Chapter 16: Geology of the Ocean

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THE WATER PLANET
16
Geology of the Ocean
When you have completed this chapter, you should be able to:
RELATE the theories of continental drift and plate tectonics to the
formation of the continents and oceans.
EXPLAIN the development of seafloor topographic features.
DESCRIBE the formation of coastal features and reef types.
16.1
Origin of the Ocean
and the Continents
16.2
The Theory of Plate
Tectonics
16.3
Ocean Floor
Topography
16.4
Coasts and
Reefs in Profile
384
The ocean is like a blanket of water that covers the seabed, or ocean
floor. But there is no true “rest” on the seabed, and Earth is not a
sleeping giant. There are many dynamic forces operating within
Earth’s solid part, or lithosphere, which influence and affect the characteristics of the liquid part, or hydrosphere. For example, the sudden emergence of a volcanic island in the middle of the ocean (such
as Surtsey, shown above) is evidence of the forces at work thousands
of meters below the surface of the sea.
The study of the development and physical characteristics of our
planet’s seafloor and continents—and of the forces that have shaped
them—makes up the field of science called geology. Scientists who
specialize in the study of geological features of the ocean are called
marine geologists. You will begin your study of the geology of Earth’s
oceans and coastlines by going back in time to learn about the beginning of Earth itself.
16.1 ORIGIN OF THE OCEAN
AND THE CONTINENTS
If you could go back before the beginning of time—back almost 20
billion years—you could observe the start of our universe and
indeed all the galaxies that make up the vast array of space. Today,
the space telescope has permitted humans on Earth to get a glimpse
back in time to the very beginnings of matter. Scientists think that
our universe formed as a result of a gigantic explosion, called the
big bang.
The Earth Forms
At the time of the big bang, all the matter in the universe was contained in one sphere. Extremely hot and dense, the sphere
exploded, sending out matter in all directions in a kind of giant
cloud. As the cloud moved out from the explosion, some of the
matter came together and formed clumps that eventually became
galaxies—including the Milky Way, the home galaxy of planet
Earth. In time, further clumping of the matter caused the formation
of stars and planets. Scientists think that Earth, and all the other
planets in our solar system, formed about 4.6 billion years ago,
about 15 billion years after the big bang.
Shortly after its formation (geologically speaking), Earth was a
hot molten mass, too hot for solid rocks to exist, too hot for water
to exist as a liquid, and much, much too hot for life to exist at all.
Evidence shows that at about four billion years ago, Earth had
cooled enough for liquid rock to become solid at Earth’s surface. But
this early Earth was not a quiet place. For many millions of years,
the solid surface of Earth was disturbed by volcanic activity that
occurred over the whole planet.
An atmosphere (the layer of gases that surrounds a planet) began
to form on Earth about 3.5 billion years ago. Earth’s first atmosphere
was very different from the atmosphere that exists today. Billions of
years ago, the atmosphere probably contained some water vapor
(water as a gas), carbon monoxide, hydrogen sulfide (a gas that
smells like rotten eggs), nitrogen (the gas that makes up most of
today’s atmosphere), and hydrogen cyanide (a deadly gas).
Geology of the Ocean
385
The Ocean Forms
Remember that oceans could not exist on early Earth because of the
high temperatures. But by about 4 billion years ago, Earth became
cool enough for water vapor within the mantle to cool. This eventually formed liquid water on the surface. As Earth cooled still more,
thunderclouds began to form. For many thousands of years, thunderstorms occurred and covered Earth with water, filling in the low
spots that were to become the early ocean.
Some of the ocean’s water came from the activity of volcanoes,
which spewed great quantities of water vapor into the atmosphere.
The impact of many meteors, which heated the surface of Earth, is
also thought to have caused the release of water vapor. In addition,
heated water in the crust boiled up to the surface and formed hot
springs. Some of the hot water emerged from the surface as a geyser,
or spray. In some places on Earth today, hot springs and geysers still
exist, which are evidence that the area beneath them is hot. (See
Figure 16-1.) Scientists call this heat (a form of energy that forms
within Earth) geothermal energy. Iceland is an island that derives
much of its energy from geothermal sources. In fact, in places in
Iceland where this underground energy comes to the surface in the
form of hot springs and steam, you can get a small glimpse of what
vast areas of early Earth must have looked like. People in Iceland
heat their homes by tapping these underground sources of heat.
Ocean water also came from molecules of water that were
bound up in compounds within Earth’s crust and released due to
Figure 16-1 Heated water
in the crust forms hot
springs and geysers.
Geyser
Overflow
Ground
386
The Water Planet
Water
Superheated
water
heating. Compounds that contain water are called hydrated compounds. An example of a hydrated compound is copper sulfate.
Hydrated copper sulfate has the formula CuSO4•5H2O. Notice that
the formula shows five water molecules attached to a molecule of
copper sulfate. When hydrated copper sulfate is heated, water is
given off (and anhydrous copper sulfate is produced), as shown in
the following reaction:
CuSO4•5 H2O
+
Heat
Hydrated
copper sulfate
→
CuSO4
Anhydrous
copper sulfate
+
5 H2O
Water
You can measure water loss from a hydrated compound by performing the lab investigation at the end of this chapter.
