11.1: Processes that Form and Modify the Coast

All coasts change in character along their length, but their general character tends to be similar for thousands of kilometers. Figure 11-2 and Figure 11-3 show just some of the various coast types of the U.S. and Canada. Worldwide, every coast is continuously changing in response to various processes that form and modify it over decades, centuries, and millennia.

Figure 11-2. Pacific and Arctic coastlines of the U.S. (clockwise from top left). (a) Permafrost beach on the Beaufort Sea in the Arctic Wildlife Refuge, Alaska. (b) Cliff and beach terrace in Olympic National Park, Washington. (c) Storm-eroded coast with tree debris in Willapa Bay, Washington. (d) Rocky coast near Monterey, California. (e) Beach and cliffs near Point Magu in southern California). (f) Lava flows on the south shore of Hawaii. (g) Island beach and cliffs near Polihale, Kauai, Hawaii. (h) Fringing coral reef in Guam. (i) Fjord in Turnagain Arm, Alaska.

Figure 11-3. Atlantic coastlines of the U.S. and Canada (clockwise from the top left). (a) High tidal range in the Bay of Fundy off of New Brunswick, Canada. (b) Rocky beach with high wave energy in York, Maine. (c) Sand dunes in Cape Cod, Massachusetts. (d) Beach and dunes near Rehoboth Beach, Delaware. (e) Barrier island beach in Virginia Beach, Virginia. (f) Mangrove forest in the Florida Everglades. (g) Barrier island beach in Galveston, Texas.

Most coasts can be classified as either erosional or depositional, depending on whether their primary features were created by erosion of land or deposition of sediments. Erosional coasts develop where the shore is actively eroded (worn away) by wave action or where rivers or glaciers caused erosion when sea level was lower than it is now. Depositional coasts develop where sediments accumulate either from a local source or after being transported to the area by rivers, glaciers or by ocean currents and waves (Fig. 11-4). Erosional coasts are often dominated by sea cliffs and rocky shores, whereas depositional coasts include deltas, mangrove swamps, salt marshes, barrier islands, and beach sand dunes.

Figure 11-4. The major sources of sediments in the coastal ocean are erosion of the shoreline and transport of particulate matter by streams, rivers, and glaciers. Many river-borne particles are deposited in estuaries or lagoons along the coastal plain to form sediment deposits. Once in the coastal marine environment, sediment particles can be transported along the coastline and deposited on beaches, barrier islands, and offshore bars. Small sediment particles can be transported offshore by currents and deposited on the deep-ocean floor. Larger particles can be transported offshore by turbidity currents that sometimes travel down submarine canyons.

Coasts can also be classified as either primary or secondary. Primary coasts are shaped predominantly by terrestrial processes, including erosion or deposition by rivers, streams, glaciers, volcanism, and tectonic movements. Secondary coasts are shaped by marine erosion or deposition caused by wave action, sediment transport by currents, and marine organisms (e.g., those that form reefs). Many coasts have characteristics of both marine and terrestrial processes.

Formation of Coasts

New coasts are formed either when the relative levels of the ocean surface and coastal landmass change or when the edge of the landmass is added to or removed. Some processes that form coasts, such as volcanic eruptions and earthquakes, can occur instantly or over a very short period, but most other processes, such as sea-level change and coral reef growth, continue slowly over centuries. Many of these processes can occur on the same coast at the same time, but at different rates or frequencies.

Tectonic Processes

Chapter 4 describes movements of lithospheric plates that create the major topographic features of the ocean floor. It also explains how these movements build mountain chains and magmatic arc and sedimentary arc islands at convergent plate boundaries, create new oceans at continental divergent plate boundaries, and create volcanic islands at hot spots and particularly-active locations on oceanic ridges. Tectonic processes take millions of years to re-form the planet. For example, the plate movements that broke apart Pangaea to form the present configuration of continents required about 150 to 200 million years.

