Download 16 - Glencoe

The Marine Environment
BIG Idea The marine
environment is geologically
diverse and contains a
wealth of natural resources.
Coral and tunicates
16.1 Shoreline Features
MAIN Idea The constant
erosion of the shoreline and
deposition of sediments by
ocean waves creates a changing
coastline.
16.2 Seafloor Features
MAIN Idea The ocean floor
contains features similar to those
on land and is covered with
sediments of several origins.
GeoFacts
• Coral reefs cover only 1 percent
of the ocean floor, yet about
25 percent of all marine fish
species are found living on
coral reefs.
Coral reef environment
• A chemical from the Caribbean
sea whip, a soft coral, is used
to make skin-care products
because it has anti-inflammatory properties.
• The blood of tunicates—softbodied organisms closely
related to vertebrates—contains high levels of the rare
metal vanadium.
436
(t)Reinhard Dirscherl/Visuals Unlimited, (b)Gary Bell/zefa/CORBIS, (bkgd)B.S.P.I./CORBIS
Start-Up Activities
Seafloor Features Make this
Foldable to diagram the major
geologic features of continental
margins and ocean basins.
LAUNCH Lab
Where does chalk form?
Although you might not live near a coast, parts
of your environment were shaped by the ocean.
For example, you might be just a few meters away
from former seafloor deposits that are now part of
the bedrock underground. One such seafloor deposit
is chalk. How can you tell that chalk formed on
the seafloor?
Procedure
1. Read and complete the lab safety form.
2. Use a mortar and pestle to grind a small
piece of natural chalk into a powder. Make
a slide of the powdered chalk.
3. Observe the chalk powder through a
microscope.
Analysis
1. Describe the powder. Are the grains irregular in shape or size? Do some of the grains
have patterns?
2. Analyze your data and hypothesize the origin of the chalk. On what evidence do you
base your conclusion?
Fold a horizontal sheet of paper in half
from side to side.
STEP 1
STEP 2 Fold into thirds.
Unfold and draw lines
lightly along the fold lines
to indicate three tabs.
Label the uncut
tabs Continent, Continental
Margin, and Ocean Basin.
Draw a diagram of these
seafloor features here.
STEP 3
Cut along the
fold lines to make three
tabs after your diagram
is complete.
STEP 4
Continent Continental Ocean
Basin
Margin
Continent Continental Ocean
Basin
Margin
FOLDABLES Use this Foldable with Section 16.2.
As you read this section, diagram the major
features of the seafloor. When you finish your
diagram, cut the tabs as described in Step 4.
Summarize the features shown in your diagram under each of the tabs.
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Chapter
Section 16
1 ••XXXXXXXXXXXXXXXXXX
The Marine Environment 437
1
Section
16
6.1
SC.912.E.6.2: Connect surface features to surface processes that are responsible for their formation. SC.912.E.6.5:
Describe the geologic development of the present day oceans and identify commonly found features. SC.912.E.7.8:
Explain how various atmospheric, oceanic, and hydrologic conditions in Florida have influenced and can influence
human behavior, both individually and collectively.
Objectives
◗ Explain how shoreline features are
formed and modified by marine
processes.
◗ Describe the major erosional and
depositional shoreline features.
◗ Identify protective structures used
near shore.
Review Vocabulary
breaker: collapsing wave that forms
when a wave enters shallow water
Vocabulary
beach
wave refraction
longshore bar
longshore current
barrier island
Shoreline Features
MAIN Idea The constant erosion of the shoreline and deposition
of sediments by ocean waves creates a changing coastline.
Real-World Reading Link Have you ever picked up a smoothly polished pebble
on the beach? A sculptor forms a masterpiece out of a chunk of wood or stone by
chipping away one tiny bit at a time. Waves are nature’s chisel, and beaches display the carefully carved features.
The Shore
Shown in Figure 16.1, the shore is the area of land between the
lowest water level at low tide and the highest area of land that is
affected by storm waves. Shores are places of continuous, often dramatic geologic activity—places where you can see geologic changes
occurring almost daily. The shoreline is the place where the ocean
meets the land. Shorelines are shaped by the action of waves, tides,
and currents. The location of the shoreline constantly changes as the
tide moves in and out. As waves erode some shorelines, they create
some of the most impressive rock formations on Earth. In other
areas, waves deposit loose material and build wide, sandy beaches.
Beaches Long stretches of U. S. coasts are lined with wide, sandy
beaches. A beach, shown in Figure 16.1, is the area in which sediment is deposited along the shore. Beaches are composed of loose
sediments deposited and moved about by waves along the shoreline.
The size of sediment particles depends on the energy of the waves
striking the coast and on the source of the sediment. Beaches
pounded by large waves or formed on rocky coasts usually consist of
coarse materials such as pebbles and cobbles.
■
Figure 16.1 The location of the shoreline changes as the tide moves in and out.
Coast
Identify How is a beach related to a shore?
Shore
Waves
Beach
High-tide shoreline
Low-tide shoreline
438
Chapter 16 • The Marine Environment
Beach
Shallow
water
Bay
Beach
■ Figure 16.2 Wave crests advance toward
the shoreline and slow down when they encounter
shallow water. This causes the wave crests to bend
toward the headlands and move in the direction of
the arrows.
Beach
Shallow
water
Headland
Bay
Cliff
Headland
Cliff
Bay
Formation of Shoreline Features
Large breaking waves can hurl thousands of metric tons of water,
along with suspended rock fragments, against a shore with such
force that they are capable of eroding solid rock.
Erosional features Waves move faster in deep water than in
shallow water. This difference in wave speed causes initially straight
wave crests to bend when part of the crest moves into shallow water,
a process known as wave refraction, illustrated in Figure 16.2.
Along an irregular coast with headlands and bays, the wave crests
bend toward the headlands. As a result, most of the breaker energy is
concentrated along the relatively short section of the shore around
the tips of the rocky headlands, while the remaining wave energy is
spread out along the much longer shoreline of the bays. The headlands thus undergo severe erosion. The material eroded from the
headlands is swept into the bays, where it is deposited to form
crescent-shaped beaches. The headlands are worn back and the
bays are filled in until the shoreline straightens.
Wave-cut platforms Many headlands have spectacular rock for-
mations. Generally, as a headland is worn away, a flat erosional surface called a wave-cut platform is formed. The wave-cut platform
terminates against a steep wave-cut cliff, as shown in Figure 16.3.
Original headland
Figure 16.3 A headland can be modified by
wave erosion. The dotted lines indicate the original
shape of the headland.
■
Wave-cut
cliff
Wave-cut platform
Section 1 • Shoreline Features 439
Elio Ciol/CORBIS
■ Figure 16.4 The sea stack and sea
arch shown here, along the coast of France,
were formed by wave refraction at a rocky
headland.
