S6 UNIT 3
Overview
There are several theories of continental drift that were developed at the beginning of the 20th century. The following are the four main theories of continental drift:
• Alfred Lothar Wegener’s theory
• Maurice Ewing’s theory
• Harry Hammond Hess’ theory
• Frank Taylor’s theory
THE ORIGIN AND DISTRIBUTION OF THE CONTINENTS
3.1. Concept and theories of continental drift
The term continental drift refers to the slow movement of the Earth’s continents(landmasses) towards and away from each other.
3.1. 2. Theories of the origin and distribution of the continents and ocean basins
There are several theories of continental drift that were developed at the beginning of the 20th century. The following are the four main theories of continental drift:
- Alfred Lothar Wegener’s theory
- Maurice Ewing’s theory
- Harry Hammond Hess’ theory
- Frank Taylor’s theory
a) Alfred Lothar Wegener’s theory
According to Wegener’s theory, there was single super continent block called Pangaea “pan JEE uh”, which means “all land” surrounded by an extensive water mass called pantalassa (pan means all and Thalassa means oceans), that moved apart in a process called continental drift. That movement took place about 200 million years ago. It has been hypothesized that the supercontinent of Pangea broke up to form Laurasia (North America, Greenland and all Eurasia, North of India subcontinent) and Gondwanaland (South America, Africa, Madagascar, India, Arabia, Malaysia, East Indies, Australia and Antarctica).
The present shapes and relative positions of the continents are the result of fragmentation of Laurasia and Gondwanaland by rifting and drifting apart of the broken landmasses following the formations of oceans and seas
These two blocks were separated by a long shallow inland sea called Tethys sea.
However, Wegener’s theory was initially criticized because he could not explain how solid continents have changed their positions. His theory has been revived by other researchers after discovering new evidences.
Maurice Ewing confirmed the existence of Mid-Atlantic Ridge which is a mountain range extending the entire length of the ocean bed which is about 1000 km wide and rises 2500 m in height. Also, Ewing’s studies argue that rocks of this range were volcanic and recent in origin. Similar ranges were later discovered on other oceans’floor.
C) Harry Hammond Hess’s Theory: Sea-Floor Spreading
The Sea-Floor Spreading theory was put forward by an American Geologist, Harry Hess. Sea-floor spreading occurs along mid-ocean-ridge; when the tectonic plates slowly moves away from each other, hot magma from the mantle comes up to the surface. As magma cools by the seawater the rock forms a new part of the crust.
d)Taylor’s theory(American) argued that originally there were two big landmasses, namely Laurasia and Gondwanaland.
Frank Taylor’s theory states that the original Laurasia was located near the current North Pole, whereas Gondwanaland was located near the South Pole. Both landmasses radially moved to the Equator. Their collision would have resulted in the formation of folded mountains, such as Atlas, Alps mountain ranges and others.
He suggested that Laurasia and Gondwanaland were forced to move from their former positions because of the moon’s tidal attraction. According to this theory, the moon came very close to the earth during the cretaceous period.
This closeness of the moon to the earth exerted powerful tidal attraction, which pulled the landmasses from their polar position towards the Equator. Where there was resistance to the outward spread of landmasses, the crust usually would fold, raising mountain ranges in front, while resulting in stretches (troughs and basins).
He explained that Pangea later split up into two super continents known as Laurasia and Gondwanaland.
Taylor’s arguments about continental drift have however been criticized:
- The theory doesn’t clearly demonstrate how the causes of the movement of continents from their polar positions ought to have been from within the earth and not outside it.
- The theory was rejected because researchers of his time doubted how the moon could ever exert enough force to pull the huge landmasses (continents) as they are known today.
- Finally, Taylor doesn’t explain the formation of earlier fold mountains like the Caledonian system of Siluro-Devonian times while explaining the possible formation of the fold mountains Atlas and Alps.
3.2 The evidence of continental drift
The following are Wegner’s evidence (indicators) of continental drifting:
- The jigsaw fit (Visual fitting) of the southern continents: Observation shows that when Africa and South America are assembled together, a nice and perfect fitting would occur. The west coast of Africa and Eastern coast of south America fit exactly each other.
