Plate tectonic theory

Geologically, the word plate means a slab of a rock made up of continental and oceanic lithosphere. The word tectonic comes from the Greek word tekton which means “a builder or to build”. The development of plate tectonic theory began in 1915 following the early criticism of Continental drift theory. Essentially, plate tectonics are geological components of study related to the origin and arrangement of the broad structural features of the earth’s surface including not only folds and faults but also mountain belts, continents and earthquake belts. The basic idea of plate tectonics is that the lithosphere (earth’s crust and the upper rigid part of the mantle) is divided into separate parts called tectonic plates, which move slowly. At present it is believed that there are seven (7) large plates and about thirteen (13) or more small plates, making a total of about twenty (20) plates or more.

The major plates

The seven major plates are:

(a) North American plate – It includes the surrounding oceanic crust up to the mid Atlantic ridge.

(b) South American plate – It includes the surrounding oceanic crust up to the mid Atlantic ridge.

(c) Pacific Plate – This plate is the largest of all covering about one-fifth of the earth’s surface, hence occupies the whole of Pacific region.

(d) Eurasian plate – It is a continental plate, thus it occupies both Europe and Asia continents and includes their surrounding areas.

(e) Antarctic Plate – It covers the Antarctica continent and the surrounding Southern Ocean.

(f) African plate – It takes all of Africa and its surrounding area to the mid Atlantic ridge.

(g) Australian–Indian plate – This includes the continental crust of India and Australia as well as the oceanic crust of India and parts of Pacific Ocean.

The minor plates

The minor plates are greater in number than major plates reaching to about 13. They include; Caribbean, Philippine, Arabian, Nazca, Cocos, Scotia, Caroline, Somali, Fuji, Burma, New Hebrides and Juan de Fuca plates (Figure 1.7). These plates move over the hot and partially molten asthenosphere though their speed is low.

Assumptions of the plate tectonic theory

The plate tectonic theory states that the earth’s crust is divided into separate parts called tectonic plates. These plates are mobile and they float on underlying semi molten mantle (the asthenosphere) and are moved by convectional currents.

The theory can also be used to be the study of the movement of the plates and their resultant landforms. The plates are divided into two types which are continental and oceanic plates.

Continental plates

These are plates which carry the continents also known as continental mass. They are composed of old rocks mainly over 1500 million years and lighter rocks of granitic type dominated by minerals rich in silica and aluminium (sial). The continental plate of Africa, North America, and South America are good examples of continental plates with old rocks.

Oceanic plates

These are made up of simatic younger and denser rock of basalt composition, dominated with silica and magnesium minerals. They are younger mainly under 200 million years. The boundary between the crust (continental or oceanic) and the underlying mantle is known as Moho or Mohorovicic discontinuity and it was named after a study conducted by Andrija Mohorovicic in 1909.

Causes of plate movement

The causes of plate movement are mantle convectional current generated by heat from the centre of the Earth, formation of new oceanic crust, cooling and sinking of oceanic crust, differences in force of gravity between the oceanic ridge and trench and oceanic topography (elevation of the mid oceanic ridge).

Mantle convectional current

Radioactivity generated heat in the upper mantle produces convectional currents which carry plates along them with the continents as passengers. The heated material in mantle expands and becomes less dense and slowly rises. When it reaches near the crust, it cools down slowly and contracts; hence it becomes dense and sinks down again (Figure 1.8). The new oceanic crust gradually cools down and thickens with age and it is pushed downhill as new magma emerges from the active zones of divergence behind it. This force is regarded as gravity force.

Formation of new oceanic crust

New oceanic crust is formed at the mid oceanic ridge as the magma comes to the earth’s crust. The magma rises through the cracks on the plates and cools to form new crust. As the newly formed crust cools down and becomes denser, thus causing the sea bed to sink to subduction zone. As it moves down into subduction zone, the crust is pushed down under another plate. It bends down and sinks into the mantle, thus keeping the plates moving.

Differences in forces of gravity between the oceanic ridge and oceanic trench

The existing differences of gravitational pull between the ridge and trench causes the ridge to be pulled down to the deep trench, hence causing the motion of the plates.

