Paleogeography

Palaeogeography or paleogeography is the study of historical geography, generally physical landscapes (visible features of an area of land). This is a brief overview of four famous paleogeographic theories: the permanence of continents, land bridges, continental drift, and plate tectonics.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)

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Paleogeography
“Palaeogeography/Paleogeography is the study of historical geography,
generally physical landscapes (visible features of an area of land).”
Four major theories are explaining the present configuration of continents with reference to
the past in paleogeography, they are as follows:
Permanence of Continents
“The theory of permanence of continents proposes that the continents are
fixed portions of the Earth’s crust.”
A geologist named Dr. Lyell proposed in 1830-1833 that the continents had the same position
in the past as they have in the present and there have been no changes in the oceanic bodies.
He stated that the features of the continents have changed over time due to physical forces
that caused the land to rise or become submerged in water but their positions never changed.
Mountain ranges were created and destroyed and the ocean invaded some low lying parts of
the land but left others dry (because they were high). The major drawback of this theory is
that it does not explain the distribution of similar animals on different continents which are
far apart from each other (and may also have barriers in between).
Theory of Land Bridges
“The theory of land bridges proposes that land-masses of continental size
connected continents in the past, allowing animals to migrate from one
continent to the other. These land bridges are hypothesized to have been
wiped out without trace after the Cenozoic era."
To explain the distribution of animals in the past, many zoogeographers proposed that land
bridges were present between the continents. This theory demands the presence of bridges
of continental size spreading under deep water, serving as a means of communication
between continents. These land bridges were present in the Cenozoic era leaving behind no
trace of their existence. This theory has now become obsolete and has been replaced by
continental drift theory and plate tectonics theory. Numerous land bridges have been
proposed by zoogeographers, some of them are as follows:
Atlantic Land Bridge: This land bridge was hypothesized to have been present between
Africa and South America. This explains the distribution of lungfish, running birds, porcupines,
etc. Although the species may be different (due to different ecological niche), yet these
animals belong to the same groups. However, evidence indicates that such a land bridge is

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unlikely to have ever existed. Moreover, in South America, Edentata is an endemic order of
mammals which includes armadillos and sloths, this order is not present in Africa.
Lemuria Land Bridge: Lemuria land bridge was thought to have been present between
Madagascar and India. It was hypothesized to explain the distribution of lemurs (arboreal and
nocturnal primates). The representatives of lemurs are present in the Oriental and Ethiopian
regions. Lorises are lemur-like animals present in the Oriental region, but the original lemurs
were left only in Madagascar. Lemuria land bridge was hypothetically destroyed in the sea,
leaving lorises behind in India. Many zoologists suggest that Madagascar separated from the
rest of the world during the Cenozoic era due to some drastic climatic and environmental
conditions, hence the Lemuria land bridge is unlikely to have ever existed.
Antarctica Land Bridge: It was thought to be present between Australia and South America.
Evidence in support of this idea is the presence of marsupials, lungfish, running birds, and tree
frogs on both continents. Fossils and other geological evidence suggest that there was no such
land bridge.
Continental Drift Theory
“The theory of continental drift proposes that total land masses remain
unchanged but change their relative positions.”
This theory was proposed by a German meteorologist Alfred Wegener in 1912. He proposed
that continents are made up of lighter material known as Sial (Aluminium Silicate) and the
floor of the sea is made up of heavy material known as Sima (Magnesium Silicate). According
to this theory, the continents of the world were once united but they drifted apart:
 All the continents were connected in the Paleozoic era (544 million years ago) to form a
large land-mass known as Pangaea.
 This large land-mass was surrounded by a sea which is known as Panthalassa.
 At the end of the Triassic period, this large land-mass split into two supercontinents,
namely: Laurasia and Gondwana land. Laurasia included small continents of the Northern
Hemisphere: Europe, Greenland, and North America. Gondwana land was named after an
Indian tribe; it includes four large continents: Africa, India, South America, and Australia.
 These two supercontinents were separated by a sea known as the Tethys Sea.
 By the end of the Cretaceous period of the Mesozoic era, these continents separated into
land masses that look like present-day continents.
The separated parts of continents fit very much if they are put together. The drifting of the
continents started soon after the Carboniferous period and continues even today. Atlantic
Ocean was fully formed during the Eocene epoch of the Cenozoic era. It separates Africa and
South America.

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)

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Figure: Continental Drift Theory – series of events (top left to bottom right): 225 million
years ago (Mesozoic era) the land-masses were held together as a giant supercontinent
called Pangaea. 150 million years ago (by the end of the Triassic period), Pangaea divided
into two continents – Laurasia (Europe, Greenland and North America) and Gondwana
(Africa, India, South America, and Australia). Later on, the continents continued to drift
apart and formed the arrangement that we see today.
Evidence of Continental Drift: The supporters of this theory argue that:
 The fit of Continents: Separated continents fit together remarkably when put close together,
i.e. when India, South America, and Australia are grouped, they have an outline
complementary to that of Africa. This complementary arrangement of the facing sides of
South America and Africa is obvious but a temporary coincidence. In millions of years, slab
pull, ridge-push, and other forces of tectonophysics will further separate and rotate these
two continents. It was this feature that inspired Wegener to propose the continental drift
theory although he did not live to see his idea generally accepted.
 Identical Geological Structures: Many deposits of coal and plants are found in India, South
America, Australia, and Antarctica. The first study of these deposits was in district Orissa
occupied by a tribe Gond, hence the name Gondwana land was extended to all continents
having those deposits. This shows the similarity in past geological structures.
 Fossil Evidence: Similar plant and animal fossils are found around the shores of different
continents, suggesting that they were once joined. Many similar fern fossils are found in
India, Australia, South America, and Antarctica. In the case of animal fossils, the fossils of
Mesosaurus spp., a freshwater reptile rather like a small crocodile, found both in Brazil

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and South Africa, are one example; another is the discovery of fossils of the land reptile
Lystrosaurus spp. in rocks of the same age at locations in Africa, India, and Antarctica.

