THE WATER PLANETtyburnscience.education/MarineBio/16.pdfThe Ocean Forms Remember that oceans could...

When you have completed this chapter, you should be able to:

RELATE the theories of continental drift and plate tectonics to theformation of the continents and oceans.

EXPLAIN the development of seafloor topographic features.

DESCRIBE the formation of coastal features and reef types.

The ocean is like a blanket of water that covers the seabed, or oceanfloor. But there is no true “rest” on the seabed, and Earth is not asleeping giant. There are many dynamic forces operating withinEarth’s solid part, or lithosphere, which influence and affect the char-acteristics of the liquid part, or hydrosphere. For example, the sud-den emergence of a volcanic island in the middle of the ocean (suchas Surtsey, shown above) is evidence of the forces at work thousandsof meters below the surface of the sea.

The study of the development and physical characteristics of ourplanet’s seafloor and continents—and of the forces that have shapedthem—makes up the field of science called geology. Scientists whospecialize in the study of geological features of the ocean are calledmarine geologists. You will begin your study of the geology of Earth’soceans and coastlines by going back in time to learn about the begin-ning of Earth itself.

384

16.1Origin of the Oceanand the Continents

16.2The Theory of PlateTectonics

16.3Ocean FloorTopography

16.4Coasts and Reefs in Profile

Geology of the Ocean

THE WATER PLANET

1616

16.1 ORIGIN OF THE OCEAN AND THE CONTINENTS

If you could go back before the beginning of time—back almost 20billion years—you could observe the start of our universe andindeed all the galaxies that make up the vast array of space. Today,the space telescope has permitted humans on Earth to get a glimpseback in time to the very beginnings of matter. Scientists think thatour universe formed as a result of a gigantic explosion, called thebig bang.

The Earth Forms

At the time of the big bang, all the matter in the universe was con-tained in one sphere. Extremely hot and dense, the sphereexploded, sending out matter in all directions in a kind of giantcloud. As the cloud moved out from the explosion, some of thematter came together and formed clumps that eventually becamegalaxies—including the Milky Way, the home galaxy of planetEarth. In time, further clumping of the matter caused the formationof stars and planets. Scientists think that Earth, and all the otherplanets 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 ahot molten mass, too hot for solid rocks to exist, too hot for waterto 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 hadcooled enough for liquid rock to become solid at Earth’s surface. Butthis early Earth was not a quiet place. For many millions of years,the solid surface of Earth was disturbed by volcanic activity thatoccurred over the whole planet.

An atmosphere (the layer of gases that surrounds a planet) beganto form on Earth about 3.5 billion years ago. Earth’s first atmospherewas very different from the atmosphere that exists today. Billions ofyears ago, the atmosphere probably contained some water vapor(water as a gas), carbon monoxide, hydrogen sulfide (a gas thatsmells like rotten eggs), nitrogen (the gas that makes up most oftoday’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 thehigh temperatures. But by about 4 billion years ago, Earth becamecool enough for water vapor within the mantle to cool. This even-tually formed liquid water on the surface. As Earth cooled still more,thunderclouds began to form. For many thousands of years, thun-derstorms occurred and covered Earth with water, filling in the lowspots 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, isalso thought to have caused the release of water vapor. In addition,heated water in the crust boiled up to the surface and formed hotsprings. Some of the hot water emerged from the surface as a geyser,or spray. In some places on Earth today, hot springs and geysers stillexist, which are evidence that the area beneath them is hot. (SeeFigure 16-1.) Scientists call this heat (a form of energy that formswithin Earth) geothermal energy. Iceland is an island that derivesmuch of its energy from geothermal sources. In fact, in places inIceland where this underground energy comes to the surface in theform of hot springs and steam, you can get a small glimpse of whatvast areas of early Earth must have looked like. People in Icelandheat their homes by tapping these underground sources of heat.

Ocean water also came from molecules of water that werebound up in compounds within Earth’s crust and released due to

386 The Water Planet

Ground Water Superheatedwater

OverflowGeyser

Figure 16-1 Heated waterin the crust forms hotsprings and geysers.

heating. Compounds that contain water are called hydrated com-pounds. An example of a hydrated compound is copper sulfate.Hydrated copper sulfate has the formula CuSO4•5H2O. Notice thatthe formula shows five water molecules attached to a molecule ofcopper sulfate. When hydrated copper sulfate is heated, water isgiven off (and anhydrous copper sulfate is produced), as shown inthe following reaction:

CuSO4•5 H2O + Heat → CuSO4 + 5 H2OHydrated Anhydrous Water

copper sulfate copper sulfate

You can measure water loss from a hydrated compound by per-forming the lab investigation at the end of this chapter.

