Introductory activity

    Observe carefully the maps provided below and answer the following questions:

    1. How many oceans do you find on map a

    2. How many continents do you see on map b

    3. How many continents do you see on map c

    4. Explain the processes which led to the separation of the unique initial landmass into various continents as they appear today.

    3.1. Concept and theories of continental drift

    Learning activity 3.1

    Make research using books and internet to explain briefly the theories related to the continental drift.

    3.1.1. Concept of continental drift

    The term continental drift refers to the study of causes and consequences of the distribution of continents and ocean basins. It is defined as a slow movement of the Earth’s continents towards and away from each other. The differential movement of the outer shell resulted into fragmentation by rifting, followed by drifting apart of individual masses of the broken outer shell.

    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 a breakup of the single super continent block called Pangaea“pan JEE uh”, which means “all land” into multiple continents, as they appear today, that moved apart in a process called continental drift. That movement took place about 200 million years ago. The map provided below fits together the continents whose breaking up resulted in today’s continents.

    The theory of continental drift traces the origin and distribution of continents through five major steps:

    i. The super continent Pangaea was surrounded by an extensive water mass called the ‘Panthalassa’ (pan means all and Thalassa means oceans) or the primeval Pacific Ocean. During the Carboniferous period (about 250 million years ago), the South Pole was near Natal (South African coast) and the North Pole was in the Pacific Ocean.

    ii. In about 200 million years, Pangaea broke up to form Laurasia (North America, Greenland, and all of Eurasia north of Indian subcontinent), and Gondwanaland (South America, Africa, Madagascar, India, Arabia, Malaysia, East Indies, Australia, and Antarctica). These two blocks were separated by a long shallow inland sea called Tethys Sea.

    iii. In about 145 million years ago, the drifting of the southern landmasses continued. India drifted northwards.

    iv. In about 65 million years ago, Australia began to separate from Antarctica.

    v. 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 (see figure 3.24). South America separated from Africa, North America separated from Europe, while Antarctica, Australia, India and Madagascar formed a single unit with South America.

    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.

    f. Maurice Ewing’s theory

    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’ floors.

    g. Harry Hammond Hess’s Theory: Sea-Floor Spreading

    The Seafloor spreading theory suggests that magma from earth’s mantle rises to the surface at mid-ocean ridges and cools to form new seafloor, which new magma pushes away from the ridge.

    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.

    The interior of the Earth is in a molten (semi-fluid) state because of great heat resulting from radioactivity within the asthenosphere. This tremendous heat causes melting, or near-melting of rocks of the interior of the Earth. The molten rocks tend to rise from within the mantle in form of convection currents.

    Material heated by radioactive elements in the earth’s interior slowly rise in the crust. This magma reaches the surface along the Mid-oceanic ridges and flows away from them, cooling and hardening to form the rigid lithosphere.

    New lava emerging from the ridges attaches itself to the near solidified older lava plates and forces them to move laterally. Hess’s studies demonstrated that after millions of years the lithospheric plates will have moved thousands of miles by constant additions of new lava at their rear.

    The leading edges were eventually forced to sink down into the lithosphere under the continental crust block thus forming deep ocean trenches along the edge of continents. In this “recycling” process, later named “seafloor spreading”, older sediments and fossils are carried off in the subduction zone, and continents are moved as new ocean crust spreads away from the ridges.

    Hess explained how the once-joined continents had separated into the seven that exist today. The newest rocks were in the center of the ocean, and were still being formed in Iceland, and that the oldest rocks were those nearest to the USA and the Caribbean. He also suggested that the Atlantic could be widening by up to 5 cm a year. This process produced by mantle convection currents was named the “Sea floor spreading”.

    h. Taylor’s theory

    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).

    The present basins of Southern Atlantic and Indian Oceans were formed in this way.

    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 SiluroDevonian times while explaining the possible formation of the fold mountains Atlas and Alps.

    Application activity 3.1

    1. Discuss the contribution of each researcher’s findings described in this section to the confirmation of continental drift.

    2. Referring to the different theories of continental drift, explain why Taylor’s theory about Moon’s tidal attraction has been rejected.

    3.2. Evidence of continental drift

    Learning activity 3.2

    Observe the map provided below and answer the following questions:

    1. Describe the edges of the continents.

    2. What suggests the distribution of the same animal and vegetation species over the different continents?

    Many evidences of continental drift exist, but they can be summarized in four major categories:

    i. Geological evidence

    A good fit of edges of continents and similar rock structures are found on different continents. For example:

    • East coast of South America and the Western Coast of Africa have good visual fits, both at the surface (1000 m) and depth (2000 m).

