THEORY OF CONTINENTAL DRIFT
The theory of continental drift was propounded by Wegner (1912 1924) According to him the whole land- mass of the world was a single super continent during Palaeozoic era. He named it as Pangaea. That super continent was surrounded by sea on all the sides which was named Panthalassa. During Mesozoic, Pangaea split up into two large landmasses: Laurasia in the north and Gondwanaland in south.
The two landmasses were separated by Tethys Sea. Du Toit (1937), however, suggested that Laurasia and Gondwanaland existed from the very beginning. The two large landmasses having characteristic flora and fauna broke up into new landmasses called continents. Laurasia gave rise to Eurasia, Greenland and North America and similarly Gondwanaland gave rise to South America, Africa, India, and Polynesia, Australia Antarctica etc.
About 135 million years ago reorientation of continents began. The continents were drifted apart by the oceans. This is called Continental Drift. The occurrence of Dinosaurs and many fossil plants lend support to the existence of Laurasia and Gondwanaland. With the separation of continents, the distribution areas of several plant and animal species got separated and gave rise to discontinuous distribution areas.
Continental drift theory was proposed by Alfred Wegener in 1912.
• It was first put forward by Abraham Ortelius in 1596 before fully being developed by Alfred Wegener.
• The theory deals with the distribution of the oceans and the continents.
• According to Wegener’s Continental Drift theory, all the continents were one single continental mass (called a Super Continent) – Pangaea and a Mega Ocean surrounded this supercontinent.
The mega ocean is known by the name Panthalassa.
• Although Wegener’s initial theory did not cover mantle convection until Arthur Holmes later proposed the theory.
• The supercontinent was named Pangaea (Pangea) and the Mega- Ocean was called Panthalassa.
• According to this theory, the supercontinent, Pangaea, began to split some two hundred million years back.
• Pangaea first split into 2 big continental masses known as Gondwanaland and Laurasia forming the southern and northern modules respectively.
• Later, Gondwanaland and Laurasia continued to break into several smaller continents that exist today.
Evidence supporting the Continental Drift Theory
1. The Matching of Continents (Jig-Saw-Fit)
• The coastlines of South America and Africa fronting each other have a remarkable and unique match.
• In 1964, Bullard created a map using a computer program to find the right fit of the Atlantic margin and it proved to be quiet.
2. Rocks of the Same Age across the Oceans
• The radiometric dating methods have helped in correlating the formation of rocks present in different continents across the ocean.
• The ancient rocks belts on the coast of Brazil match with those found in Western Africa.
• The old marine deposits found in the coasts of South America and Africa belong to the Jurassic Age. This implies that the ocean never existed before that time.
3. Tillite
• It is the sedimentary rock made from glacier deposits.
• The Gondwana system of sediments from India is recognized as having its counterparts in 6 different landmasses in the Southern Hemisphere.
• Counterparts of this series are found in Madagascar, Africa, Antarctica, Falkland Island, and Australia not to mention India.
• At the base, the system has thick tillite signifying widespread and sustained glaciation.
• Generally, the similarity of the Gondwana type sediments shows that these landmasses had exceptionally similar origins.
• The glacial tillite gives clear evidence for paleoclimates and the drifting of continents.
4. Placer Deposits
• The presence of abundant placer deposits of gold along the Ghana coast and the complete lack of its source rocks in the area is a phenomenal fact.
• The gold-bearing veins are present in Brazil, and it is evident that the gold deposits of Ghana in Africa are obtained from the Brazil plateau from the time when the two continents were beside each other.
• The widespread distribution of Perma-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica, and Australia was one of the major pieces of evidence for the theory of continental drift.
• The continuity of glaciers, inferred from oriented glacial striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift.
5. Distribution of Fossils
• The interpretations that Lemurs occur in India, Africa, and Madagascar led to the theory of a landmass named “Lemuria” connecting these 3 landmasses.
• Mesosaurus was a tiny reptile adapted to shallow brackish water.
• The skeletons of these creatures are found in the Traver formations of Brazil and Southern Cape Province of South Africa.
Forces for Continental Drift
• Wegener proposed that the movement accountable for the drifting of the continents was instigated by tidal force and pole-fleeing force.
• The polar-fleeing force relates to the rotation of the earth.
• The shape of the earth
• The second force that was proposed by Wegener, the tidal force.
• Though, most of the scholars considered these forces to be insufficient.
Drawbacks of Continental Drift Theory
• Wegener failed to explain why the drift began only in Mesozoic era and not before.
• The theory doesn’t consider oceans.
• Proofs heavily depend on assumptions that are generalist.
• Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents.
• Modern theories (Plate Tectonics) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift.
THEORY OF TOLERANCE
Range of distribution of plants is limited by their tolerances. Each plant species has a range of climatic and edaphic conditions. Therefore, tolerance of a large taxon is the sum of tolerances of its constituent species.
Tolerances have a Genetic basis. The response of plants to environment is governed by their genetic
makeup. Many of the crops through breeding and genetic changes have been made to grow in wider range of environmental conditions. In nature, hybrid plants have been found to have wider range of tolerances than their parents.
Different ontogenetic phases have different tolerances. Different developmental stages of plants show different degree of tolerances, as for example seeds and mature plants are more tolerant to temperature and moisture variations than their seedlings.
Condition is abiotic environmental factor that varies in space and time and affects the performance of organism. Example is temperature can affect the reproduction of organism because to matting and lay the egg, need a warm temperature.
Resource is all things consumed (used up) by organisms (space, nutrients, water, prey, holes for refuge, etc.) and it can divide into
Interactive resources. An organism does not need certain resources independently. For example, one
that gets more heat might need more water, or less nitrogen.
non-interactive resources. Independent resources that an organism needs. For example, a human being needs both iron and calcium, though ingesting one will not affect the need for the other.
Liebig’s law of the minimum: Growth of a plant is dependent on the amount of foodstuff which is
1840. This law of the minimum is less applicable under ‘transient state’ conditions when the amounts, and hence the effects, of many constituents are rapidly changing.
Shelfords’s law of tolerance:
The existence, abundance, and distribution of a species in an ecosystem are determined by whether the levels of one or more physical or chemical factors fall within the range tolerated by that species. In other words, there are minimum and maximum limits for physical conditions (such as temperature) and concentrations of chemical substances-called tolerance limits-beyond which no members of a particular species can survive.
The degree of tolerance is (1) Steno is that has narrow range and (2) Eury is that has a comprehensive range.
Limiting Factors are factor that is most deficient determine the presence or absence of any given
organism. The presence and success of an organism or a group of organisms depends upon a complex of conditions. Any condition which approaches or exceeds the limits of tolerance is said to be a limiting condition or a limiting factor.
References
• www.britinica.com
• www.biologydiscussion.com
• www.livescience.com
[The information, including the figures, will be used solely for academic purpose.]