Zoogeography – Plate tectonic and Continental drift theory

Continental Drift Theory

• The Continental Drift Theory was introduced by Alfred Wegener in 1915.
• Prior to Wegener, it was commonly believed that the continents were fixed and immovable.
• Wegener proposed that the Earth was once a single supercontinent called “Pangaea” surrounded by a large ocean called “Panthalassa.”
• Over time, Pangaea broke apart into smaller continents, namely Laurasia and Gondwanaland.
• According to Wegener, the continents are not fixed but instead float and drift on the Earth’s surface.
• Wegener’s theory was based on the observation of symmetrical coasts on the Atlantic Ocean’s sides.
• The movement of continents across the ocean bed is known as continental drift, a process that takes millions of years to complete.
• The concept of continental drift provided the foundation for the modern theory of plate tectonics.
• Plate tectonics explains the movement of Earth’s lithospheric plates, which includes the drifting of continents.
• Wegener’s theory of continental drift revolutionized the understanding of Earth’s geological history and the formation of continents and ocean basins.

Different Stages of Continental Drift Theory

• First Stage: During the Carboniferous period, a supercontinent called Pangea existed, surrounded by a mega-ocean called Panthalassa.
• Second Stage: Around 200 million years ago in the Jurassic period, Pangaea started to break apart. It divided into two major landmasses known as Laurasia (northern component) and Gondwanaland (southern component).
• Third Stage: The Tethys Sea gradually filled the space between Laurasia and Gondwanaland during the Mesozoic epoch. It expanded over time.
• Fourth Stage: Approximately 100 million years ago, North and South America began drifting westward, leading to the formation of the Atlantic Ocean. The movement resulted in the formation of the Rocky Mountains and the Andes.
• Orogenetic Stage: The fifth stage is characterized by mountain-building activity. The Himalayas and the Alps were formed through the folding of sedimentary deposits in the Tethys Sea.

Different Forces Responsible for Continental Drift

• Factors Responsible for Continental Drift:
  1. Gravitational Forces: The combined action of gravitational forces played a role in the equatorward movement of continental drift. The planet’s shape, with a bulge at the equator, contributed to this force.
  2. Pole-Fleeing Force: The increase in centrifugal force from the poles towards the equator, known as the pole-fleeing force, contributed to the equatorward movement of continental drift.
  3. Buoyancy Force: The buoyancy force, related to the density of the continental crust, also influenced the movement of continents.
  4. Tidal Currents: Tidal currents caused by the rotation of the Earth played a role in the westward movement of continental drift.

Note: It is important to mention that these factors were later found to be insufficient to fully explain continent drifting, which led to criticism of Wegener’s theory.

Evidence Supporting Continental Drift Theory

Evidence Supporting Continental Drift Theory:

1. Matching of Continents (Jig-Saw-Fit):

• Shorelines of South America and Africa exhibit a similarity when facing each other.
• Africa, Madagascar, and India’s east coast can be fitted together.

2. Rocks of Same Age Across the Oceans:

• Radiometric dating techniques show correlation of rock formations across continents.
• Ancient rocks off the coast of Brazil (2,000 million years old) correspond to mountain ranges in Western Africa.
• Similarities between the Caledonian and Appalachian mountains.
• Early marine deposits along Africa’s and South America’s coastlines date back to the Jurassic period, suggesting a time before the existence of the ocean.

3. Tillite:

• Tillite, a sedimentary rock formed by glacier deposits, found in the Gondwana system of sediments from India.
• Parallels between the Gondwana system and landmasses in the Southern Hemisphere, including Africa, the Falkland Islands, Madagascar, Antarctica, Australia, and India.

4. Placer Deposits:

• Gold placer deposits found along the Ghana coast in West Africa, but no source rock in the vicinity.
• Presence of gold-bearing veins in Brazil, suggesting a source for Ghana’s gold reserves when the continents are laid side by side.

5. Distribution of Fossils:

• Identical species and animals found on both sides of the marine barrier.
• Example: Mesosaurus, a freshwater crocodile-like reptile, found only in Southern Africa and Eastern South America, dating back to 286-258 million years ago.

Criticisms of Continental Drift Theory

1. Insufficient Mechanisms:

• Buoyancy, tidal currents, and gravity were considered by Wegener, but deemed insufficient to move continents.

2. Movements in All Directions:

• Wegener favored westward or equatorial travel, but movements have been observed in various directions.

3. Inability to Explain Pre-Carboniferous Period:

• Wegener couldn’t explain why continental drift started in the Mesozoic era and not earlier, leaving a gap in understanding the pre-carboniferous period.