Oceanic topography

Oceanic topography is also known as elevation of the mid oceanic ridges. When the elevated parts of the oceanic floor is subjected to extra weight it results into sinking of the materials which in turn lead to plate motion. As plates move, they can either collide, move apart or slide past each other and result into the formation of various landforms such as mountains, trenches, earthquakes, volcanoes, and mid oceanic ridges.

Plate movement and resulting landforms

Tectonic plates move slowly over time. Some plates move towards each other forming a convergent movement. At a time, some plates move away from each other forming divergent movement, while other plates may also move horizontally and past each other and form a transform movement. The movement of plates creates plate wrinklings or tectonic boundaries from which landforms formation occur.

However, due to its relatively low density, continental crust does not sink but floats permanently on a denser oceanic plate.

Oceanic crust is being formed and destroyed continuously. Continental plates, such as the Eurasian plate, may consist of both continental and oceanic crust, the Earth is neither expanding nor shrinking in size. Thus, when new oceanic crust is being formed in one place, older oceanic crust must be destroyed in another part. Continental crust may extend far beyond the margins of the land masses. Plates cannot overlap. This means that either they must be pushed upwards on impact to form mountains or one plate must be forced downwards into the mantle and destroyed.

Moreover, plate movement is slow, though not in geological terms, and is usually continuous. Sudden movements are detected as earthquakes. Most significant landforms (fold mountains, volcanoes, island arc, deep sea trenches and batholith intrusions) are found at plate boundaries. Very little change occurs in plate centres (shield lands).

The plate margins

These are important regions, zones or boundaries where plates either meet, slide or move apart from one another. The margins are also active seismically and tectonically. Plate margins are characterized by a combination of tectonic and topographic features such as oceanic ridges, Benioff zones, young fold mountains, and transform faults.

Plate boundaries can be classified into three main types which are divergence, convergence and transform/conservative/shear boundaries.

Divergence plate boundary

This boundary (zone) is also known as constructive zone. It is a zone whereby two plates move away from each other. They are called constructive zones because new materials from the interior part of the earth are added to the surface and form new features like the mid-ocean ridge, island arcs, and new oceanic crust. A divergence zone can be found among continental plates (Figure 1.9a), oceanic plates (Figure 1.9b) and continental and oceanic plates. When divergence occurs in the ocean floor no gap is left on the ocean floor as new crustal material from mantle fills them.

Moreover, when divergence occurs on the continent, the continent breaks up and drift apart and the central block subsides. For instance, the land giant through such as the Great Rift Valley in Africa were formed when plates moved apart. The continent to continent divergence zones are associated with different features.

Formation of rift valley occurs if the divergence boundary is on the continental land. A good example is the Great East African rift valley. Volcanic features like volcanic mountains, such as Mount Kilimanjaro in Tanzania, are formed as magma forces along the faults.

A divergent zone on oceanic to oceanic boundary is associated with formation of oceanic ridges if two oceanic plates move away from each other. An example of this formation is the Mid Atlantic Ridges which are 1000 km long and 2500 metres high.

The formation of a rift valley at the mid-oceanic ridges happens when the oceanic ridge breaks at the centre. The Atlantic Oceanic trench is an example of seafloor spreading. It was formed due to the divergence of South America and African plates. Moreover, volcanic islands are formed from the rising submarine volcano at divergent zone that solidified to form an island. Volcanicity and shallow earthquakes are also resulting features at divergent zones on oceanic boundary.

Convergence plate boundary

The convergence plate boundary is also known as a destructive or consuming plate margin. This occurs when two plates move towards each other and result into the formation of trenches such as Marianas, fold mountains like Himalaya and volcanic islands like Japan. Convergence boundary is divided into three main categories namely;

• continental–oceanic,
• continental–continental, and
• oceanic–oceanic convergence boundaries.

Continental–oceanic convergence boundary

The continental–oceanic convergence boundary forms when oceanic floors (plate) and continental plate meet. As they meet, the heavier rocks (oceanic floor) will be forced to sink (making subduction zone) while the lighter continental rock will be forced to rise. The sinking oceanic rock will be destroyed in hot mantle and come out in the form of volcano (Figure 1.10). The subduction zone is active in volcanicity and earthquake zones. Examples of these include Java Trench and Mariana Trench which were formed when the Philippine Plate subducted under the Pacific Plate.

