Marine coastal landforms. Landforms, coastline Coastal landforms

22.10.2020

Shore- this is the boundary of land and sea where their interaction occurs. It is observed within a relatively wide band called coastal zone. This zone includes the surface part of the coast and the underwater coastal slope.

The coastal zone is affected by the following forces: sea waves; wave currents and tidal phenomena. The following also take part in the formation of coasts: rivers connecting delta coasts; tectonic movements; alive organisms.

Debris material in the coastal zone, transported by waves and surf, is usually called marine sediments. The accumulation of sediment in the zone of the surf flow is usually called beach. Usually the beach is composed of larger sediments than the underwater coastal slope. The largest debris accumulates near the wave breaking zone, as maximum speeds flow - at the beginning of its movement.

Beaches are classified according to morphological characteristics full and incomplete profile.

Full profile beach is formed if there is free space ahead of the forming sediments. Then the beach takes on the appearance of a coastal rampart, which has a gentle and wide sea slope and a steeper slope facing the shore. If the beach is formed at the foot of the ledge, then sloping beach or incomplete profile beach, with one slope facing the sea.

With the transverse movement of sediment, various underwater and coastal accumulative forms arise. This underwater shafts, representing accumulative forms composed of sandy material and stretched along the coast parallel to each other. Usually there are 2-3 shafts, less often 5-6 shafts. Their height is from 1 to 4 meters and their length is from several hundred meters to several kilometers.

The origin of underwater shafts is associated with partial destruction of waves or the so-called by drilling. In this case, the shafts lose some of their energy and the material they carry is deposited on the bottom in the form of an underwater shaft. The zone of partial destruction of waves with a shallow bottom near the shore can be quite wide. They call her bubbling zone. The large number of underwater swells is due to the fact that waves of different magnitudes experience surfacing at different depths. Underwater swells are formed in places of the underwater slope, where partial destruction of waves of a certain magnitude occurs. Beaches, coastal and underwater levees are elementary accumulative forms. Larger accumulative formations include coastal bars or barriers.

Shore bars composed of material of bottom origin, usually shells and coral sand. Οʜᴎ stretch for tens and hundreds of kilometers along low-lying sea coasts and usually separate the coastal water area from the sea, which is usually called lagoon. The foot of the sea bars are located at a depth of 10-20 m, and above the water they rise 5-7 m, sometimes up to several tens of meters. Such high altitude achieved through dunes. Bars are widespread and are found along 10% of the world's ocean shores. It can be assumed that the formation of bars is associated with a rise in sea level in post-glacial times. At the same time, the surfaces of the flooded accumulative plains turn out to be too sloping and the waves carry large masses of sand towards the shore. Excess transported sediment falls out during movement and forms an underwater bar, which becomes an obstacle to sediment arriving from the underwater coastal slope. Their deposition on the sea side of the bar leads to its expansion in width. At the same time, the bar grows in height due to the accumulation of sediment on the ridge and the general movement of the bar to shallower depths. It can be assumed that the formation of coastal or island bars is associated with changes in the level of the world ocean in recent times.

Longitudinal movement of sediment. When waves approach the shore at an oblique angle, longitudinal, or alongshore sediment movement. Due to wave vibrations, sediment particles travel along a zigzag path and travel a certain distance along the coast. The surf flow, running up the beach, at the beginning maintains the direction of the wave movement, then deviates more and more from it under the influence of gravity. The reverse flow runs in the direction of the greatest slope. It describes an asymmetrical trajectory, resembling a parabola, and moves sediment particles along the coastline. The speed of such movement depends on the angle of approach of the wave to the shore. In this case, its optimal value is 45°. Under certain conditions, massive sediment movement occurs on the beach and on the underwater coastal slope. Such movement in one direction over a long period of time, for example a year, is usually called sediment flow. The flow is characterized by power, capacity and saturation.