Sea stacks Differential erosion, the removal of weaker rocks
or rocks near sea level, produces many of the other characteristic
landforms of rocky headlands. As shown in Figure 16.4, a sea
stack is an isolated rock tower or similar erosional remnant left on
a wave-cut platform. A sea arch, also shown in Figure 16.4, is
formed as stronger rocks are undercut by wave erosion. Sea caves
are tubelike passages blasted into the headlands at sea level by the
constant assault of the breakers.
Longshore currents Suppose you stood on a beach at the edge
of the water and began to walk out into the ocean. As you walked,
the water might get deeper for a while, but then it would become
shallow again. The shallow water offshore lies above a sandbar,
called a longshore bar, that forms in front of most beaches, as illustrated in Figure 16.5. Waves break on the longshore bar in the area
known as the surf zone. The deeper water between the shore and
longshore bar is called the longshore trough.
The waves striking the beach are almost parallel to the shoreline, although the waves seaward of the longshore bar are generally
not parallel to the shore. This is another case of wave refraction.
The slowing of the waves in shallow water causes the wave crests
to bend toward the shore.
Figure 16.5 A typical beach profile
includes longshore troughs and bars within the
surf zone.
Explain What is the difference between
a longshore trough and a longshore bar?
■
Dunes
Shore
Longshore
trough
440
Chapter 16 • The Marine Environment
Surf zone
Longshore
bar
Waves approach
shore at an angle.
Path of sand particles
Direction of sediment
movement
Longshore current
■ Figure 16.6 Longshore currents are
driven by incoming waves.
Interactive Figure To see an
animation of longshore currents,
visit glencoe.com.
As water from incoming breakers spills over the longshore bar, a
current flowing parallel to the shore, called the longshore current, is
produced. This current varies in strength and direction from day to
day. Over the course of a year, because of prevailing winds and wave
patterns, one direction usually dominates.
Movement of sediments Longshore currents, shown in
Figure 16.6, move large amounts of sediment along the shore. Fine-
grained material, such as sand, is suspended in the turbulent, moving
water, and larger particles are pushed along the bottom by the current.
Additional sediment is moved back and forth on the beach by incoming and retreating waves. Incoming waves also move sediment at an
angle to the shoreline in the direction of wave motion. Overall, the
transport of sediment is in the direction of the longshore current.
On both the Atlantic and Pacific coasts of the United States, longshore
transport moves generally toward the south.
Reading Check Explain how the longshore current moves sediments.
Rip currents Wave action also produces rip currents, which flow
out to sea through gaps in the longshore bar. Rip currents, shown in
Figure 16.7, return the water spilled into the longshore trough to the
open ocean. These dangerous currents can reach speeds of several kilometers per hour. If you are ever caught in a rip current, you should not
try to swim against it, instead swim parallel to shore to get out of it.
Figure 16.7 Rip currents return water
through gaps in the longshore bar out to sea.
Rip currents spread out and weaken beyond
the longshore bar.
■
Shoreline
Longshore
bar
Longshore
current
Rip current
Longshore
bar
Section 1 • Shoreline Features 441
Lagoon
Lagoon
Bay
Spit
Spit
Mainland beach
Baymouth bar
Longshore
Barrier islands
current
Tombolo
Figure 16.8 Depositional features of coastlines include tombolos,
spits, baymouth bars, lagoons, and barrier islands.
■
Depositional features As a result of wave erosion, longshore
transport, and sediment deposition, most seashores are in a constant
state of change. Sediments are eroded by large storm waves and
deposited wherever waves and currents slow down. Sediments
moved and deposited by longshore currents build various characteristic coastal landforms, such as spits and barrier islands, illustrated
in Figure 16.8.
Spit A narrow bank of sand that projects into the water from a
bend in the coastline is called a spit. A spit, which forms where
a shoreline changes direction, is protected from wave action.
When a growing spit crosses a bay, a baymouth bar forms.
■ Figure 16.9 This barrier island off
the coast of North Carolina is one of
many that line the eastern coast.
Barrier islands Long ridges of sand or other sediment, deposited
or shaped by the longshore current, that are separated from the
mainland are called barrier islands. Barrier islands, like the ones
shown in Figure 16.9, can be several kilometers wide and tens
of kilometers long. Most of the Gulf Coast and the eastern coast
south of New England is lined with an almost continuous chain
of barrier islands.
Baymouth bars A baymouth bar, shown in Figure 16.10,
forms when a spit closes off a bay. The shallow, protected bodies of
water behind baymouth bars and barrier islands are called lagoons,
which are saltwater coastal lakes that are connected to the open sea
by shallow, restricted outlets.
Tombolo Another coastal landform is a tombolo, shown in
Figure 16.10. A tombolo is a ridge of sand that forms between the
mainland and an island and connects the island to the mainland.
When this happens, the island is no longer an island, but is the tip of
a peninsula.
Reading Check Describe how a tombolo is formed.
442 Chapter 16 • The Marine Environment
Dr. Frank M. Hanna/Visuals Unlimited
(tl)Joseph Melanson/Aero Photo Inc/www.skypic.com, (tr)Joseph Melanson/Aero Photo Inc/www.skypic.com, (bl)T. Michot/USGS, (bc)T. Michot/USGS
Baymouth bar
Tombolo
Natural and human effects on the coast All of these
depositional coastal landforms, including large barrier islands, are
unstable and temporary. Occasionally, major storms sweep away
entire sections of barrier islands and redeposit the material elsewhere. Figure 16.11 shows the existence of South Gosier Island, a
barrier island off the coast of Louisiana in August of 2004, a month
before Hurricane Ivan passed over it. The island was completely
destroyed by the strong waves generated by Hurricane Ivan. Even
in the absence of storms, however, changing wave conditions can
slowly erode beaches and rearrange entire shorelines. For example,
the shoreline of Cape Cod, Massachusetts, is retreating by as much
as 1 m per year.
People are drawn to coastal areas for their rich natural resources,
mild climate, and recreation opportunities. Coastal areas in the United
States make up only 17 percent of contiguous land areas, but are currently home to over half of the population in the nation. Increasing
population has caused substantial coastal environmental changes,
including pollution, shoreline erosion, and wetland and wildlife-habitat loss. These coastal changes in turn increase the susceptibility of the
communities to natural hazards caused by hurricanes and tsunamis.
Before
After
■ Figure 16.10 Baymouth bars and tombolos are examples of features formed by the
deposition of sediments.
■ Figure 16.11 The island of South Gosier,
a barrier island off the coast of Louisiana, was
completely washed away by Hurricane Ivan in
September, 2004.
Section 1 • Shoreline Features 443
Groins
Erosion
of sand
Deposition
of sand
Sand
movement
Erosion
of sand
Jetties
Figure 16.12 Seawalls deflect the energy of waves on the beach. Groins
and jetties deprive downshore beaches of sand.