Figure: Distribution of similar fossils in different continents of the Gondwana land (as
explained by the continental drift theory).
 Distribution of Organisms: There is also living evidence, with the same animals being found
on two continents. Some earthworm families (such as Ocnerodrilidae, Acanthodrilidae,
Octochaetidae) are found in both South America and Africa. Similarly, marsupials
(pouched mammals) are found in the Australian region and the Americas (North, Central,
and South), which are separated by vast oceans.
 Variations of Climate in the Past: Many sedimentary rocks are found in Greenland having
fossils of corals and deposits of coal from plants. The climate of Greenland is very cold
today, while corals are found in a tropical climate. These rocks indicate that the climate
was tropical in the past.
Drawbacks: Although it is a very important and valid theory, it is criticized for several reasons:
 It does not explain the mechanism of continental drift.
 It does not explain the main source of energy.

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)

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 It does not explain the abundance of animals (mammals and reptiles) in Africa and their
absence in South America.
 It does not explain the fauna of South America and Africa in the Paleozoic era which seems
to be dissimilar.
 Continental fit is very strong evidence but some scientists state that the outlines of
continents are continuously modified by erosional processes, hence it is not a valid proof.
Plate Tectonics Theory
“The plate tectonics theory proposes that fragments of the lithosphere called
tectonic plates move over the asthenosphere and interact with each other.”
The surface of Earth is composed of lithospheric (outermost shell of Earth / crustal) plates
that have moved over the asthenosphere (which is underneath it) throughout the geological
time and have occupied the present-day position of continents.

Figure: Diagram of the internal layering of the Earth showing the lithosphere above the
asthenosphere.
“A plate is a fragment of lithosphere that can move over the surface of the
asthenosphere."
Plate tectonics theory explains the locomotion and interaction of these plates. It indicates the
location of volcanic activity, earthquakes, and mountain building. The seafloor is constantly
moving and spreading at the center and sinking at the edges. Convection currents, below the
surface of Earth, are responsible for the movement of these plates. The source of energy is
probably the radioactivity in the mantle. Lithospheric plates consist of continental crust and

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oceanic crust and the upper part of the mantle that moves over weaker plastic shell known
as the asthenosphere. The Continental crust is 32 km deep while the oceanic crust is 10 km
deep.
Types of Tectonic Plates: There are two types of tectonic plates; oceanic and continental.
Oceanic plates cover approximately 71 percent of Earth’s surface, while continental plates
cover 29 percent. Their details are as follows:
 Oceanic Plate: It is a tectonic plate at the bottom of the oceans. It is primarily made of rocks
rich in iron and magnesium. It is thinner than the continental crust, generally less than 10
kilometers thick; and denser. It is also younger than the continental crust. When they
collide, the oceanic plate moves underneath the continental plate because of its density.
As a result, it melts in the mantle and reforms. The oldest oceanic rocks are less than 200
million years old.
 Continental Plate: It is the thick part of the Earth's crust which forms the large land-masses.
They are mostly made up of rocks rich in silica, aluminum, sodium, and potassium.
Continental plates take a long time to form but are rarely destroyed. Its oldest rocks seem
to be 4 billion years old. Continents have a comparatively lower density.
Major Plates of the World: There are eight major plates, excluding several smaller plates:
 Eurasian plate
 Indian plate
 N. American plate
 Antarctic plate
 S. American plate
 African plate
 Pacific plate
 Australian plate

Figure: Tectonic plates of the world.
Plate Boundaries: Tectonic plates often interact with each other in multiple ways:

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“Plate boundary is the location where two plates meet.”
Plate boundaries are commonly associated with geological events such as earthquakes and
the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and
oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries,
with the Pacific Plate's Ring of Fire being the most active and widely known today. There are
three main types of boundaries:
Divergent Plate Margins / Boundaries (Constructive): When two plates move away from each
other, a divergent plate margin is formed. The place where this boundary occurs is called a
rift. Magma from the mantle (underneath) pushes up and cools off forming new land. These
margins make earthquakes and trenches. Examples include: mid-ocean ridges and Africa's
Great Rift Valley.

Figure: Divergent plate margin formation. Two plates move away from each other, forming
an oceanic ridge, where magma from underneath seeps out to form new crust.
Convergent Plate Margins / Boundaries (Destructive): This boundary is formed when plates move
towards each other. Sometimes one plate will move under the other. This is called
subduction. When an oceanic plate collides with a continental plate, the oceanic plate will
move underneath the continental because it is denser. Convergent boundaries can make
mountains and volcanoes, for example: the Andes mountain range in South America, the
Japanese island arc, and the Pacific Ring of Fire (volcano).

8. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)

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Figure: Convergent plat margin formation. The oceanic plate moves underneath the
continental plate to make an oceanic trench, as a result, mountains and volcanoes are
formed.
Transform Plate Margins: In this case, the two plates moving parallel to each other but in the
opposite direction, slip past each other (with friction). These margins cause earthquakes. The
San Andreas Fault in California is an example of a transform boundary.

Figure: Transform plate margin formation. Two plates, moving in the opposite direction, slip
past each other (with friction) and cause an earthquake, but no volcanoes are formed.