Origin of the Continents

In 1912, Alfred Wegener, a German meteorologist, proposed ahypothesis that caused a great deal of controversy in the scientificcommunity. He suggested that the continents were not alwayslocated in their present positions, that over time they had moved.Wegener had noticed that the continents fit together like the piecesof a jigsaw puzzle. (See Figure 16-2 on page 388.) He suggested thatabout 200 million years ago the present continents formed one largelandmass he called Pangaea, surrounded by a single huge ocean. Atthat time, Pangaea began to break up into smaller continents thatmoved over the surface of Earth, ultimately reaching their presentpositions. What is the evidence that the present-day continents orig-inated in the single landmass Pangaea? Wegener cited the similaritiesof fossils and rock formations on different continents (especially oneither side of the Atlantic) along with other technical evidence tosupport his hypothesis, which he called the theory of continentaldrift. His theory was not well received by most geologists at thattime. In fact, scientists were quick to point out that Wegener was noteven a geologist; he was a meteorologist—a scientist who normallystudies the weather. However, studies of the seafloor that were con-ducted in the 1950s provided more evidence to confirm Wegener’stheory, and it is now generally accepted by the scientific community.

What caused the single landmass to break apart into severalcontinents? And what forces caused the continents to drift apart?


Wegener was ridiculed because he was not able to provide an exactmechanism for the movement of the continents. (See Figure 16-3.)However, we now know that powerful forces inside Earth’s interiorcaused the breakup of the continents. Look at the diagram of Earth’sinterior, shown in Figure 16-4. The interior of our planet is com-posed of several layers. At Earth’s center is the inner core, sur-rounded by the outer core, the mantle, and then the crust. Muchof Earth’s interior is in a hot, molten state. The inner core has thehighest temperatures, with a range of about 6200 to 6600°C. Themantle, which is a region of geologic activity between the core andthe 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—


NorthAmerica

SouthAmerica

Africa

India

Madagascar Australia

Antarctica

Greenland

EuropeAppalachianMountains

CaledonianMountains

Rockformations

Evidence ofglaciation

Figure 16-2 Evidence thatthe continents were oncejoined.

consists of hot, molten material. This molten material within themantle is called magma. The churning of the magma creates a forcethat generates great pressures upward into Earth’s surface layer, orcrust. Earth’s crust is only about 40 km thick. If enough force is gen-erated, it cracks. Then the ground trembles and moves, producingan earthquake. Sometimes magma flows out of a crack in the crust,


THE EARTH 250 MILLION YEARS AGO THE EARTH 135 MILLION YEARS AGO

THE EARTH 45 MILLION YEARS AGO THE EARTH AT PRESENT

NorthAmerica

NorthAmerica

SouthAmerica

NorthAmerica

SouthAmerica Atlantic

Ocean

IndianOcean

PacificOcean

IndiaIndia

AfricaAfrica

Eurasia

Pangaea

Asia

AntarcticaAntarcticaAustralia

Australia

Eurasia

Gondwanaland

Europe

Crust

2900 km

Mantle2188 km

1255 km

Outer core

Innercore

Figure 16-3 The breakupof Pangaea and continen-tal drift over time.

Figure 16-4 The layers ofEarth’s interior.

producing a volcanic eruption. Magma that flows out of the crustonto Earth’s surface is called lava. Disturbances (vibrations) inEarth’s crust, such as volcanic eruptions and earthquakes, are exam-ples of seismic activity.

Seismic activity was involved in the breakup of the superconti-nent Pangaea. In the next section, you will learn about the mecha-nism 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 continentsoriginated from a single landmass.

16.2 THE THEORY OF PLATE TECTONICS

Satellite photos show that the continents are moving at a rate ofapproximately one centimeter per year. The Atlantic Ocean is get-ting wider and the Pacific Ocean is getting narrower. What exactly iscausing the continents to drift? Research on Earth’s interior hasrevealed that its crust is divided into segments called plates, whichfloat like rafts on the molten interior layer. The continents ride ontop of these plates. This idea that Earth’s crust is divided into seg-ments that drift about has been developed into the theory of platetectonics.