■
Protective Structures
In many coastal areas, protective structures such as seawalls,
groins, jetties, and breakwaters are built in an attempt to prevent
beach erosion and destruction of oceanfront properties. However,
these artificial structures interfere with natural shoreline processes
and can have unexpected negative effects.
■ Figure 16.13 A breakwater slows
the movement of water against the
coast, which results in sediment being
deposited behind the structure.
Seawalls Structures called seawalls, shown in Figure 16.12,
are built parallel to shore, often to protect beachfront properties
from powerful storm waves. Seawalls reflect the energy of such
waves back toward the beach, where they worsen beach erosion.
Eventually, seawalls are undercut and have to be rebuilt larger and
stronger than before.
Groins and jetties Groins, shown in Figure 16.12, are walllike structures built into the water perpendicular to the shoreline for
the purpose of trapping beach sand. Groins interrupt natural longshore transport and deprive beaches down the coast of sand. The
result is aggravated beach erosion down the coast from groins.
Similar effects are caused by jetties, which are walls of concrete built
to protect a harbor entrance from drifting sand. Jetties are also
shown in Figure 16.12.
Breakwaters Structures like the one shown in Figure 16.13 that
are built to provide anchorages for small boats or a calm beach area
are called breakwaters. Breakwaters are built parallel to the shoreline.
When the current slows down behind a breakwater and is no longer
able to move sediment, it deposits sediment behind the breakwater.
If the accumulating sediment is left alone, it can eventually fill an
anchorage. To prevent this, anchorages have to be dredged regularly.
444 Chapter 16 • The Marine Environment
(t)Sexto Sol/Getty Images, (b)Tom Bean/CORBIS
Deposition
of sand
Changes in Sea Level
VOCABULARY
At the height of the last ice age, approximately 20,000 years ago, the
global sea level was about 130 m lower than it is at present. Since
that time, the melting of most of the ice-age glaciers has raised the
ocean to its present level. In the last 100 years, the global sea level
has risen 10 to 15 cm. It continues to rise slowly; estimates suggest
a rise in sea level of 3 mm/year.
Many scientists contend that this continuing rise in sea level is the
result of global warming. During the last century, Earth’s average surface temperature has increased by approximately 0.5°C. As Earth’s surface temperature rises, seawater warms up and expands, which adds to
the total volume of the seas. In addition, higher temperatures on
Earth’s surface cause glaciers to melt, and the meltwater flowing into
the oceans increases their volume.
ACADEMIC VOCABULARY
Estimate
a rough or approximate calculation
The estimate for the new roof was
$3000.
Effects of sea level changes If Earth’s remaining polar ice
sheets in Greenland and Antarctica melted completely, their meltwater
would raise sea level by another 70 m. This rise would completely
flood some countries, such as the Netherlands, along with some
coastal cities in the United States, such as New York City, and lowlying states, such as Florida and Louisiana. Measurements from
NASA’s GRACE satellite, launched in 2002, have indicated that the
Greenland ice sheet is melting at an accelerating rate. The West
Antarctic Ice Sheet has also been thinning according to a decade of
satellite measurements. A complete melt of the Greenland Ice Sheet
would lead to a sea level rise of about 6 to 7 m.
Many of the barrier islands of the Atlantic and Gulf Coasts
might be former coastal dunes that were drowned by rising sea levels. Other features produced by rising sea levels are the fjords of
Norway, shown in Figure 16.14. Fjords (fee ORDZ) are deep
coastal valleys that were scooped out by glaciers during the ice age
and later flooded when the sea level rose.
Figure 16.14 Fjords are flooded
U-shaped valleys that can be up to 1200 m
deep.
■
Section 1 • Shoreline Features 445
Anders Blomqvist/Lonely Planet Images
Dr. Marli Miller/Visuals Unlimited
Figure 16.15 Elevated marine terraces are former wave-cut platforms that
are now well above the current sea level.
This elevated marine terrace is near Half
Moon Bay, California.
■
Effects of tectonic forces Other processes that affect local
sea levels are tectonic uplift and sinking. If a coastline sinks, there
is a relative rise in sea level along that coast. A rising coastline,
however, produces a relative drop in sea level. As a result of tectonic forces in the western United States, much of the West Coast
is being pushed up much more quickly than the sea level is rising.
Because much of the West Coast was formerly under water, it is
called an emergent coast.
Emergent coasts tend to be relatively straight because the
exposed seafloor topography is smoother than typical land surfaces
with hills and valleys. Other signs of an emergent coast are former
shoreline features such as sandy beach ridges located far inland.
Among the most interesting of these features are elevated marine
terraces — former wave-cut platforms that are now dry and well
above current sea level. Figure 16.15 shows a striking example of
such a platform. Some old wave-cut platforms in Southern California are hundreds of meters above current sea level.
1
Section
16.1
6.1
Assessment
SC.912.E.6.2, SC.912.E.6.5, SC.912.E.7.8
Section Summary
Understand Main Ideas
◗ Wave erosion of headlands produces
wave-cut platforms and cliffs, sea
stacks, sea arches, and sea caves.
1.
◗ Wave action and longshore currents
move sediment along the shore and
build depositional features.
3. Apply How do jetties and groins affect the longshore current?
◗ Artificial protective structures interfere with longshore transport.
Think Critically
◗ Sea levels in the past were 130 m
lower than at present.
446 Chapter 16 • The Marine Environment
MAIN Idea Shorelines have headlands and bays. Which experiences the most
severe erosion by breakers? Why?
2. Describe What are sea stacks, and how are they formed?
4. Analyze What effect does a seawall have on a beach?
5. Evaluate why resort communities built on barrier islands spend thousands of
dollars each year to add sand to the beaches along the shoreline.
MATH in Earth Science
6. The city of Orlando, Florida, is 32 m above sea level. If the sea level continues to
rise at the highest estimated rate of 3 mm/y, in how many years might Orlando be
under water?
Self-Check Quiz glencoe.com
SC.912.E.6.2: Connect surface features to surface processes that are responsible for their formation. SC.912.E.6.5:
Describe the geologic development of the present day oceans and identify commonly found features. ALSO COVERS:
SC.912.N.1.1, MA.912.S.1.2, MA.912.S.3.2
1
Section
16.
6 .2
Objectives
Seafloor Features
◗ Describe the major geologic
features of continental margins.
◗ Identify the major geologic
features of ocean basins.
◗ Describe the different types of
marine sediments and their origin.
MAIN Idea The ocean floor contains features similar to those on
land and is covered with sediments of several origins.
Real-World Reading Link Why do you shake a container of orange juice
before pouring a glass? The juice appears thicker at the bottom of the container
because the pulp sinks to the bottom. Similarly, gravity causes fine grains of
sand to settle to the bottom of the ocean.