Flow power- this is the amount of sediment that actually moves along the coast per year. Capacity- it is customary to call the amount of sediment that waves are capable of moving. If the power is equal to the capacity, this means that all the wave energy is spent only on transport. Neither bank erosion nor sediment deposition occurs in this case. For this reason saturation flow should be called the power-to-capacity ratio. If this ratio is less than 1, then the flow is unsaturated. In this case, part of the energy free from the work of transporting material will be directed to erosion of the coast. If the flow capacity is less than the sediment supply to a given area, we can talk about an increase in the intensity of sediment supply above the flow capacity. As a result, some of the material stops moving and is deposited, forming accumulative forms.

Accumulative forms during longitudinal movement of sediment. As the angle of approach of the waves to the shore decreases, the flow capacity decreases and the accumulation of material begins. This creates: accumulative forms of filling the shore contour. This category includes various accumulative underwater terraces at the tops of bays. Accumulative braids, which are formed when a flow bends around a shore protrusion. In this case, the wave front spreads and its energy decreases. The braids are attached to the shore only by their root part, and their growing end remains free. For this reason, such braids are called free accumulative forms. If the coast on the sea side is protected by a cape, then a closing form is formed at the entrance to the bay, which is usually called a bay-bar.

Abrasion. The destructive work of the sea is commonly called abrasion. It can be mechanical, chemical and thermal.

Mechanical abrasion- this is the destruction of rocks that make up the shores under the influence of impacts from surf waves and rock fragments carried by waves and surf.

Chemical abrasion- destruction of rocks due to their dissolution sea water.

Thermal abrasion- destruction of coastlines composed of frozen rocks or ice as a result of the warming effect of sea water.

With a steep slope of the coastal slope, the waves come to the shore with a large supply of energy. They have a direct impact on the area adjacent to coastline. For this reason, a recess is formed here, which is usually called wave-breaking niche. When it deepens, the cornice collapses. The mass of rock entering the water further accelerates the destruction of the shore. The process of cornice collapse is repeated repeatedly, which leads to the formation of a vertical ledge, that is abrasion cliff or clif. In front of the cliff, a platform is developed that is slightly inclined towards the sea, called Bench. The bench begins at the very foot of the cliff and continues below sea level. Expansion of the bench during abrasion leads to flattening of the profile and attenuation of the processes of bank destruction.

Straightening the coastline. The destruction of coastlines and the formation of accumulative coastal forms lead to the leveling of the coastline. Its initial outlines are currently determined by the penetration of sea water into relief depressions after the retreat of the glacier. Such shores are called ingressive. Among them are:

1. Fiord (fjord) shores, formed during the flooding of glacial valleys. Fjords- narrow and long winding bays (Norway, Canada, New Zealand).

2. Skerry shores, which were formed during the flooding of low glacial plains. Skerries- these are small rocky islands, which look like submerged sheep's foreheads, separated by narrow straits.

3. Rias shores, arising from the flooding of river valleys in mountainous countries. Rias are narrow, winding bays. For example, Sevastopol Bay.

4. Estuary shores, formed during the flooding of river valleys of the coastal plains. The bays that arise in this case - estuaries. ( North-West Black Sea region).

5. Dalmatian-type shores, which arose during the flooding of folded structures with a strike close to the general direction of the coast. In this case, bizarre archipelagos of islands are formed, stretched along the coast.

6. The banks of fault-block lobate dissection, which are formed during the flooding of tectonic depressions of grabens (Greek coast of the Aegean Sea).

All types of coasts are in different stages of leveling, which is associated with the unequal nature of the initial dissection and different geological structure. At the same time, some sections of the coast turned out to be leveled, others are being leveled, and others have become even more dismembered during the leveling. This happened due to the formation of bays or irrigation in place of rocks more susceptible to erosion, as well as during the formation of closing accumulative forms.

Some banks retain their original dissection. This applies to rias and fiord shores, as well as to the shores of tectonic dissection, composed of durable igneous rocks. Such shores are called shores, not changed by the sea. Their development occurs mainly under the influence of slope processes. They can be called denudation, and with great influence of the sea - abrasion-denudation.