- Similarities of flora (Vegetation) and fauna (animals): Studies have shown that vegetation types and animal species on both the west African and North west American coast are the same.
- Positions of climatic zones: It has been observed through time that the positions of climatic zones have fundamentally changed. For example, the presence of temperate climatic features such as glaciated highlands in south America and Africa, conflicts with their location in the tropics where climate is purely tropical.
- Also the presence of tropical lateritic soils in North America, Britain, China and Germany, contradicts the location of these countries in the temperate regions whose climate does not favour the formation of laterites.
America, Europe and China were enjoying a tropical climate at the equator, which encouraged laterites to form.
- The formation of the Mid Atlantic Ridge: The continuous accumulation of new volcanic rock materials at the mid of the Atlantic and Pacific oceans indicate that Africa and South America are drifting apart, molten rock fills the trench between them hence forming a ridge in the middle of the ocean.
- The widening Eastern Arm of the East African Rift Valley: Research has shown that the eastern arm of the East African Rift Valley is widening and that Somalia and Arabia are consequently separating at the rate of 2cm per year implying that continents are still drifting.
- Geological evidence: Similarities have been noted to exist in the rock structures of Africa, South America and the Indian subcontinent. The strata(layers) of rocks along the coast of South America are found to be similar.
- The existence of glacial erosional features: There are hanging valleys and truncated spurs in the tropics and low lying areas of Africa, all suggest that Africa must have been nearer to the southern pole during glaciations.
3.3 Effects of the continental drift on the evolution of the physical features
- Pangaea split apart into a southern landmass “GONDWANLAND” and the northern landmass called “LAURASIA”, later the two super continents split again into landmasses that are present day continents.
- It has also affected the earth’s climate. The climate of different part of the world has changes throughout the year.
- Collision of earth’s crusts the Indian subcontinent and Asian continent created the Himalayan mountain range.
- Formation of rift valleys
- Continental drift is the major cause of earthquakes, volcanoes, oceanic trenches, mountain range formation and other geologic phenomenon which created the new landscapes on the earth’s surface.
3.4.1. The concept of tectonics plate
The concept suggests that earth’s crust and upper mantle (lithosphere) are broken into sections, called plates that are slowly move on the mantle.
The upper surface of the earth’s crust (SIAL) is made up six major and twenty other minor blocks called tectonic plates. The whole mechanism of the evolution, nature and motion of plates and resultant reactions is called Plate tectonics.
The word tectonic comes from the Greek word “TECTONIKOS” meaning building or construction. Used in this context it refers to the deformation of earth’s crust as a result of internal forces resulting in various structures in the lithosphere.
Tectonic processes include
Tension when plates diverge and
Compression when plates converge. These processes result in deformation of the earth crust.
Tension causes fracturing and faulting of the crust while compression produces folds and over thrust faults.
3.4.2. Types of Plate Tectonics
There are two types of plate tectonics: continental plate and oceanic plate.
- Continental crust is composed of older, lighter rock of granitic type: Silicon and Aluminum (SIAL).
- Oceanic crust consists of much younger, denser rock of basaltic composition: Silicon and Magnesium (SIMA). The major differences between the two types of plates are summarized in the table below: Difference between continental plate and oceanic plate
Factor | Continental plate (SIAL) | Oceanic plate (SIMA) |
Thickness of rock | 35-40 km on average, reaching 60-70 km under mountain chains | 6-10 km on average |
Age of rocks | Very old, | Very young, |
Weight of rocks | Lighter, with an average density of 2.6gm/cc | Heavier, with an average density of 3.0gm/cc |
Nature of rocks | Light in color, many contain silica and aluminum; numerous types, granite is the most common | Dark in color; many contain silica and magnesium; few types, mainly basalt |
3.4.3. Boundaries and movement of tectonic plates
i. Tectonic Plate boundaries
Boundaries of plate tectonic include the subduction zone, the mid-ocean ridge and the transform boundary.
