Fires Within: Igneous Activity
The following statements summarize the primary objectives presented in the chapter.
- 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 stratovolcanoes, are large, nearly symmetrical structures built of interbedded lavas and pyroclastic deposits. Composite cones represent the most violent type of volcanic activity.
- Other than volcanoes, regions of volcanic activity may contain volcanic necks (rocks that once occupied the vents of volcanoes but are now exposed because of erosion), craters (steep walled depressions at the summit of most volcanoes), calderas (craters that exceed one kilometer in diameter), 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. 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.
- Active areas of volcanism are found along the oceanic ridges, adjacent to ocean trenches, as well as the interiors of plates themselves. Most active volcanoes are associated with plate boundaries.