Shores of tidal seas. The seashores are affected not only by waves, but also by tides. At the same time, tides on deep shores intensify abrasion, since with increasing depth near the shore, the waves act more energetically on the cliff. The foot of the cliff on such shores is at high tide level. At low tide, only part of the suspended material is carried away by the ebb current. As a result, accumulative forms are formed near the shore, which are called dryings or watts. Gradually, the drying surface becomes higher than the tidal level, vegetation settles on it and a soil cover is formed - marches appear.

However, the accumulative activity of tides generally leads to the buildup of land. Within the coastal shallow waters, underwater accumulative forms can form: sand ridges and sand waves.

Sand ridges- these are large linear forms up to several kilometers long and 1-2 km wide. Their height is up to 20 m. They are located along the coast in the direction of tidal currents.

Sand waves- these are formations that arose on the slopes of sand ridges and are oriented frontally in relation to the direction of tidal currents. Their sizes range from several hundred meters to several kilometers in length and up to several meters in height. Οʜᴎ resemble enlarged wave ripple marks.

Coral Shores and islands. In the formation of seashores on the coasts of tropical seas, some marine reef building organisms, which absorb lime from sea water. When they die, it forms coral or reef limestone. Accumulative forms built from such limestone are called coral reefs.

The following types of coral structures are distinguished:

- fringing or shore reefs;

- barrier reefs;

- ring or intralagoonal reefs.

Fringing reefs these are underwater coral-limestone terraces adjacent to the shore. Their outer side is covered with living colonies of corals. The surface of the reef is usually called reef flat. With distance from the outer zone, this surface is increasingly covered with sediment cover of gravel and sand. Near the shore it is bordered by a sand and gravel beach. The thickness of the fringing reef on tectonically stable shores usually does not exceed 50 m, which is due to the habitat conditions of reef-forming corals. Coral polyps live in symbiosis with unicellular green algae, which needs good lighting.

Barrier reefs these are coral-limestone ridges located considerable distances from the coast. The thickness of barrier reefs is many times greater than that of fringing reefs. It has been established that they arise during tectonic subsidence of their outer edge. The largest such reef is the Great Barrier Reef, which stretches along the northeastern edge of Australia for more than 2,300 km. If a barrier reef forms around a small subducting island, it becomes a ring-shaped reef, or atoll. In this case, a coral lagoon is formed inside the atoll, in which intralagoonal reefs can arise. In most cases, they take the form of columns, or giant pedestals, randomly scattered inside the lagoon. By merging with each other, the columns form large formations in area, which are called banks. Coral islands are often found in tropical seas. They are usually located on accumulative forms formed due to the activity of sea waves and consisting of coral sediments - sand, gravel, pebbles, and sometimes blocks of reef limestone.

As a result of repeated changes in ocean depth during glacial and interglacial eras, peculiar relief forms were formed in the coastal zones of the seas, which are called ancient coastlines.Οʜᴎ can sometimes be located on land and correspond to a higher sea position than at present. Ancient coastlines corresponding to more low level, are now flooded by the sea.

Elevated coastlines are expressed as sea terraces. These are steps stretched along the shore. In each terrace the following are distinguished: the surface of the terrace; ledge; edge; back seam.Οʜᴎ record the position of the ancient coastline.

Taking into account the dependence of the reconstruction, the following are distinguished:

1. Accumulative terraces, that is, completely composed of coastal-marine sediments;

2. Abrasion terraces, which are composed only of bedrock;

3. Basement terraces, having a bedrock base covered by marine sediments.

To identify the history of coastal development, so-called spectra of terraces, which make it possible to compare different sections of the coast and contain information about neotectonic movements.

Marine coastal landforms - concept and types. Classification and features of the category "Marine coastal landforms" 2017, 2018.

(lakes, rivers) is called the shore.

The shores are divided depending on their steepness (sloping, steep) and the nature of the materials composing them (mud, sand, pebbles, rocky). On the side of the water area, a strip of seabed adjoins the shore, which is constantly exposed to wave movements of water. This strip is called the underwater coastal slope.

