Download SUMMARY KEY TERMS APPLYING THE CONCEPTS

484
Chapter Nineteen: The Earth
What is generally accepted about the plate tectonic theory
is the understanding that the solid materials of the earth are
engaged in a continual cycle of change. Oceanic crust is subducted, melted, then partly returned to the crust as volcanic
igneous rocks in island arcs and along continental plate boundaries. Other parts of the subducted crust become mixed with the
upper mantle, returning as new crust at diverging boundaries.
The materials of the crust and the mantle are thus cycled back
and forth in a mixing that may include the deep mantle and the
core as well. There is more to this story of a dynamic earth that
undergoes a constant change. The story continues in the next
chapters with different cycles to consider.
Locate and label the major plates of the lithosphere on an
outline map of the world according to the most recent findings
in plate tectonics. Show all types of boundaries and associated
areas of volcanoes and earthquakes.
15 percent of the earth’s total volume and about a third of its total
mass. The mantle is the middle part of the earth’s interior that
accounts for about two-thirds of the earth’s total mass and about 80
percent of its total volume. The Mohorovicic discontinuity separates
the outer layer, or crust, of the earth from the mantle. The crust of
the continents is composed mostly of less dense granite-type rock.
The crust of the ocean basins is composed mostly of the more dense
basaltic rocks.
Another way to consider the earth’s interior structure is to consider the weak layer in the upper mantle, the asthenosphere that extends
around the entire earth. The rigid, solid, and brittle layer above the
asthenosphere is called the lithosphere. The lithosphere includes the
entire crust, the Moho, and the upper part of the mantle.
Evidence from the ocean floor that was gathered in the 1950s and
1960s revived interest in the idea that continents could move. The evidence for seafloor spreading came from related observations concerning oceanic ridge systems, sediment and fossil dating of materials outward from the ridge, and from magnetic patterns of seafloor rocks.
Confirmation of seafloor spreading led to the plate tectonic theory.
According to plate tectonics, new basaltic crust is added at diverging
boundaries of plates, and old crust is subducted at converging boundaries. Mountain building, volcanoes, and earthquakes are seen as
related geologic features that are caused by plate movements. The force
behind the movement of plates is uncertain, but it may involve convection in the deep mantle.
SUMMARY
The elements silicon and oxygen make up 75 percent of all the elements
in the outer layer, or crust, of the earth. The elements combine to make
crystalline chemical compounds called minerals. A mineral is defined as
a naturally occurring, inorganic solid element or compound with a
crystalline structure.
About 92 percent of the minerals of the earth’s crust are composed
of silicon and oxygen, the silicate minerals. The basic unit of the silicates
is a tetrahedral structure that combines with positive metallic ions or
with other tetrahedral units to form chains, sheets, or an interlocking
framework. The ferromagnesian silicates are tetrahedral structures combined with ions of iron, magnesium, calcium, and other elements. The
ferromagnesian silicates are darker in color and more dense than other
silicates. The nonferromagnesian silicates do not have irons or magnesium ions and they are lighter in color and less dense than the ferromagnesians. The nonsilicate minerals do not contain silicon and are carbonates, sulfates, oxides, halides, sulfides, and native elements.
A rock is defined as an aggregation of one or more minerals that
have been brought together into a cohesive solid. Igneous rocks formed
as hot, molten magma cooled and crystallized to firm, hard rocks. Sedimentary rocks are formed from sediments, accumulations of weathered
rock materials that settle out of the atmosphere or out of water. Sediments become sedimentary rocks through a rock-forming process that
involves both the compaction and cementation of the sediments. Metamorphic rocks are previously existing rocks that have been changed by
heat, pressure, or hot solution into a different kind of rock without
melting. The rock cycle is a concept that an igneous, a sedimentary, or a
metamorphic rock is a temporary stage in the ongoing transformation
of rocks to new types.
The earth has a layered interior that formed as the earth’s materials underwent differentiation, the separation of materials while in
the molten state. The center part, or core, is predominantly iron with
a solid inner part and a liquid outer part. The core makes up about
KEY TERMS
asthenosphere (p. 475)
convergent boundaries
(p. 481)
core (p. 474)
crust (p. 473)
divergent boundaries (p. 481)
igneous rocks (p. 469)
island arcs (p. 482)
lithosphere (p. 475)
mantle (p. 474)
metamorphic rocks (p. 470)
mineral (p. 463)
Mohorovicic discontinuity
(p. 473)
new crust zone (p. 481)
oceanic ridges (p. 478)
oceanic trenches (p. 478)
plate tectonics (p. 479)
rift (p. 478)
rock (p. 468)
seafloor spreading (p. 478)
sedimentary rocks (p. 469)
silicates (p. 465)
subduction zone (p. 481)
transform boundaries
(p. 482)
APPLYING THE CONCEPTS
1. Based on its abundance in the earth’s crust, most rocks will
contain a mineral composed of oxygen and the element
a. sulfur.
b. carbon.
c. silicon.
d. iron.