Origin of the Continents
In 1912, Alfred Wegener, a German meteorologist, proposed a
hypothesis that caused a great deal of controversy in the scientific
community. He suggested that the continents were not always
located in their present positions, that over time they had moved.
Wegener had noticed that the continents fit together like the pieces
of a jigsaw puzzle. (See Figure 16-2 on page 388.) He suggested that
about 200 million years ago the present continents formed one large
landmass he called Pangaea, surrounded by a single huge ocean. At
that time, Pangaea began to break up into smaller continents that
moved over the surface of Earth, ultimately reaching their present
positions. What is the evidence that the present-day continents originated in the single landmass Pangaea? Wegener cited the similarities
of fossils and rock formations on different continents (especially on
either side of the Atlantic) along with other technical evidence to
support his hypothesis, which he called the theory of continental
drift. His theory was not well received by most geologists at that
time. In fact, scientists were quick to point out that Wegener was not
even a geologist; he was a meteorologist—a scientist who normally
studies the weather. However, studies of the seafloor that were conducted in the 1950s provided more evidence to confirm Wegener’s
theory, and it is now generally accepted by the scientific community.
What caused the single landmass to break apart into several
continents? And what forces caused the continents to drift apart?
Geology of the Ocean
387
Figure 16-2 Evidence that
the continents were once
joined.
Greenland
Caledonian
Mountains
North
America
Appalachian
Mountains
Europe
Africa
South
America
Rock
formations
India
Madagascar
Evidence of
glaciation
Australia
Antarctica
Wegener was ridiculed because he was not able to provide an exact
mechanism for the movement of the continents. (See Figure 16-3.)
However, we now know that powerful forces inside Earth’s interior
caused the breakup of the continents. Look at the diagram of Earth’s
interior, shown in Figure 16-4. The interior of our planet is composed of several layers. At Earth’s center is the inner core, surrounded by the outer core, the mantle, and then the crust. Much
of Earth’s interior is in a hot, molten state. The inner core has the
highest temperatures, with a range of about 6200 to 6600°C. The
mantle, which is a region of geologic activity between the core and
the crust, has a temperature range of about 1200 to 5000°C.
The high temperatures inside Earth are hot enough to melt rock.
In fact, Earth’s interior—from the inner core to the upper mantle—
388
The Water Planet
THE EARTH 250 MILLION YEARS AGO
THE EARTH 135 MILLION YEARS AGO
Eurasia
North
America
Gondwanaland
Pangaea
THE EARTH 45 MILLION YEARS AGO
THE EARTH AT PRESENT
Eurasia
North
America
North
America
Europe Asia
Pacific
Ocean
Africa
Africa
India
South
America
South
America
Antarctica
Australia
Atlantic
Ocean
India
Indian
Ocean
Antarctica
Australia
consists of hot, molten material. This molten material within the
mantle is called magma. The churning of the magma creates a force
that generates great pressures upward into Earth’s surface layer, or
crust. Earth’s crust is only about 40 km thick. If enough force is generated, it cracks. Then the ground trembles and moves, producing
an earthquake. Sometimes magma flows out of a crack in the crust,
Figure 16-3 The breakup
of Pangaea and continental drift over time.
Figure 16-4 The layers of
Earth’s interior.
Crust
Outer core
2900 km
2188 km
Mantle
1255 km
Inner
core
Geology of the Ocean
389
producing a volcanic eruption. Magma that flows out of the crust
onto Earth’s surface is called lava. Disturbances (vibrations) in
Earth’s crust, such as volcanic eruptions and earthquakes, are examples of seismic activity.
Seismic activity was involved in the breakup of the supercontinent Pangaea. In the next section, you will learn about the mechanism that is responsible for making the continents drift apart.
16.1 SECTION REVIEW
1. How did the stars and planets come into being?
2. What were the sources of the ocean’s water?
3. Describe some of the evidence that indicates the continents
originated from a single landmass.
16.2 THE THEORY OF PLATE TECTONICS
Satellite photos show that the continents are moving at a rate of
approximately one centimeter per year. The Atlantic Ocean is getting wider and the Pacific Ocean is getting narrower. What exactly is
causing the continents to drift? Research on Earth’s interior has
revealed that its crust is divided into segments called plates, which
float like rafts on the molten interior layer. The continents ride on
top of these plates. This idea that Earth’s crust is divided into segments that drift about has been developed into the theory of plate
tectonics.
To understand the theory of plate tectonics, look at the world
map in Figure 16-5. Earth’s crust is divided into seven major plates
and about a dozen minor plates. Locate the North American plate.
The eastern border lies in the middle of the Atlantic Ocean, and the
western border runs through California. The North American plate
is drifting westward. To understand what causes the plates to move,
look at the profile of the mantle shown in Figure 16-6. There is a
big difference in temperature between the upper mantle (about
1400°C) and the lower mantle (about 2600°C). Scientists think that
this difference in temperature creates convection currents. A convection current is a transfer of heat in a liquid or gas that causes
390
The Water Planet
n
utia
Ale ch
n
Tre
San
Andreas
Fault
Eurasian
Plate
North
American
Plate
ma
la y
Arabian
Plate
aT
Mountains
Nazca
Plate
as
J av
African
Plate
A n de s
Pacific
Plate
Hi
re
nc
h
Indian
Plate
South
American
Plate
Antarctic
Plate
the molten magma to rise up through the mantle and into the crust,
forming an oceanic ridge. Off the east coast of North and South
America, it is the Mid-Atlantic Ridge that is formed; this ridge runs
the entire length of the Atlantic Ocean, dividing it in half. Magma
that breaks through Earth’s oceanic crust as lava can also accumulate to form mid-ocean islands.