Motions of the lithospheric plates are slow but continuous, modifying coasts at plate boundaries and hot spots. At hot spots, new coast is formed when volcanoes erupt. For example, several hundred hectares were added to the island of Hawaii by an eruption of the Kilauea Volcano’s East Rift Zone that started in 1983 and continued until 2018. Lava from East Rift Zone eruptions often flows into the sea, hardens, and extends the coastline (Fig. 11-2f). Tens of thousands of years from now, a new island, Loihi, will emerge south of Hawaii (Chap. 4).

New coast is also formed by volcanic eruptions in magmatic arcs at convergent plate boundaries. Indonesia and the Aleutian Islands are good examples (Chap. 4). At convergent plate boundaries or at transform faults, new coast is formed when earthquakes uplift a continent edge as the oceanic plate is subducted beneath it, or when earthquakes uplift ocean sediments at a sedimentary arc. Although earthquakes along these boundaries that uplift land to create new coast are infrequent, coastal erosion processes are also slow. Therefore, uplifted coast is formed at some convergent plate boundaries periodically but almost instantaneously, much faster than the coast is modified by ocean processes. For example, the Loma Prieta earthquake in October 1989 raised the coastal mountains and coast near Santa Cruz, California, by as much as 1.5 to 2 m. A strong earthquake in Alaska in 1964 raised parts of the seafloor of Prince William Sound by as much as 8 m, creating a new strip of land several hundred meters wide from the former seafloor.

Coasts can also be destroyed by tectonic processes. For example, coasts of what is now northern India were destroyed as India collided with Asia (Chap. 4). Earthquakes at convergent plate boundaries or at transform faults can also cause sections of coastal land to move vertically downward, although such changes at these boundaries are often temporary because subsequent earthquakes may uplift this same section as the often complex subduction process continues.

Landslides 

The simultaneous destruction of old coast and formation of new is particularly dramatic where volcanoes form islands, such as Hawaii, whose underwater flanks are much steeper than most other terrestrial margins. The steep flanks can become unstable as lava accumulates from continuing eruptions or when the island cools and sinks isostatically after moving off the hot spot. When this happens, a section of the island can break loose and slide down to the deep-ocean floor like a giant avalanche, destroying the old coast and creating a new coastline where the break occurs. Huge sections of the Hawaiian Islands have apparently broken off in this way in the past. As much as 10% to 20% of Oahu has instantly broken loose at one time. There is evidence that about 70 more such landslides have occurred around the Hawaiian Islands during the past 20 million years. The remains of these giant landslides are littered over vast areas of seafloor extending more than 200 km around the islands (Fig. 11-5).

Figure 11-5. Large areas of the ocean floor surrounding the Hawaiian Islands are covered by slumps, fields of large debris, and sediments that originated from the islands. The data depicted on this map were obtained by the extensive use of precision sonar and three-dimensional sonar and the collection of many sediment samples. The slumps are areas where large blocks of the side of the volcano slipped downward. Slippage may occur both by a slow creeping motion and by periodic surges of several meters that cause large earthquakes. The debris fields and sediments are the result of catastrophic landslides in which huge blocks of the islands, up to 10 km or more in size, break loose instantaneously in massive avalanches that slide and flow down the steep volcano sides to the deep-sea floor. Although not recorded in recent history, these landslides must cause massive turbidity currents and, probably, massive tsunamis that may be hundreds of meters high when they reach the adjacent islands.

Little is known about these monster landslides or the probability of another one occurring on Hawaii or other volcanic islands. However, in November of 2000, a 20-km-long by 10-km-wide section of the southeast slope of the Kilauea Volcano on Hawaii slipped about 10 cm in only 36 h, millions of times faster than most tectonic plate motions. This occurrence may have been a forewarning of an imminent (in geological time) collapse of this section of the island. Such a slide not only could destroy a large section of the islands and their inhabitants, but also could cause a huge tsunami (Chap. 9), which might be several tens of meters high when it impacted the west coast of North America. Fortunately, such large slides apparently occur only at intervals of about 100,000 years or more in Hawaii and about once in every 10,000 years on average worldwide.

Although not as dramatic in size or impact, landslides smaller than those observed around Hawaii are important processes of coastline modification on most uplifted coasts. On eroding coastlines such as those found in southern California, many homes built years ago and tens of meters from the then existing cliff edge have been destroyed as the cliffs have progressively collapsed.