To understand the theory of plate tectonics, look at the worldmap in Figure 16-5. Earth’s crust is divided into seven major platesand about a dozen minor plates. Locate the North American plate.The eastern border lies in the middle of the Atlantic Ocean, and thewestern border runs through California. The North American plateis drifting westward. To understand what causes the plates to move,look at the profile of the mantle shown in Figure 16-6. There is abig difference in temperature between the upper mantle (about1400°C) and the lower mantle (about 2600°C). Scientists think thatthis difference in temperature creates convection currents. A con-vection current is a transfer of heat in a liquid or gas that causes


the molten magma to rise up through the mantle and into the crust,forming an oceanic ridge. Off the east coast of North and SouthAmerica, it is the Mid-Atlantic Ridge that is formed; this ridge runsthe entire length of the Atlantic Ocean, dividing it in half. Magmathat breaks through Earth’s oceanic crust as lava can also accumu-late to form mid-ocean islands.


JavaTrench

Himalayas

SanAndreasFault

PacificPlate Nazca

PlateSouth

AmericanPlate

NorthAmerican

Plate

AfricanPlate

EurasianPlate

ArabianPlate

AntarcticPlate

IndianPlate

Aleutian

Trench

An

des Mountains

Magma

Risingmagma(Hot)

Oceaniccrust

Mid-oceanridge

Continentalcrust

Trench

(Cool)

Heat currents in mantle

(Cool)

Volcano

Figure 16-5 Earth’s majorcrustal plates.

Figure 16-6 Convectioncurrents in the mantlecause seafloor spreading.

Seafloor Spreading

The upward movement of magma under the Mid-Atlantic Ridgecauses seafloor spreading, which is the moving apart of the plates.As you can see in Figure 16-5, the North American and South Amer-ican plates move westward, while the Eurasian and African platesmove eastward. As the hot magma rises under the Mid-AtlanticRidge, cooler magma moves in to take its place. This sets up a con-tinuous circulation pattern similar to the circulation of warm air ina room.

The movement of the plates causes them to collide with oneanother. Notice, in Figure 16-5, that the North American plate col-lides with the Pacific plate. When this occurs, one plate overridesthe other plate. Crust from the Pacific plate plunges downwardunder the North American plate in a process called subduction. ThePacific plate slides under part of the North American plate becauseoceanic plates are denser than continental plates. Subductiondestroys old plates as the crust descends into the mantle to becomemolten magma. This process occurs in several areas around theworld (at subduction zones) and forms trenches, the deepest andsteepest depressions found on the ocean floor. (See Figure 16-7.)


AsiaNorth

America

SouthAmerica

Puerto RicoTrench

Peru-ChileTrench

Mid-AmericaTrench

PACIFIC OCEAN

TongaTrench

KermadecTrench

JavaTrench

Mindanao Trench

Mariana Trench

JapanTrench

Aleutian Trench

Hawaiian Islands

Australia

Figure 16-7 Deep-seatrenches are formed atsubduction zones.

The theory of plate tectonics helps to explain various geologicalphenomena. At the margin of the plates, a crack occurs in Earth’scrust that is called a fault. For example, the San Andreas fault,which cuts through California, forms the boundary between theNorth American and Pacific plates. Earthquakes and volcanic activ-ity tend to occur along the margins of plates, where there is move-ment of, and friction between, the adjoining plates.

Ocean Floor Formation

Plate tectonics also explains how the ocean floor was formed. Asmagma continued to rise up to form the Mid-Atlantic Ridge, theNorth American and Eurasian plates moved farther apart, creating anew ocean floor. This process, which has been occurring for manymillions of years, is recorded in the symmetrical, parallel bands ofbasalt (the volcanic rock that makes up the ocean floor) that spreadout along either side of the ridge. The spreading apart of the Atlanticseafloor is an ongoing process.

Further evidence that the seafloor is spreading comes from thestudy 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 theoldest rocks are farther away. Scientists discovered that the magneticproperties of rocks on both sides of the ridge were as symmetrical as


Mantle

Rift valley

Age in millions of years

40 30 20 0 20 30 40

Mid-ocean ridgeOcean

Oceanic crust Continental crust

Magma

Figure 16-8 Parallel bandsof rock increase in ageaway from the ridge.

the bands of rocks. When the magma hardened, the magnetic miner-als in the rock aligned in the direction of Earth’s magnetic field. Dur-ing Earth’s history, the magnetic poles have reversed several times.The pattern of polarity on either side of the ridge was identical andreflected these pole reversals. Scientists concluded that, as the magmahardened, half moved to one side of the oceanic ridge and the otherhalf moved to the other side. This pattern of magnetic bands providedstrong evidence for ocean floor spreading. (See Figure 16-9.)

The theory of plate tectonics is called a unifying theory becauseit explains the origin of, and connections between, such phenom-ena as earthquakes, volcanic activity, faults, continental drift, andseafloor spreading. A knowledge of plate tectonics is also helpful inexplaining 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 theprocess is related to seafloor spreading.