Figure 16-5 Earth’s major
crustal plates.
Figure 16-6 Convection
currents in the mantle
cause seafloor spreading.
Oceanic
crust
Mid-ocean
ridge
Trench
Volcano
Continental
crust
Magma
Heat currents
(Cool)
in mantle
Rising
magma
(Hot)
(Cool)
Geology of the Ocean
391
Seafloor Spreading
The upward movement of magma under the Mid-Atlantic Ridge
causes seafloor spreading, which is the moving apart of the plates.
As you can see in Figure 16-5, the North American and South American plates move westward, while the Eurasian and African plates
move eastward. As the hot magma rises under the Mid-Atlantic
Ridge, cooler magma moves in to take its place. This sets up a continuous circulation pattern similar to the circulation of warm air in
a room.
The movement of the plates causes them to collide with one
another. Notice, in Figure 16-5, that the North American plate collides with the Pacific plate. When this occurs, one plate overrides
the other plate. Crust from the Pacific plate plunges downward
under the North American plate in a process called subduction. The
Pacific plate slides under part of the North American plate because
oceanic plates are denser than continental plates. Subduction
destroys old plates as the crust descends into the mantle to become
molten magma. This process occurs in several areas around the
world (at subduction zones) and forms trenches, the deepest and
steepest depressions found on the ocean floor. (See Figure 16-7.)
Figure 16-7 Deep-sea
trenches are formed at
subduction zones.
Asia
North
America
Aleutian Trench
Japan
Trench
Puerto Rico
Trench
Hawaiian Islands
Mariana Trench
Mid-America
Trench
Mindanao Trench
PACIFIC OCEAN
Java
Trench
Tonga
Trench
Australia
Kermadec
Trench
392
The Water Planet
South
America
Peru-Chile
Trench
The theory of plate tectonics helps to explain various geological
phenomena. At the margin of the plates, a crack occurs in Earth’s
crust that is called a fault. For example, the San Andreas fault,
which cuts through California, forms the boundary between the
North American and Pacific plates. Earthquakes and volcanic activity tend to occur along the margins of plates, where there is movement of, and friction between, the adjoining plates.
Ocean Floor Formation
Plate tectonics also explains how the ocean floor was formed. As
magma continued to rise up to form the Mid-Atlantic Ridge, the
North American and Eurasian plates moved farther apart, creating a
new ocean floor. This process, which has been occurring for many
millions of years, is recorded in the symmetrical, parallel bands of
basalt (the volcanic rock that makes up the ocean floor) that spread
out along either side of the ridge. The spreading apart of the Atlantic
seafloor is an ongoing process.
Further evidence that the seafloor is spreading comes from the
study of rock samples taken from both sides of the mid-ocean ridge.
(See Figure 16-8.) The youngest rocks are closer to the ridge and the
oldest rocks are farther away. Scientists discovered that the magnetic
properties of rocks on both sides of the ridge were as symmetrical as
Age in millions of years
Figure 16-8 Parallel bands
of rock increase in age
away from the ridge.
40 30 20 0 20 30 40
Ocean
Rift valley
Mid-ocean ridge
Magma
Mantle
Oceanic crust
Continental crust
Geology of the Ocean
393
Figure 16-9 Identical
magnetic bands are
seen on each side of
the ridge.
North
Ridge
Axis
Basalt
Lava
Spreading
Spreading
Magma
South
the bands of rocks. When the magma hardened, the magnetic minerals in the rock aligned in the direction of Earth’s magnetic field. During Earth’s history, the magnetic poles have reversed several times.
The pattern of polarity on either side of the ridge was identical and
reflected these pole reversals. Scientists concluded that, as the magma
hardened, half moved to one side of the oceanic ridge and the other
half moved to the other side. This pattern of magnetic bands provided
strong evidence for ocean floor spreading. (See Figure 16-9.)
The theory of plate tectonics is called a unifying theory because
it explains the origin of, and connections between, such phenomena as earthquakes, volcanic activity, faults, continental drift, and
seafloor spreading. A knowledge of plate tectonics is also helpful in
explaining how some structures on the ocean floor were formed.
(See Figure 16-10, which shows a profile of the Atlantic Ocean floor.)
16.2 SECTION REVIEW
1. What process causes the continents to drift apart? How?
2. How was the Mid-Atlantic Ridge formed? Explain how the
process is related to seafloor spreading.
3. Why is the theory of plate tectonics called a unifying theory?
Figure 16-10 A profile of
the Atlantic Ocean floor.
United
States
Bermuda
(Sea level)
Mid-Atlantic
Ridge
Horizontal scale: 2.5 cm = approx. 1390 km
Vertical scale: Greatly exaggerated
394
The Water Planet
Africa
16.3 OCEAN FLOOR TOPOGRAPHY
What do the features of the ocean bottom look like? The average
depth of the ocean is about 3636 meters—much too deep for scuba
divers to explore. However, oceanographers can obtain a profile of
the ocean floor (without submerging in an underwater vehicle) by
using sonar. Modern ships are equipped with sonar. A ship’s sonar
device beams a continuous sound signal downward. After the sound
wave hits the bottom, the returning signal, called an echo, is
received by a depth recorder in the ship. This produces a line tracing
of the ocean floor. (See Figure 16-11.) Notice that the depth recording of the ocean floor shows a bottom that varies from fairly smooth
to jagged, or irregular. The irregular part could be debris dumped on
the ocean floor, a sunken ship, or a natural feature. Recall that the
Titanic and other sunken ships have been located by using sonar.
