Marine terraces

Marine terraces are ancient coastlines at elevations well above or well below present-day sea level. Global cycles of sea-level change, tectonic uplift, and coast subsidence create multiple sets of parallel marine terraces.

Coastlines

The coastlines of continents and islands represent a fundamental boundary between the Earth's solid landmass and the constructional and destructional energy of the sea. The landforms in coastal areas result from continuous dynamic interaction between these competing geological agents. Marine processes of erosion and sedimentation construct a shoreline profile on the edge of the landmass that defines the strandline, a narrow zone of wave- and tide-washed coast. The coastal strandline usually exhibits a gently sloping platform with its top at an elevation between high and low tides; it is often bounded by adjacent steeper slopes and is a reliable indicator of sea level. Familiar strandline features are beaches, coral reefs, and wave-cut platforms—strandline platforms cut into the bedrock by wave erosion.

If erosion and sedimentation were the only active geological conditions, most coasts would have a single type of strandline landform whose position remained constant through time. However, the volume of seawater in the world's oceans fluctuates directly with changes in the ice volume in continental glaciers, causing sea level to rise and fall in response. Glacial ice melts (sea-level rises) or accumulates (sea-level falls) in response to temperature changes in the Earth's atmosphere. The temperature of the surface ocean reacts similarly: Warming causes seawater expansion (sea-level rise), and cooling results in contraction (sea-level fall). It has been estimated from oxygen isotope evidence in deep-sea cores that the worldwide sea level has been 130 to 160 meters lower than today several times during the last 2 to 3 million years of Earth's history. There is also abundant physical evidence for large-scale sea-level excursions. On the Atlantic coast of the United States, elephant teeth 25,000 years old have been dredged up by fishermen from more than forty locations on the continental shelf, some as far as 120 kilometers offshore. Three-thousand-year-old oak tree stumps and many cultural artifacts used by coastal American Indians have also been found below sea level in this region.

The landmasses are also surprisingly dynamic, rising or falling in elevation in response to various tectonic factors. On some coasts, isostatic adjustment results in uplift and subsidence. The weight of increasing thicknesses of overlying glacial ice or seawater will cause the Earth's lithosphere to be depressed downward; when the weight is removed, the landmass will rebound upward. Volcanic activity will cause expansion (uplift) when the Earth's lithosphere is heated; contraction (subsidence) occurs as the lithosphere cools. An extreme example of this effect is the guyot, an inactive volcanic seamount that subsides far below sea level as the hot lithosphere on which it formed cools and contracts. Guyots have been beveled flat by wave erosion and rimmed with banks of dead coral reefs when their tops were in shallow water. Coastal landmasses also undergo uplift and subsidence in response to the forces of plate tectonic movement. Although usually a slow, incremental process, a sudden, rapid coastal uplift occurred in Prince William Sound, Alaska, during the 1964 Good Friday earthquake, which resulted in the elevation of a wave-cut platform 400 meters wide.

Fossil Coasts

During the Pleistocene epoch, strandline features formed on coasts worldwide, strongly influenced by these complex and rapidly changing geologic conditions. Most coasts are not tectonically stable (located at one elevation) for a very long period of geologic time, and landscape erosion and decay processes often act much more slowly than sea-level and land-level change rates. Abandoned strandline features, or “fossil coasts,” are common in many areas of the world. First recognized in nineteenth-century Europe, parallel sets of these distinctive landforms are present on cliffs and coastal hills at elevations high above present-day sea level and the ocean floor far below present-day sea level. Many contain marine fossils—demonstrating their marine origin—and create a steplike topographic profile of the terrain leading up the seaward-facing hills. These features are referred to as marine terraces.