The shore and the underwater coastal slope together form coastal zone of the sea, within which the complex interaction of the lithosphere, hydrosphere, atmosphere and biosphere constantly takes place. This zone is characterized by the variability of relief forms and their various combinations within even small areas. The work of sea water is manifested in the destruction of shores - abrasion, as a result of which they retreat inland, as well as in the deposition of destruction products - accumulation, which leads to a change in the underwater relief of the coastal zone and the formation of new types of shores. Shores formed primarily as a result of the destructive action of waves are called abrasive, and shores created by sediment deposition are called accumulative.

The main factor in the formation of abrasion shores is the destructive action of breaking waves, as a result of which a depression is formed at the base of the slope - wave-breaking niche. Over time, this niche deepens more and more; the overhanging parts of the slope fall into the sea, breaking up into a mass of fragments, with the help of which breaking waves continue further destruction of the coastal ledge.

The creative work of the sea is expressed in the accumulation of materials thrown out by the sea (sand, pebbles, shells of sea animals, etc.) off the coast. Pebbles and sand on the surface of the abrasion platform are constantly moving within its boundaries under the influence of the surf. As a result, relief forms of accumulative origin are created.

As a result of repeated changes in ocean depth during glacial and interglacial eras, peculiar relief forms were formed in the coastal zones of the seas, which are called ancient coastlines. They can sometimes be located on land and correspond to a higher sea position than at present. The ancient coastlines corresponding to the lower level are now submerged by the sea.

Elevated coastlines are expressed as sea terraces. These are steps stretched along the shore.

In each terrace the following are distinguished: the surface of the terrace; ledge; edge; back seam. They record the position of the ancient coastline.

Depending on the structure there are:

Accumulative terraces, that is, completely composed of coastal-marine sediments;
Abrasion terraces, which are composed only of bedrock;
Basement terraces, having a bedrock base covered by marine sediments.

To identify the history of coastal development, so-called spectra of terraces, which make it possible to compare different sections of the coast and contain information about neotectonic movements.

Types of banks (according toD. G. Panov)

(a – rias, b – fiord, c – skerry, d – estuary, e – Dalmatian, f – watt (1 – watts, 2 – runoff hollows), g – thermal abrasion, h – coral, i – volcanic).

Literature.

Smolyaninov V. M. General geoscience: lithosphere, biosphere, geographical envelope. Educational manual / V.M. Smolyaninov, A. Ya. Nemykin. – Voronezh: Origins, 2010 – 193 p.

Shore- a narrow zone within which the interaction of land and sea occurs. The processes that shape the coast include waves, currents and tidal phenomena. The coastal zone consists of the coast itself - its surface part and the underwater coastal slope. The lower boundary of the shore is a depth equal to half the wavelength; it is at this depth that the wave’s impact on the shore begins. The upper limit is a line drawn along the tops of the wave splashes.

Coast- a strip of land that includes modern and ancient coastlines. The main process that determines the uniqueness of coastal relief forms is waves. The wave produces destructive and accumulative work in the coastal zone, causing the development of abrasive and accumulative forms. The destructive work of waves is called abrasion. There are mechanical, chemical and thermal abrasion. Mechanical abrasion- This is the destruction of rocks under the influence of waves and surf and bombardment with debris. Chemical abrasion occurs when rocks are dissolved by sea water. Thermal abrasion is the destruction of coastlines composed of frozen rocks as a result of the warming influence of sea waters. The predominance of abrasion or accumulation in the coastal zone depends on the steepness of the coast and the properties of the rocks composing it. On a steep slope composed of durable rocks, abrasion forms of relief predominate. In this case, a wave with high energy acts on the shore, and at the level of the coastline a wave-breaking niche is formed.