Continental–continental convergence boundary

Continental–continental convergence boundary is the zone where two similar plates (continental and continental) collide such that neither of them sinks below the other; rather they are forced to bend, leading to the formation of fold mountains such as the Himalayas, Alps and Atlas (Figure 1.11). It is also known as a destructive zone because the plates tend to lose the materials at their margins where collision occurs.

Oceanic–oceanic convergence boundary

Oceanic–oceanic convergence boundary occurs when two oceanic plates converge and one is subducted beneath the other along an oceanic–oceanic plate boundary. Specifically, it happens when one oceanic plate is slightly denser than the other (Figure 1.12). On the non-subducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate. A good example is an island arc of Japan and West Indies.

Transform boundary

Transform boundary is also known as conservative or tear (shear) plate boundary. This occurs when two lithospheric plates slide past each other along a transform fault (Figure 1.13). During that process neither plate is created nor destroyed.

As a result, transform faults which are accompanied by tremors and earthquakes may occur. Examples of faults in the transform boundaries are the San Andreas Fault in California and the North Anatolian Fault in Turkey.

Isostasy theory

This theory was proposed by Clarence Dutton, an American Geologist in 1889.

The term isostasy is derived from two Greek words isos which means equal and stasis which refers to the state of equilibrium or balance or standing still. Literally, isostasy refers to equal standing. Generally, the theory explains the tendency of the earth’s crust to attain an equilibrium state and the distribution of the materials in the earth’s crust which conforms to the observed gravity values.

Assumption of the Isostasy theory

The theory assumes that the earth’s crust is floating on the molten rocks of the upper mantle, like a raft floating in the water rather than resting on the mantle like a raft sitting on the ground.

The theory states that, the lithosphere which has a constant density of 2.7 grams per cubic centimetre, floats in the asthenosphere which has a constant density of 3.3 grams per cubic centimetre. It further states that, there is a state of gravitational equilibrium between the earth’s lithosphere and asthenosphere, in such a way that the tectonic plates floating at an elevation depend on their thickness and density (Figure 1.14). This means that, a continental rock (sial – silica and aluminium) has low density, underlie denser oceanic rocks (sima – silica and magnesium) which results to a state of balance.

The theory further states that, where the continental masses (Sial) rise to form a mountain, the penetration of ice below is greater to compensate the excess mass. Moreover, where the continental mass is thin, like Sima layer draws near the surface or reaches the surface of the Earth and form the ocean floor.

The principle of buoyant force is proportional to the depth of its root. Hence, the deeper the root the higher the mountain. This state or balance between different parts of the earth’s crust is called isostasy where equal mass underlies equal surface area. This concept helps to explain the existence of different topographic heights on the earth’s surface.

The theory further states that any disturbance or restoration in the earth’s crust will cause a sinking and uplifting movement of the Earth in order to balance itself. The disturbance can either be erosion or melting of the accumulated snow while restoration can be deposition or accumulation of large snow or a volcanic hill. The theory therefore describes the state of balance achieved between erosion and deposition.

Analogy of Isostasy theory

Analogy of Isostasy theory may be made with things like; floated icebergs, floated pieces of wood, and loaded and unloaded ships.

(a) Floated iceberg

An iceberg or ice mountain is a large piece of freshwater ice that has broken off a glacier or an ice shelf and it is floating freely in open water. It may subsequently become frozen into pack ice (see “crust of sea ice”). An iceberg always floats with a certain proportion of its mass below water surface.

The portion below water surface is compensation portion (Figure 1.15). If a layer of ice is somehow sliced off on the top of the iceberg, the remaining iceberg will rise. Similarly, the earth’s lithosphere floats on the asthenosphere the same as the iceberg floats on water.

Floated pieces of wood

The idea of isostasy may be grasped by considering a series of wooden blocks of different heights floating in water. If such woods are immersed in water, the longer one will sink down more than the short one (Figure 1.16). Therefore, when wooden blocks are submerged, the height of the submerged part of the pieces is proportional to their respective emerged parts.