Isostatic and Eustatic Sea-Level Changes

If sea level rises, coasts are drowned and a new coastline is formed inland from the previous location. Similarly, if sea level falls, the ocean floor is exposed and becomes the new coastline. Sea level can change on a particular section of coast because the continent edge rises or sinks isostatically, while global sea level remains the same (Chap. 4, CC2). Sea level can also change eustatically if the volume of water in the oceans changes or the volume of the ocean basins themselves changes (CC2). Eustatic changes take place at the same time throughout the world’s oceans, whereas isostatic leveling occurs locally or regionally. At present, worldwide sea level is rising slowly because of eustatic processes (melting of glaciers and the warming and expansion of ocean water), but sea level is not observed to be rising on all coasts. Some coasts are rising isostatically as fast as, or faster than, the rate of eustatic sea-level rise. The net result is that the observed sea level is stable or falling on these coasts. Other coasts are sinking isostatically, and the observed sea level on these coasts is rising faster than the rate of eustatic sea-level rise.

Eustatic sea-level changes have led to major shifts in the locations of continental coastlines. During approximately the past 19,000 years, sea level has risen about 120 m (Fig. 6-18). The history of sea-level change during this 19,000-year period can be determined by, for example, studies of the ages of relict sediments or buried sediments on the continental shelf (Chap. 6). Determining the history of sea level before 19,000 years ago is more difficult, but sea level seems to have oscillated many times during the present spreading cycle, from about 130 m or more below its present level, to about 40 to 50 m above the present level. Consider where the coastline would be if no isostatic changes had occurred during this oscillation of sea level. At the highest sea level, the Gulf of Mexico would extend across the central plains states of the U.S. as far north as southern Canada. At the lowest sea level, the Texas coastline would be about 150 km farther south in the present-day Gulf of Mexico and the Florida Peninsula would be about twice as wide.

During the past 4000 years, the eustatic rise in sea level has been slower than in the immediately preceding period. Therefore, most present-day coasts were formed several thousand years ago as sea level rose rapidly over what is now the continental shelf. Sea level is expected to rise more rapidly in the future as a result of global climate changes caused by enhancement of the greenhouse effect. In any event, the relatively slow sea-level change of the past 4000 years cannot continue indefinitely. If sea level does rise more quickly, the types of coasts at various locations will change because the rate of sea-level rise greatly affects the formation and migration of coastal features such as barrier islands and estuaries. Coastal changes may disrupt the Earth’s ecosystems, thereby possibly causing more damage than even that caused by the flooding of coastal cities.

Because sea level is expected to rise more rapidly in response to climate change induced by the enhanced greenhouse effect, oceanographers are currently mounting intensive studies of coasts. Critical questions that remain to be answered include how fast sea level is rising eustatically, whether it will continue to rise, whether the rate of rise will continue to accelerate, how isostatic changes will enhance or mitigate eustatic changes in sea level on specific coasts, and how coasts will change if the rate of eustatic sea-level rise increases.

Glaciers

As glaciers flow, they scour out steep-sided valleys (Fig. 11-6a). Rocks and smaller particles that have been eroded from the valley walls and floor are carried by the glacier and deposited where the ice melts at the glacier’s end. During the last 19,000 years, the Earth’s climate warmed and the glaciers retreated, each one leaving one or more sedimentary deposits called moraines at the former location of the glacier’s end. At the same time, sea level was rising as ocean waters warmed and expanded, and as more water entered the oceans from melting glaciers. The rising sea inundated many steep-sided valleys cut by glaciers and created deep, narrow fjords, many of which are partially closed off from the ocean by a submerged sill, which is usually a moraine (Fig. 11-6b,c).

