The following statements summarize and describe many of the key terms and concepts 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. Rhyolitic 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 that 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).
- Successive eruptions of lava from a central vent result in a mountainous
accumulation of material known as a volcano. Located at the summit of many
volcanoes is a steep-walled depression called a crater. Shield
cones are broad, slightly domed volcanoes built primarily of fluid, basaltic lava.
Cinder cones have steep slopes composed of pyroclastic material.
Composite cones, or stratovolcanoes, are large, nearly
symmetrical structures built of interbedded lavas and pyroclastic deposits. Composite cones
produce some of the most violent volcanic activity. Often associated with a violent eruption
is a nuée ardente, a fiery cloud of hot gases infused with incandescent
ash that races down steep volcanic slopes. Large composite cones may also generate a type of
mudflow known as a lahar.
- Most volcanoes are fed by conduits or pipes. As erosion
progresses, the rock occupying the pipe is often more resistant and may remain standing
above the surrounding terrain as a volcanic neck. The summits of some
volcanoes have large, nearly circular depressions called calderas that result
from collapse following an explosive eruption. Calderas also form on shield volcanos by
subterranean drainage from a central magma chamber, and the largest calderas form by the
discharge of colossal volumes of silica-rich pumice along ring fractures. Although volcanic
eruptions from a central vent are the most familiar, by far the largest amounts of volcanic
material are extruded from cracks in the crust called fissures. The term
flood basalts describes the fluid, waterlike, basaltic lava flows that cover
an extensive region in the northwestern United States known as the Columbia Plateau. When
silica-rich magma is extruded, pyroclastic flows consisting largely of ash and
pumice fragments usually result.
- 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 (tablelike) 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 make up the cores of
- Magma originates from essentially solid rock of the crust and mantle. In addition to
a rock's composition, its temperature, depth (confining pressure), and water content
determine whether it exists as a solid or liquid. Thus, magma can be generated by
raising a rock's temperature, as occurs when a hot mantle plume "ponds"
beneath crustal rocks. A decrease in pressure can cause decompression
melting. Further, the introduction of volatiles (water) can lower a
rock's melting point sufficiently to generate magma. Because melting is generally not
complete, a process called partial melting produces a melt made of the
lowest-melting-temperature minerals, which are higher in silica than the original rock.
Thus, magmas generated by partial melting are nearer to the granitic (felsic) end of the
compositional spectrum than are the rocks from which they formed.
- Most active volcanoes are associated with plate boundaries. Active
areas of volcanism are found along oceanic ridges where seafloor spreading is occurring
(divergent plate boundaries), in the vicinity of ocean trenches where one
plate is being subducted beneath another (convergent plate boundaries), and in
the interiors of plates themselves (intraplate volcanism). Rising plumes of
hot mantle rock are the source of most intraplate volcanism.