- Divergent boundary (Mid-ocean ridge): It is an underwater mountain range which is formed when forces within earth spread the seafloor apart. It is created when convection currents rise in the mantle beneath where two tectonic plates meet at a divergent boundary, thus forming the oceanic ridge.
- Transform boundary (Transform fault): It is a boundary which exists between two plates that are sliding horizontally past one another, thus forming the transform faults (see the figure below).
- Convergent boundary (Subduction zone): This is the area where an ocean floor plate collides with a continental plate and the denser oceanic plate sinks under the less dense continental plate, thus forming the oceanic trench.
ii. Tectonic plate movements
Plate movements include convergence, divergence and way past movement along the transform fault.
- Convergence is a movement whereby two crustal plates are colliding or one subsiding beneath the other. The margin where this process occurs is known as a destructive plate boundary. This boundary is a region of active deformation.
- Divergence is a movement whereby two crustal plates are moving away from each other. The margin where this process occurs is known as a constructive plate boundary. It initially produces rifts which eventually become rift valleys.
- Way past is plates’ movement predominantly horizontal, where crust is neither produced nor destroyed as the plates slide horizontally past each other.
The plate movements are characterized by the following:
- Due to its relatively low density, continental crust does not sink; but it is the oceanic crust which is denser that can sink. Oceanic crust is then formed and destroyed, continuously;
- Continental plates, such as the Eurasian plate, may consist of both continental and oceanic crust;
- Continental crust may extend far beyond the margins of the landmass;
- Plates cannot overlap. This means that either they must be pushed upwards on impact to form mountains, or one plate must be forced to downwards into the mantle;
- No gap may occur on the earth’s surface so, if two plates are moving apart new oceanic crust originating from the mantle is formed;
- The Earth is neither expanding nor shrinking in size. Thus, when the new oceanic crust is being formed in one place, older oceanic crust is being destroyed in another;
- Plate movement is slow and is usually continuous. Sudden movements are detected as earthquakes;
- Most significant landforms (folded mountains, volcanoes, insular arcs deep sea trenches, and batholith intrusion) are found at plate boundaries.
Major landforms resulting from plate movements:
Plate movement | Description of changes | Example of landform |
Divergent | Spreading: Two plates move away from each other, new oceanic crust appears, forming mid-oceanic ridges with volcanoes | Mid-Atlantic Ridge formed by American plates, moving away from Eurasian and African plates. |
Convergent | Subduction: Oceanic crust moves towards continental crust but, being denser, sinks and is destroyed to form deep sea trench and islands arcs with volcanoes, | Andes fold mountain chain formed by Nazca which sinks under South American Plate Rocky mountain chain formed by Juan de Fuca, sinks under North Americas Plate, Island arcs of the West Indies and Aleutians Examples of trenches: Mariana trench, PeruChile-trench (Pacific ocean), Puerto-Rico trernch in the Atlantic ocean. |
Convergent | Collision: two continental crust collide and, as neither can sink, are forced up into fold mountains | Himalayas formed by Indian plate collided with Eurasian Plate, Alp mountains formed by African Plate collided with Eurasian Plate, |
Transform | Lateral sliding: Two plates move sideways past each other. Land is neither formed nor destroyed | San Andreas fault in California |
3.4.4 Characteristics of plate tectonics
Tectonic plates are characterized by the construction and destruction of landforms at margins of plates. However, at some boundaries, the construction or destruction may not occur. These are called passive margins or conservative boundaries.
i. Constructive landforms
Constructive landforms occur where two plates diverge, or move away from each other, and a new crust is created at the boundary. They are formed in the following ways:
- This occurs when a continent ruptures and the two new plates move apart and create a new ocean.
- The crust is uplifted and stretched apart, causing it to break into blocks that become tilted on faults. Eventually a long narrow rift valley appears.
- Magma rises up from the mantle to continually fill the widening crack at the center (The magma solidifies to form new crust in the rift valley floor.