  

  

Figure 11-6. Glaciers and fjord formation. (a) The Franz Josef Glacier on the South Island of New Zealand has retreated substantially in recent decades. Note the very steep-sided valley that the glacier has left behind as it has receded. (b) When sea level was lower and glaciers more extensive, the glaciers cut deep, steep-sided valleys down to the ancient sea level. Rock fragments carried by the glaciers were deposited to form one or more glacial moraines at the foot of each glacier. (c) Geirangerfjord, Norway where rising sea level has filled a glacial valley that now allows ocean cruise ships to navigate to the head of the fjord, which is 60 km inland. (d) As the Earth’s climate warmed and sea level rose, the glaciers retreated, and their steep-sided valleys were filled by seawater to form fjords. The glacial moraines left behind by the glaciers formed shallow submerged sills at the mouths of many fjords.

Because fjords are long, narrow inlets, they are generally well protected from erosion by ocean waves, and their shores are little altered from the original sides of the glacial valley. Many high-latitude areas where glaciers cut through coastal mountain ranges have extensive fjord systems. Excellent examples are found on the South Island of New Zealand, in Scandinavia, on the Pacific coast of Canada and Alaska, and in Patagonia, Chile.

River-Borne Sediments

New coasts are formed where large amounts of river-borne sediments are deposited. The extended delta of the Mississippi River (Fig. 11-7) and similar deltas elsewhere are examples of coasts formed by river-borne sediments. Deltas, discussed in more detail later in this chapter, are present at the mouths of relatively few rivers. Most of the world’s rivers flow across a gradually sloping coastal plain before reaching the sea. The sediment load of the river is deposited in the river valley as the flow slows in this flatter area.

  

Figure 11-7. The Mississippi River delta. (a) This satellite image shows the extensive delta with its many distributary channels. (b) The Mississippi River periodically changes its course through the coastal plain and delta, distributing its sediments across the entire area. Flood-control levees now prevent this natural process.

Only a few rivers other than the Mississippi carry such large sediment loads that their river valleys have filled enough for large quantities of sediment to be transported to the sea. Most rivers that flow across coastal plains, such as those on the Atlantic coast of North America, carry considerably less sediment than the Mississippi. In addition, rivers emptying to the Atlantic Ocean have only recently (in geological time) begun to flow toward that ocean. In the region now drained by the rivers emptying to the Atlantic Ocean, rivers flowed away from the Atlantic Ocean until about 100 million years ago, when the newly formed passive margin of the Atlantic coast sank isostatically sufficiently far to reverse the slope (Chap. 4).

Rivers that flow across tectonically active coastal margins generally flow through steep coastal mountain ranges. Because they drain relatively small land areas and flow through steep valleys to the sea, many carry relatively little sediment, but most of it is transported to the oceans. The continental shelf is steeper and narrower on these active coastal margins, so sediment is transported to the deep-sea floor more easily than at passive margins.

Biological Processes 

Reef-building corals cannot grow and build reefs unless they are underwater. However, reefs grow fastest in shallow waters where light intensity is high (Chap. 15), and some reef tops emerge above water at low tides. Although they are not truly land, these reefs constitute an important feature of the coast because ocean waves break on them and lose much of their energy. Fringing reefs and barrier reefs are present on many coasts in tropical and subtropical regions. A small drop in sea level or a small coastal margin uplift can raise coral reefs above the sea surface, where the corals cannot survive.

When corals die, they leave behind their hard “skeletons.” Many tropical and subtropical islands and coasts are characterized by rocks composed of old coral reefs. These coral-rock shores are eroded to form a jagged surface, often called “ironshore,” that makes walking difficult (Fig. 11-8). The Cayman Islands in the Caribbean Sea have excellent examples of reef-dominated shore. In fact, these islands are predominantly uplifted coral reefs. The ironshore on part of one island is so jagged that the local community has been named “Hell.”

  

Figure 11-8. An uplifted and eroded coral reef forms a jagged shore often called “ironshore.” (a) Coastlines of coral islands, like this island in Palau, often have interspersed sandy beaches and eroding ironshore. (b) The jagged rocks of this ironshore segment in Fiji are razor-sharp.