Modern fishing boats also use sonar to locate schools of fish. Sonar
Figure 16-11 Sonar is
used to obtain a profile of
the ocean floor.
Soun
d wa
ves
0
Depth in meters
Source
600
1200
Receiver
1800
2400
0
1
2
3
4
5
Kilometers
Depth in meters
0
600
1200
1800
2400
5 kilometers
Geology of the Ocean
395
is very useful to help ships navigate in shallow waters. For example,
a reef may be located only several meters below the surface—close
enough to make ships cautious when they pass by.
Sonar, Ocean Depth, and Topography
Ocean depth is calculated automatically by sonar. Two pieces of
information are needed to calculate the depth—the speed of sound
in water (1454 meters per second) and the time it takes for the signal to reach the bottom. If a signal takes one second (after being
sent) to return to the ship, then it takes one-half second to travel
to the bottom. Since sound travels 1454 meters per second underwater, it travels 1454 divided by 2 in one-half second. Therefore, the
depth of the water is 727 meters. The following formula is useful in
calculating ocean depth using sonar:
Depth (D) = 1454 meters per second ✕ time (t)2
In this example,
D = 1454 ✕ 12
D = 727 meters
Ships equipped with sonar have been crisscrossing the oceans
for a number of years in an attempt to map as much of the ocean
floor as possible. Thousands of sonar tracings have been made. The
data from the ships’ sonar maps have been combined to create a
single map that shows all seafloor elevations and depressions. The
study of Earth’s surface features, such as elevations and depressions,
on the land and the ocean floor is called topography. Seafloor
topography includes some very dramatic depressions and elevations. (See Figure 16-12.)
Figure 16-12 Features of
the ocean floor.
396
The Water Planet
Mid-ocean
Continental
Deep ocean
Continental
ridge
slope
floor
Continent
shelf
Seamount
Deep
ocean
Continental
Continent
floor
slope
Island
Trench
Seafloor Features
Recently, the U.S. Navy released a treasure trove of formerly classified data on the oceans that were collected during the Cold War.
These images of the ocean floor were obtained via satellite readings.
The satellites Seasat and Geosat used radar to measure sea level. They
measured bumps and depressions on the ocean’s surface that
reflected the pull of gravity (on the water) exerted by seafloor
objects. A variety of features were uncovered. High ridges off the
coast of Oregon were formed when plates collided. Off the west
coast of Florida, the seafloor has steep-walled elevations 2 km high.
Off New Jersey’s coast, there is a continental slope with deep
canyons that were most likely produced by submarine avalanches. A
continental slope is the area where the seafloor drops steeply at the
outer edge of the continental shelf. (See Figure 16-13.) In the Gulf of
Mexico, off the Louisiana coast, the images show a moonlike, pockmarked surface, formed when the gulf was dry and filled with evaporated sea salt. Sediments from the Mississippi River covered the
salt. When the sea level rose, the weight of ocean water on these
sediments created the odd shapes.
Cutting through the continental shelf and slope are steep
V-shaped depressions called submarine canyons. Some of these
canyons are as large as the Grand Canyon. Two well-known submarine canyons along the U.S. coasts are the Monterey Canyon off California and the Hudson (River) Canyon off New York. (See Figures
16-14a and 16-14b on page 398.) How did these huge canyons
form? Many submarine canyons are extensions of sunken river valleys from the adjoining continent. During the last ice age, when the
sea level was much lower, many canyons existed as river valleys.
When the sea level rose at the end of the ice age, these valleys were
submerged, creating the submarine canyons. The deeper parts of
United States (San Diego)
0
Slope
Depth in meters
Shelf
1500
South America
Shelf
Figure 16-13 Continental
shelves and slopes.
Slope
3000
4500
6000
Geology of the Ocean
397
Figure 16-14a The
Hudson (River)
Canyon.
New Jersey
New York City
Long Island
Hudson
Canyon
Continental shelf
Continental slope
submarine canyons are formed by swift undersea currents. Smaller
valleys that cut through a slope are formed as a result of erosion by
mudslides, which move from the shelf out to the ocean basin. The
accumulation of mudslide sediments at the base of a slope creates a
slightly elevated region called the continental rise.
Sonar maps of the ocean floor often reveal small submarine
mountains called seamounts (shown in Figure 16-12). Seamounts
Figure 16-14b A sonar
scan of the Hudson (River)
Canyon.
398
The Water Planet
are produced in regions of intense volcanic activity, where magma
(lava) pushes through the crustal plate and piles up on the seafloor.