    • Both Africa and South America are composed of rocks of varying ages and there is a convincing boundary joint across the two continents between Accra and Sao Louis in Brazil and, dividing Pan-African rocks and Elaurean rocks. This evidence constitutes what is commonly known as “matching geology”

    • Parts of Appalachian Mountains of the United States of America are similar to those found in Greenland and Western Europe;

    • The fact that rock particles have magnetic properties allowed geophysicists to reconstruct the position of the poles in past times and also the probable climatic lay belts of the past. From this, it appears that Southern Africa and South America lay within the Arctic circle of Permian and carboniferous times and that during the Triassic period, the continents had moved some 40° closer to the Equator.

    ii. Biological evidence

    There is similarity in the fossils and vegetation remains found on the eastern coast of South America and the Western coast of Africa. For example;

    • Mesosaurus was small reptile living in Permian time (280 million of years before present); its remains have been found only in South Africa and Brazil.

    • Remains of Glossopteris, a plant which existed when coal was being formed has only been located in India and Antarctica. These animals and plants could not have swum across oceans if continents were separated by water bodies, so continents must have been close together for them to occur on different continents which probably had a similar climate.

    ii. Climatic evidence

    Coal formed under warm and wet conditions was found beneath the Atlantic ice-cap, and evidence of carboniferous glaciation had been noted in tropical and central India. For example;

    • Coal could not have been formed in Britain with its present climate.

    • Peninsular India, Australia and Antarctica further prove the unification of all landmasses in one landmass (Pangaea) during carboniferous period.

    • Groves curved on rocks by glaciers in the southern parts of landmasses forming Gondwanaland shown by arrows on the figure below provided evidence for continental drift.

    iv. Geodetic evidence

    Geodetic evidence has revealed that Greenland is drifting westward at the rate of 20 cm per year. This is one of the scientific evidences arising from measurement and representation of the earth that confirm the spread of the sea floor.

    Application activity 3.2

    1. Describe the rocks at the edge of the continents and show how all continents formed a unique block.

    2. Using some examples, compare the fossils of animal species and vegetation species found on different continents by showing how they indicate the continental drift.

    3.3. Effects of continental drift on the evolution of physical features

    Learning activity 3.3

    Make a research and describe at least four major effects of continental drift.

    The continental drift has had many effects on the evolution of physical features but the most important are the following:

    • Pangaea split apart into a southern landmass, Gondwanaland and the northern landmass called Laurasia; later the two super continents split again into land masses that look like present day continents.

    • Continental drift has also affected the earth’s climate. The climate of different part of the world has changes throughout the year;

    • Continental drift has affected the evolution of animals. The rearrangement and displacement of huge landmasses has helped create the diversity which we see present in modern day animals.

    • Collision of earth crusts. The collision of the Indian subcontinent and Asian continent created the Himalayan mountain range, home to the world’s high-est mountain peaks.

    • Formation of rift valleys. Rift valleys are sites where a continental landmass is ripping itself apart. Africa, for example, will eventually split along the western Great Rift Valley system.

    • Continental drift is the major cause of earthquakes, volcanoes, oceanic trench-es, mountain range formation, and other geologic phenomenon which created the new landscapes on the earth’s surface;

    Application activity 3.3

    Explain the effects of continental drift on the evolution of physical landscape of the earth.

    3.4. Plate Tectonics

    Learning activity 3.4

    Observe the illustration below and answer the following questions:

    1. Identify the types of crust found on the map

    2. Describe the difference between lithosphere and asthenosphere

    3. Differentiate collision, constructive, and destructive processes

    4. Determine the position of plate movements

    5. Explain how convection cells cause the movement of plates

    3.4.1. The concept of plate tectonics

    The concept suggests that earth’s crust and upper mantle (lithosphere) are broken into sections, called plates that slowly move on the mantle.

    The word tectonic comes from the Greek word ‘tektonikos’ meaning building or construction; this means how the earth crust is constructed. Therefore, plate tectonics refers to the deformation of the earth’s crust, because of internal forces, which can form various structures in the lithosphere.

    The plate size can vary greatly, from a few hundred to thousands of kilometers across. Plates are moved by the energy originating from the earth interior. This energy is a result of convection currents which form convection cells. Tectonic plates are irregularly shaped slabs of solid rocks, generally presenting two types: Continental crust and Oceanic crust as shown on the figure below.

    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.

    i. Continental crust is composed of older, lighter rock of granitic type: Silicon and Aluminum (SIAL).

    ii. 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

    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 land forms (folded mountains, volcanoes, insular arcs deep sea trenches, and batholith intrusion) are found at plate boundaries.

    Major land forms resulting from plate movements:

    3.4.4 Characteristics of plate tectonics

    Tectonic plates are characterized by the construction and destruction of land forms 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 land forms

    Constructive land forms 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 (A) as presented on figure below.

    • 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, floored by new oceanic crust (B) as presented on figure below.

    • 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.