Because their formation requires time, strandline landforms usually form when coastal uplift/subsidence and sea-level change are in balance, and the position of the strandline is relatively stable for thousands of years, a situation referred to as a sea-level stillstand. In many areas, sea level tends to vary in cycles separated by stillstand events, during which marine terraces can develop. Occasionally, the processes of terrace formation will destroy or obscure an older terrace, so most coasts exhibit only one or two terraces. An unbroken flight of marine terraces climbing inland indicates a coast that is being continuously uplifted. One of the most striking and well-developed sequences of marine terraces in the world is on the seaward slopes of the Palos Verdes Peninsula of Southern California. Thirteen distinct terrace levels, from 15 to 448 meters in elevation, form a complete record of coastal uplift and sea-level change spanning several hundred thousand years.

Submerged terraces are usually better preserved than elevated marine terraces because they are less affected by erosion and landscape degradation processes. Less is known about them because they are obscured from view and more difficult to study than terraces on land. The most distinctive submerged wave-cut platforms are present off the southwest coast of Great Britain, northern Australia, and the Southern California coast and its nearshore islands from Santa Barbara to San Diego. Marine terraces have been studied to document sea-level variations and the tectonic uplift and subsidence of coastal landmasses through time and to estimate the magnitude of these changes.

Coastal Erosion

There are several types of marine terraces, differing in their mode of formation. Processes of coastal erosion form marine terraces on coasts where the shoreline is a steep, irregular cliff, where the sediment supply is too limited to build beaches, and where nearshore waters are so shallow that waves break directly on the sea cliff. The base of the cliff is continuously pounded by the full force of the waves, resulting in very large impact pressures. Beach cobbles and abrasive, sand-laden water are hurled at the base of the cliff. The force with which these stones attack the cliff is difficult to exaggerate; there are reports of beach cobbles thrown 45 meters above sea level.

Wave-cut platforms result from the rapid horizontal cutting away of rock at the base of a sea cliff. Wave erosion quarries a notch into the base of the sea cliff, which undercuts and oversteepens the cliff until landsliding causes the cliff to collapse; the waves rapidly carry away this loose rubble, which is a source of new rocks to continue the wave attack. In some places, notch formation is aided by the bioabrasion activities of marine organisms that live attached to and that feed on rocky shores and secrete chemicals to help dissolve the rock or abrade it away with rasping feeding appendages. Dissolution of the rock is sometimes aided by groundwater seeping out of the base of the sea cliff. The net effect of this process is the landward retreat of the sea cliff and the formation of a platform that slopes gently seaward. The boundary between the “step” of a wave-cut terrace and the steeper “riser” of the sea cliff that terminates its landward edge is called the inner edge point. The elevation of this point is the position of the highest sea level during the formation of the feature, and it is used to calculate the height of ancient sea levels in marine terraces.

The platform is continually abraded by the waves transporting sand down the coast, and the amount of landward sea cliff erosion determines its width. The rate of sea-cliff retreat can be surprisingly rapid. Average rates on the California coast exceed 15 centimeters per year for hard rock cliffs and up to 1 meter per year in cliffs composed of soft, unconsolidated sediment. The coast of East Anglia, Great Britain, experiences up to 4 meters of sea-cliff retreat annually. Continuous records are available for the Huntcliff coast of Yorkshire, Great Britain, at a former Roman signal station where the cliff has retreated 30 meters in eight hundred years.

Overlying many elevated wave-cut platforms is a mantle of unconsolidated sediment called coverhead. This material, deposited by terrestrial and marine sedimentary processes, buries the abraded platform surface and may be as much as 30 meters thick. Typical coverhead deposits include a basal layer of well-sorted beach sand, rounded beach cobbles, and marine fossils—the shoreline sediments left on the platform as the sea level retreated. These basal strandline sediments are often covered by a heterogeneous deposit composed of poorly sorted rock debris, soil, and stream gravel deposited by sediment washing or landsliding down from the sea cliff slopes above and by coastal streams building their alluvial fans toward the coast.