If the lava breaks the surface of the ocean, then a volcanic island is
formed. The Hawaiian Islands were formed from a chain of
seamounts. The main island of Hawaii, at the eastern end of the
chain, is the youngest (formed about 800,000 years ago) and most
geologically active of the five Hawaiian Islands. The progressively
older seamounts stretch in an arc to the northwest. (The oldest
Hawaiian Island is about 4 to 6 million years old.) Such a chain of
islands is formed when a crustal plate moves over an area of intense
activity in the mantle, called a hot spot. The area over the hot spot
develops a seamount as lava pours through its crust. As the plate
moves along, the hot spot breaks through the next area of crust,
forming a new seamount. (See Figure 16-15.)
Some seamounts actually may be former islands that have sunk
beneath the surface. Erosion by waves and currents can cause the
tops of seamounts to become flattened, forming structures called
guyots (pronounced GEE-ohz), shown in Figure 16-15. Trenches, as
mentioned above, are another topographical feature of the ocean
floor. Recall that trenches are found at the margins (subduction
zones) of crustal plates, where one plate descends into the mantle
below the other plate. (See again Figure 16-12.) The deepest ocean
trench is the Mariana Trench, located in the western Pacific Ocean.
The Mariana Trench is 10,958 meters deep, which is deep enough to
contain Mt. Everest, the tallest terrestrial mountain on Earth.
Figure 16-15 A
chain of islands
forms over a hot
spot.
Oldest
Youngest
Guyots
Sea level
Oceanic plate
Mantle
Hot spot
Geology of the Ocean
399
TECHNOLOGY
Loihi on the Rise
The inhabitants of the main island of Hawaii
must be among the bravest people in the
world. These islanders live about 30 km from
the most active volcano on Earth. This volcano,
named Loihi (meaning “long one” in Hawaiian)
is so active that it produced nearly 1500 earthquakes in just one week! Even though the volcano is more than 3 km high and about 40 km
long, local Hawaiians cannot see it. This is
because Loihi rises from the ocean floor, with its
peak about 1 km below the ocean surface.
Marine geologists predict that Loihi will be the
next Hawaiian island to emerge from the
ocean—about 50,000 to 100,000 years from
now.
Loihi has been erupting continuously for
about 20 years. Most of the earthquakes it produces are relatively weak. However, they seem
to be increasing in magnitude and frequency,
causing local authorities to be concerned for the
safety of the islanders. Some recent quakes have
been recorded at magnitudes between 4 and 5
on the Richter scale. (Either the Richter or the
magnitude scale is used to measure earthquake
intensity.) Seismologists (scientists who study
earthquakes) fear that an underwater earthquake with a magnitude of 6.8 might produce a
huge wave a that would reach the big island in
just 15 minutes—not enough time for people
along the coast to prepare for emergency evacuation to higher ground.
Several federal agencies are monitoring the
volcano’s activity. Local officials have decided to
set up an early warning system that will give
island residents more time to evacuate in the
event of a serious earthquake. The Hawaiian
Undersea Research Laboratory at the University
of Hawaii is using submersibles and robots to
monitor and videotape the eruptions underwater, thus giving scientists the opportunity to
observe and record never-before-seen events
going on inside the crater.
QUESTIONS
1. Explain why the inhabitants of Hawaii cannot see Loihi’s eruptions.
2. Why is Loihi’s seismic activity potentially dangerous for Hawaiians?
3. Describe the technology scientists are using to monitor Loihi’s activity.
400
The Water Planet
Island arc of
volcanoes
Ocean trench
Magma
Figure 16-16 Volcanic island arcs
form over trenches.
Sea level
Oceanic plate
Mantle
Ocean crust
Associated with the trenches are groups of volcanic islands that
form an arc in the ocean, called island arcs. (See Figure 16-16.) Most
trenches, and their island arcs, are located on the periphery of the
Pacific Ocean (that is, along the west coasts of North and South
America and the east coast of Asia). Many of these islands are still
volcanically active, so the area bordering the Pacific is called the
Ring of Fire. Frequent earthquakes also occur along the Ring of Fire,
due to the movement of subducting plates.
The Mid-Ocean Ridge
When magma rises up from the mantle through the oceanic crust, it
forms ridges. The prominent mid-ocean ridge is the continuous
undersea volcanic mountain range that encircles the globe, marking the boundaries of several crustal plates. As noted above, the part
of the ridge that runs through the middle of the Atlantic Ocean is
called the Mid-Atlantic Ridge. (See Figure 16-17 on page 402.)
The Mid-Atlantic Ridge rises about 3030 meters above the ocean
floor; its highest underwater ridge lies about 900 meters below the
surface of the ocean. Iceland, which rises above the surface, is a volcanic island formed by lava that poured out of the Mid-Atlantic
Ridge. Other smaller volcanic islands have formed near Iceland,
such as Surtsey, which was “born” in 1963. Eruptions continued on
this island for several years, causing Surtsey to increase in size. (See
photograph on page 384.)
Geology of the Ocean
401
Figure 16-17 The
Mid-Atlantic Ridge is
part of the mid-ocean
ridge.
Europe
North America
Ocean
Mid-Atl a n
Atlantic
tic
Africa
R
i
d
ge
South America
A depression called the central rift valley runs along the crest of
the mid-ocean ridge (including the Mid-Atlantic Ridge). The rift is a
crack in the seafloor through which molten rock from the mantle is
expelled. The rift zone is of special interest to oceanographers
because that is the place where new seafloor is continually being
created. When the hot magma pours out on the ocean floor, it cools
and solidifies to form new ocean crust. Lateral movements on both
sides of the rift then cause the seafloor to spread apart. About 250
million years ago, all the continents were joined together at the
Mid-Atlantic Ridge. Over geologic time, hot molten matter rising
up through the mantle and into the crust caused the continents to
split apart. The continuing flow of magma pushed the continents
farther apart and created a space between them, called an ocean
basin. This ocean basin filled up with water and became the Atlantic
Ocean.