- Crustal blocks on either side slip down along a succession of steep faults, creating mountains.
- A narrow ocean is formed
- The ocean basin can continue to widen until a large ocean has been formed and the continents are widely separated.
- The ocean basin widens, while the passive continental margins subside and receive sediments from the continents.
- As the plates diverge, molten rock or magma rises from the mantle to fill any possible gaps between them, creating new oceanic crust
- Destructive landforms
Destructive landforms occur where continental and oceanic plates converge. They are formed in the following ways:
- The oceanic plate that is denser is forced to dip downwards at an angle to form a subduction zone with its associated deep-sea trench.
- The sunk plate will melt and transformed into magma as the pressure and the temperature rise.
- The newly created magma will try to rise to the earth’s surface. Where it does rich surface volcanoes will occur. This process will either create a long chain of fold mountains (e.g. the Andes) or, if the eruptions take place off shore, an Island arc will be created (e.g. Japan, Caribbean).
- Passive or conservative margins
- The areas which are lacking active plate boundaries at the contact of continental crust with oceanic crust.
- The transform faults which are large cracks produced at right-angles to the plate boundary because neither landform is constructed nor destroyed
3.5. Major plates and effects of plate tectonics
The following are the major tectonic plates of the world:
- The Pacific plate which covers a large part of the basin of Pacific Ocean.
- The Eurasian plate located between the northern mid-ocean ridge of the Pacific Ocean and the Pacific and Philippines Plates margins.
- The North American plate bordered by the eastern margin of the Pacific plate in the West and mid-ocean ridge of the Atlantic Ocean in the East. iv. The South American Plate located between the subduction zone of Nazca plate in the West and the mid-ocean ridge of the Atlantic Ocean in the East.
- The African plate located between the mid-ocean ridge of the Atlantic Ocean in the West and the mid-ocean ridge of Indo-Australian plate in the East.
- The Indo-Australian plate extends around the Australian subcontinent, between the Pacific plate and the African Plate.
- The Antarctic plate corresponds with the Antarctic continent around the South Pole.
- The Nazca Plate which is located between the Pacific plate and the South American plate.
However, several minor plates, about 20 have been identified (e.g. Arabian plate, Bismarck plate, Caribbean Plate, Carolina plate, Cocos plate, Juan de Fuca plate, Nazca or East Pacific plate, Philippines plate, Scotia plate among others).
3.5.2. Effects of plate tectonics
- Earthquakes
- Volcanic eruption and
- Tsunamis
3.6. The theory of Isostasy
3.6.1. Meaning of Isostasy
The concept of Isostasy comes from “iso” = equal, and “stasis” = equilibrium. It describes how various continental and oceanic crusts, stay in equilibrium over the asthenosphere.
The main characteristic of isostasy
- By isostasy, the lighter crust must float on the denser underlying mantle.
- It explains how different topographic heights can exist on the earth’s surface.
- Isostatic equilibrium is an ideal which states where the crust and mantle would settle in equilibrium in absence of disturbing forces.
- Isostasy theory is concerned with vertical movements of plates which depend on lithospheric masses.
- The loading of crust by ice or sediments may cause the subsidence of lithosphere, whereas the discharge resulting from ice melting or erosion may cause the uplift of lithospheric compartment.
- The waxing and waning of ice sheets erosion, sedimentation, and extrusive volcanism are examples of processes that perturb isostasy.
- Isostasy controls the regional elevations of continents and ocean floors in accordance with the densities of their underlying rocks.
Main theories of Isostasy
There are two main theories which have been developed to explain how Isostasy acts to support mountain masses.
Pratt’s theory: The theory stipulates that there are lateral changes in rock density across the lithosphere (crust). If the mantle below is uniformly dense, the less dense crustal blocks float higher to become mountains, whereas the denser blocks form basins and lowlands.
Airy’s theory: According to Airys’s theory, the rock density across the lithosphere is approximately the same but the crustal blocks have different thicknesses. Therefore, mountains that shoot up higher also extend deeper base into the denser material beneath.