3. Why is the theory of plate tectonics called a unifying theory?


Spreading SpreadingMagma

South

North

RidgeAxis

BasaltLava

UnitedStates Bermuda

Mid-AtlanticRidge Africa

Horizontal scale: 2.5 cm = approx. 1390 kmVertical scale: Greatly exaggerated

(Sea level)

Figure 16-9 Identicalmagnetic bands areseen on each side ofthe ridge.

Figure 16-10 A profile ofthe Atlantic Ocean floor.

16.3 OCEAN FLOOR TOPOGRAPHY

What do the features of the ocean bottom look like? The averagedepth of the ocean is about 3636 meters—much too deep for scubadivers to explore. However, oceanographers can obtain a profile ofthe ocean floor (without submerging in an underwater vehicle) byusing sonar. Modern ships are equipped with sonar. A ship’s sonardevice beams a continuous sound signal downward. After the soundwave hits the bottom, the returning signal, called an echo, isreceived by a depth recorder in the ship. This produces a line tracingof the ocean floor. (See Figure 16-11.) Notice that the depth record-ing of the ocean floor shows a bottom that varies from fairly smoothto jagged, or irregular. The irregular part could be debris dumped onthe ocean floor, a sunken ship, or a natural feature. Recall that theTitanic and other sunken ships have been located by using sonar.Modern fishing boats also use sonar to locate schools of fish. Sonar


2400

1800

1200

600

0

Dep

th in

met

ers

0 1 2 3 4 5Kilometers

5 kilometers

Source Receiver

2400

1800

1200

600

0

Dep

th in

met

ers

Sou

nd w

aves

Figure 16-11 Sonar isused to obtain a profile ofthe ocean floor.

is very useful to help ships navigate in shallow waters. For example,a reef may be located only several meters below the surface—closeenough to make ships cautious when they pass by.

Sonar, Ocean Depth, and Topography

Ocean depth is calculated automatically by sonar. Two pieces ofinformation are needed to calculate the depth—the speed of soundin water (1454 meters per second) and the time it takes for the sig-nal to reach the bottom. If a signal takes one second (after beingsent) to return to the ship, then it takes one-half second to travelto the bottom. Since sound travels 1454 meters per second under-water, it travels 1454 divided by 2 in one-half second. Therefore, thedepth of the water is 727 meters. The following formula is useful incalculating ocean depth using sonar:

Depth (D) = 1454 meters per second ✕ time (t)�2

In this example,

D = 1454 ✕ 1�2

D = 727 meters

Ships equipped with sonar have been crisscrossing the oceansfor a number of years in an attempt to map as much of the oceanfloor as possible. Thousands of sonar tracings have been made. Thedata from the ships’ sonar maps have been combined to create asingle map that shows all seafloor elevations and depressions. Thestudy of Earth’s surface features, such as elevations and depressions,on the land and the ocean floor is called topography. Seafloortopography includes some very dramatic depressions and eleva-tions. (See Figure 16-12.)


Continentalshelf

Continentalslope

Continentalslope

ContinentContinent

Deep oceanfloor

Deep oceanfloor

Mid-oceanridge

Seamount

IslandTrench

Figure 16-12 Features ofthe ocean floor.

Seafloor Features

Recently, the U.S. Navy released a treasure trove of formerly classi-fied 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. Theymeasured bumps and depressions on the ocean’s surface thatreflected the pull of gravity (on the water) exerted by seafloorobjects. A variety of features were uncovered. High ridges off thecoast of Oregon were formed when plates collided. Off the westcoast of Florida, the seafloor has steep-walled elevations 2 km high.Off New Jersey’s coast, there is a continental slope with deepcanyons that were most likely produced by submarine avalanches. Acontinental slope is the area where the seafloor drops steeply at theouter edge of the continental shelf. (See Figure 16-13.) In the Gulf ofMexico, off the Louisiana coast, the images show a moonlike, pock-marked surface, formed when the gulf was dry and filled with evap-orated sea salt. Sediments from the Mississippi River covered thesalt. When the sea level rose, the weight of ocean water on thesesediments created the odd shapes.

Cutting through the continental shelf and slope are steepV-shaped depressions called submarine canyons. Some of thesecanyons are as large as the Grand Canyon. Two well-known subma-rine canyons along the U.S. coasts are the Monterey Canyon off Cal-ifornia and the Hudson (River) Canyon off New York. (See Figures16-14a and 16-14b on page 398.) How did these huge canyonsform? Many submarine canyons are extensions of sunken river val-leys from the adjoining continent. During the last ice age, when thesea level was much lower, many canyons existed as river valleys.When the sea level rose at the end of the ice age, these valleys weresubmerged, creating the submarine canyons. The deeper parts of


6000

4500

3000

1500

0Shelf Shelf

Slope

Slope

United States (San Diego) South America

Dep

th in

met

ers

Figure 16-13 Continentalshelves and slopes.

submarine canyons are formed by swift undersea currents. Smallervalleys that cut through a slope are formed as a result of erosion bymudslides, which move from the shelf out to the ocean basin. Theaccumulation of mudslide sediments at the base of a slope creates aslightly elevated region called the continental rise.