Its further deepening causes the collapse of the cornice and the development of a vertical ledge - clif. As the cliff retreats towards the shore, a platform grows at its foot - bench. The bench begins at the foot of the cliff and continues below sea level; at low tide the bench can become dry. The rate of abrasion on banks composed of clays and marls can reach several meters per year. Accumulative forms are formed on shallow shores; their originality depends on the angle of approach of the wave to the shore. There are transverse and alongshore movement of sediments. If a wave approaches perpendicular to the shore, a transverse movement of sediment occurs. Gradually, the coast, composed of sediments of the same size, takes on the form of dynamic equilibrium. This happens as follows. At a depth equal to half the wavelength, the impact of the wave on the shore begins. The advantage of forward speeds (toward the shore) is still small compared to the reverse ones. But since the particle is on an inclined surface, it moves down the slope a little. The closer to the shore, the greater the forward speeds; at the neutral point they become equal to the reverse speeds. At the neutral point, the particle performs only oscillatory movements. Higher up the slope, the particle will move toward the shore, causing the accumulation of material on the shore; below the neutral point, material will be carried down the slope. During the transverse movement of sediment, the material delivered to the shore from the bottom is mainly sand, pebbles, and shells. Landforms created by the lateral movement of sediment include submarine shore bars, submarine and island bars, beaches and terraces. At depth X/2, wave destruction begins, material accumulates in the blistering zone at the bottom, and accumulative underwater shore wall. There can be several underwater shafts; they are located parallel to each other and the shore. The height of the shafts reaches 1 - 4 m with a length of up to several kilometers. The formation of several rows of underwater coastal ridges is explained by the approach of waves of different lengths, so their formation is observed at different depths. When material accumulates, the shafts are converted into underwater bars. The crest of an underwater bar may appear on the surface, in which case the bar becomes island and represents? is a chain of islands stretching along the coast. It is believed that island bars can only arise if the World Ocean Level changes. The bars stretch for hundreds of kilometers along the low-lying sea coasts and separate the coastal waters, called the lagoon, from the sea. The bases of the bars are located at a depth of 10 - 20 m; they rise above the water by 5 -7 m. Bars are very common in sea shores, 10% of the coastline is on shores bordered by bars. On the above-water part of the coast, with the transverse movement of sediments, a beach is formed. Based on morphological characteristics, beaches with a complete and incomplete profile are distinguished. A full profile beach is formed in free space. Then the beach looks like a coastal rampart with gentle slopes.

Beach of an incomplete profile is formed at the foot of the ledge; it has one slope facing the sea. If sea level drops, the beach becomes accumulative sea terrace. When waves approach the shore at an angle of less than 90°, an alongshore sediment flow is formed. Sediment moves along the coast towards an obtuse angle and consists of products of coastal destruction and river alluvium supplied to the coast. The optimal angle of wave approach to the shore is 45°. At this angle of approach, the maximum amount of sediment is transported. When the shore contour changes, the rate of material supply turns out to be excessive and accumulation begins. An accumulative terrace forms at the tops of the bays near the concave coast. Since the landform is adjacent to the shore along its entire length, it is called adjacent. When going around the shore protrusion, the speed of movement of the material drops, and an accumulative form is formed in this place - braid. The spit is attached to the shore in only one part, its end remains free. This form is called free. On a section of the coast protected by an island, the accumulation of material leads to the appearance tombolo (take over). If the coast is protected from the sea by a far protruding cape, a overspray. As it grows, the bay bar can reach the opposite shore and block the bay. In this case, the accumulative form is called closing. Depending on the outlines of the coastline and the complex of processes occurring on the banks, they are divided into several types.

1. Primary dissected shores, slightly modified by the sea (ingression). The dismemberment of the coast is created by non-wave processes; sea waters only fill depressions in the relief. Such shores include fiord shores - formed during the flooding of glacial and trough valleys, skerry shores - formed during the flooding of the relief of “curly” rocks (ram’s foreheads).

Such shores are typical for Scandinavia and the northern coasts of Canada, while skerries are developed in the Baltic Sea. Rias shores arise when the mouths of mountain rivers are flooded by the sea; such shores include the coast of the Iberian Peninsula. The Dalmatian coasts are formed when negative folded structures parallel to the coast are flooded by the sea. This creates chains of islands stretching along the coast and long narrow bays. This kind of shore is typical for Adriatic Sea. Estuary banks are formed due to the flooding of the mouths of river valleys on low-lying coastal plains. Typical estuaries are characteristic of the Don and Dnieper rivers.

