels molten rock from the vent of a volcano.

•The materials associated with a volcanic eruption include lava flows (pahoehoe and aa flows for basaltic lavas), gases (primarily in the form of water vapor), and pyroclastic material (pulverized rock and lava fragments blown from the volcano’s vent, which include ash, pumice, lapilli, cinders, blocks, and bombs).

•Shield cones are broad, slightly domed volcanoes built primarily of fluid, basaltic lava. Cinder cones have very steep slopes composed of pyroclastic material. Composite cones, or statovolcanoes, are large, nearly symmetrical structures built of interbedded lavas and pyroclastic deposits. Composite cones represent the most violent type of volcanic activity. Often associated with violent eruptions are nuée ardente, a fiery cloud of hot gases infused with incandescent ash that races down steep volcanic slopes. Large composite cones often generate a type of mudflow known as a lahar.

•Other than volcanoes, regions of volcanic activity may contain craters (steep walled depressions at the summit of most volcanoes), calderas (craters that exceed one kilometer in diameter), volcanic necks (rocks that once occupied the vents of volcanoes, but are now exposed because of erosion), lava plateaus produced from fissure eruptions (volcanic material extruded from fractures in the crust), and pyroclastic flows.

•Igneous intrusive bodies are classified according to their shape and by their orientation with respect to the host rock, generally sedimentary rock. The two general shapes are tabular (sheetlike) and massive. Intrusive igneous bodies that cut across existing sedimentary beds are said to be discordant, whereas those that form parallel to existing sedimentary beds are concordant.

•Dikes are tabular, discordant igneous bodies produced when magma is injected into fractures that cut across rock layers. Tabular, concordant bodies, called sills, form when magma is injected along the bedding surfaces of sedimentary rocks. In many respects sills closely resemble buried lava flows. Laccoliths are similar to sills but form from less—fluid magma that collects as a lens-shaped mass that arches the overlying strata upward. Batholiths, the largest intrusive igneous bodies with surface exposures of more than 100 square kilometers (40 square miles), frequently compose the cores of mountains.

•Temperature, pressure, and partial melting influence the formation of magma. By raising its temperature, solid rock will melt and generate magma. One source of heat is that which is released by the decay of radioactive elements found in Earth’s mantle and crust. Secondly, a drop in confining pressure can lower the melting temperature of rock sufficiently to trigger melting. Thirdly, igneous rock, which contains several different minerals’ melts over a range of temperatures, with the lower temperature minerals melting first. This process, known as partial melting, produces most, if not all, magma. Partial melting often results in the production of a melt with a higher silica content than the parent rock.

•Active areas of volcanism are found along the oceanic ridges (spreading center volcanism), adjacent to ocean trenches (subduction zone volcanism), as well as the interiors of plates (intraplate volcanism) themselves. Most active volcanoes are associated with plate boundaries.