Review Vocabulary
sediment: solid particles deposited
on Earth’s surface that can form
sedimentary rocks by processes
such as weathering, erosion,
deposition, and lithification
The Continental Margin
If you were asked to draw a map of the seafloor, what kind of topographic features would you include? Until recently, most people
had little knowledge of the features of the ocean floor. However,
modern oceanographic techniques, including satellite data, reveal
that the topography of the ocean bottom is as varied as that of the
continents.
The topography of the seafloor is surprisingly rough and irregular, with numerous high mountains and deep depressions. The
deepest place on the seafloor, the Marianas Trench in the Pacific
Ocean, is about 11 km deep.
Study Figure 16.16. Notice that the continental margin is the
area where edges of continents meet the ocean. It consists of continental crust, covered with sediments, that eventually meets oceanic
crust. Continental margins represent the shallowest parts of the
ocean. As shown in Figure 16.16, a continental margin includes the
continental shelf, the continental slope, and the continental rise.
Vocabulary
continental margin
continental shelf
continental slope
turbidity current
continental rise
abyssal plain
deep-sea trench
mid-ocean ridge
seamount
guyot
Figure 16.16 A cross section of the ocean
reveals a diverse topography with many features
that are similar to those on land.
■
Continental shelf The shallowest part of a continental margin
extending seaward from the shore is the continental shelf.
Continental shelves vary greatly in width, averaging 60 km wide.
On the Pacific coast of the United States, the continental shelf is
only a few kilometers wide, whereas the continental shelf of the
Atlantic coast is hundreds of kilometers wide.
Continental margin
Continental margin
Submarine
canyon
Land
Continental
shelf Continental
Seamount
Continental
slope
Abyssal
plain
slope
Continental
crust
Trench
Continental
shelf
Mid-ocean
ridge
Oceanic crust
Land
Continental
crust
Continental
rise
Section 2 • Seafloor Features 447
VOCABULARY
SCIENCE USAGE V. COMMON USAGE
Shelf
Science usage: the sloping border of a
continent or island
Common usage: a flat piece of
material attached to a wall on which
objects are placed
The average depth of the water above continental shelves is about
130 m. Recall that sea level during the last ice age was approximately
130 m lower than at present; therefore, most of the world’s continental shelves must have been above sea level at that time. As a result,
present-day coastlines are radically different from the way they were
during the last ice age. At that time, Siberia was attached to North
America by the Bering land bridge, Great Britain was attached to
Europe, and a large landmass existed where today there are only
the widely scattered islands of the Bahamas.
When Earth’s surface began to warm after the last ice age, and the
continental ice sheets began to melt, the sea gradually covered up the
continental shelves. Beaches and other coastal landforms from that
time are now submerged and located far beyond the present shoreline.
Commercially valuable fishes now inhabit the shallow, nutrient-rich
waters of the continental shelves. In addition, the thick sedimentary
deposits on the shelves are significant sources of oil and natural gas.
Reading Check List the resources that can be found on the
continental shelf.
Continental slope Beyond the continental shelves, the seafloor drops away quickly to depths of several kilometers, with
slopes averaging nearly 100 m/km. These sloping regions are the
continental slopes. To marine geologists, the continental slope is
the true edge of a continent because it generally marks the edge of
the continental crust. In many places, this slope is cut by deep submarine canyons, which are shown in Figure 16.17. Submarine
canyons are similar to canyons on land and some are comparable
in size to the Grand Canyon in Arizona.
These submarine canyons were cut by turbidity currents, which
are rapidly flowing water currents along the bottom of the sea that
carry heavy loads of sediments, similar to mudflows on land. Turbidity currents, shown in Figure 16.18, might originate as underwater
landslides on the continental slope that are triggered by earthquakes,
or they might originate from sediment stirred up by large storm waves
on the continental shelf. Turbidity currents can reach speeds exceeding
30 km/h and effectively erode bottom sediments and bedrock.
Figure 16.17 Submarine canyons
are similar to canyons on land. These
deep cuts in the continental slope vary
in size and can be as deep as the Grand
Canyon.
Explain how submarine canyons
are formed.
■
448 Chapter 16 • The Marine Environment
Jerome Neufeld/The Experimental Nonlinear Physics Group/The University of Toronto
Continental rise The gently sloping accumulation of deposits
from turbidity currents that forms at the base of the continental
slope is called a continental rise. A continental rise can be several
kilometers thick. The rise gradually becomes thinner and eventually merges with the sediments of the seafloor beyond the continental margin. In some places, especially around the Pacific Ocean,
the continental slope ends in deeper depressions in the seafloor,
known as deep-sea trenches. In such places, there is no continental
rise at the foot of the continental margin.
Reading Check Differentiate between the continental slope and the
continental rise.
SC.912.E.6.5: Describe the geologic development
of the present day oceans and identify commonly
found features. MA.912.S.1.2: Determine
appropriate and consistent standards of
measurement for the data to be collected in a
survey or experiment.
P NO a
PROBLEM-SOLVING
VI
Lab
L
Interpret Graphs
How do surface elevations compare? A useful comparison of the heights of the continents
to the depths of the oceans is given by the
curve in the graph below.
Elevation
(km)
Surface Elevations on Earth
Depth
(km)
b
Figure 16.18 Turbidity currents flow
along the seafloor because the seawatersediment mixture of the current is denser
than seawater alone. Scientists study turbidity
currents in the lab by simulating them in
glass tanks, as shown.
■
10
8
6
4
2
0
2
4
6
8
10
Land
29.2%
Ocean
70.8%
Mountains
Analysis
1. Approximately how tall is the highest mountain on Earth’s surface in kilometers?
2. At about what depth would you begin to
find trenches on the ocean floor?
3. What percentage of Earth’s surface is above
current sea level?
4. What percentage of Earth’s surface is represented by the continental margin?
Ocean surface
Continents
Trenches
Continental
margins
Ocean basins
0
20
40
60
80
100
Think Critically
5. Calculate The oceanic crust is the part of
the crust that is at a depth of 2 km or more
below sea level. What percentage of Earth’s
surface lies above the oceanic crust?
Percentage of Earth’s surface
Section 2 • Seafloor Features 449
Master Page used: NGS
Visualizing the Ocean Floor
Figure 16.19 The ocean floor has topographic features, including mid-ocean ridges, trenches, abyssal
plains, and seamounts.
Fracture zones run perpendicular to mid-ocean
ridges.
Abyssal plains are
smooth, flat areas of
the deep ocean covered
with fine-grained
sediments.
Seamounts are submerged volcanoes
that are more than
1 km high.
Mid-ocean ridges
run through all
ocean basins.
Trenches are the
deepest areas of
the ocean.
To explore more about the ocean
floor, visit glencoe.com.
450 Chapter 16 • The Marine Environment
Marie Tharp
(t)Marie Tharp, (b)Science VU/NGDC/Visuals Unlimited
Deep-Ocean Basins
Beyond the continental margin are ocean basins, which represent
about 60 percent of Earth’s surface and contain some of Earth’s most
interesting topography. Figure 16.19 shows the topography of the
ocean basin beneath the Atlantic Ocean.
