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:

10.1 Discuss the factors that create and influence surface ocean currents and describe the influence these currents have on climate.

10.2 Explain the processes that produce coastal upwelling and the ocean’s deep circulation.

10.3 Explain why the shoreline is considered a dynamic interface. List the factors that influence the height, length, and period of a wave and describe the motion of water within a wave.

10.4 Explain how waves erode and move sediment along the shore.

10.5 Describe the features typically created by wave erosion and those resulting from sediment deposited by longshore transport processes.

10.6 Distinguish between emergent and submergent coasts. Contrast the erosion problems faced on the Atlantic and Gulf coasts with those along the Pacific coast.

10.7 Summarize the ways in which people deal with shoreline erosion problems.

10.8 Explain the cause of tides and the monthly tidal cycle. Describe the horizontal flow of water that accompanies the rise and fall of tides.

Concept Checks 

10.1  What is the primary driving force of surface-ocean currents? How does the Coriolis effect influence ocean currents? Name the five subtropical gyres and identify the main surface currents in each. How do ocean currents influence climate? Provide at least three examples.

10.2 Describe the process of coastal upwelling. Why is an abundance of marine life associated with these areas? Why is deep-ocean circulation referred to as thermohaline circulation? How does the ocean’s surface salinity change when sea ice forms? How does this change influence density?

10.3 Why is the shoreline described as being an interface? List three factors that determine the height, length, and period of a wave. Describe the motion of a floating object as a wave passes. How do a wave’s speed, wavelength, and height change as it moves into shallow water and breaks?

10.4 What is a beach? Why do waves approaching the shoreline often bend? What is the effect of wave refraction along an irregular coastline? Describe the two processes that contribute to longshore transport.

10.5 How is a marine terrace related to a wave-cut platform? Describe the formation of each labeled feature in Figure 10.22. Along which of America’s coastal areas are barrier islands common?

10.6 Are estuaries associated with submergent or emergent coasts? Explain. What observable features would lead you to classify a coastal area as emergent? How might building a dam on a river that flows to the sea affect a coastal beach?

10.7 List two examples of hard stabilization and describe what each is intended to do. How does each affect the distribution of sand on a beach? What are two alternatives to hard stabilization, and what potential problems are associated with each?

10.8 Explain why an observer can experience two unequal high tides during one day. Distinguish between neap tides and ebb tides. Contrast flood current and ebb current.

Concepts In Review: The Restless Ocean

10.1 The Ocean’s Surface Circulation

Discuss the factors that create and influence surface ocean currents and describe the influence these currents have on climate.

Key Terms: gyre, Coriolis effect

*The ocean’s surface currents follow the general pattern of the world’s major wind belts. Surface currents are parts of huge, slowly moving loops of water called gyres that are centered in the subtropics of each ocean basin. The positions of the continents and the Coriolis effect also influence the movement of ocean water within gyres. Because of the Coriolis effect, subtropical gyres move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Generally, four main currents comprise each subtropical gyre.

*Ocean currents can have a significant effect on climate. Poleward-moving warm ocean currents moderate winter temperatures in the middle latitudes. Cold currents exert their greatest influence during summer in middle latitudes and year-round in the tropics. In addition to cooler temperatures, cold currents are associated with greater fog frequency and dryness.

QUESTION: Assume that arrow A represents prevailing winds in a Northern Hemisphere ocean. Which arrow, A, B, or C, best represents the surface-ocean current in this region? Explain.

10.2 Upwelling and Deep-Ocean Circulation

Explain the processes that produce coastal upwelling and the ocean’s deep circulation.

Key Terms: upwelling, thermohaline circulation

*Upwelling, the rising of colder water from deeper layers, is a wind-induced movement that brings cold, nutrient-rich water to the surface. Coastal upwelling is most characteristic along the west coasts of continents.

*In contrast to surface currents, deep-ocean circulation is governed by gravity and driven by density differences. The two factors that are most significant in creating a dense mass of water are temperature and salinity, so the movement of deep-ocean water is often termed thermohaline circulation.

*Most water involved in thermohaline circulation begins in high latitudes at the surface, when the salinity of the cold water increases as a result of sea ice formation. This dense water sinks, initiating deep-ocean density currents.

10.3 The Shoreline and Ocean Waves

Explain why the shoreline is considered a dynamic interface. List the factors that influence the height, length, and period of a wave and describe the motion of water within a wave.

Key Terms: shoreline, interface, wave height, wavelength, wave period, fetch, surf

*The shoreline is a transition zone between marine and continental environments. It is a dynamic interface, a boundary where land, sea, and air meet and interact.

*Energy from waves plays an important role in shaping the shoreline, but many factors contribute to the character of particular shorelines.

*Waves are moving energy, and most ocean waves are initiated by wind. The three factors that influence the height, wavelength, and period of a wave are (1) wind speed, (2) length of time the wind has blown, and (3) fetch, the distance that the wind has traveled across open water. Once waves leave a storm area, they are termed swells, which are symmetrical, longer-wavelength waves.

*As waves travel, water particles transmit energy by circular orbital motion, which extends to a depth equal to about one-half the wavelength (the wave base). When a wave enters water that is shallower than the wave base, it slows down, which allows waves farther from shore to catch up. As a result, wavelength decreases and wave height increases. Eventually the wave breaks, creating turbulent surf in which water rushes toward the shore.

10.4 Beaches and Shoreline Processes

Explain how waves erode and move sediment along the shore.

Key Terms: beach, abrasion, wave refraction, beach drift, longshore current

*A beach is composed of any locally derived material that is in transit along the shore.