Sediment Deposition

Marine terraces are also formed by sediment deposition, which usually occurs on relatively flat coasts with a wide continental shelf where the energy of approaching waves is dissipated by friction with the shallow sea bottom. In areas such as North America's Atlantic and Gulf coasts, rivers transport large quantities of mud and sand into coastal waters. Waves and wind currents pile sand into a long, narrow strandline feature known as a barrier island, a common landform found fringing low-lying coasts worldwide. Once formed during a stillstand of several thousand years, a barrier island will migrate landward as the sea level rises. Waves and tidal currents cause barrier sand to wash over the top of the island or around its ends, moving it grain-by-grain landward with the rising sea. When the sea level begins to fall, the barrier does not migrate seaward but is left behind inland of the new strandline, where it remains a record of the former high stand of the sea level; this feature is usually referred to as a beach ridge. After the sea level falls and the strandline moves seaward, river and marsh sediment are often deposited on the seaward side of the abandoned barrier. When the sea level again rises to its former position, a new barrier island migrates with it; this island is, in turn, abandoned on the coast at the highest position of sea level. If the new barrier migrates far enough inland to reach the older remnant barrier, it will be welded onto it, forming a wide composite barrier island composed of two or more barrier islands.

Sediment deposition has widened the Atlantic and Gulf coastal plains, and repeated cycles of sea-level rise and fall have formed concentric arcs of abandoned barrier islands stretching inland nearly 50 kilometers from the present-day coast. These beach ridge barrier islands are a type of marine terrace, each recording an ancient high stand of sea level. One of the most prominent is Trail Ridge in southern Georgia, a sand ridge nearly 60 kilometers long that encloses a low swampy area on its landward side known as the Okefenokee Swamp. Submarine beach ridges are less common than their inland counterparts but have been reported from western Brittany.

Corals flourish in tropical areas where the coastal waters are warm all year and clear of suspended mud, and they often form massive reefs. Reefs growing during a long stillstand will become large and well-developed, with a gently sloping top corresponding to sea level. These coastal depositional features are also found stranded above the shoreline when the sea level falls, or the landmass rises, resulting in reef terraces. This type of marine terrace is most common on island coasts and the Mediterranean and Red Seas coasts.

In the twenty-first century, scientists continue to make discoveries of new marine terraces, such as the submerged sequence of marine terraces in the Bay of Biscay off the coasts of Spain and France. These discoveries offer insights into the ancient shorelines of the regions. Advances in technology have made the study of marine terraces more efficient and accurate. These technologies, such as optically stimulated luminescence dating, have allowed for more specific dating of marine terraces and a more complete understanding of past sea levels. Seismic-stratigraphic analysis is another new technology scientists employ to better understand marine terraces. Finally, because marine terraces are intrinsically linked to sea levels, they offer insight into the importance of sea levels to global climate change. 

Principal Terms

barrier island: a long, low sand island parallel to the coast and separated from the mainland by a salt marsh and lagoon

bioabrasion: physical and chemical erosion or removal of rock as a result of the activities of marine organisms

guyot: an oceanic volcano, presently submerged far below sea level, with a top that has been beveled flat by wave erosion

isostatic readjustment: rapid tectonic uplift or subsidence of continental areas in response to the addition or removal of the weight of overlying deposits of glacial ice or seawater

lithosphere: the outer shell of the Earth, where the rocks are less dense but more brittle and coherent than those in the underlying layer (asthenosphere)

notch or nip: an erosional feature found at the base of a sea cliff, the result of undercutting by wave erosion, bioabrasion from marine organisms, and dissolution of rock by groundwater seepage

Pleistocene epoch: the time of Earth's history, from about 2.58 million to 11,000 years ago, during which the Earth experienced cycles of warming and cooling, resulting in cycles of glaciation and sea-level change

strandline: the position or elevation of the portion of the shoreline between high and low tide (at sea level); usually synonymous with “beach” and “shoreline”

subsidence: the sinking of a block of the Earth's lithosphere because of a force pushing it down; coastal areas undergoing rapid subsidence tend to be submerged below sea level

tectonic: pertaining to large-scale movements of the Earth's lithosphere

uplift: the rising of a block of the Earth's lithosphere because of a force pushing it up; coastal areas undergoing uplift tend to emerge above sea level

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