Features of the Rift Zone
The oceanic crust is very porous in the area of a rift zone. Ocean
water seeps down through cracks in the crust and gets heated from
the hot magma below. The hot water rises up through the crust, dis402
The Water Planet
Figure 16-18 Mineral-rich
“black smokers” (such as
the one shown here) form
at hydrothermal vents.
solving minerals out of the rock as it flows. When the hot water
emerges from the seafloor, it makes contact with the cold ocean
water. Then the minerals dissolved in the hot water form a cloud that
looks like smoke coming from a chimney. Oceanographers call these
springs of mineral-laden waters “black smokers.” (See Figure 16-18.)
The area in the rift zone where these hot springs emerge is
called a hydrothermal vent. Submersibles have visited the hydrothermal vents, some more than 2400 meters deep. But the submersibles cannot get too close to a vent. The temperature can be as
high as 371°C. (Recall that water boils at 100°C.) The water was so
hot at one vent that it melted the first thermometer used to record
the temperature.
In 1977, scientists aboard the Alvin, an American submersible,
made an amazing discovery while investigating some hydrothermal
vents. Parts of the seafloor near the vents were carpeted by a thick
growth of living things. There were clusters of giant tube worms,
large clams, albino crabs, deep-sea fishes, and other organisms.
Oceanographers wondered how so much life could exist at such a
great depth, where there is no light. Since 1977, the Alvin and other
submersibles have made many more dives to study the vents and
to bring back water samples and specimens. The water was found
to be very rich in minerals, particularly the compound hydrogen
sulfide (H2S). Researchers also found that the water had a high concentration of bacteria. It turns out that these bacteria use the hydrogen sulfide to produce their food.
Geology of the Ocean
403
Some types of bacteria share the food they make in a symbiotic
relationship with giant tube worms and other deep-sea creatures.
You may recall that this form of food making by bacteria, which is
not based on photosynthesis, is called chemosynthesis. (Refer to
Chapter 8 for a review of how chemosynthetic bacteria use hydrogen sulfide to make their food.) Marine scientists continue to collect water samples from the areas around hydrothermal vents. They
recently discovered large masses of heat-loving bacteria, called thermophilic bacteria, which live on the outside of the vents in water at
or near the boiling point! In addition, many new and unique species
of crabs, mussels, octopus, shrimp, and fish have been discovered
near the vents.
16.3 SECTION REVIEW
1. Calculate the depth of a sunken ship if a sonar signal takes two
seconds to return to the research ship after it is emitted.
2. How were the Hawaiian Islands formed? Are they all the same
age?
3. What important geological process occurs in a rift zone?
16.4 COASTS AND REEFS IN PROFILE
Approximately 70 percent of the U.S. population lives within 80 km
of a coast. The coast, or shore, is the boundary between land and
sea. As you have learned in previous chapters, some coasts are rocky,
while others have sandy beaches. A beach is a part of the shore that
contains loose sediments eroded from the land.
Beaches
Our coastal states have an abundance of sandy beaches and rocky
shores. In Hawaii, there are both sandy beaches and rocky coasts of
volcanic origin. Volcanic rock originates from the molten lava that
pours out of volcanoes and flows down to the sea. When the lava
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Figure 16-19 A typical
sandy beach is produced
by erosion of rocks and is
mildly sloped.
reaches the ocean, it boils the water into steam, and the lava hardens into rock. Sand on a beach is mainly the product of erosion
from rocks along the shore. Waves pound on the rocky shores, and
pieces of rock break off and fall into the surf. Tides move the rocks
back and forth, wearing them down into pebbles. Over time, the
pebbles are ground into sand by rubbing against one another as
they are tossed by the waves. (See Figure 16-19.)
Beach sand may also come from eroded rocks from mountains
located hundreds of km away. Rivers and streams wear down the
rocks. Sediments from the rocks are transported downstream to the
ocean, where they are deposited as sand on the beach. (You already
examined sand grains under the microscope to determine their origin in the lab investigation in an earlier chapter.) The erosion of
volcanic rock produced the black sands found on some Hawaiian
beaches. The white and pink sandy beaches of many other tropical
islands are largely composed of fine sediments eroded from offshore
coral reefs. Sand may also contain shell or bone fragments, fish
scales, and other debris from marine animals. (See Figure 16-20 on
page 406.)
Coasts have differently shaped profiles. On sandy shores with
heavy surf, the crashing waves erode the sand, forming a steeply
sloped beach. On beaches where large rivers empty into a calm sea,
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405
Figure 16-20 Beach
sand and seafloor
sediments come
from eroded rock
and other debris.
Dust particles
and cinders
Wind
Meteoric
dust
Running
water
Gravel
and sand
Silt
Mud
Ooze
Shells
Bedrock
Sea floor
Deep sea
zone
sediments carried by the river are deposited along the shore, producing a fan-shaped feature called a delta. (See Figure 16-21.) The
Nile River, which flows into the Mediterranean, and the Mississippi
River, which empties into the Gulf of Mexico, both form deltas. (See
Figure 16-22.)