2. Non-wave shores. Such shores are created by tides, rivers, organisms, slope or tectonic processes. TO tidal shores include watts - they are flooded twice a day by the lowest quadrature tide, marches - are rarely flooded, only by high spring tides. Deposition large quantity alluvium on the banks causes the creation of a deltaic coast.

There are large deltas near the Lena, Volga, and Nile rivers. On the coasts of tropical seas, organisms, especially corals, play an active role in the formation of coasts. Organogenic coral shores are formed here. In tectonically active zones, tectonic shores can form; sometimes tectonics activates slope processes, and then talus and landslide shores are formed.

3. The actual wave banks. Leveled abrasion banks appear where abrasion is actively occurring. As a rule, these shores are steep, composed of easily eroded rocks. Due to the high speed of retreat, the banks quickly level out, forming a leveled abrasion coast. Leveled accumulative shores are characteristic of gentle underwater slopes. On this shore, the process of accumulation comes first. Transitional types of shores include bay and lagoon shores. Abrasion is observed on bay shores on capes, and accumulation is observed in bays. On the lagoon coast, the separation of the lagoon by the growing spit has not yet finished; therefore, the formation of a leveled accumulative coast continues.

Relief of the ocean floor

In the relief of the ocean floor there are four geotextures. Three geotextures are located entirely within the ocean floor: ocean floor, transition zone, mid-ocean ridges; the latter - the underwater edge of the continent - is part of the geotexture - the continental protrusion.

Continental ledge. A significant part of the continental ledge (about 35%) is flooded by ocean waters. This part is called underwater margins of continents. Approximately 2/3 of it is in the Northern Hemisphere and only 1/3 is in the Southern Hemisphere.

The underwater edge of the continent has a continental type of crust. As the sea level rises, the area of the underwater part increases, and as the level decreases, the share of land increases. The underwater continental margin consists of shelf, or continental shallows, continental slope And continental foothills.

The coastal, relatively shallow part of the underwater margin, directly adjacent to the shore, is called the shelf, or continental shoal. In the polar regions, the shelf topography is complicated by glacial morphosculpture; in temperate latitudes and on the equator, river valleys have been preserved on the shelf. In tropical and equatorial latitudes, coral reefs are very typical on the shelf.

Below the shelf edge there is a continental slope. It is characterized by a noticeable increase in slope up to 5 - 7°, sometimes up to more than 50°. Very often the continental slope has a stepped profile. If the steps have significant areas, they are called marginal plateaus (Blake Submarine Plateau off the Florida Peninsula).

Submarine canyons are widespread within the continental slope. These are deeply incised hollows with steep slopes, the depth of the incision reaches 2000 m. Underwater canyons resemble river valleys in mountainous countries and are often their underwater extensions.

At a depth of about 2.5 km, the continental slope smoothly turns into the continental foot. It looks like a sloping plain adjacent to the base of the slope. If it is impossible to distinguish the shelf, slope and foot, then such areas are called borderland(California coast). Within the oceans there are underwater and surface protrusions. They are separated from the continents by a wide strip of bottom with an oceanic type of crust. Similar

formations are called microcontinents. For example, Seychelles, underwater margin of New Zealand, underwater rise of the Naturalist, etc.

ocean bed. This geotexture consists of deep-sea abyssal basins and separating them underwater ridges And volcanic mountains The ocean bed has an oceanic type of crust. It is most widespread, especially in Pacific Ocean, have hilly plains, the topography of which is complicated by seamounts and swell-like uplifts of various sizes. Among them there are oceanic ridges, mainly of tectonic origin, chains of volcanic mountains and individual volcanoes. At the bottom of the ocean there are flat-topped mountains - guyots. The rate of deposition of material on the ocean floor is several centimeters per year.