4. Neglecting Oceans:

• The theory didn’t take oceans into account, failing to explain their role in continental drift.

5. Unexplained Oceanic Features:

• Oceanic ridges and island arcs were not adequately explained by the theory.

6. Impossibility of Large-scale Motions:

• The rigidity of the Earth’s crust led critics to argue that large-scale motions, as proposed by Wegener, were impossible.

7. Lack of Convincing Mechanism for Displacement of Larger Masses:

• Wegener’s theories didn’t provide a convincing mechanism for supporting the displacement of larger masses during long voyages.

What is Tectonic Plate?

• A tectonic plate, also referred to as a lithospheric plate, is a massive and irregularly shaped slab of solid rock comprising both continental and oceanic lithosphere. These plates vary in size, ranging from a few hundred to thousands of kilometers in diameter. Among the largest plates are the Pacific Plate and the Antarctic Plate.
• The thickness of tectonic plates can vary significantly. Young oceanic lithosphere, for instance, can be less than 15 kilometers thick, while ancient continental lithosphere, such as the interior regions of North and South America, can exceed 200 kilometers in thickness. This discrepancy in thickness is due to the differences in composition and nature of the continental and oceanic crust.
• The continental crust is primarily composed of granitic rocks, which consist of relatively lightweight minerals like quartz and feldspar. In contrast, the oceanic crust is made up of basaltic rocks, which are denser and heavier. These variations in composition contribute to the differing plate thicknesses observed in nature.
• The study of plate motions within the lithosphere is known as plate tectonics. The term “tectonics” originates from the Greek word “tektonikos,” meaning “building or construction,” and signifies the deformation of the Earth’s crust caused by internal forces. Plate tectonics theory recognizes the existence of seven major and twenty minor types of lithospheric plates.
• These tectonic plates are in a continuous state of motion in relation to one another. It is important to note that it is not the continents themselves that move, as was once believed by Wegener, but rather the sections of the plates. Therefore, what actually moves is the plate, and this movement leads to various geological phenomena and processes such as earthquakes, volcanic activity, and the formation of mountains.

Prelude to the Theory of Plate Tectonics

The theory of plate tectonics, which revolutionized our understanding of the Earth’s geological processes, was developed based on several key developments. These significant advancements played a crucial role in formulating the concept of plate tectonics. Here are six pivotal factors that led to the emergence of this groundbreaking theory:

1. Development of mid-oceanic ridges and seafloor spreading: The exploration of the world’s oceans revealed the presence of vast underwater mountain ranges known as mid-oceanic ridges. It was observed that these ridges acted as zones where new oceanic crust was continuously formed through volcanic activity. This concept of seafloor spreading suggested that the oceanic crust was moving away from these ridges, leading to the notion of lithospheric plates.
2. Palaeomagnetism: Scientists discovered that certain rocks possessed a remnant magnetization aligned with the Earth’s magnetic field at the time of their formation. By studying the magnetic properties of rocks, it was observed that the Earth’s magnetic field had undergone reversals throughout history. This palaeomagnetic data provided evidence of the movement of continents and supported the concept of moving lithospheric plates.
3. Findings of the age of ocean floors: Through radiometric dating techniques, scientists determined that the age of oceanic crust increased with distance from mid-oceanic ridges. The rocks closer to the ridges were found to be much younger than those farther away. This observation further reinforced the idea of seafloor spreading and the lateral movement of tectonic plates.
4. Discoveries of island arcs and submarine trenches: Extensive surveys of the ocean floor unveiled the existence of island arcs, which are curved chains of volcanic islands, and submarine trenches, which are deep depressions in the ocean floor. These features were associated with the convergence of tectonic plates. The observation of such geological formations provided evidence for plate boundaries and their interactions.
5. Precise documentation of volcanoes and earthquakes: With improved monitoring and recording techniques, scientists were able to document the occurrence and distribution of volcanoes and earthquakes more accurately. By mapping these occurrences, specific regions susceptible to seismic activity and volcanic eruptions were identified. This data played a crucial role in understanding the boundaries and behavior of tectonic plates.
6. Identification of hotspots: Through geological and geophysical studies, scientists identified certain areas on Earth’s surface known as hotspots. Hotspots are stationary plumes of molten material originating from deep within the mantle. The observation of volcanic activity unrelated to plate boundaries helped to confirm the existence of plate movement. Additionally, the movement of lithospheric plates over these hotspots explained the formation of long volcanic chains, such as the Hawaiian Islands.