Sonar maps of the ocean floor often reveal small submarinemountains called seamounts (shown in Figure 16-12). Seamounts


New Jersey

Long Island

New York City

Continental shelf

HudsonCanyon

Continental slope

Figure 16-14a TheHudson (River)Canyon.

Figure 16-14b A sonarscan of the Hudson (River)Canyon.

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 isformed. The Hawaiian Islands were formed from a chain ofseamounts. The main island of Hawaii, at the eastern end of thechain, is the youngest (formed about 800,000 years ago) and mostgeologically active of the five Hawaiian Islands. The progressivelyolder seamounts stretch in an arc to the northwest. (The oldestHawaiian Island is about 4 to 6 million years old.) Such a chain ofislands is formed when a crustal plate moves over an area of intenseactivity in the mantle, called a hot spot. The area over the hot spotdevelops a seamount as lava pours through its crust. As the platemoves 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 sunkbeneath the surface. Erosion by waves and currents can cause thetops of seamounts to become flattened, forming structures calledguyots (pronounced GEE-ohz), shown in Figure 16-15. Trenches, asmentioned above, are another topographical feature of the oceanfloor. Recall that trenches are found at the margins (subductionzones) of crustal plates, where one plate descends into the mantlebelow the other plate. (See again Figure 16-12.) The deepest oceantrench is the Mariana Trench, located in the western Pacific Ocean.The Mariana Trench is 10,958 meters deep, which is deep enough tocontain Mt. Everest, the tallest terrestrial mountain on Earth.


GuyotsSea level

YoungestOldest

Mantle Hot spot

Oceanic plate

Figure 16-15 Achain of islandsforms over a hotspot.

TECHNOLOGYLoihi on the Rise


The inhabitants of the main island of Hawaiimust be among the bravest people in theworld. These islanders live about 30 km fromthe most active volcano on Earth. This volcano,named Loihi (meaning “long one” in Hawaiian)is so active that it produced nearly 1500 earth-quakes in just one week! Even though the vol-cano is more than 3 km high and about 40 kmlong, local Hawaiians cannot see it. This isbecause Loihi rises from the ocean floor, with itspeak about 1 km below the ocean surface.Marine geologists predict that Loihi will be the

next Hawaiian island to emerge from theocean—about 50,000 to 100,000 years fromnow.

Loihi has been erupting continuously forabout 20 years. Most of the earthquakes it pro-duces are relatively weak. However, they seemto be increasing in magnitude and frequency,causing local authorities to be concerned for thesafety of the islanders. Some recent quakes havebeen recorded at magnitudes between 4 and 5on the Richter scale. (Either the Richter or themagnitude scale is used to measure earthquakeintensity.) Seismologists (scientists who studyearthquakes) fear that an underwater earth-quake with a magnitude of 6.8 might produce ahuge wave a that would reach the big island injust 15 minutes—not enough time for peoplealong the coast to prepare for emergency evac-uation to higher ground.

Several federal agencies are monitoring thevolcano’s activity. Local officials have decided toset up an early warning system that will giveisland residents more time to evacuate in theevent of a serious earthquake. The HawaiianUndersea Research Laboratory at the Universityof Hawaii is using submersibles and robots tomonitor and videotape the eruptions underwa-ter, thus giving scientists the opportunity toobserve and record never-before-seen eventsgoing on inside the crater.

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.

QUESTIONS

Associated with the trenches are groups of volcanic islands thatform an arc in the ocean, called island arcs. (See Figure 16-16.) Mosttrenches, and their island arcs, are located on the periphery of thePacific Ocean (that is, along the west coasts of North and SouthAmerica and the east coast of Asia). Many of these islands are stillvolcanically active, so the area bordering the Pacific is called theRing 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, itforms ridges. The prominent mid-ocean ridge is the continuousundersea volcanic mountain range that encircles the globe, mark-ing the boundaries of several crustal plates. As noted above, the partof the ridge that runs through the middle of the Atlantic Ocean iscalled the Mid-Atlantic Ridge. (See Figure 16-17 on page 402.)