Abyssal plains The flattest parts of the ocean floor 5 or 6 km
below sea level are called abyssal plains. Abyssal plains, shown in
Figure 16.19, are plains covered with hundreds of meters of finegrained muddy sediments and sedimentary rocks that were deposited on top of basaltic volcanic rocks.
Deep-sea trenches The deepest parts of the ocean basins are
the deep-sea trenches, which are elongated, sometimes arc-shaped
depressions in the seafloor several kilometers deeper than the
adjacent abyssal plains. Many deep-sea trenches lie next to chains
of volcanic islands, such as the Aleutian Islands of Alaska, and
most of them are located around the margins of the Pacific Ocean.
Deep-sea trenches are relatively narrow, about 100 km wide,
but they can extend for thousands of kilometers. The Peru-Chile
trench, shown in Figure 16.20, is almost 6000 km long and has
an average width of 40 km.
Mid-ocean ridges The most prominent features of the ocean
basins are the mid-ocean ridges, which run through all the ocean
basins and have a total length of more than 65,000 km—a distance
greater than Earth’s circumference. Mid-ocean ridges have an average
height of 1500 m, but they can be thousands of kilometers wide. The
highest peaks in mid-ocean ridges are over 6 km tall and emerge from
the ocean as volcanic islands. Mid-ocean ridges are sites of frequent
volcanic eruptions and earthquake activity. The crests of these ridges
often have valleys called rifts running through their centers. Rifts can
be up to 2 km deep.
Mid-ocean ridges do not form continuous lines. They are broken
into a series of shorter, stepped sections, which run at right angles
across each mid-ocean ridge. The areas where these breaks occur
are called fracture zones, shown in Figure 16.21. Fracture zones are
about 60 km wide, and they curve gently across the seafloor, sometimes for thousands of kilometers.
■ Figure 16.20 The Peru-Chile
trench runs along the west coast of
South America.
FOLDABLES
Incorporate information
from this section into
your Foldable.
■ Figure 16.21 There are many fracture zones along the Mid-Atlantic Ridge.
Fracture zone
Fracture zone
Section 2 • Seafloor Features 451
Black smoker
White smoker
Hydrothermal vents A hydrothermal vent is a hole in the seafloor
through which fluid heated by magma erupts. Most hydrothermal
vents are located along the bottom of the rifts in mid-ocean ridges.
When the heated fluid that erupts from these vents contains metal
oxides and sulfides, they immediately precipitate out of the fluid and
produce thick, black, smokelike plumes. This type of hydrothermal
vent, known as a black smoker, ejects superheated water with temperatures of up to 350°C. Figure 16.22 illustrates the black smokers
found in the rift valley of a mid-ocean ridge.
A second type of vent, known as a white smoker, also shown in
Figure 16.22, ejects warm water. Smokers are caused by seawater
circulating through the hot crustal rocks in the centers of midocean ridges. The fundamental cause of mid-ocean ridges and the
volcanic activity associated with them is plate tectonics, which you
will read about in Chapter 17.
Reading Check Identify where most hydrothermal vents are located.
Seamounts and guyots Satellite data have revealed that the
ocean floor is dotted with tens of thousands of solitary mountains.
These mountains are not located near areas of active volcanism.
How, then, did they form? You have learned that the ocean basins
are volcanically active at mid-ocean ridges and fracture zones. The
almost total absence of earthquakes in most other areas of the seafloor suggests that volcanism in those areas must have ceased a
long time ago. Thus, most of the mountains on the seafloor are
probably extinct volcanoes.
Investigations of individual volcanoes on the seafloor have
revealed that there are two types: seamounts and guyots (GEE ohz).
Seamounts are submerged basaltic volcanoes more than 1 km high.
Many linear chains of seamounts, such as the Emperor seamount
chain, are stretched out across the Pacific Ocean basin in roughly
the same direction. Guyots are large, extinct, basaltic volcanoes with
flat, submerged tops.
452
Chapter 16 • The Marine Environment
(tc)B. Murton/Southampton Oceanography Centre/Photo Researchers, (tr)NOAA/www.oceanexplorer.noaa.gov
■ Figure 16.22 Black smokers form when
metal oxides and sulfides precipitate out of
fluid heated by magma. White smokers form
when elements such as calcium and barium
precipitate out of warm water ejected from rifts
in mid-ocean ridges.
Steve Gschmeissner/Photo Researchers
Marine Sediments
The sediments that cover the ocean floor come from a variety of
sources, but most come from the continents. Land-derived sediments
include mud and sand washed into the oceans by rivers, as well as dust
and volcanic ash blown over the ocean by winds. Much of the coarser
material supplied by rivers settles out near shorelines or on beaches,
but fine-grained material such as silt and clay settles so slowly through
water that some tiny particles take centuries to reach the bottom.
Terrigenous sediments Ocean currents disperse fine silt, clay,
and volcanic ash from land, called terrigenous sediments, throughout
the ocean basins. Thus, the dominant type of sediment on the deep
ocean floor is fine-grained, deep-sea mud. Deep-sea mud usually has a
reddish color because the iron present in some of the sediment grains
becomes oxidized during the descent to the ocean bottom. Closer to
land, the sediments become mixed with coarser materials such as
sand, but some sandy sediments occasionally reach the abyssal plains
in particularly strong turbidity currents.
Biogenous sediments Deep-sea sediments that come from
biological activity are called biogenous sediments. When some marine
organisms die, such as the diatoms shown in Figure 16.23, their
shells settle on the ocean floor. Sediments containing a large percentage of particles derived from once-living organisms are called oozes.
Most of these particles are small and consist of either calcium carbonate or silica. The oozes and deep-sea mud of the deep ocean typically
accumulate at a rate of only a few millimeters per thousand years.
■ Figure 16.23 The shells of microscopic marine organisms, such as diatoms, as well as shell fragments and
hard parts from larger marine organisms
make up biogenous sediments.
SEM: magnification unknown
SC.912.E.6.5: Describe the geologic development
of the present day oceans and identify commonly
found features. MA.912.S.3.2: Collect, organize,
and analyze data sets, determine the best format
for the data and present visual summaries from
the following: bar graphs, line graphs, stem and
leaf plots, circle graphs, histograms, box and
whisker plots, scatter plots, cumulative frequency
(ogive) graphs.
Measure Sediment Settling Rates
How fast do sediment grains sink?
Procedure
1. Read and complete the lab safety form.
2. Obtain sediment grains with approximate diameters of 0.5 mm, 1 mm, 2 mm, 5 mm, and 10 mm.
3. Draw a data table with these headings: Type of Particle, Diameter (mm), Distance (cm), Time (s),
and Settling Speed (cm/s).
