25
The Face of Earth
floats on top of the deformable rocks of the mantle at a higher level than oceanic crust for
the same reason that a large, empty (less dense) cargo ship rides higher than a small, loaded
(more dense) one.
Major Features of the Continents
The largest features of the continents can be grouped into two distinct categories: (1) extensive, flat stable areas that have been eroded nearly to sea level and (2) uplifted regions of
deformed rocks that make up present-day mountain belts. Notice in FIGURE 1.28 that the
young mountain belts tend to be long, narrow features at the margins of continents, and
the flat, stable areas are typically located in the interior of the continents.
MOUNTAIN BELTS. The most prominent topographic features of the continents are linear
mountain belts. Although the distribution of mountains appears to be random, this is not
the case. When the youngest mountains are considered (those less than 100 million years
old), we find that they are located principally in two major zones. The circum-Pacific belt
(the region surrounding the Pacific Ocean) includes the mountains of the western Americas and continues into the western Pacific in the form of volcanic island arcs. Island arcs
are active mountainous regions composed largely of volcanic rocks and deformed sedimentary rocks. Examples include the Aleutian Islands, Japan, the Philippines, and New Guinea
(Figure 1.27).
The other major mountainous belt extends eastward from the Alps through Iran and
the Himalayas and then dips southward into Indonesia. Careful examination of mountainous terrains reveals that most are places where thick sequences of rocks have been squeezed
and highly deformed, as if placed in a gigantic vise. Older mountains are also found on the
continents. Examples include the Appalachians in the eastern United States and the Urals in
Russia. Their once lofty peaks are now worn low, the result of millions of years of erosion.
THE STABLE INTERIOR. Unlike the young mountain belts, which have formed within the
last 100 million years, the interiors of the continents have been relatively stable (undisturbed) for the last 600 million years or even longer. Typically, these regions were involved
in mountain-building episodes much earlier in Earth’ s history.
Within the stable interiors are areas known as shields, which are expansive, flat regions
composed of deformed crystalline rock. Notice in Figure 1.28 that the Canadian Shield is
exposed in much of the northeastern part of North America. Age determinations for various
shields have shown that they are truly ancient regions. All contain Precambrian-age rocks
that are more than 1 billion years old, with some samples approaching 4 billion years in age
(see Figure 1.8 to review the geologic time scale). These oldest-known rocks exhibit evidence of enormous forces that have folded and faulted them and altered them with great
heat and pressure. Thus, we conclude that these rocks were once part of an ancient mountain system that has since been eroded away to produce these expansive, flat regions.
Other flat areas of the stable interior exist in which highly deformed rocks, like those
found in the shields, are covered by a relatively thin veneer of sedimentary rocks. These
areas are called stable platforms. The sedimentary rocks in stable platforms are nearly
horizontal except where they have been warped to form large basins or domes. In North
America a major portion of the stable platforms is located between the Canadian Shield
and the Rocky Mountains (Figure 1.28).
Major Features of the Ocean Basins
If all water were drained from the ocean basins, a great variety of features would be seen,
including linear chains of volcanoes, deep canyons, extensive plateaus, and large expanses
of monotonously flat plains. In fact, the scenery would be nearly as diverse as that on the
continents (see Figure 1.27).
During the past 60 years, oceanographers using modern depth-sounding
equipment have gradually mapped significant portions of the ocean floor. From these
studies they have defined three major
regions: continental margins, deep-ocean
basins, and oceanic (mid-ocean) ridges.
CONTINENTAL MARGINS. The
continental margin is that portion of the
seafloor adjacent to major landmasses. It
may include the continental shelf, the
continental slope, and the continental rise.
Although land and sea meet at the
shoreline, this is not the boundary between
the continents and the ocean basins.
Rather, along most coasts a gently sloping
platform, called the continental shelf,
extends seaward from the shore. Because it
is underlain by continental crust, it is considered a flooded extension of the continents. A glance at Figure 1.27 shows that
the width of the continental shelf is variable. For example, it is broad along the East
and Gulf coasts of the United States but
relatively narrow along the Pacific margin
of the continent.
The boundary between the continents
and the deep-ocean basins lies along the
continental slope, which is a relatively
steep dropoff that extends from the outer
edge of the continental shelf to the floor of
the deep ocean (Figure 1.27). Using this
as the dividing line, we find that about
60 percent of Earth’ s surface is represented
by ocean basins and the remaining
40 percent by continents.
D I D Y O U K N O W ?
Ocean depths are often expressed in
fathoms. One fathom equals 1.8 m or
6 ft, which is about the distance of a
person’s outstretched arms. The term is
derived from how depth-sounding lines
were brought back on board a vessel by
hand. As the line was hauled in, a
worker counted the number of arm
lengths collected. By knowing the
length of the person’s outstretched
arms, the amount of line taken in could
be calculated. The length of one fathom
was later standardized to 6 ft.