The Mid-Atlantic Ridge rises about 3030 meters above the oceanfloor; its highest underwater ridge lies about 900 meters below thesurface of the ocean. Iceland, which rises above the surface, is a vol-canic island formed by lava that poured out of the Mid-AtlanticRidge. Other smaller volcanic islands have formed near Iceland,such as Surtsey, which was “born” in 1963. Eruptions continued onthis island for several years, causing Surtsey to increase in size. (Seephotograph on page 384.)


Magma

MantleOcean crust

Ocean trench

Island arc of volcanoes

Oceanic plate

Sea level

Figure 16-16 Vol-canic island arcsform over trenches.

A depression called the central rift valley runs along the crest ofthe mid-ocean ridge (including the Mid-Atlantic Ridge). The rift is acrack in the seafloor through which molten rock from the mantle isexpelled. The rift zone is of special interest to oceanographersbecause that is the place where new seafloor is continually beingcreated. When the hot magma pours out on the ocean floor, it coolsand solidifies to form new ocean crust. Lateral movements on bothsides of the rift then cause the seafloor to spread apart. About 250million years ago, all the continents were joined together at theMid-Atlantic Ridge. Over geologic time, hot molten matter risingup through the mantle and into the crust caused the continents tosplit apart. The continuing flow of magma pushed the continentsfarther apart and created a space between them, called an oceanbasin. This ocean basin filled up with water and became the AtlanticOcean.

Features of the Rift Zone

The oceanic crust is very porous in the area of a rift zone. Oceanwater seeps down through cracks in the crust and gets heated fromthe hot magma below. The hot water rises up through the crust, dis-


North America

Atlantic Ocean

South America

Europe

Africa

Mid

-At la

n t icR

id ge

Figure 16-17 TheMid-Atlantic Ridge ispart of the mid-oceanridge.

solving minerals out of the rock as it flows. When the hot wateremerges from the seafloor, it makes contact with the cold oceanwater. Then the minerals dissolved in the hot water form a cloud thatlooks like smoke coming from a chimney. Oceanographers call thesesprings of mineral-laden waters “black smokers.” (See Figure 16-18.)

The area in the rift zone where these hot springs emerge iscalled a hydrothermal vent. Submersibles have visited the hydro-thermal vents, some more than 2400 meters deep. But the sub-mersibles cannot get too close to a vent. The temperature can be ashigh as 371°C. (Recall that water boils at 100°C.) The water was sohot at one vent that it melted the first thermometer used to recordthe temperature.

In 1977, scientists aboard the Alvin, an American submersible,made an amazing discovery while investigating some hydrothermalvents. Parts of the seafloor near the vents were carpeted by a thickgrowth 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 agreat depth, where there is no light. Since 1977, the Alvin and othersubmersibles have made many more dives to study the vents andto bring back water samples and specimens. The water was foundto be very rich in minerals, particularly the compound hydrogensulfide (H2S). Researchers also found that the water had a high con-centration of bacteria. It turns out that these bacteria use the hydro-gen sulfide to produce their food.


Figure 16-18 Mineral-rich“black smokers” (such asthe one shown here) format hydrothermal vents.

Some types of bacteria share the food they make in a symbioticrelationship with giant tube worms and other deep-sea creatures.You may recall that this form of food making by bacteria, which isnot based on photosynthesis, is called chemosynthesis. (Refer toChapter 8 for a review of how chemosynthetic bacteria use hydro-gen sulfide to make their food.) Marine scientists continue to col-lect water samples from the areas around hydrothermal vents. Theyrecently discovered large masses of heat-loving bacteria, called ther-mophilic bacteria, which live on the outside of the vents in water ator near the boiling point! In addition, many new and unique speciesof crabs, mussels, octopus, shrimp, and fish have been discoverednear the vents.

16.3 SECTION REVIEW

1. Calculate the depth of a sunken ship if a sonar signal takes twoseconds to return to the research ship after it is emitted.

2. How were the Hawaiian Islands formed? Are they all the sameage?

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 kmof a coast. The coast, or shore, is the boundary between land andsea. As you have learned in previous chapters, some coasts are rocky,while others have sandy beaches. A beach is a part of the shore thatcontains loose sediments eroded from the land.

Beaches

Our coastal states have an abundance of sandy beaches and rockyshores. In Hawaii, there are both sandy beaches and rocky coasts ofvolcanic origin. Volcanic rock originates from the molten lava thatpours out of volcanoes and flows down to the sea. When the lava


reaches the ocean, it boils the water into steam, and the lava hard-ens into rock. Sand on a beach is mainly the product of erosionfrom rocks along the shore. Waves pound on the rocky shores, andpieces of rock break off and fall into the surf. Tides move the rocksback and forth, wearing them down into pebbles. Over time, thepebbles are ground into sand by rubbing against one another asthey are tossed by the waves. (See Figure 16-19.)