These six developments provided substantial evidence for the theory of plate tectonics. They allowed scientists to comprehend the dynamic nature of the Earth’s lithosphere, with the crustal plates constantly moving, interacting, and shaping the Earth’s surface through various geological phenomena.

Theory of Plate Tectonics

• The theory of plate tectonics provides an explanation for the large-scale movements of the Earth’s lithosphere, which encompasses the crust and the uppermost portion of the mantle. The term “plate” was first introduced by JT Wilson in 1965. While the theory was initially proposed by Harry H. Hess in 1962, it was further developed and scientifically explained by other notable thinkers such as Morgan, McKenzie, Parker, and Holmes.
• Considered the most comprehensive and intricate hypothesis regarding continental drift and the expansion of the seafloor, the theory of plate tectonics represents a significant advancement over Wegener’s earlier concept. According to this theory, the Earth’s crust is divided into numerous large and small fragments referred to as plates. These lithospheric plates are approximately 100 kilometers thick and rest upon the semi-molten asthenosphere.
• Tectonic plates come in a range of sizes, from small to substantial, and can be either continental (e.g., the Arabian Plate) or oceanic (e.g., the Pacific Plate), with some plates consisting of a combination of both continental and oceanic crust (e.g., the Indo-Australian Plate). Oceanic plates tend to be thinner and composed mostly of Simatic crust, while continental plates are denser and primarily made up of Sialic crust.
• The movement of these crustal plates, driven by convection currents in the mantle, leads to the formation of various landforms and the occurrence of geological phenomena. The upper mantle’s convection currents are believed to be responsible for the movement of the tectonic plates. Along the boundaries where these plates interact, phenomena such as seafloor spreading, volcanic eruptions, crustal deformation, mountain building, and continental drift take place. These processes shape the Earth’s surface and contribute to the dynamic nature of the planet.

The seven major plates are

The seven major plates identified in the theory of plate tectonics are:

1. North American plate: This plate includes the western Atlantic floor and is separated from the South American plate, encompassing the Caribbean islands.
2. South American plate: The South American plate includes the western Atlantic floor and is separated from the North American plate, including the Caribbean islands.
3. Pacific plate: The Pacific plate is the largest plate among the seven major plates.
4. Antarctica and surrounding oceanic plate: This plate consists of Antarctica and the surrounding oceanic crust.
5. Eurasian plate: The Eurasian plate encompasses Eurasia (Europe and Asia) along with the adjacent oceanic crust.
6. African plate: The African plate includes the eastern Atlantic floor.
7. India-Australia-New Zealand plate: This plate is composed of India, Australia, and New Zealand.

These major plates are fundamental components of the theory of plate tectonics and play a significant role in shaping the Earth’s surface through their interactions and movements.

Minor plates

Here are some of the minor plates identified in the theory of plate tectonics:

1. Caribbean Plate
2. Cocos Plate
3. Caroline Plate
4. Juan de Fuca Plate
5. Juan Fernandez micro Plate
6. Iranian Plate
7. South Sandwich Plate
8. Myanmar Plate
9. Anatolian Plate
10. Nazca Plate
11. Nubian Plate
12. Philippines Plate
13. Okhotsk Plate
14. Scotian Plate
15. Eastern micro Plate
16. Somalian Plate
17. Arabian Plate
18. Solomon Plate
19. Fiji Plate
20. Bismarck Plate

These minor plates, though smaller in size compared to the major plates, also contribute to the overall dynamics of plate tectonics and play a role in shaping the Earth’s geology through their interactions and movements.

Rate of Plate Movements

Here are the key points regarding the rate of plate movements:

1. Harry H. Hess’s explanations did not cover the features of ocean floors or the movement of continents.
2. Tectonic plates exhibit varying rates of movement, ranging from less than 6 feet per 100 years to 66 feet per 100 years (1.83–20.1 m/100 years).
3. It is important to note that the rates of plate movement might have been faster in the ancient past.
4. On average, a tectonic plate moves at a rate of 33 feet per 100 years (about 10 cm/year).
5. With this average rate, a tectonic plate can cover a distance of 62.5 miles (about 100 km) in 1 million years.

Understanding the rates at which tectonic plates move is essential for studying and predicting various geological phenomena, including the formation of landforms, earthquakes, and volcanic activity.