    • The magma initially forms submarine volcanoes which may in time grow above sea-level. Volcanic islands are created by the submarine volcanism at the vertical of oceanic ridge, e.g. Iceland (see the figure below).

    ii. Destructive land forms

    Destructive land forms 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).

    iii. Passive or conservative margins

    Passive continental margins are:

    • 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 land form is constructed nor destroyed.

    Application activity 3.4

    1. Describe SIAL and SIMA in terms of thickness, age, weight and nature of rocks

    2. Explain the difference between convergent movement, divergent movement and way past movement

    3. Describe the subduction, collision, spreading processes and give their effects and corresponding motions in relation to plate tectonic movements.

    4. Explain the processes that lead to constructive and destructive land forms

    3.5. Major plates and effects of plate tectonics

    Learning activity 3.5

    1. Make research using books and a printed hand out and represent on the world map the major tectonic plates.

    2. Identify the effects of the plate tectonic?

    3.5.1. Major tectonic plates of the world

    The following are the major tectonic plates of the world:

    i. The Pacific plate which covers a large part of the basin of Pacific Ocean.

    ii. The Eurasian plate located between the northern mid-ocean ridge of the Pacific Ocean and the Pacific and Philippines Plates margins.

    iii. 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.

    v. 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.

    vi. The Indo-Australian plate extends around the Australian subcontinent, between the Pacific plate and the African Plate.

    vii. The Antarctic plate corresponds with the Antarctic continent around the South Pole.

    viii. 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

    The following are the main effects of plate tectonics:

    i. Earthquake

    This is a series of vibrations induced in the earth’s crust by the abrupt separation and echo of rocks in which elastic strain has been slowly accumulating. This sudden violent shaking of the ground typically causes great destruction, because of movements of seismic waves within the earth’s crust.

    Most earthquakes occur as the result of the sudden movement along a fault line between two adjacent tectonic plates. These have several impacts like landscape modification, destruction of houses, tsunamis, etc.

    ii. A volcanic eruption

    A volcanic eruption occurs when hot materials (molten materials) are thrown out of a volcano. Lava, rocks, dust, and gas compounds are some of these materials which are ejected out during volcanic eruption. Volcanic eruption take place when a plate moves over the top of another plate, then the energy and friction melt the rock and push it upwards.

    iii. Tsunamis

    Tsunamis are giant waves, often generated at destructive plate margins that can cross oceans. They occur when a sudden, large scale change in the area of an ocean bed leads to the displacement of a large volume of water and the subsequent formation of one or more huge waves.

    When a major seismic tremor occurs underneath a body of water, the energy from that tremor is released into the surrounding liquid. The energy spreads out from its original site, traveling through the water in the form of a wave.Tsunamis have exceptionally long wave-length up to 10 km and can cross oceans at speeds of up to 700 km/hour but can sometime be imperceptible when their magnitude is low.

    Application activity 3.5

    1. Conduct your own research to identify the minor tectonic plates of the world and locate them geographically.

    2. Apart from the distribution of the continent, what are other effects of plate tectonics.

    3. Identify the major seismic and volcanic zones in the world and explain the impact of those natural hazards referring to the tectonic plates.

    4. Our country, Rwanda, is in a region which is tectonic ally active and subjected to earthquakes events. The more documented earthquake is the one which occurred on 3rd and 4th February 2008. It occurred on Sunday about 09h31 with the magnitude of 6.1 and 5, and on Monday the 4th February 2008 and affected mostly Nyamasheke and Rusizi Districts, Western Province. 37 people died, and 643 injured including 367 traumatized. Many houses were destroyed in these two Districts where 1,201 families were rendered homeless:

    5. Knowing the causes of the earthquake, explain how Rwandans can cope with it and its impacts and other resulting natural hazards.

    3.6. The theory of Isostasy

    Learning activity 3.6

    1. Make research and explain the isostasy theory.

    2. Explain isostasy based on the figure below.

    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 following are the main characteristics 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.

    3.6.2. Main theories of Isostasy

    There are two main theories which have been developed to explain how Isostasy acts to support mountain masses

    i. 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.

    ii. 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.

    Both theories predict a relative deficiency of mass under high mountains. Airy’s theory is now known to be a better explanation of mountains within continental regions, whereas Pratt’s theory essentially explains the difference between continents and oceans, since the continent crust is largely of granitic composition which is less dense than the basaltic ocean basin.

    Application activity 3.6

    Referring to Pratt’s theory and Airy’s theory, explain the principle of Isostasy.

    End unit assessment

    1. What is the contribution of Wegner’s theory and others on the distribution of continents?

    2. Basing on the knowledge acquired in this unit, explain the relationship between the earthquakes which occur in the region of the western rift valley of Africa where Rwanda is located, with the continental drift.

    3. Using a map represent graphically the main tectonic plates of the world map.

    4. Discuss the consequences of the plate tectonics on population in some specific areas of the world.