Beach sand may also come from eroded rocks from mountainslocated hundreds of km away. Rivers and streams wear down therocks. Sediments from the rocks are transported downstream to theocean, where they are deposited as sand on the beach. (You alreadyexamined sand grains under the microscope to determine their ori-gin in the lab investigation in an earlier chapter.) The erosion ofvolcanic rock produced the black sands found on some Hawaiianbeaches. The white and pink sandy beaches of many other tropicalislands are largely composed of fine sediments eroded from offshorecoral reefs. Sand may also contain shell or bone fragments, fishscales, and other debris from marine animals. (See Figure 16-20 onpage 406.)

Coasts have differently shaped profiles. On sandy shores withheavy surf, the crashing waves erode the sand, forming a steeplysloped beach. On beaches where large rivers empty into a calm sea,


Figure 16-19 A typicalsandy beach is producedby erosion of rocks and ismildly sloped.

sediments carried by the river are deposited along the shore, pro-ducing a fan-shaped feature called a delta. (See Figure 16-21.) TheNile River, which flows into the Mediterranean, and the MississippiRiver, which empties into the Gulf of Mexico, both form deltas. (SeeFigure 16-22.)

Rocky Coasts

Compared to sandy beaches, rocky coasts are often very steep. (SeeFigure 16-23.) How are they formed? The rocky coast of Maine wasformed 12,000 years ago, toward the end of the last Ice Age. As theclimate warmed, the glacier that covered much of North Americaretreated, carving out troughs, or valleys, which later became rivervalleys. When the glaciers melted, the sea level rose. The ocean


Lagoon Delta Hook

Offshore barsSpit

Dust particlesand cinders

Runningwater

Sea floor

Shells

OozeMudSiltGravel

and sand

Bedrock

Meteoricdust

Wind

Deep seazone

Figure 16-20 Beachsand and seafloorsediments comefrom eroded rockand other debris.

Figure 16-21 Typicalshoreline features, includ-ing a delta.

invaded the land, filling in the eroded valleys left by the retreatingglaciers. Coasts eroded by glaciers are found in such places as Alaska,Chile, Greenland, Norway, and Scotland, where they are known asfjords. A fjord (pronounced FEE-yord) is a narrow inlet from the seathat is both steep and deep. For example, one of the fjords in Chileis more than 1210 meters deep. (See Figure 16-24 on page 408.)


Figure 16-22 The fan-shaped Mississippi Deltaextends into the Gulf ofMexico.

Figure 16-23 A typicalrocky coastline is usuallyquite steep.

Types of Reefs

You may recall that a coral reef is a limestone structure that is builtby coral polyps that live on the reef’s surface. There are three kindsof reefs: fringing reefs, barrier reefs, and atolls. (See Figure 16-25.) A fringing reef lies a few kilometers offshore and is parallel to themainland. 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 theocean side of the reef because there is greater water circulation,which brings more food and oxygen to the coral. Fringing reefs aretypically found in the Florida Keys and in the Caribbean.

A reef that grows farther offshore is called a barrier reef. Mostbarrier reefs lie approximately 25 km offshore and are separatedfrom the island by a channel. The world’s most famous barrier reefis the Great Barrier Reef, which is actually a series of reefs that liebetween 16 and 160 km off the northeast coast of Australia. TheGreat Barrier Reef is 2000 km long.

Fringing reefs and barrier reefs grow right up to the surface ofthe ocean. At low tide, the tops may extend above the water. Waves


Figure 16-24 Fjords, suchas this one, are very deepand steep; they are foundalong coasts eroded byglaciers.

and currents break off pieces of the coral; these chunks of coralstone accumulate on the seafloor. If enough coral piles up, smallislands called keys or cays are formed. The Florida Keys and theCayman Islands are formed from coral stone.

Scattered throughout the South Pacific and the Indian Oceanare coral structures called atolls. An atoll is a string of coral islandsthat forms a circle. In the middle is a shallow lagoon that may varyin width from 1 to 12 km. In 1837, the naturalist Charles Darwinobserved these islands as he sailed on the research vessel H.M.S. Bea-gle, and he wondered why the islands form a circle. He hypothe-sized that the circular shape represents the last stage in reefevolution, which is associated with the sinking of a volcanic island.According to Darwin, a fringing reef appears first along the shorelineof a volcanic island. While the island begins slowly to sink or erode,the fringing reef continues to grow upward and outward to form abarrier reef. Finally, the island sinks completely below the surface,leaving only a circular fringe of reefs, that is, the atoll. Scientistshave confirmed Darwin’s hypothesis by drilling through the corallimestone and discovering a foundation of volcanic rock beneaththe reef. There are many well-known coral atoll islands in thePacific, such as Wake, Midway, Bikini, and Eniwetok.