*Wave erosion is caused by wave impact pressure and abrasion (the sawing and grinding action of water armed with rock fragments).

*As they approach the shore, waves refract (bend) to align nearly parallel to the shore. Refraction occurs because a wave travels more slowly in shallower water, allowing the part that is still in deeper water to catch up. Wave refraction causes wave erosion to be concentrated against the sides and ends of headlands and dispersed in bays.

*Wind-generated waves provide most of the energy that modifies shorelines. Each time a wave hits, it can impart tremendous force. The impact of waves, coupled with abrasion from the grinding action of rock particles, erodes material exposed along the shoreline.

*Waves that approach the shore at an angle transport sediment parallel to the shoreline. On the beach face, this longshore transport is called beach drift, and it is due to the fact that the incoming swash pushes sediment obliquely upward, whereas the backwash pulls it directly downslope. Longshore currents are a similar phenomenon in the surf zone, capable of transporting very large quantities of sediment parallel to a shoreline.

QUESTION:  What process is causing wave energy to be concentrated on the headland? Predict how this area will appear in the future.

10.5 Shoreline Features

Describe the features typically created by wave erosion and those resulting from sediment deposited by longshore transport processes.

Key Terms: wave-cut cliff, wave-cut platform, marine terrace, sea arch, sea stack, spit, bay, mouth bar, tombolo, barrier island

*Erosional features include wave-cut cliffs (created by the cutting action of the surf against the base of coastal land), wave-cut platforms (relatively flat surfaces left behind by receding cliffs), and marine terraces (uplifted wave-cut platforms). Erosional features also include sea arches (formed when a headland is eroded and two sea caves from opposite sides unite) and sea stacks (formed when the roof of a sea arch collapses).

*Depositional features that form when sediment is moved by beach drift and longshore currents include spits (elongated ridges of sand that project from the land into the mouth of a bay), baymouth bars (sandbars that completely cross a bay), and tombolos (ridges of sand that connect an island to the mainland or to another island). The Atlantic and Gulf coastal region is characterized by offshore barrier islands, which are low narrow ridges of sand that parallel the coast.

*Over time, irregular, rocky shorelines are modified by erosion and deposition to become smoother and straighter.

QUESTION: Identify the lettered features in this diagram.

10.6 Contrasting America’s Coasts

Distinguish between emergent and submergent coasts. Contrast the erosion problems faced on the Atlantic and Gulf coasts with those along the Pacific coast.

Key Terms: emergent coast, submergent coast, estuary

*Coasts may be classified by their changes relative to sea level. Emergent coasts are sites of either land uplift or sea-level fall. Marine terraces are features of emergent coasts. Submergent coasts are sites of land subsidence or sea-level rise. One characteristic of submergent coasts is drowned river valleys called estuaries.

*The Atlantic and Gulf coasts are lined in many places by barrier islands—dynamic expanses of sand that see a lot of change during storm events. Many of these low and narrow islands have also been prime sites for real estate development.

*The Pacific coast’s big issue is the narrowing of beaches due to sediment starvation. Rivers that drain to the coast (bringing it sand) have been dammed, resulting in reservoirs that trap sand before it can make it to the coast. Narrower beaches offer less resistance to incoming waves, often leading to erosion of bluffs behind the beach.

QUESTION: Is this an emergent coast or a submergent coast? Provide an easily seen line of evidence to support your answer. Is the location more likely along the coast of North Carolina or California? Explain.

10.7 Stabilizing the Shore

Summarize the ways in which people deal with shoreline erosion problems.

Key Terms: hard stabilization, groin, breakwater, seawall, beach nourishment

*Local factors that influence shoreline erosion are (1) the proximity of a coast to sediment-laden rivers, (2) the degree of tectonic activity, (3) the topography and composition of the land, (4) prevailing winds and weather patterns, and (5) the configuration of the coastline and near-shore areas.

*Hard stabilization refers to any structures built along the coastline to prevent movement of sand. Groins are oriented perpendicular to the coast, with the goal of slowing beach erosion by longshore currents. Offshore, breakwaters are constructed parallel to the coast to blunt the force of incoming ocean waves, often to protect boats. Like breakwaters, seawalls are oriented parallel to the coast, but they are built on the shoreline itself. Hard stabilization measures often lead to increased erosion elsewhere along the shore.

*Beach nourishment is an expensive alternative to hard stabilization. Sand is pumped onto the beach from some other area to temporarily replenish the beach. Another alternative to hard stabilization is relocating buildings away from high-risk areas and leaving the beach to be shaped by natural processes.

QUESTION: Based on their position and orientation, identify the three types of hard stabilization illustrated in this diagram.

10.8 Tides

Explain the cause of tides and the monthly tidal cycle. Describe the horizontal flow of water that accompanies the rise and fall of tides.

Key Terms: tide, spring tide, neap tide, tidal current, tidal flat

*Tides are daily changes in ocean surface elevation. They are caused by gravitational pull on ocean water by the Moon and, to a lesser extent, the Sun. When the Sun, Earth, and Moon all line up about every 2 weeks (full moon or new moon), the tides are most exaggerated (spring tide). When a quarter moon is in the sky, the Moon is pulling on Earth’s water at a right angle relative to the Sun, and the daily tidal range is minimized as the two forces partially counteract one another (neap tide).

*Tides are strongly influenced by local conditions, including the shape of the local coastline and the depth of the ocean basin.

*A flood current is the landward movement of water during the shift between low tide and high tide. When high tide transitions to low tide again, the movement of water away from the land is an ebb current. Ebb currents may expose tidal flats to the air.