Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

9.1 Discuss the extent and distribution of oceans and continents on Earth. Identify Earth’s four main ocean basins.

9.2 Define salinity and list the main elements that contribute to the ocean’s salinity. Describe the sources of dissolved substances in seawater and causes of variations in salinity.

9.3 Discuss temperature, salinity, and density changes with depth in the open ocean.

9.4 Define bathymetry and summarize the various techniques used to map the ocean floor.

9.5 Compare a passive continental margin with an active continental margin and list the major features of each.

9.6 List and describe the major features associated with deep-ocean basins.

9.7 Summarize the basic characteristics of oceanic ridges. 

9.8 Distinguish among three categories of seafloor sediment and explain why some of these sediments can be used to study climate change.

Concept Checks 

9.1  How does the area of Earth’s surface covered by oceans compare with the area covered by continents?Contrast the distribution of land and water in the Northern Hemisphere and the Southern Hemisphere. Excluding the Southern Ocean, name the four main ocean basins. Contrast them in terms of area and depth. How does the average depth of the oceans compare to the average elevation of the continents?

9.2 What is salinity, and how is it usually expressed? What is the average salinity of the ocean? What are the two most abundant elements dissolved in seawater? What are the primary sources for the elements that comprise the dissolved components in seawater? List several factors that cause salinity to vary from place to place and from time to time. Is the pH of the ocean increasing or decreasing? What is responsible for the changing pH?

9.3 Contrast temperature variations with depth in the high and low latitudes. Why do high-latitude waters generally lack a thermocline? What two factors influence seawater density? Which one has the greater influence on surface seawater density? Contrast density variations with depth in the high and low latitudes. Why do high-latitude waters generally lack a pycnocline? Describe the ocean’s layered structure. Why does the three-layer structure not exist in high latitudes?

9.4 Define bathymetry. Describe how satellites orbiting Earth can determine features on the seafloor without being able to directly observe them beneath several kilometers of seawater. List the three major provinces of the ocean floor.

9.5 List the three major features of a passive continental margin. Which of these features is considered a flooded extension of the continent? Which one has the steepest slope? Describe the differences between active and passive continental margins. Where is each type found? How are active continental margins related to plate tectonics?

9.6 Explain how deep-ocean trenches are related to plate boundaries. Why are abyssal plains more extensive on the floor of the Atlantic than on the floor of the Pacific? How does a flat-topped seamount, called a guyot, form? What features on the ocean floor most resemble basalt plateaus on the continents?

9.7 Write a brief summary describing oceanic ridges. List some ways that oceanic ridges are different from mountains on continents. What is the primary reason for the elevated position of oceanic ridges?

9.8 List and describe the three basic types of seafloor sediments. Give at least one example of each. Why are seafloor sediments useful in studying past climates?

Concepts In Review

9.1 The Vast World Ocean

Discuss the extent and distribution of oceans and continents on Earth. Identify Earth’s four main ocean basins.

Key Term: oceanography

Oceanography is an interdisciplinary science that draws on the methods and knowledge of biology, chemistry, physics, and geology to study all aspects of the world ocean. 

*Earth’s surface is dominated by oceans. Nearly 71 percent of the planet’s surface area is oceans and marginal seas. In the Southern Hemisphere, about 81 percent of the surface is water.

*Of the three major oceans—the Pacific, Atlantic, and Indian—the Pacific Ocean is the largest, contains slightly more than half of the water in the world ocean, and has the greatest average depth—3940 meters (12,927 feet).

QUESTION: Name the four principal oceans identified with letters on this world map.

9.2 Composition of Seawater

Define salinity and list the main elements that contribute to the ocean’s salinity. Describe the sources of dissolved substances in seawater and causes of variations in salinity.

Key Term:salinity

*Salinity is the proportion of dissolved salts to pure water, usually expressed in parts per thousand (‰). The average salinity in the open ocean is about 35‰. The principal elements that contribute to the ocean’s salinity are chlorine (55%) and sodium (31%). The primary sources for the elements in sea salt are chemical weathering of rocks on the continents and volcanic outgassing on the ocean floor.

*Variations in seawater salinity are primarily caused by changing the water content. Natural processes that add large amounts of freshwater to seawater and decrease salinity include precipitation, runoff from land, iceberg melting, and sea ice melting. Processes that remove large amounts of freshwater from seawater and increase salinity include the formation of sea ice and evaporation.

QUESTION: How are emissions from this coal-burning power plant influencing the acidity of seawater? Explain.

9.3 Variations in Temperature and Density with Depth

Discuss temperature, salinity, and density changes with depth in the open ocean.

Key Terms: thermocline, density, pycnocline

*The ocean’s surface temperature is related to the amount of solar energy received and varies as a function of latitude. Low-latitude regions have relatively warm surface water and distinctly colder water at depth, creating a thermocline, which is a layer of rapid temperature change. No thermocline exists in high-latitude regions because there is little temperature difference between the top and bottom of the water column.

*The density of seawater depends mainly on its temperature and secondarily on salinity. Cold, high-salinity water is densest. Low-latitude regions have distinctly denser (colder) water at depth than at the surface, creating a pycnocline, which is a layer of rapidly changing density. No pycnocline exists in high-latitude regions because there is little density difference between the top and bottom of the water column.