The Face of Earth
floats on top of the deformable rocks of the mantle at a higher level than oceanic crust for
the same reason that a large, empty (less dense) cargo ship rides higher than a small, loaded
(more dense) one.
Major Features of the Continents
The largest features of the continents can be grouped into two distinct categories: (1) extensive, flat stable areas that have been eroded nearly to sea level and (2) uplifted regions of
deformed rocks that make up present-day mountain belts. Notice in FIGURE 1.28 that the
young mountain belts tend to be long, narrow features at the margins of continents, and
the flat, stable areas are typically located in the interior of the continents.
MOUNTAIN BELTS. The most prominent topographic features of the continents are linear
mountain belts. Although the distribution of mountains appears to be random, this is not
the case. When the youngest mountains are considered (those less than 100 million years
old), we find that they are located principally in two major zones. The circum-Pacific belt
(the region surrounding the Pacific Ocean) includes the mountains of the western Americas and continues into the western Pacific in the form of volcanic island arcs. Island arcs
are active mountainous regions composed largely of volcanic rocks and deformed sedimentary rocks. Examples include the Aleutian Islands, Japan, the Philippines, and New Guinea
(Figure 1.27).
The other major mountainous belt extends eastward from the Alps through Iran and
the Himalayas and then dips southward into Indonesia. Careful examination of mountainous terrains reveals that most are places where thick sequences of rocks have been squeezed
and highly deformed, as if placed in a gigantic vise. Older mountains are also found on the
continents. Examples include the Appalachians in the eastern United States and the Urals in
Russia. Their once lofty peaks are now worn low, the result of millions of years of erosion.
THE STABLE INTERIOR. Unlike the young mountain belts, which have formed within the
last 100 million years, the interiors of the continents have been relatively stable (undisturbed) for the last 600 million years or even longer. Typically, these regions were involved
in mountain-building episodes much earlier in Earth’ s history.
Within the stable interiors are areas known as shields, which are expansive, flat regions
composed of deformed crystalline rock. Notice in Figure 1.28 that the Canadian Shield is
exposed in much of the northeastern part of North America. Age determinations for various
shields have shown that they are truly ancient regions. All contain Precambrian-age rocks
that are more than 1 billion years old, with some samples approaching 4 billion years in age
(see Figure 1.8 to review the geologic time scale). These oldest-known rocks exhibit evidence of enormous forces that have folded and faulted them and altered them with great
heat and pressure. Thus, we conclude that these rocks were once part of an ancient mountain system that has since been eroded away to produce these expansive, flat regions.
Other flat areas of the stable interior exist in which highly deformed rocks, like those
found in the shields, are covered by a relatively thin veneer of sedimentary rocks. These
areas are called stable platforms. The sedimentary rocks in stable platforms are nearly
horizontal except where they have been warped to form large basins or domes. In North
America a major portion of the stable platforms is located between the Canadian Shield
and the Rocky Mountains (Figure 1.28).
Major Features of the Ocean Basins
If all water were drained from the ocean basins, a great variety of features would be seen,
including linear chains of volcanoes, deep canyons, extensive plateaus, and large expanses
of monotonously flat plains. In fact, the scenery would be nearly as diverse as that on the
continents (see Figure 1.27).
During the past 60 years, oceanographers using modern depth-sounding
equipment have gradually mapped significant portions of the ocean floor. From these
studies they have defined three major
regions: continental margins, deep-ocean
basins, and oceanic (mid-ocean) ridges.
CONTINENTAL MARGINS. The
continental margin is that portion of the
seafloor adjacent to major landmasses. It
may include the continental shelf, the
continental slope, and the continental rise.
Although land and sea meet at the
shoreline, this is not the boundary between
the continents and the ocean basins.
Rather, along most coasts a gently sloping
platform, called the continental shelf,
extends seaward from the shore. Because it
is underlain by continental crust, it is considered a flooded extension of the continents. A glance at Figure 1.27 shows that
the width of the continental shelf is variable. For example, it is broad along the East
and Gulf coasts of the United States but
relatively narrow along the Pacific margin
of the continent.
The boundary between the continents
and the deep-ocean basins lies along the
continental slope, which is a relatively
steep dropoff that extends from the outer
edge of the continental shelf to the floor of
the deep ocean (Figure 1.27). Using this
as the dividing line, we find that about
60 percent of Earth’ s surface is represented
by ocean basins and the remaining
40 percent by continents.
D I D Y O U K N O W ?
Ocean depths are often expressed in
fathoms. One fathom equals 1.8 m or
6 ft, which is about the distance of a
person’s outstretched arms. The term is
derived from how depth-sounding lines
were brought back on board a vessel by
hand. As the line was hauled in, a
worker counted the number of arm
lengths collected. By knowing the
length of the person’s outstretched
arms, the amount of line taken in could
be calculated. The length of one fathom
was later standardized to 6 ft.