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?


Channel

Atoll

Lagoon

Barrier reef

Fringing reef

Figure 16-25 The threetypes of coral reef struc-tures also represent themain stages in reefevolution.

Laboratory Investigation 16


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 evaporat-ing 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 burneror 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 note-book, 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 theevaporating dish. Record the weight (mass) of the dish plus the copper sul-fate in the table.

5. Place the evaporating dish that contains the copper sulfate onto a hot plateor over the Bunsen burner. Heat gently for five minutes, until the blue colorof the copper sulfate disappears.

6. Use tongs to transfer the dish to a cooling pad and wait a minute for it tocool. Place the dish on the balance and record the weight (mass) in the table.

TABLE 16-1 WEIGHTS (MASSES) OF COPPER SULFATE

Evaporating dish (empty): ________________ grams

Evaporating dish plus copper sulfate (before heating): ________________ grams

Evaporating dish plus copper sulfate (after heating): ________________ grams

Getting Water from a “Stone”

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) obtainedin 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 water�weight of hydrate ✕ 100.

4. You can calculate the number of water molecules in the hydrate by usingthe equation

Number of water molecules = weight of hydrate�weight of water.

OBSERVATIONS AND ANALYSES

1. Compare your answer for calculation 4 with those of the other students. Youranswers may vary. How can you explain these differences? (The correct num-ber of water molecules is five.)

2. Describe what happened—physically and chemically—to the copper sulfatehydrate when it was heated.

3. What is the important difference between the copper sulfate before it washeated and the copper sulfate after it was heated?


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, continentalslope, 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, riftvalley, seafloor spreading, seamounts, subduction, submarinecanyons, 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 inFigure 16-8 on page 393, and on your knowledge of marine science.

9. Based on this profile of the ocean floor, which statement iscorrect? a. The sides of the ridge are moving away from eachother. b. The sides of the ridge are moving toward each other.

Chapter 16 Review


c. The bands of rocks closest to the center of the ridge are theoldest. d. A deep-sea trench is being formed at the mid-ocean ridge.

10. The magma that flows through the mid-ocean ridge comes upfrom the a. continental crust b. oceanic crust c. mantled. rift valley.

11. What is an accurate statement regarding the data in thisdiagram? a. Seafloor spreading began about 20 million yearsago. b. The ocean is deepest where it covers the rift valley.c. The two sides of the ridge are very different from eachother. d. The topographic features on both sides of the ridgeare very similar.

Multiple Choice

Choose the response that best completes the sentence or answers thequestion.

12. The fan-shaped feature that is formed by a river depositingsediments near the shore is a a. barrier reef b. continentalrise c. delta d. plate.

13. The structure labeled “C” in thediagram is the a. crustb. mantle c. inner cored. outer core.

14. The largest area of ocean floor isthe a. ocean basinb. continental slopec. continental shelf d. continental rise.

15. Deep-sea trenches are caused by a. faulting b. subductionc. volcanic eruptions d. turbidity currents.

16. A feature of the seafloor that provides evidence for the theoryof plate tectonics is a. sedimentary layers b. coral reefsc. 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 Americancontinent? a. ocean basin, slope, shelf b. shelf, slope,ocean basin c. trench, slope, basin d. ocean basin, shelf,rise


A

B

C

D

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 ofcontinental drift is a. Alfred Wegener b. Charles Darwinc. 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 arefixed 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 millionyears ago is called a. Loihi b. Atlantis c. Antarcticad. Pangaea.

22. If sound travels 1454 meters per second in water, how deep isthe ocean floor if the echo of a ship’s signal takes one secondto return to the surface? a. 1454 meters b. 727 metersc. 2181 meters d. 484 meters

23. Darwin hypothesized that the last stage in coral reef evolutionis the a. fringing reef b. barrier reef c. coral atolld. key.

24. What natural process is illustrated in the following diagram?a. the after-effects of a tidal wave b. a rise in sea levelc. ecological succession on a volcanic island d. theevolution of a coral (reef) atoll.

Research/Activity

Use different colors of modeling clay to make a model of seafloorspreading, an oceanic ridge, a subduction zone, or Earth’s plates.Label the structures that you have represented in your model.


Channel

Atoll

Lagoon

Barrier reef

Fringing reef

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