Types of Plate Boundaries

1. Divergent Plate Boundaries

1. Divergent plate boundaries occur when plates move away from each other consistently.
2. This type of interaction leads to the formation of mid-ocean ridges, such as the Mid-Atlantic Ridge.
3. Seafloor spreading occurs at divergent plate boundaries, where basaltic magma erupts and separates, forming new oceanic crust.
4. The East African Rift Valley is a significant landform resulting from the divergence of the African and Somali plates, making it a prominent geomorphological feature on the continent.
5. Divergent plate boundaries are often referred to as constructive margins because they are locations where new crust is formed. Volcanic landforms are common along these boundaries.
6. Earthquakes with shallow focal depths are prevalent along diverging margins.

2. Convergent Plate Boundaries:

1. Convergent plate boundaries occur when continental and oceanic plates collide.
2. The denser and thinner oceanic plate is forced beneath the less dense and thicker continental plate.
3. This process is known as “subduction,” where the oceanic plate is driven down into the mantle.
4. As the oceanic plate descends, it enters higher temperature environments.
5. The increase in temperature causes the materials in the subducting plate to approach their melting temperatures at a depth of around 100 miles (160 kilometers), leading to partial melting.
6. The Himalayan Mountain Range is an active example of a continental convergent plate boundary.
7. Approximately 55 million years ago, the collision between the Indian and Asian plates formed the Himalayas, which is the world’s largest mountain range.
8. The collision between the Indian and Eurasian plates is ongoing and continues to shape the Himalayan region.

3. Transform Plate Boundaries

1. Transform plate boundaries occur when two plates slide past each other horizontally, with no creation or destruction of landforms. Instead, the existing landforms are deformed.
2. Transform faults are planes of separation that commonly occur in the ocean and are often perpendicular to mid-ocean ridges.
3. One prominent example of a transform plate boundary on land is the San Andreas Fault, located on the western coast of the United States. This fault is a transcurrent edge where the Pacific Plate and the North American Plate slide past each other.
4. The San Andreas Fault is significant, particularly for its proximity to Silicon Valley, a major technological hub situated close to the faultline.

Evidences that supports the Plate Tectonic Theory

1. Paleomagnetism: The orientation of iron grains in earlier rocks indicates the existence of the South Pole, which was originally located between present-day Africa and Antarctica. This phenomenon, known as polar wandering, provides crucial evidence supporting plate tectonics.
2. Age of Rocks: The age of rocks provides compelling evidence for plate tectonics. Continents consist of older rocks, some dating back up to 3.5 billion years, while the youngest rocks are found on the ocean floor, with the oldest being around 75 million years old. As we approach oceanic ridges, we find progressively younger rocks, indicating the effective spreading of the seafloor along these ridges.
3. Gravitational Anomalies: Gravity measurements reveal variations in gravitational forces along plate boundaries. Subduction zones, where one plate is forced beneath another, show weaker gravitational constants (g) due to material loss. For example, the presence of significant gravity anomalies along the oceanic trench bordering Indonesia supports the occurrence of subduction.
4. Earthquakes and Volcanoes: Plate boundaries are known for their association with earthquakes and volcanic eruptions. The occurrence of seismic activity and volcanic events predominantly along plate boundaries provides further evidence for the theory of plate tectonics.

These evidences, such as paleomagnetism, the age distribution of rocks, gravitational anomalies, and the occurrence of earthquakes and volcanoes along plate boundaries, collectively support the fundamental principles of plate tectonics. They provide crucial insights into the dynamic nature of the Earth’s lithosphere and the interactions between tectonic plates.

Significance of Plate Tectonics

1. Formation of Landforms: Plate tectonics is responsible for the creation of almost all significant landforms on Earth. Processes such as volcanic activity, mountain building, and the formation of oceanic trenches and ridges are driven by the movements and interactions of tectonic plates.
2. Mineral Resources: Plate boundaries are often associated with magmatic activity and the formation of economically valuable minerals. Near plate borders, minerals such as copper and uranium can be discovered, providing important resources for various industries.
3. Geological Predictions: The understanding of plate tectonics allows scientists to make projections about the future shape and configuration of landmasses. By studying current knowledge of crustal plate movement, it becomes possible to anticipate changes in the Earth’s geography over time.
4. Future Landmass Configurations: Based on the current trends of plate movements, it is predicted that significant changes will occur in the configuration of landmasses. For example, it is anticipated that North and South America will eventually separate, the east coast of Africa will be divided by a narrow strip of land, and Australia will become more closely connected with Asia.

Understanding plate tectonics is essential for studying and predicting geological phenomena, identifying valuable mineral resources, and gaining insights into the dynamic nature of the Earth’s surface. It provides a framework for comprehending the processes that have shaped and continue to shape our planet.

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