Many coral reefs are located on islands or submerged pinnacles that are sinking isostatically (Fig. 4-29). Isostatic sinking and sea-level rise both tend to increase the depth of the water column over a reef. If the combined rate of these deepening processes exceeds the rate at which a coral reef can build upward, the top of the reef becomes progressively deeper and may eventually become too deep to sustain the photosynthesis on which the corals depend. Sea level has been rising for about the past 19,000 years, and many coral reefs appear to have been drowned in this way. If global climate change leads to an increase in the rate of sea-level rise, many more coral reefs may die, primarily those with lower maximum growth rates, even if they can survive ocean acidification.

The maximum upward growth rate of coral reefs varies with latitude, depth, and water clarity. Upward growth is slower at higher latitudes, where water temperatures are cooler, and they are lower at deeper depths or in less clear waters, where the light levels are reduced. However, the maximum upward growth rate is about 1 to 10 mm per year at 10 m depth, and the current rate of sea-level rise is estimated to be about 3 to 4 mm per year.

Modification of the Coast

All shores and coasts are continuously, but slowly, modified by waves, tides, winds, and biological processes. The present form of coasts represents a balance between modification processes and formation processes. Older coasts and coasts with higher wind, wave, and tide energies are generally more extensively modified. The extent of modification also depends on the type of rock constituting the coast and, in some cases, on the types of vegetation on the coastal land and in the nearshore zone.

Waves

The breaking of waves on the shore is the principal coast-modifying process. On rocky coasts, breaking waves progressively erode the rock away. Soft sedimentary rocks are eroded much faster than harder volcanic rocks. In addition, erosion is faster on coasts that are exposed to greater wave action. Wave action is greater in areas of frequent storms or where the coastline is impacted by waves that travel far across the ocean.

As discussed in Chapter 9, wave energy is focused on headlands and spreads out along the interior shores of bays. Consequently, on an indented coastline, erosion occurs fastest at headlands, and the products of erosion (sand) accumulate within the intervening bays. Preferential erosion of headlands along a coastline tends to straighten the coast progressively until no headlands remain. This process is complete on many coasts, particularly where rocks are easily eroded. The New Jersey coast is an example (Fig. 11-9).

Figure 11-9. The New Jersey coastline has long, straight beaches and barrier islands. Headlands that used to exist along the northern shore have been eroded away, providing sand for the formation and maintenance of the spits to the north and south.

On rocky coasts, waves cut away rock between the high-tide lines and low-tide lines. As rock is cut away, the land becomes unstable and breaks away or slumps, leaving behind a cliff that may be nearly vertical (Figs. 11-2g, 11-10). The debris from the cliff temporarily alters the shape and nature of the beach where the cliff face has collapsed. However, these rocks fall into the wave-breaking zone, where they are eroded away relatively quickly. The beach is thus restored, and the waves renew their attack on the base of the cliff. In many locations, there is no beach at the base of the cliffs because wave energy is too high or wave erosion has not continued long enough for sand to accumulate (Fig. 11-11).

  

Figure 11-10. Wave erosion on steep coastlines can create cliffs. (a) As rock is eroded by waves, the land is undercut, becomes unstable, and slumps. The slumped material is eroded and transported away by the wave action. Thus, a gently sloping wave-cut platform is maintained at the base of the cliff. (b) Cliff and beach near Monterey, California.

Figure 11-11. Wave erosion, combined with boring and dissolution by marine organisms, undercut this rocky coastline on the island of Palau in the Pacific Ocean. Here, erosion is confined to a very narrow height range because the tidal range is nearly zero.

As waves cut into coastal cliffs and headlands, they encounter rocks of variable resistance to erosion. Rapid erosion of the less resistant rock often leads to the formation of sea caves at the base of a cliff (Fig. 11-12a,b). Sea caves that are cut into either side of a headland can continue to be eroded until they meet under what remains of the headland, resulting in the formation of a sea arch (Fig. 11-12b,c). As the headland erodes further, the arches collapse, and the remaining pinnacles of rock, called “stacks” (Fig. 11-12b,d), are eventually eroded away.