4. Measure and record the diameters of each specimen using a set of sieves.
5. Fill a 250-mL graduated cylinder with cooking oil. Measure the height of the cooking oil.
6. Drop the largest specimen into the oil. Use a stopwatch to measure and record the time it takes for
the specimen to sink to the bottom of the cylinder.
7. Repeat Step 6 for the remaining specimens.
Analysis
1. Calculate the settling speed for each specimen, and fill in your data table.
2. Plot the settling speed (cm/s) against particle diameter (mm) on a graph.
3. Explain How do settling speeds change as particle sizes decrease?
Section 2 • Seafloor Features 453
Inst. of Oceanographic Sciences/NERC/Photo Researchers
Figure 16.24 Metals, such as manganese, precipitate directly from seawater.
These manganese nodules consist of manganese and iron and range in size from a
few centimeters to 10 cm across.
■
Hydrogenous sediments While terrigenous sediments are
derived from land and biogenous sediments are derived from
biological activity, another sediment type, called hydrogenous
sediments, is derived from elements in seawater. For example,
manganese nodules, shown in Figure 16.24, consist of oxides of
manganese, iron, and copper and other valuable metals that have
precipitated directly from seawater. The precipitation happens
slowly, therefore growth rates of manganese nodules are extremely
slow. Growth rates of manganese nodules are measured in millimeters per million years.
Manganese nodules cover huge areas of the deep-sea floor.
Although some have tried to mine manganese nodules for their
valuable metals, the difficulty and cost involved in removing them
from a depth of 5000 to 6000 m has made progress slow.
1
Section
16.2
6.2
Assessment
SC.912.E.6.2, SC.912.E.6.5, LA.910.2.2.3
Section Summary
Understand Main Ideas
◗ The oceans cover parts of the continental crust as well as oceanic crust.
1.
◗ A continental margin consists of the
continental shelf, the continental
slope, and the continental rise.
3. Summarize the differences between deep-sea mud and oozes.
◗ Deep-ocean basins consist of abyssal
plains, trenches, mid-ocean ridges,
seamounts, and guyots.
◗ Most deep-sea sediments are finegrained and accumulate slowly.
MAIN Idea
Describe the features of deep-ocean basins.
2. Identify which sediment sinks faster — pebbles or sand grains.
4. Compare and contrast the characteristics of the three major areas of the
continental margin.
Think Critically
5. Suggest If there is little volcanic activity on abyssal plains, yet they are dotted
with thousands of seamounts, from where did these extinct volcanoes come?
Earth Science
6. Suppose you are taking side-scan sonar readings as your ship moves across the
Pacific Ocean from east to west. Describe how you would interpret the sonar data
according to the features you would expect to find beneath the surface.
454
Chapter 16 • The Marine Environment
Self-Check Quiz glencoe.com
Ralph White/CORBIS
eXpeditions!
SC.912.E.6.5: Describe the geologic development of the
present day oceans and identify commonly found features.
ON SITE:
Surveying
the Deep
Ocean
Floor
here are a variety of vessels that can
Tcollect
data on the deep ocean floor
ranging from human-occupied submersible vehicles to free-moving autonomous
robots to remotely operated vehicles.
These vessels are used by oceanographers
to learn more about the topography, biology, and chemistry of the deep ocean.
Submersibles Alvin is one of several humanoccupied deep-sea submersibles used worldwide.
In Alvin, the trip to the bottom of the ocean takes
about two hours, and it takes two hours to return
to the surface. Under the immense pressure of
thousands of kilograms of water and in the darkness and cold of the deep ocean, scientists have
about four hours of bottom time to record video
and collect water, sediment, and biological samples. Using Alvin, scientists got their first view of
hydrothermal vents in 1977, a major discovery for
both geologists and biologists.
Autonomous underwater vehicles The
Autonomous Benthic Explorer (ABE) has a mass
of over 500 kg and is 2 m long, but, thanks to
computer systems, the bulky vehicle is able
to dive into narrow trenches and zip around seamounts at depths as deep as 5000 m. Autonomous
underwater vehicles are able to move around
independent of a pilot, ship, or submersible.
Alvin is a human-occupied submersible vehicle that is used to
study hydrothermal vents and other deep-sea features.
The ABE has completed over 150 dives to the
deep seafloor, recording videos of the deep sea
environment and taking temperature and water
chemistry samples.
Remotely operated vehicles Jason/Medea
is a remotely operated vehicle (ROV) system that
can collect data at depths as deep as 6000 m.
An ROV is deployed from a ship, but unlike the
ABE, it needs a pilot, an engineer, and a navigator to control its operations from the ship. In
2006, Jason/Medea explored part of the western
Pacific Ocean near the Marianas Trench, collecting data on water chemistry near active undersea
volcanoes and hydrothermal vents. Each dive
provided scientists with views—from glowing,
red, hot lava pouring out a vent from a volcano,
to black-smoker hydrothermal vents and the
unique organisms that live near them—that
were never disappointing.
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455
MAPPING: IDENTIFY COASTAL LANDFORMS
Background: Topographic maps of coastal areas
show a two-dimensional representation of coastal
landforms. You can identify an emergent coast by the
landforms along the coastline as well as landforms
found inland.
Question: How can you identify and describe the coastal
landforms of an emergent coast on a topographic map?
Materials
metric ruler
drafting compass
graph paper
calculator
Procedure
1. Read and complete the lab safety form.
2. Determine the map scale and the contour interval.
3. On the inset map, plot a west-east cross section of
the coast just north of Coon Creek from sea level
depth contour to a point 2000 ft inland. Use a horizontal scale of 1:24,000.
4. Use both maps to answer the following questions.
Analyze and Conclude
1. Describe What kind of coastal landform is the
Morro Rock Peninsula?
2. Explain What kind of feature is Pillar Rock, and
how was it formed?
3. Interpret On what coastal feature is Morro Bay
State Park located? How was the feature formed?
4. Infer What is the direction of the longshore transport along Morro Bay?
5. Apply Your west-east cross section shows an elevated flat area next to the shoreline. What kind of
coastal landform is this? How was it formed?
Morro Rock is located off the coast of California.
6. Draw Conclusions If sea level dropped 10 m, how
would the shoreline change? How far would it move
seaward? Would it become more regular or irregular?
What would happen to Morro Bay?
7. Suggest three major changes that could occur to the
coastal region if sea level rose 6 m.
APPLY YOUR SKILL
Compare and contrast this coastal section with a section of the Texas coast between Corpus Christi and
Galveston. Which coastal features are similar? Which
are different?