*At low latitudes, most regions of the open ocean exhibit a three-layered structure based on water density. The shallow surface mixed zone has warm and nearly uniform temperatures. The transition zone includes a prominent thermocline and associated pycnocline. The deep zone is continually dark and cold and accounts for 80 percent of the water in the ocean. In high latitudes, the three-layered structure does not exist in temperate latitudes, it is present during the summer.

QUESTION: The accompanying graph depicts variations in ocean water density and temperature with depth for a location near the equator. Which line represents temperature, and which represents density? Explain.

9.4 An Emerging Picture of the Ocean Floor

Define bathymetry and summarize the various techniques used to map the ocean floor.

Key Terms: bathymetry, sonar, echo sounder

*The measurement of ocean depths and charting of the ocean floor is called bathymetry.

*Seafloor mapping is done with sonar using shipboard instruments that emit pulses of sound that “echo” off the bottom.

*Satellites with gravity-sensing instruments are used to map the ocean floor by measuring slight variations in sea level that result from differences in the gravitational pull of features on the seafloor. Maps of seafloor topography can be made by combining data from these sources.

*Mapping efforts have revealed three major areas of the ocean floor: continental margins, deep-ocean basins, and oceanic ridges.

9.5 Continental Margins

Compare a passive continental margin with an active continental margin and list the major features of each.

Key Terms: continental margin, passive continental margin, continental shelf, continental slope, continental rise, deep-sea fan, submarine canyon, turbidity current, active continental margin, accretionary wedge, subduction erosion

*Continental margins are transition zones between continental and oceanic crust. Active continental margins occur where a plate boundary and the edge of a continent coincide, usually on the leading edge of a plate. Passive continental margins are on the trailing edges of continents, far from plate boundaries.

*Heading out to sea from the shoreline of a passive margin, a submarine traveler would first encounter the gently sloping continental shelf and then the steeper continental slope, marking the end of the continental crust and the beginning of the oceanic crust.

*Beyond the continental slope is the gently sloping continental rise: Here sediment transported by turbidity currents through submarine canyons and piled up in deep-sea fans atop the oceanic crust.

*Submarine canyons are deep, steep-sided valleys that originate on the continental slope and may extend to the deep-ocean basin. Many submarine canyons have been excavated by turbidity currents (downslope movements of dense, sediment-laden water).

*At an active continental margin, material may be added to the leading edge of a continent in the form of an accretionary wedge (common at shallow-angle subduction zones), or material may be scraped off the edge of a continent by subduction erosion (common at steeply dipping subduction zones).

QUESTION: What type of continental margin is depicted by this diagram? Name the feature indicated by the question mark.

9.6 Features of Deep-Ocean Basins

List and describe the major features associated with deep-ocean basins.

Key Terms: deep-ocean basin, deep-ocean trench, volcanic island arc, continental volcanic arc, abyssal plain, seamount, guyot, oceanic plateau

*The deep-ocean basin makes up about half of the ocean floor’s area. Much of it is abyssal plain (deep, featureless sediment-draped crust). Subduction zones and deep-ocean trenches also occur in deep-ocean basins. Paralleling trenches are volcanic island arcs (if the subduction goes underneath oceanic lithosphere) or continental volcanic arcs (if the overriding plate has continental lithosphere on its leading edge).

*There are a variety of volcanic structures on the deep-ocean floor. Seamounts are submarine volcanoes; if they pierce the surface of the ocean, we call them volcanic islands. Guyots are old volcanic islands that have had their tops eroded off before sinking below sea level. Oceanic plateaus are unusually thick sections of oceanic crust formed by massive underwater eruptions of lava.

9.7 The Oceanic Ridge System

Summarize the basic characteristics of oceanic ridges.

Key Terms: oceanic ridge or rise (mid-ocean ridge), rift valley

*The oceanic ridge system is the longest topographic feature on Earth, wrapping around the world through all major ocean basins. It is a few kilometers tall, a few thousand kilometers wide, and up to tens of thousands of kilometers long. The crest or axis is the place where new oceanic crust is generated, often marked by a rift valley.

*Oceanic ridges are elevated features because they are warm and therefore less dense than older, colder oceanic lithosphere. As oceanic crust moves away from the ridge crest, heat loss causes the oceanic crust to become colder and denser and eventually to subside. After 80 million years, crust that was once part of an oceanic ridge is in the deep ocean basin, far from the ridge.

9.8 Seafloor Sediments

Distinguish among three categories of seafloor sediment and explain why some of these sediments can be used to study climate change.

Key Terms: terrigenous sediment, biogenous sediment, hydrogenous sediment, proxy data, paleoclimatology

*There are three broad categories of seafloor sediments. Terrigenous sediment consists primarily of mineral grains that were weathered from continental rocks and transported to the ocean; biogenous sediment consists of shells and skeletons of marine organisms; and hydrogenous sediment includes minerals that crystallize directly from seawater through various chemical reactions.

*Seafloor sediments are one source of proxy data that are helpful in studying worldwide climate change because they often contain the remains of organisms that once lived near the sea surface. The numbers and types of these organisms change as the climate changes, and their remains in seafloor sediments record these changes.