Transition zone. Several transition zones are located along the eastern edge of the Eurasian continent, two zones are observed off the coast of Northern and South America. The transition zone consists of basins of the marginal sea, island arc And deep-sea trench.

There is a certain connection between the depths of the basins and the thickness of sediments at the bottom: the deeper the sea, the less the thickness of the sediments. In the Sea of Okhotsk at a depth of 3.5. km, the thickness of precipitation is 5 km. In the Bering Sea, which has a depth of 4 km, the thickness of precipitation decreases to 2.5 km.

Deep-sea trenches are narrow depressions - deflections in the earth's crust, having the shape of an arc in plan. Currently, 35 deep-sea trenches are known, 28 of them in the Pacific Ocean. Five trenches have a depth of more than 10,000 m, the Mariana Trench is 034 m. The steepness of the slopes increases towards the bottom: in the upper part of the slope it is 5 - 6°, in the lower part it can increase to 25°. The slopes are stepped and dissected by underwater canyons. The sources of earthquakes are confined to deep-sea canyons.

Island arcs are huge ridges, usually located on the inside of a deep-sea trench. Island arcs are characterized by volcanism and high seismic activity. Island arcs can be double, they have inner and outer ridges separated by depressions ( Kurile Islands). At a certain stage of development

island arcs merge with each other, forming large island or peninsula (Kamchatka, Japanese islands). Sometimes at the edge of a deep-sea trench there is only an underwater rise, there are no islands.

Mid-ocean ridges. They are the largest underwater rises elongated in the meridional direction. Mid-ocean ridges can reach 2000 km in width and 6 km in relative height. Mid-ocean ridges form a single system stretching across all oceans. IN Atlantic Ocean the ridge is located almost in the center; in the Pacific Ocean it approaches the coast of both Americas, in Indian Ocean runs along the coast of Africa. Based on relief and tectonic activity, rift and non-rift ridges are distinguished. The relief of rift ridges is complex, rugged: rift valleys, narrow mountain ranges, giant transverse faults. Underwater and surface volcanoes and islands are often found. Non-rift ridges are characterized by the absence rift valley and less difficult terrain. For example, most of The rift valley does not have a Pacific arch uplift. Mid-ocean ridges are cut by a grandiose system of transverse faults, called transform faults, along which blocks move relative to each other. The ridges correspond to a riftogenic type of the earth's crust.

Abrasion is the destructive work of the sea. There are mechanical (basic), chemical and thermal abrasion. Mechanical abrasion occurs under the influence of mechanical impact of waves and dragged debris. Chemical abrasion is the destruction of shores composed of soluble rocks (carbonates, sulfates). Under the influence of salts and carbon dioxide contained in chemically aggressive sea water, dissolution and leaching of rocks occurs. Thermal abrasion occurs mainly in the polar zones on the shores of seas composed of loose permafrost rocks or ice. Here the coast is destroyed not only under the influence of the mechanical energy of waves, but also as a result of the fact that the temperature of sea water is higher than the temperature of frozen rocks, and the latter thaw, loosen and are more easily destroyed. Chemical and thermal abrasion are always accompanied by mechanical, which is ultimately the main one.

Abrasion occurs most intensely on deep coastlines, that is, where there is a steep underwater slope. Then the waves approach the shore without wasting their energy, as happens when waves overturn on shallow shores, and all the kinetic energy of the waves goes to destroy the shore. First, at the edge of the sea, a depression is formed at the base of the coastal cliff - an abrasion, or wave-breaking niche, and then, with its further deepening and the collapse of the cornice of bedrock overhanging it, a steep abrasion ledge, or cliff, is formed. If the coast is composed of hard rocks, then large fragmentary material formed during their destruction remains at the foot of the cliff, and smaller reverse currents carried out to sea and deposited on an underwater slope, gradually forming an underwater accumulative terrace. As the cliff is destroyed and retreats inland, a slightly inclined seaward or horizontal abrasion platform - a bench - is formed in front of its foot (Fig. 17).