456
GeoLab
Davis Barber/Photo Edit
SC.912.N.1.1: Define a problem based on a specific body of knowledge, for example:
biology, chemistry, physics, and earth/space science, and do the following: 1. pose
questions about the natural world, 2. conduct systematic observations, 3. examine books
and other sources of information to see what is already known, 4. review what is known
in light of empirical evidence, 5. plan investigations, 6. use tools to gather, analyze, and
interpret data (this includes the use of measurement in metric and other systems, and
also the generation and interpretation of graphical representations of data, including data
tables and graphs), 7. pose answers, explanations, or descriptions of events, 8. generate
explanations that explicate or describe natural phenomena (inferences), 9. use appropriate
evidence and reasoning to justify these explanations to others, 10. communicate results of
scientific investigations, and 11. evaluate the merits of the explanations produced by others.
GeoLab 457
BIG Idea The marine environment is geologically diverse and contains a wealth of
natural resources.
Vocabulary
Key Concepts
Section 16.1 Shoreline Features
• barrier island (p. 442)
• beach (p. 438)
• longshore bar (p. 440)
• longshore current (p. 441)
• wave refraction (p. 439)
MAIN Idea
•
•
•
•
The constant erosion of the shoreline and deposition of sediments by ocean waves creates a changing coastline.
Wave erosion of headlands produces wave-cut platforms and cliffs, sea
stacks, sea arches, and sea caves.
Wave action and longshore currents move sediment along the shore and
build depositional features.
Artificial protective structures interfere with longshore transport.
Sea levels in the past were 130 m lower than at present.
Section 16.2 Seafloor Features
• abyssal plain (p. 451)
• continental margin (p. 447)
• continental rise (p. 449)
• continental shelf (p. 447)
• continental slope (p. 448)
• deep-sea trench (p. 451)
• guyot (p. 452)
• mid-ocean ridge (p. 451)
• seamount (p. 452)
• turbidity current (p. 448)
458 Chapter 16
X ••Study
StudyGuide
Guide
MAIN Idea
•
•
•
•
The ocean floor contains features similar to those on land and
is covered with sediments of several origins.
The oceans cover parts of the continental crust as well as oceanic crust.
A continental margin consists of the continental shelf, the continental
slope, and the continental rise.
Deep-ocean basins consist of abyssal plains, trenches, mid-ocean ridges,
seamounts, and guyots.
Most deep-sea sediments are fine-grained and accumulate slowly.
Vocabulary
PuzzleMaker
glencoe.com
Vocabulary
PuzzleMaker
biologygmh.com
(t)Joseph Melanson/Aero Photo Inc/www.skypic.com
Download quizzes, key
terms, and flash cards
from glencoe.com.
SC.912.E.6.5, LA.910.4.2.2
Vocabulary Review
Write the definition for each vocabulary term listed
below.
1. barrier island
2. beach
3. longshore bar
4. wave refraction
Complete the sentences below using vocabulary terms
from the Study Guide.
5. ________ can create submarine canyons by cutting
through bottom sediments and bedrocks of
seafloors.
6. The deepest part of an ocean basin is a(n) ________.
7. The ________ is the part of a continent that is
submerged under the ocean.
8. Submerged basaltic volcanoes more than 1 km
high are ________.
Understand Key Concepts
9. Which coastal features are usually found in the
bays along irregular coasts with headlands?
A. sea stacks
C. wave-cut platforms
B. wave-cut cliffs
D. beaches
10. Which coastal landform is not produced by longshore transport?
A. barrier island
C. baymouth bar
B. sand spit
D. sea stack
11. Which is the correct order of features on the continental margin moving from land out to sea?
A. continental slope, continental shelf,
continental rise
B. continental rise, continental trench,
continental shelf
C. continental shelf, continental slope,
continental rise
D. continental slope, continental shelf,
continental trench
Chapter Test glencoe.com
12. What percentage of Earth’s surface is represented
by ocean basins?
A. 10 percent
B. 30 percent
C. 50 percent
D. 60 percent
13. What do the sediments of the abyssal plains mostly
consist of?
A. sand and gravel
B. salt
C. seashells
D. mud and oozes
14. Where are most deep-sea trenches located?
A. in the Atlantic Ocean
B. in the Pacific Ocean
C. in the Indian Ocean
D. in the Arctic Ocean
15. Which runs through all the oceans?
A. abyssal plains
B. the mid-ocean ridges
C. deep sea trenches
D. seamounts and guyots
Use the diagram below to answer Questions 16 and 17.
A
B
C
D
16. Which letter indicates the continental shelf?
A. A
B. B
C. C
D. D
17. Which feature is indicated by the letter A?
A. guyot
B. continental slope
C. continental rise
D. trench
Chapter 16 • Assessment
459
18. Which marks the true edge of a continent?
A. submarine canyon C. continental shelf
B. continental slope D. abyssal plain
19. Which seafloor feature can be found along rifts in
the mid-ocean ridges?
A. hydrothermal vents
B. manganese nodules
C. deep-sea trenches
D. seamounts
25. Explain the relationship between oozes and the
sedimentary rock known as chalk.
26. Illustrate the effect that wave refraction has on
incoming wave crests that approach the coast at an
angle.
Use the diagram below to answer Question 27.
20. Which represents the flattest part of Earth’s
surface?
A. deep-sea trenches
B. continental margins
C. abyssal plains
D. mid-ocean ridges
Use the diagram below to answer Question 21.
Original headland
Wave-cut
cliff
Wave-cut platform
27. Apply Copy the diagram above onto a sheet of
paper, and label the following features: shore,
beach, low-tide shoreline, high-tide shoreline, coast,
and waves.
28. Illustrate how incoming waves along a shoreline
create the longshore current.
29. Apply If a coast has elevated marine terraces, is it
more likely to be rising or sinking? Explain.
21. Which features are not caused by erosion?
A. wave-cut platforms C. original headlands
B. wave-cut cliffs
D. sea stacks
22. Which features of the seafloor are cut by turbidity
currents?
A. longshore bars
C. abyssal plains
B. submarine canyons D. baymouth bars
23. Which are not associated with mid-ocean ridges?
A. black smokers
C. fracture zones
B. guyots
D. hydrothermal vents
30. Differentiate between a baymouth bar and a
tombolo.
31. Discuss the processes that can cause sea level
changes.
32. Compare and contrast seamounts and guyots.
How are they formed?
33. Analyze Why does deep-sea mud usually have
a reddish color?
34. Compare and contrast the origins of marine
sediments.
Constructed Response
24. Explain Is global sea level currently rising, falling,
or staying the same? During the last ice age, was the
sea level higher, lower, or the same as at present?
460
Chapter 16 • Assessment
Think Critically
.
35. Infer Form an inference to explain why seamounts are extinct volcanoes.
Chapter Test glencoe.com
36. Suppose you are surfing at a beach in California.
Describe which near-shore currents might affect
your position relative to where you start out on
the beach after each cycle of surfing. Will you
come back to the same point?
37. Suggest Evidence shows that woolly mammoths
moved from Siberia to North America during the
last ice age. How could they have crossed the two
continents?