Loading and unloading ships

Floating ships show how isostasy affects the behavior of the crust as it floats on the heavy mantle rock. For instance, when cargo is transferred from ship “A” to ship “B”, ship “A” floats higher in the water and ship “B” floats lower. This happens as a response to the process of loading and unloading of cargo (Figure 1.17).

The Earth maintains its balance in the same way as ships. The process of loading and unloading of the eroded materials of the earth’s surface disturbs the balance on the Earth, therefore, the process of compensation takes place to restore it. The restoration process is extremely slow as it happens over thousands of years.

Isostatic movements and resulting landforms

There are two main movements of isostasy namely; vertical and horizontal movements. Vertical movement is also known as the movement of weight while horizontal movement is referred to as the movement of heat accumulation.

Vertical movement

It involves upward and downward movements of materials which enhance isostatic readjustment. The process is slow and takes thousands of years in maintaining the balance between oceanic mass and continental rocks.

The existence of vertical movements can be indicated in various parts of the world, especially in the poles in connection to the formation of ice (ice age) and its melting. Therefore, it is evidenced that, when the great ice caps are formed, the weight of ice causes the sinking or sagging of the crust, and when the ice melts, recovery gradually takes place, thus producing considerable changes in the level of the land relative to that of the sea.

Examples of vertical movement can be evidenced in different parts of the world including, the continental shelf of Antarctica which is deeper than others as its water depth is about 750 metres compared with 180 metres around other continents. This may be a result of the weight of the present ice sheet.

Another example is the ice cap of Greenland whose landscape is covered by enormous weight of ice. Unless otherwise, the surface of underlying plateaus would be 1100 metres higher.

Furthermore, changes in weather and climate in northern Europe caused disappearance of ice sheets which caused a raised land and formed a beach in Scandinavia.

As the weight of continental rocks increased due to restoration, it resulted into extra weight which disrupted the balance in the area and finally caused the sinking.

Several reasons cause extra weight including volcanic eruption, deposition of sediments, accumulation of snow and ice, the decrease of continental rock weight as well as denudation and melting of ice.

Volcanic eruption

The outpouring of lava due to volcanic eruption that forms volcanic mountain/arch may cause extra weight, hence a sinking movement.

Deposition of sediment

When large amounts of sediments are deposited on a particular region, the enormous weight of the new sediments may cause the crust below to sink.

Accumulation of snow and ice

The formation of ice sheets and the falling of snow for instance in the northern hemisphere like Arctic, Alaska, Greenland, Siberia or Antarctica in the South Pole have caused the earth’s surface to sink.

The decrease in continental rock weight

If the weight of continental rocks is decreased because of erosion/denudation at the mountain and melting of ice in one area, it results in the uplifting of a given area.

Denudation

When large amounts of materials are eroded away from a region, the land may rise to compensate the lost height and weight. Therefore, as a mountain range is eroded away, the reduced range rebounds upwards to a certain extent (Figure 1.18 a, b and c).

(a) Mountain showing eroded high elevation

(b) Eroded mountain materials deposited on the base of sea or on the side of mountain peak

(c) Regained balance

Melting of ice

Isostatic post glacial rebound is observed in areas once covered by ice sheets which have now melted. Example are found around the Baltic-sea and Hudson Bay. As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels. In this way it is possible to find former sea cliffs and associated wave-cut platforms hundreds of metres above present day sea level. The rebound movements are slower such that the uplifting caused by the ending of the last glacial period continues.

Horizontal movement

This occurs when there is a horizontal flow of material from the depressed area to the uplifted area for adjustment. The adjustment involves a horizontal flow or convection current that involves accumulation of heat. It takes place in the simatic layer, especially in the asthenosphere where there is maximum plasticity (Figure 1.19). There is a continuous radioactive disintegration within the simatic layer specifically in the asthenosphere. The disintegration releases heat which may be accumulated and prevented by overlying more solid layer from escaping the earth’s surface through radiation. The heat, therefore, makes the sima mobile resulting to the sinking of the sialic continents, once periodic ocean transgression. On the other hand, if it happens that, the accumulated heat is degenerated due to various processes such as widespread volcanic activities, the sima becomes less fluidity and continents begin to rise. Therefore, it produces a regression of the sea, hence the exposure of former seafloor. These major transgressions and regressions in the Earth are affected by the warping and tilting of the sialic blocks.