Rice. 17

Depending on the climate, geological structure and recent tectonic movements, the relationship between marine and subaerial processes, many types are distinguished sea coasts. The most typical types of coasts are the following (Fig. 18):

1. Estuary shores are characteristic of low-lying plain shores that experience subsidence. Estuaries are bays, straight or winding in plan, with low banks, formed as a result of flooding by the sea (or lake) of the mouths of lowland rivers and gullies. Characteristic, for example, of the northwestern coast of the Black Sea.

2. The lagoon type is formed in cases of coastal evolution when bays and estuaries are separated from open sea. The entrance to the bay or estuary can be blocked after the formation of a spit and its transformation into a bay that connects the opposite sides of the bay. The formation of a bar on a gentle underwater slope and its attachment to prominent shore protrusions leads to the formation of a lagoon. The lagoon gradually becomes shallow due to sediments entering it. The source of sediment supply is the river, at the mouth of which a delta is formed. Then the sediment is brought tidal currents through the ducts in the bar.

3. Fjord shores are characteristic of mountainous regions, cut up by narrow, deep bays that protrude far into the land - fjords, which are glacial valleys flooded by the sea. The length of the fjords is tens and a few hundred kilometers, they stretch far along the shelf, the width is a few kilometers, the depth is hundreds of meters. Many of them are straightforward, because they are confined to discontinuous faults, others have a branching shape. All of them have steep, rocky abrasive shores. Fjords are characteristic of the shores of the North, Norwegian, Barents Seas, the coasts of Alaska, Canada, and Iceland.

4. Rias banks are formed when the mouths of river valleys in mountainous areas are flooded by the sea. Rias are long, narrow, winding bays that arose as a result of the ingression of the sea into the mouths of river valleys. Developed in the north-west of the Iberian Peninsula, the coasts of China, Korea, Primorye, the southern part of Sakhalin Island, etc.

5. Skerry shores are also associated with glaciations. Skerries are small rocky islands and groups of underwater rocks, smoothed by a glacier (“curly”), representing large “ram’s foreheads” flooded by a shallow sea. Sometimes accumulative glacial and water-glacial forms - drumlins, kamas, eskers - are also flooded by the sea. This type of coastline is typical for Finland, Sweden, Norway, Alaska, and Canada.

6. The Dalmatian type of coast is due to the peculiarities of the geological structure. It represents submerged folded structures that usually extend parallel to the shore or obliquely relative to it. At the same time, anticlines appear in the form of peninsulas and islands, and bays are confined to synclines. Both islands and bays stretch along the coast. The coastline has a rugged character. An example is the Dalmatian coast of the Adriatic Sea.

Rice. 18 Coast types

In addition to the above types of coasts, there are many others - deltaic, volcanic, biogenic, thermal abrasion, etc.

(lakes, rivers) is called the shore.

As a result of repeated changes in ocean depth during glacial and interglacial eras, peculiar relief forms were formed in the coastal zones of the seas, which are called ancient coastlines. They can sometimes be located on land and correspond to a higher sea position than at present. The ancient coastlines corresponding to the lower level are now submerged by the sea.

Elevated coastlines are expressed as sea terraces. These are steps stretched along the shore.

In each terrace the following are distinguished: the surface of the terrace; ledge; edge; back seam. They record the position of the ancient coastline.

Depending on the structure there are:

Accumulative terraces, that is, completely composed of coastal-marine sediments;
Abrasion terraces, which are composed only of bedrock;
Basement terraces, having a bedrock base covered by marine sediments.

To identify the history of coastal development, so-called spectra of terraces, which make it possible to compare different sections of the coast and contain information about neotectonic movements.

Types of banks (according toD. G. Panov)

(a – rias, b – fiord, c – skerry, d – estuary, e – Dalmatian, f – watt (1 – watts, 2 – runoff hollows), g – thermal abrasion, h – coral, i – volcanic).

Literature.

Smolyaninov V. M. General geoscience: lithosphere, biosphere, geographical envelope. Educational manual / V.M. Smolyaninov, A. Ya. Nemykin. – Voronezh: Origins, 2010 – 193 p.