Rocky Coasts
Compared to sandy beaches, rocky coasts are often very steep. (See
Figure 16-23.) How are they formed? The rocky coast of Maine was
formed 12,000 years ago, toward the end of the last Ice Age. As the
climate warmed, the glacier that covered much of North America
retreated, carving out troughs, or valleys, which later became river
valleys. When the glaciers melted, the sea level rose. The ocean
Figure 16-21 Typical
shoreline features, including a delta.
Lagoon
Delta
Spit
Offshore bars
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Hook
Figure 16-22 The fanshaped Mississippi Delta
extends into the Gulf of
Mexico.
invaded the land, filling in the eroded valleys left by the retreating
glaciers. Coasts eroded by glaciers are found in such places as Alaska,
Chile, Greenland, Norway, and Scotland, where they are known as
fjords. A fjord (pronounced FEE-yord) is a narrow inlet from the sea
that is both steep and deep. For example, one of the fjords in Chile
is more than 1210 meters deep. (See Figure 16-24 on page 408.)
Figure 16-23 A typical
rocky coastline is usually
quite steep.
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407
Figure 16-24 Fjords, such
as this one, are very deep
and steep; they are found
along coasts eroded by
glaciers.
Types of Reefs
You may recall that a coral reef is a limestone structure that is built
by coral polyps that live on the reef’s surface. There are three kinds
of reefs: fringing reefs, barrier reefs, and atolls. (See Figure 16-25.)
A fringing reef lies a few kilometers offshore and is parallel to the
mainland. On the shore side of the reef, the water is shallow; on
the ocean side, it is deep. A fringing reef grows most rapidly on the
ocean side of the reef because there is greater water circulation,
which brings more food and oxygen to the coral. Fringing reefs are
typically found in the Florida Keys and in the Caribbean.
A reef that grows farther offshore is called a barrier reef. Most
barrier reefs lie approximately 25 km offshore and are separated
from the island by a channel. The world’s most famous barrier reef
is the Great Barrier Reef, which is actually a series of reefs that lie
between 16 and 160 km off the northeast coast of Australia. The
Great Barrier Reef is 2000 km long.
Fringing reefs and barrier reefs grow right up to the surface of
the ocean. At low tide, the tops may extend above the water. Waves
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Channel
Barrier reef
Lagoon
Fringing reef
Atoll
and currents break off pieces of the coral; these chunks of coral
stone accumulate on the seafloor. If enough coral piles up, small
islands called keys or cays are formed. The Florida Keys and the
Cayman Islands are formed from coral stone.
Scattered throughout the South Pacific and the Indian Ocean
are coral structures called atolls. An atoll is a string of coral islands
that forms a circle. In the middle is a shallow lagoon that may vary
in width from 1 to 12 km. In 1837, the naturalist Charles Darwin
observed these islands as he sailed on the research vessel H.M.S. Beagle, and he wondered why the islands form a circle. He hypothesized that the circular shape represents the last stage in reef
evolution, which is associated with the sinking of a volcanic island.
According to Darwin, a fringing reef appears first along the shoreline
of a volcanic island. While the island begins slowly to sink or erode,
the fringing reef continues to grow upward and outward to form a
barrier reef. Finally, the island sinks completely below the surface,
leaving only a circular fringe of reefs, that is, the atoll. Scientists
have confirmed Darwin’s hypothesis by drilling through the coral
limestone and discovering a foundation of volcanic rock beneath
the reef. There are many well-known coral atoll islands in the
Pacific, such as Wake, Midway, Bikini, and Eniwetok.
Figure 16-25 The three
types of coral reef structures also represent the
main stages in reef
evolution.
16.4 SECTION REVIEW
1. What are three common sources of beach sand?
2. What are deltas and how are they formed?
3. Why are some rocky coasts (with fjords) so steep?
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409
Laboratory Investigation 16
Getting Water from a “Stone”
PROBLEM: How can we show that Earth’s crust contains water?
SKILLS: Heating and measuring a chemical compound; calculating weight
(mass) and percentages.
MATERIALS: Safety glasses, Bunsen burner or hot plate, porcelain evaporating dish, hydrated copper sulfate, spatula, tongs, triple-beam balance,
cooling pad.
PROCEDURE
1. Put on the safety glasses. Heat the evaporating dish over a Bunsen burner
or on a hot plate for a minute to evaporate possible moisture from the dish.
2. Use tongs to transfer the dish to a cooling pad for a few minutes.
3. After it cools, transfer the dish to the balance to be weighed. In your notebook, record the weight (mass) in a copy of Table 16-1.
4. Use a spatula to measure out 2 grams of copper sulfate and put it into the
evaporating dish. Record the weight (mass) of the dish plus the copper sulfate in the table.
5. Place the evaporating dish that contains the copper sulfate onto a hot plate
or over the Bunsen burner. Heat gently for five minutes, until the blue color
of the copper sulfate disappears.
6. Use tongs to transfer the dish to a cooling pad and wait a minute for it to
cool. Place the dish on the balance and record the weight (mass) in the table.