The evidence of isostatic movement on earth’s surface

The evidences of isostatic movements can be seen in different parts of the world as explained below:

Presence of raised beach

The raised beach in Scandinavia which was covered by a vast ice sheet about 10000 years ago is still rebounding isostatically at the rate of up to 1 metre per century. The result is the formation of raised beaches which lie between 8–30 metres above present beaches. Similarly, the Ballyhillin beach in Ireland is rebounding isostatically to form raised beaches.

The evidence of coastal cities in Scandinavia as old constructed dock, have been uplifted rapidly enough and the docks which were constructed several centuries ago are now far from the shore. The coastal areas are rebounding to maintain balance due to the areas covered by vast ice sheet during ice age. Moreover, in East Africa, this raised beach was witnessed through past studies. For example, studies conducted in the 1970’s marked some evidence of the raising shore line in Mombasa coastal area, in Kenya.

Depression formation

This is another evidence which means landform sunk below the surface area. Depressions may be formed by various mechanisms such as erosion-related. With this one, features like blowouts are created by wind erosion typically in either a partially vegetated sand dune ecosystem or dry soils. Moreover, area of subsidence caused by the collapse of an underlying structure such as sinkholes in karst terrain. In Greenland and Antarctica, the surface has been depressed below sea level by the weight of a glacial ice so as to maintain balance.

Land uplift

This is another evidence with which isostatic rebound has also occurred in different areas. For example in the eastern and central Canada and southern Ontario where the land has risen as much as 100 metres in the last 600 years. During the ice age, the weight of the glaciers depressed the surface of the land. The depressed land began to rise when the glacier melted because of the isostatic readjustment.

Deformity of continental shelf

An example of this evidence is the continental shelf around Antarctica, converted with water to a depth of about 500 feet (750 metres) compared with 600 feet (180 metres) around other continents. This may be a result of the weight of the present ice. The current Antarctic ice sheet acts as surface load causing deformation of the Earth. This explains why continental shelf of Antarctica is deeper than that of the entire Antarctica continent including the subsided continental shelf due to the presence of Antarctic ice sheet as a significant load on the earth’s surface compared to other continental shelf in the world. The Antarctic continent is almost isostatic equilibrium.

Existence of mountain ranges

Despite the continuous processes of erosion, several mountains have maintained their heights. For instance, Mt. Kilimanjaro has maintained its height of about 5895 metres above sea level for thousands of years.

Existence of water bodies

Despite the continuous deposition activity which accumulate and deposit millions of tons of materials, water bodies like Indian Ocean, Lake Victoria, and Lake Tanganyika have not disappeared. There is also, the emergence of emerged coast formed as a result of land uplift around the coast of Scandinavian countries.

Significance of Isostasy theory

Several implications of the Isostasy theory can be provided. However, the most significant ones are; the theory signifies the state of equilibrium in the earth’s crust with equal mass underlining equal surface area. Therefore, it gives an understanding on the earth’s crust dynamic state as it is influenced by internal and external forces. Also, the relationship which is made as the crust floats on mantle just like the iceberg on the ocean or seawater is important in the understanding of plate tectonic and continental drift theories. Precaution can be taken depending on the nature of the climatic condition of a given area especially in the occurrence of ice sheet and the melting of ice. Furthermore, it provides a basis for predicting the future of the crustal state at any place on the earth’s surface and understanding how different landforms are formed and the mechanisms involved in their formation.

The effects of isostatic movements

The isostatic movements causes several effects on the earth’s surface. The vertical and horizontal movements of isostasy result into occurrences of faulting and folding of the earth’s crust. Faulting and folding have a prolonged impact of causing rift valleys, mountains as well as volcanic eruptions with their associated features. Despite the geomorphological effects of the Isostasy theory, still it is criticized due to its failure to explain the formation of inland and undersea mountains or the presence of volcanic islands.