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UNIT 2 :THE ORIGIN AND DISTRIBUTION OF THE CONTINENTS
Key Unit competence:
By the end of this unit, I should be able to discuss theories of the origin and
the distribution of continents.
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.
2.1. Concept and theories of continental drift
Learning activity 2.1
• Make research using books and internet to explain the theory of
Alfred Wegener on the continental drift.
2.1.1. 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.
2.1. 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 and Frank Taylor’s theory.
The theory of the origin and distribution of the continents and ocean
basins according to Alfred Wegener
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 break-up of Pangaea and periods of disintegration
The theory of continental drift traces the origin and distribution of continents
through five major steps:
i) The supercontinent 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 above). 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.
Application activity 2.1
1) Explain the concept of continental drift
2) Explain why Taylor’s theory on the origin and distribution of the
continents and ocean basins was initially criticized.
2.2. Evidences of continental drift
Learning activity 2.2
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.
iii) 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 2.2
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.
2.3. Effects of continental drift on the evolution of physical
features
Learning activity 2.3
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 parts 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 highest 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
trenches, mountain range formation, and other geologic phenomenon
which created the new landscapes on the earth’s surface;
Application activity 2.3
Explain the effects of continental drift on the evolution of physical landscape
of the earth.
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
2.4.1. 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.
2.4.2. 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:
2.4.3. 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.
i) 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:
Application activity 2.4
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.
2.5. Major plates and effects of plate tectonics
Learning activity 2.5
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.
2.5.1. 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).
2.5.2. Effects of plate tectonicsThe 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 likelandscape 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 frictionmelt 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 sometimes be imperceptiblewhen their magnitude is low.
Application activity 2.5
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 continents, 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 tectonically 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:
Knowing the causes of the earthquake, explain how Rwandans cancope with it and its impacts and other resulting natural hazards.
Skills Lab
Basing on the distribution of continents and oceans basins, discuss thegeological evidences of continental drift.
End unit assessment
1) What is the contribution of Wegner’s theory 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 insome specific areas of the world.