TABLE 16-1 WEIGHTS (MASSES) OF COPPER SULFATE
410
Evaporating dish (empty):
________________
grams
Evaporating dish plus copper sulfate (before heating):
________________
grams
Evaporating dish plus copper sulfate (after heating):
________________
grams
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CALCULATIONS
1. To find the weight (mass) of the copper sulfate, subtract the weight (mass)
obtained in step 3 from that of step 4.
2. To find the weight (mass) of the water, subtract the weight (mass) obtained
in step 6 from that of step 4 (line 3 from line 2 in the table).
3. To calculate the percentage of water in the hydrate, use the equation
Percentage of water = weight of waterweight of hydrate ✕ 100.
4. You can calculate the number of water molecules in the hydrate by using
the equation
Number of water molecules = weight of hydrateweight of water.
OBSERVATIONS AND ANALYSES
1. Compare your answer for calculation 4 with those of the other students. Your
answers may vary. How can you explain these differences? (The correct number of water molecules is five.)
2. Describe what happened—physically and chemically—to the copper sulfate
hydrate when it was heated.
3. What is the important difference between the copper sulfate before it was
heated and the copper sulfate after it was heated?
Geology of the Ocean
411
Chapter 16 Review
Answer the following questions on a separate sheet of paper.
Vocabulary
The following list contains all the boldface terms in this chapter.
atoll, barrier reef, continental drift, continental rise, continental
slope, convection current, crust, delta, fault, fjord, fringing reef,
guyots, hot spot, hydrothermal vent, island arcs, keys (cays),
magma, mantle, mid-ocean ridge, plate tectonics, plates, rift
valley, seafloor spreading, seamounts, subduction, submarine
canyons, topography, trenches
Fill In
Use one of the vocabulary terms listed above to complete each sentence.
1. The theory of ____________________ explains continental drift.
2. A ____________________ is an area of intense activity in the mantle.
3. The crust of one plate plunges below another during
____________________.
4. Molten material within the mantle is called ____________________.
5. A ____________________ is where black smokers emerge in a rift zone.
Think and Write
Use the information in this chapter to respond to these items.
6. What forces caused the supercontinent Pangaea to split apart?
7. How do coral atolls form? How are they related to reefs?
8. Explain how seamounts are related to guyots and islands.
Inquiry
Base your answers to questions 9 through 11 on the information in
Figure 16-8 on page 393, and on your knowledge of marine science.
9. Based on this profile of the ocean floor, which statement is
correct? a. The sides of the ridge are moving away from each
other. b. The sides of the ridge are moving toward each other.
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c. The bands of rocks closest to the center of the ridge are the
oldest. d. A deep-sea trench is being formed at the midocean ridge.
10. The magma that flows through the mid-ocean ridge comes up
from the a. continental crust b. oceanic crust c. mantle
d. rift valley.
11. What is an accurate statement regarding the data in this
diagram? a. Seafloor spreading began about 20 million years
ago. b. The ocean is deepest where it covers the rift valley.
c. The two sides of the ridge are very different from each
other. d. The topographic features on both sides of the ridge
are very similar.
Multiple Choice
Choose the response that best completes the sentence or answers the
question.
12. The fan-shaped feature that is formed by a river depositing
sediments near the shore is a a. barrier reef b. continental
rise c. delta d. plate.
13. The structure labeled “C” in the
diagram is the a. crust
b. mantle c. inner core
d. outer core.
D
C
A
B
14. The largest area of ocean floor is
the a. ocean basin
b. continental slope
c. continental shelf d. continental rise.
15. Deep-sea trenches are caused by a. faulting
c. volcanic eruptions d. turbidity currents.
b. subduction
16. A feature of the seafloor that provides evidence for the theory
of plate tectonics is a. sedimentary layers b. coral reefs
c. small canyons d. the mid-ocean ridge.
17. Which topographical features are in the proper sequence,
going from the Mid-Atlantic Ridge to the North American
continent? a. ocean basin, slope, shelf b. shelf, slope,
ocean basin c. trench, slope, basin d. ocean basin, shelf,
rise
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413
18. Which of the following items is not a topographical feature?
a. seamount b. oil deposits c. trenches d. guyots
19. The scientist most responsible for formulating the theory of
continental drift is a. Alfred Wegener b. Charles Darwin
c. Jacques Cousteau d. Sir Charles Thompson.
20. Which is not an accurate statement about plate tectonics?
a. Some continents are drifting apart. b. The continents are
fixed in position. c. The continents ride on crustal plates.
d. There is seismic activity at the margins of crustal plates.
21. The original supercontinent that existed about 200 million
years ago is called a. Loihi b. Atlantis c. Antarctica
d. Pangaea.
22. If sound travels 1454 meters per second in water, how deep is
the ocean floor if the echo of a ship’s signal takes one second
to return to the surface? a. 1454 meters b. 727 meters
c. 2181 meters d. 484 meters
23. Darwin hypothesized that the last stage in coral reef evolution
is the a. fringing reef b. barrier reef c. coral atoll
d. key.
24. What natural process is illustrated in the following diagram?
a. the after-effects of a tidal wave b. a rise in sea level
c. ecological succession on a volcanic island d. the
evolution of a coral (reef) atoll.
Channel
Barrier reef
Lagoon
Fringing reef
Atoll
Research/Activity
Use different colors of modeling clay to make a model of seafloor
spreading, an oceanic ridge, a subduction zone, or Earth’s plates.
Label the structures that you have represented in your model.
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