  

 

  

Figure 11-12. (a) Wave energy is concentrated on headlands by wave refraction. Because the seafloor topography is rough and varies as slumps occur from different parts of the headland, wave energy is distributed unevenly. In addition, the rocks of the headland differ in composition and susceptibility to erosion. As a result, the wave action causes the headlands to be eroded unevenly. (b) The uneven erosion of the headland can cause sea caves and sea arches to be formed. (c) This headland on the island of Molokai in Hawaii has been eroded by the intense wave action to form sea caves and sea stacks. (d) There are many sea stacks on the Pacific coast of North America, including these on the Big Sur coast south of Monterey, California.

Coasts that have been substantially eroded usually have beaches. Off these coasts, waves transport and distribute particles (e.g., sand, silt, or pebbles) that make up the beach. These processes are discussed later in the chapter. Beaches help protect the coast from wave erosion.

Tides

The shore is defined as the area along the coast that lies between the lowest point exposed at the lowest tide and the highest point reached by storm waves. The shore consists of the foreshore, which is the area between the low-tide line and the high-tide line, and the backshore, which is the area above the high-tide line that is affected by storm waves (Fig. 11-13). Tidal range determines the height range over which wave erosion occurs and, thus, the width of the shore. Where the tidal range is large, wave-erosion energy is spread over a large vertical range. Generally, coasts with small tidal ranges are eroded faster because wave energy is concentrated in a narrow zone. The swift currents associated with large tidal ranges have little erosional effect because they are slow in relation to the orbital speed of water in waves (CC4).

Figure 11-13. The coastline is separated into several zones that reflect different exposures to erosion and sediment movement due to the variation in waves and tides. The backshore is covered by water only during storms, but it is subject to seawater wind spray constantly. The foreshore is the region between the high- and low-tide lines, and the nearshore is the region between the low-tide line and the depth at which wave action no longer affects the seafloor. (a) When the shoreline is more resistant to weathering, wave action may cut steep cliffs with a notch underneath and a terrace offshore. (b) Longshore bars and troughs may be present on the seafloor within the nearshore zone. The foreshore is often separated from the backshore by a berm. Also, a scarp may be present in the foreshore at the height reached during a recent high tide by waves that are not strong storm waves but somewhat higher and steeper than waves that reached the beach during previous days or weeks and on the backshore at the base of sand dunes (see also Figs. 11-14, 11-15c,d).

Tides are particularly important in shaping and maintaining wetlands. Tidal motions in shallow bays and estuaries transport and redistribute sediments. In some bays and estuaries, extensive mudflats form just below the high-tide line. These mud flats are usually dissected by drainage channels. During the tidal cycle, the mudflats of tidal wetlands are alternately exposed and covered by shallow water.

Winds and Weather

Onshore winds can carry sand from beaches and deposit it on the backshore above the highest point reached by waves. This process leads to the formation of the sand dunes (Fig. 11-14) that characterize many coastlines. Conversely, sand dunes and soils can be eroded by offshore winds and deposited in the oceans. Just as they are elsewhere, coastal rocks, sand dunes, and soils are eroded by water in streams and rainfall, and by various processes, including alternate cycles of freezing and thawing and dissolution in acidic rainwater. Wind-driven particles and ocean spray may also erode rocks or soils.

  

Figure 11-14. Sand dunes are characteristic of many coasts. (a) Dunes usually separate the beach from a low marshy area, as they do on this Aruba coast. (b) Dunes can be 10 m or more high, like these in Monterey Bay, California. Note the patches of vegetation on the dunes that help reduce erosion.

Vegetation

The type and extent of vegetation on the coast affect the rate at which winds, streams, and storm waves erode the land. Grasses are particularly important in protecting sand dunes from erosion. Similarly, rooted plants that grow in the water, including sea grasses and mangroves (Chap. 15), help reduce erosion of mudflats by waves and currents. In contrast, tree roots and animal activities, such as burrowing, contribute to the continuous erosion of rocks and soils of land near the coast just as they do elsewhere. In addition, especially on rocky coastlines, many animals that live in the zone between high and low tides erode the rocks as they bore or chemically dissolve their way into the rock or probe into cracks in the rocks, either to find food or shelter or to be able to “hold on” to the rock against the power of the waves.