38. Suppose that you are swimming in the ocean
toward the shoreline, and you suddenly find that,
despite all your efforts to swim ahead, you remain
in the same location. Explain why this might happen, and describe the path in which you should
swim to get yourself out of this situation.
Additional Assessment
43.
Earth Science Write an editorial
article for a newspaper that discusses some of the
risks involved in living in a low-lying coastal area.
Document–Based Questions
Data obtained from: National assessment of shoreline change: part 1
historical shoreline changes and associated land loss along the U.S.
Gulf of Mexico. Report 2004–1043. U.S. Geological Survey.
The graph below shows the average annual sea level
based on tide data for a 100-year period for two cities
on the Gulf Coast of the United States.
Sea Level Changes
39. Assess How is it possible to have a coast that is
sinking when global sea level is falling?
Sea Level (mm)
Use the diagram below to answer Question 40.
7800
7600
Key West, FL
7400
7200
7000
6800
Galveston, TX
6600
1905
1925
1945
1965
1985
2005
Year
40. Evaluate The arrow is pointing to what ocean floor
feature? What is the significance of this feature?
44. Estimate the sea level change for Key West,
Florida, and Galveston, Texas, by roughly fitting
a straight best-fit line through the data points.
Concept Mapping
45. The global mean sea level rise for the past century
is estimated to be about 0.18 m. Which location
has a sea level rise closer to the global mean?
41. Use the following terms to construct a concept
map about the continental margin: continental
shelf, continental slope, continental rise, turbidity
currents, and submarine canyons. Refer to the
Skillbuilder Handbook for more information.
46. What is one possible cause for global sea level rise?
What is one reason that a local sea level change
can be much larger than the global average?
Challenge Question
47. What are the four most common foliated metamorphic rocks? (Chapter 6)
42. Investigate the causes and effects of a natural
disaster that occurred in the past five years and
resulted in coastal erosions in the United States
or in any other country.
Chapter Test glencoe.com
Cumulative Review
48. Describe the process that creates supercell thunderstorms. (Chapter 13)
Chapter 16 • Assessment 461
Standardized Test Practice
Multiple Choice
1. Which ocean movement is slow-moving and occurs
in deep waters?
SC.912.E.6.5
A. surface currents C. density currents
B. upwelling
D. gyres
6. If the northern hemisphere is experiencing long
hours of darkness and cold weather, what season is
SC.912.E.7.4
it in the southern hemisphere?
A. spring
C. winter
B. summer
D. fall
Use the illustration below to answer Questions 2 and 3.
Use the illustration below to answer Questions 7–9.
B
2. What instrument is shown?
A. thermometer
C. barometer
B. hygrometer
D. rain gauge
SC.912.E.7.5
3. If a barometer records high pressure, what type of
SC.912.E.7.3
weather can be expected?
A. storms
C. fair weather
B. cloudy weather
D. hot weather
4. Why does the silica content of seawater in the
Pacific and Atlantic Oceans increase as the depth of
the water increases?
SC.912.E.6.5
A. The chemical makeup of the water at the surface
of the oceans dissolves more silica.
B. When shelled organisms die, their shells fall to
the floor of the ocean and dissolve, releasing silica into the water.
C. Wave action at the surface of the oceans dilutes
the silica.
D. Oxygen entering the surface water pushes silica
down into deeper water.
5. How might volcanic activity affect changes in the
SC.912.E.7.4
climate?
A. Volcanic dust blocks incoming radiation causing
lower global temperatures.
B. Volcanic dust traps radiation in the Earth’s atmosphere raising global temperatures.
C. Lava running along Earth’s surface increases the
temperature thus increasing global temperature.
D. Lava thrown into the air increases the atmospheric
temperature thus increasing global temperatures.
462 Chapter 16 • Assessment
C
7. What shoreline feature is indicated by B?
A. a tombolo
C. a lagoon
SC.912.E.6.5
B. a spit
D. a beach
8. The material in the section indicated by the B is
very coarse. What conclusions can be drawn about
the waves and sediment source?
SC.912.E.6.5
A. The area is rocky and pounded by heavy waves.
B. The area is rocky and hit with light waves.
C. The area consists of fine-grained material that
has been hit by heavy waves.
D. The area consists of fine-grained material that
has been hit by light waves.
9. Which forms the area indicated by C?
A. large storm waves
B. gaps in the longshore bar
C. the longshore current
D. a rip current
SC.912.E.6.5
10. Limestone that is exposed to enough heat and
pressure is transformed into
SC.912.E.6.2
A. marble
B. slate
C. quartzite
D. gneiss
Standardized Test Practice glencoe.com
Reading for Comprehension
Short Answer
Use the illustration below to answer Questions 11 and 12.
Underwater Hot Spots
Scientists think that underwater hot spots might
host unique, previously unknown forms of life.
Hydrothermal vents at ocean ridges are an essential
part of the chemical balance of seawater. They support ecosystems not found anywhere at the surface
and are thought to have been the sites of the early
formation and evolution of life. The study of these
organisms will give scientists insight to the flexibility
and adaptability of life. The water that spews forth
from hydrothermal vents can reach temperatures of
350°C and is rich in chemicals such as sulfur and
salt. Some microorganisms have adapted to the environments on these vents creating rich underwater
ecosystems that some scientists think might represent some of the earliest lifeforms on Earth.
11. Describe the process that is occurring in the
SC.912.P.10.4
illustration.
12. How does a greenhouse mimic Earth’s atmosphere?
SC.912.P.10.4
13. How does a greenhouse benefit gardeners?
SC.912.P.10.4
14. Describe the hypothesis of how comets might have
SC.912.E.6.5
created Earth’s early oceans.
15. Discuss a location where an analog forecast could
be more beneficial than a digital forecast.
SC.912.E.7.5
16. If a sonar signal takes 3 s to be emitted and
received by an oceanographer mapping the ocean
floor, what is the ocean depth in that location?
(Hint: sound travels 1500 m/s through water.)
SC.912.E.6.5
17. If air is rising due to uneven heating of Earth’s
surface within an air mass, what can you expect
to occur?
SC.912.E.7.6
Article obtained from: Roach, J. Hydrothermal vents found in Arctic Ocean. National
Geographic News. January 23, 2003.
18. Which is NOT an importance of hydrothermal vents?
A. They support unique ecosystems.
LA.910.4.2.2
B. They create rich underwater environments.
C. They help underwater creatures to adapt to
extremely hot temperatures.
D. They are an essential part of the chemical balance of seawater.
19. Why is it rare to find life-forms around hydrothermal vents?
LA.910.4.2.2
A. They are extremely hot.
B. They are along ocean ridges.
C. They are not chemically balanced with the rest of
the ocean.
D. No life-form is able to survive due to the sulfur
in the area.
20. Why is studying these hydrothermal vents so
important to scientists?
LA.910.4.2.2
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